PART 11: NOTES ON SOME
DIFFICULTIES IN CALIBRATING THE AIRSPEED PROBE, AND NOTES ON SOME DIFFICULTIES IN INTERFACING WITH THE GARMIN ETREX GPS
APPENDIX 1: About the tests
APPENDIX 2: Related articles
PART 1: INTRODUCTION
This article is the result of a project to better
understand the functioning of my Brauniger IQ Comp GPS variometer. I've found
that many functions of the vario are not fully explained in the standard
Brauniger manual (specifically, the English version, 2003 edition), and some
sections of the manual contain errors. This article is intended to describe, in
detail, all the vario functions that are not fully and accurately explained in the manual.
This article will also explore the advantages and disadvantages of using the
vario's goal-related features such as the approach altimeter in various
real-world situations. Despite the title of this article, it is not really
intended to serve as a replacement for the standard Brauniger manual for this
vario: many features that are adequately explained in the manual are not
explored here. Readers will need to already have some familiarity with the
vario, or have a copy of the standard Brauniger manual on hand, in order to
fully benefit from these additional notes. The Brauniger manual for the IQ Comp GPS variometer may be
downloaded from Brauniger's website at www.brauniger.com.
This article is based entirely on a series of careful in-flight experiments
designed to explore the various features of the Brauniger IQ Comp GPS vario.
This article is not based on input from Brauniger staff or any other person.
It's likely that a few of the more unusual features detailed here represent
inadvertent software bugs rather than intentional design features.
Our goal here is to be as complete
as possible. We haven't made any effort
to cull out the more esoteric bits of information, so the article is a long
one. Each major part of the article is designed to be somewhat
self-sufficient, so there is a fair amount of repetition of certain key ideas
from one section to the next. Readers
are encouraged to skim quickly through the entire article before choosing areas
of special relevance for extra attention.
Part Two ("One pilot's
preferred settings") is intended to help new users to quickly "get
off the ground" without going through all the different vario functions in
detail. This part covers my preferred
settings for general soaring, and my preferred settings for collecting polar
data and other experimental situations.
Readers seeking a deeper understanding will probably
get the greatest benefit from this article if they first download the standard Brauniger manual for this
vario, and then read Part Three of this article which is a list of comments
that are keyed directly to the Brauniger manual (English version, 2003
edition). This will shed some light on
the vario features that are incompletely or inaccurately covered in the
Brauniger manual, and provide a good foundation for understanding the more
detailed discussions in the later parts of this article.
Part Four, entitled "A simple way to use the Brauniger IQ Comp GPS vario, describes the rationale for the settings given in Part Two.
Part Five is the "philosophical" section
of this article. It compares the
advantages of using the vario's digital current-glide-ratio display with
the advantages of using the vario's approach altimeter display, in various
real-world situations. These displays
are never both available at the same time. Part 5 also gives an overview of all the glide-ratio-related functions and displays of the vario.
Parts Six and Seven of this article explores many of the vario functions in great detail. Readers with a
specific question about a specific vario function will likely be able to find
the answer here. Many of the features function differently when the vario is
connected only to a GPS, when the vario is connected to an airspeed probe as
well as a GPS, and when the vario is connected only to an airspeed probe, so
separate descriptions are given for each case. On the first pass through,
readers may want to focus only on the sections of Parts Six and Seven that specifically
address the way that they plan to use the vario in flight. There is a lot of repetition in Parts Six and Seven--they aren't meant to be read straight through, but rather are meant to help the reader find the answer to a given question as efficiently as possible
Parts Eight, Nine, and Ten are new sections introduced in summer 2006, and are meant to save the reader some aggravation by conveying some lessons that I had to learn "the hard way"! The titles of these sections should be self-explanatory: "Tips on avoiding accidentally overwriting barograph data", "Notes on downloading data to PC Graph version 1.5", and "More on PC Graph version 1.5, with notes on backing up data".
Part Eleven of this article
describes some difficulties I encountered in calibrating the airspeed probe and
interfacing the vario with a Garmin Etrex GPS. Appendix 1 describes the experiments I performed to explore the various
functions of the vario, and Appendix 2 lists some related articles on this
website.
Although this article uses some terminology that is specific to hang
gliding--for example, we'll occasionally say that the pilot "pulls in the
bar" when we mean that he increases the airspeed--all of the content is
equally applicable to paragliding. We'll give special attention to exploring
how the vario works when the user chooses to fly without with an airspeed
probe; those sections of this article may be more interesting to paraglider
pilots than to hang glider pilots.
Note: this article makes frequent references to the "digital
current-glide-ratio" and "digital glide-ratio-to-target"
displays, which share the same display window as the digital time-averaged
vertical speed or netto display. These digital glide ratio functions are not
present in older software versions for the Brauniger IQ Comp GPS vario, and so
some readers may find these functions absent from their own varios. Any IQ Comp
GPS can be upgraded by the factory to the newest software. My own vario was
upgraded in summer 2003 to a software version which included these digital
glide ratio functions, and also included the new "A3" cumulative
climb altimeter, and also included the capability to interface with the Garmin
Etrex series of GPS's. To see if your own Brauniger IQ Comp GPS has this newer
software version with the digital glide ratio displays, go into the
"memo" mode and then see if you can scroll to a display labeled
"A3" by pushing several times on the button labeled "A1 A2
SF". If you cannot scroll to the "A3" display when you are in
"memo" mode, then you have an older software version and should
disregard the many portions of this article that refer to the vario's
"digital current-glide-ratio" and "digital
glide-ratio-to-target" displays. You will find that your older software
version will not work with some GPS's, such as the Garmin Etrex series.
Users may wonder whether some of the descriptions in this article also apply
to newer variometers such as the Galileo or Compeo. I can't shed any light on
this at present, as I'm not familiar with these other varios. A careful reading
of this article may provide some food for thought to anyone planning a thorough
and systematic evaluation of these newer varios.
Please feel free to contact me if you have further insights into the
workings of the Brauniger IQ Comp GPS variometer, or if you are interested in
learning more about how the tests of the different vario functions were carried
out.
A note on printing this
article: as with many of the articles on this website, entering a url with suffix
of "html" rather than "shtml", or adding the suffix of "html" if no suffix is present, will omit the background
graphics and site navigation tabs and provide a more compact text layout.
PART 2: ONE PILOT'S PREFERRED SETTINGS
This section is intended to help new users quickly "get off the ground" without going through all the different vario functions in detail. See the other sections of this article for detailed descriptions of the various functions of the vario. See also p. 21 of the standard Brauniger instruction manual for this vario (English version, 2003 edition) for more on the different options in the setup menu.
For general soaring, here are my preferred settings when I'm using the Brauniger IQ Comp GPS vario in conjunction with an Etrex Vista GPS:
No.1--QNH--I don't set this value, because the vario automatically selects an
appropriate QNH value whenever I set the A1 altimeter to reflect an accurate
MSL altitude. QNH can be thought of a value that relates to the atmospheric pressure. The QNH value is independent of altitude, but will change as meteorological
conditions change.
*No.2--Barograph recording interval--1 second, or 5 seconds if I expect the flight to last longer than 5 hours, or a longer interval on a multi-day flying trip with no opportunity to download data. The 5 second interval yields over 25 hours of recording time, the 15 second interval yields over 75 hours of recording time, and the 25 second interval yields over 125 hours of recording time.
*No.3--Sink tone start--500 feet/minute
*No.4--Stall alarm--8 mph. This particular
value (or 15 km/hour) disables the stall alarm. Pilots considering using the stall alarm as an aid to flare
timing should bear in mind that the stall alarm will tend to sound too late
during landings at high density-altitudes (hot, high, and/or humid
conditions).
*No.5--Total energy compensation--50%
*No.6--First polar value--I'm still experimenting with this; currently using 200 ft/min, 20 mph.
*No.7--Second polar value--I'm still experimenting with this; currently using 400
ft/min, 40 mph. Note: the experiments
described in this article to explore the various features of the vario were
designed to be completely independent of the accuracy of the programmed polar
curve in relation to the test aircraft.
*No.8--Digital vario display time constant--15 seconds.
*No.9--Digital vario display mode--3. This
option displays the digital time-averaged vertical speed whenever the glider is
climbing and the digital current-glide-ratio whenever the vario is descending,
unless the attached GPS is in "goto" mode with a properly coded
waypoint, in which case the digital glide-ratio-to-target is displayed at all
times (including during climbs) and the digital current-glide-ratio and digital
time-averaged vertical speed displays are not available. I generally avoid using properly coded waypoint names, so even when the attached GPS is in "goto" mode, the
vario continues to display the digital time-averaged vertical speed in climbs
and the digital current-glide-ratio during descents. I read the digital glide-ratio-to-target value directly off my GPS rather than off the vario. Setting option no. 9 to mode "0" would display the digital
time-averaged vertical speed at all times. Setting option no. 9 to mode "1" would cause the vario to display a digitally time-averaged "netto" value at all times. (The "netto" value represents the vertical speed of the airmass, and is based on the actual measured vertical speed plus or minus the glider's polar-derived theoretical sink rate value for the current measured airspeed.) Setting option no. 9 to mode "2" would cause the vario to display the digital time-averaged vertical speed during climbs and the digital time-averaged "netto" value during descents.
Nos.10-12--Time, date, year.
*No.13--McCready switching time--10 seconds. It is important that this parameter not be set to "zero", or the McCready acoustic will be heard even during climbing flight, and the normal climb acoustics will not be available.
No.14--Printer--this is not a settable parameter.
No.15--Temperature units--F
No.16--A1 Altimeter and digital vertical speed units--feet and feet/min. (The other option is to display the A1 altimeter and the digital vertical speed units in meters and
meters/second).
No.17--Speed units--mph
No.18--Pilot's name
*No.19--Airspeed probe calibration--170. (100 is the neutral setting). Before I sent the vario back to Brauniger for adjustment, the airspeed probe read too low even with the calibration set to the maximum value of 255. Each calibration unit represent a .2% change in the airspeed reading, so the maximum calibration value of "255" setting boosts the airspeed reading by 31% above the unadjusted value.
*No.20--Analog vario display time constant--1.0 seconds--this is the fastest
setting for the analog vario display and accompanying acoustics.
*No.21--Safety margin for final glide calculator--zero. (Note that because I usually don't use properly coded waypoint names on my GPS, the approach altimeter is usually not active.)
*No.22--Speed deviation from airspeed for flattest glide path to display one
correction arrow--2 mph.
*No.23--Speed deviation from airspeed for flattest glide path to display two correction arrows--6 mph. These correction arrows provide speed-to-fly
guidance when the pilot has not switched on the McCready display. There seems to be some sort of rounding error involved in entering parameters No. 22 and No. 23--the parameter that the vario accepts is often 1 mph different from the parameter that the pilot has
attempted to enter.
*No.24--Relationship between A1 and A2 units--set vario to display "m"
icon only. This means that the A2 altimeter will be displayed in the same units as were chosen for the A1 altimeter display in setting mode No. 16. There is also an option to display the A2 altimeter in which ever system of units (English or metric) was NOT chosen for the A1 altimeter and the digital vertical speed display in setting mode no. 16.
No.25--choose Glider 1 or Glider 2. Note that some setting options are always the same for both gliders and some may be set independently for Glider 1 and Glider 2.
The setting options that are always the same for both gliders are No.1 (QNH), Nos.
10-12 (time, date, year), No. 15 (temperature units), No. 16 (A1 altimeter and
digital vertical speed units), No. 17 (speed units), No. 18 (pilot's name), and
No. 26 (ceiling for stall alarm). Adjustments made to any of these parameters
while No. 25 is set for "Glider 1" will remain in effect when No. 25
is set for "Glider 2", and vs. vs..
The options that may be set independently for each glider are marked with an
asterisk in the listing above. These parameters are No. 2 (barograph recording interval), No. 3 (sink tone start), No. 4 (stall alarm), No. 5 (total energy compensation), Nos. 6 and 7 (polar values), No. 8 (digital vario display time constant), No. 9 (digital vario
display mode), No. 13 (McCready switching time), no. 19 (airspeed probe
calibration), No. 20 (analog vario time constant), No. 21 (safety margin for
final glide calculator), Nos. 22 and 23 (speed deviation from airspeed for
flattest glide path to display 1 and 2 correction arrows), and No. 24
(relationship between A1 and A2 units). Before adjusting any of these parameters the pilot should verify that No. 25 is set to the correct glider.
No.26--ceiling for stall alarm feature--zero. (I don't use the stall alarm).
For collecting polar data, spiral dive experiments, and other experimental situations, I use the same settings as listed above, except for the following changes:
*No.2--Barograph recording interval--1 second
*No. 5--Total energy compensation-zero
*No. 8--Digital vario display time constant--adjust as desired; 15 seconds is a
good setting for a well-smoothed digital vertical speed display that is good
for estimating the average vertical speed in prolonged maneuvers; smaller
values give a faster-responding digital vertical speed display.
*No. 9--Digital vario display mode--0. This displays the digital time-averaged
vertical speed value at all times (climbing or descending).
PART 3: SOME NOTES KEYED TO
THE BRAUNIGER IQ COMP GPS INSTRUCTION BOOKLET, English version, 2003 edition
These notes are keyed to the standard Brauniger manual for this vario (English version, 2003 edition), which may be downloaded from www.brauniger.com.
(table of contents is page 1)
Page 2:"Altimeter and barometric pressure"--the instructions
include a description of how the user can enter "memo" mode and then
use the "A1 A2 SF" button to scroll between the A1, A2, and A3
displays. The A3 display is only visible in "memo" mode and is a
cumulative vertical climb display. If you have cannot scroll to this A3 display
in "memo" mode, then you have an older software version. This older
software version will also not include the digital glide ratio displays that
appear in the "upper window" area (see below). This older software
version will also have problems interfacing with some GPS's such as the Garmin
Etrex series. Any Brauniger IQ Comp GPS vario can be upgraded by the factory to
the newest software version; I upgraded my vario in the summer of 2003 to a
software version that included the A3 display, the digital glide ratio display,
and the ability to interface with my Garmin Etrex series GPS.
Page 3:"Digital vario"--we'll call this the "upper window"
display. This space is used for the digital time-averaged vertical speed or netto display and also for the digital current-glide-ratio and digital glide-ratio-to-target displays. Only one of these displays can be shown in the "upper window" at any given time. The instructions make almost no mention of the digital glide ratio displays. If the digital glide ratio functions are switched on by selecting option "3" in mode 9 of the setup menu, then the netto display will not be available, and the digital
time-averaged vertical speed display will only be available when both these of
conditions are satisfied: 1) the attached GPS is not locked onto a waypoint
with a properly coded name, and 2) the glider is climbing, flying horizontally,
or descending at a very low rate. If the digital glide ratio functions are switched on in the setup menu, then whenever the attached GPS is not locked onto to a waypoint with a properly coded name and the glider is descending at a significant rate, the "upper
window" display will show the digital current-glide-ratio. If the digital glide ratio functions are switched on in the setup menu, then whenever the attached GPS is locked onto to a waypoint with a properly coded name, the "upper window" will show the digital glide-ratio-to-target and neither the digital current-glide-ratio
nor the digital time-averaged vertical speed display will be available, regardless of whether the glider is climbing or descending.
See Parts 5 through 7 for more on the digital current-glide-ratio and digital glide-ratio-to-target displays.
Page 4: "Acoustics and volume: descent acoustics off-on"--the
"descent acoustics" key is an on/off toggle for the sink alarm, and
also for the McCready acoustics. The sink alarm can only be toggled
"on" or "off" when the McCready pointers have not been
switched on. See below for more on how the sink alarm behaves when the McCready
pointers have been switched on.
Page 5: "Stall alarm"--it's interesting that the manual states that the "stall alarm" has been found to be helpful as an aid to flare timing. In standard aviation terminology, the airspeed flywheel essentially measures "true
airspeed", not "indicated airspeed" or "calibrated
airspeed". The "true airspeed" at the point of stall will vary
strongly according to density altitude, so any "stall alarm" keyed to
the airspeed flywheel will tend to sound too late whenever the density altitude
is high (hot, high, and/or humid conditions).
Page 5: "Printer"--this function is especially handy if the
vario's calendar has been accidentally reset at some point. The printer will
list the flights in the order in which they were made, just as when one scrolls
through the memo functions, whereas in PC Graph the flights are automatically
sorted by date. If there were a problem with the vario's calendar setting, it
would be difficult to re-sort the flights into actual sequential order after
downloading them into PC Graph.
Page 6: "Printout of the instrument settings"--this printout includes some, but not all, of the parameters that the user can select in the setup menu. This printout also includes a polar in the form of 13 airspeed/sink rate data pairs, rather than in the form of the two data pairs that were originally entered to define the polar. This can be handy if you wish to sketch the polar by hand and don't have access to PC
Graph.
Page 7: "Memo (flight memory)"--each time the "baro" is
turned on or off, the old "flight" is ended and a new
"flight" is begun.
Page 12: "Optimized nominal flight according to McCready"-- The
McCready speed-to-fly pointers cannot be switched on if the airspeed probe is
absent. Also, the McCready pointers cannot be switched on unless the barograph
is active. The McCready pointers are switched on with the same switch that is
used to adjust the altimeter reading up or down; the altimeter cannot be
adjusted when the barograph is active. The McCready speed-to-fly pointers
continue to function even when the vario is not connected to a GPS, although in this case the vario cannot take the wind into consideration. See Part 6 for more on the
McCready pointers and acoustics.
Page 13: "Average thermal climb"--oddly, if the "McCready
delay" is set to zero in setting mode number 13, then the normal climb
acoustic is omitted and the McCready acoustic is heard even while climbing.
Most pilots would find this extremely bothersome. See Part 6 for more on the
McCready pointers and acoustics, including the peculiar behavior of these
features during climbing flight. I always ignore the "active McCready pointer"
during climbing flight.
Page 13:"Average thermal climb"--the instructions describe how the
McCready acoustics can be toggled on and off with the "descent
acoustics" button. When the McCready functions have been switched on, this
button no longer toggles the sink alarm on and off, it only toggles the
McCready acoustics on and off. If the McCready functions are active and the
McCready acoustics have been toggled "off", then the sink alarm will
be available, or not, depending on whether the sink alarm was last toggled to
"on" or "off". However the McCready acoustics have priority
over the sink alarm--the sink alarm will never be heard when the McCready
acoustics are active.
Page 13: "Connection to a GPS receiver"--my older IQ-Comp GPS
vario wouldn't work with my Garmin Etrex Vista GPS until I returned the vario
to the factory to have the software upgraded in summer 2003. See Part 11 for
more.
Page 14: "Wind speed and direction"--the vario only displays the wind direction to the nearest 30-degree increment (30, 60, 90, etc).
Page 14: "Final approach computer"--the instructions state that
when a waypoint has been activated with a "goto" function on the GPS,
the upper segmented bar displays the best glide ratio that can be flown with
the current winds, and no vertical air movement.
This section is a bit confusing--in reality the upper segmented bar often
displays something other than the best glide angle that be flown with the
current winds, particularly when the glider is descending. The upper segmented
bar is not affected in any way by whether or not a waypoint has been activated
by a "goto" function on the GPS. The upper segmented bar always
relates to the glide ratio that can be expected with the current headwind or
tailwind component, in whatever direction the glider happens to be travelling
at the current moment. If the glider is descending, and a GPS and an airspeed
probe are connected to the vario, then the upper bar displays the expected
glide performance in air with no vertical movement, at the current measured
airspeed, not at some optimal gliding airspeed. If the glider is climbing, and
a GPS and an airspeed probe are connected to the vario, the upper bar displays
the glide performance that would be expected in air with no vertical movement
if the pilot were to fly at the optimal cross-country racing airspeed, adjusted
as per McCready theory to optimize the glider's racing performance given the
wind conditions and the recent average climb rates. (If the recent climb rates
have been low, then this will be almost the same as the airspeed that yields
that flattest possible glide angle, considering the existing winds). If a GPS, but
no airspeed probe, is connected to the vario, during a glide the upper bar
displays the glide performance that would be expected, given the current winds
and assuming no vertical motion in the airmass, if the pilot were to fly at the
still-air best-glide-ratio-airspeed for the polar that has been entered into
the vario. If a GPS, but no airspeed probe, is connected to the vario, during a
climb the upper segmented bar functions much as it would if an airspeed probe
were connected to the vario, except that the airspeed that the vario forecasts
that the pilot will use on glide is no longer optimized for wind, only for the
recent average climb rate. (Unless the recent average climb rate has quite
high, this adjusted airspeed will still be nearly equal to the still-air
best-glide-ratio-airspeed.) If an airspeed probe, but no GPS, has been
connected to the vario, then the upper segmented bar reflects the actual,
measured ground speed divided by the actual, measured sink rate, which yields
the glider's actual, current glide ratio, including the effects of any updrafts
or downdrafts are present. This is the same number that is given in digital
form in the upper window display.
See Parts 5 through 7 for more on the upper segmented bar display.
Page 15: "Final approach computer"--the illustration and text give
the impression that once the glider goes on glide, the approach altimeter shows
the glider's altitude above or below an idealized McCready glide path that
computed was while the glider was climbing in the thermal. In reality, once the
glider goes on glide, the past thermal climb rate plays no role in the approach
altimeter calculations. Once the glider begins descending, if a GPS and
airspeed indicator are both connected to the vario, the approach altimeter
shows the pilot's expected arrival height at the target based on the glider's
current position in space, and on the glider's expected performance at the
current measured airspeed, given the current apparent headwind or tailwind
component.
See Parts 5 through 7 for more on the approach altimeter.
Page 16: "Final approach computer"--the manual states that when a
glider is flying without an airspeed probe, the approach altimeter assumes that
the glider is flying at the airspeed that will yield the best glide, from the
polar. It would be more precise to state that when the glider is descending,
with no airspeed probe connected to the vario, the approach altimeter (and the
upper segmented bar display) both assume that the glider is flying at the
still-air best-glide-ratio airspeed for the polar that has been entered into
the vario. When the pilot is climbing without an airspeed probe, the approach
altimeter (and the upper segmented bar display) make an upward adjustment in
the airspeed that they assume that pilot will choose to use when he goes on
glide, according to the recent climb rate, as per McCready speed-to-fly theory
for best cross-country performance in thermal conditions. (Unless the recent
average climb rate has been quite high, this adjusted airspeed will still be
nearly equal to the still-air best-glide-ratio-airspeed.) When the pilot stops
climbing and actually goes on glide, without an airspeed probe, he must fly at
the still-air best-glide-ratio airspeed if he wants the approach altimeter and
the upper segmented bar glide ratio display to read accurately. When no
airspeed probe is connected to the vario, the approach altimeter and the upper
segmented bar display both assume that the pilot is not adjusting his choice of
airspeed to optimize his performance in a headwind or tailwind.
See Parts 5 through 7 for more on the approach altimeter and the upper segmented bar
display.
Page 17: "Glide ratio (= L/D ratio)"--the instructions here give
only a brief explanation of the upper segmented bar display, and give an even
briefer explanation of the digital glide ratio display, and imply that the
digital glide ratio display is simply a digital presentation of the same
information that is given by the upper segmented bar display. This is not the
case at all.
The instructions on this page imply that the upper segmented bar display is
based on the glider's actual, measured sink rate, so that the display is
affected by updrafts and downdrafts. In reality, this is only the case when an
airspeed indicator, but no GPS, is connected to the vario. At any time when a
GPS is connected to the vario, the upper segmented bar expected-glide-ratio
display is based on a theoretical, polar-derived sink rate. In contrast, the
digital current-glide-ratio display is always based on the glider's actual,
measured sink rate.
The instructions on this page seem to imply that the upper segmented bar
display changes in some way when the vario goes into "goto" mode,
i.e. when the attached GPS is locked onto a waypoint that is named in a way
that the vario can recognize. This is not the case. The upper segmented bar
expected-glide-ratio display always shows the expected glide ratio in whatever
direction the glider is travelling at the current moment. The upper segmented
bar never displays the glide ratio that is required to reach the target.
However, when the vario goes into "goto" mode, i.e. when the attached
GPS is locked onto a waypoint that is named in a way that the vario can
recognize, then the lower segmented bar display transforms from a battery-life
indicator to a glide-ratio-to-target display, and the digital glide ratio
display transforms from a current-glide-ratio display to a
glide-ratio-to-target display.
Note the description of how an upward or downward trend in the digital
glide-ratio-to-target display will indicate that the glider is likely to fall
short of the target or overfly the target, respectively, as we'll explore in Part
5.
See Parts 5 through 7 for much more on the behavior of the upper segmented bar
expected-glide-ratio display and the digital current-glide-ratio and digital
glide-ratio-to-target displays.
PART 4: A SIMPLE WAY TO USE THE IQ COMP GPS VARIOMETER
Despite the fact that I’ve
analyzed the features of the Brauniger IQ Comp GPS variometer “ad nauseum”, I
only normally actually only pay attention to a handful of the features of the
variometer. I normally connect my
variometer to a Garmin GPSmap 76S, which I configure with 6 different numerical
data fields on the map screen. I devote
these fields to “speed” (groundspeed), “heading” (direction of travel over the
ground), “bearing” (to target waypoint), “distance” (to
target waypoint), “current glide ratio”, and “glide ratio to target”. Therefore I don’t really need to see any of
these values on my variometer. However,
I like the vario’s digital current-glide-ratio feature better than the
“current glide ratio” feature on the GPS, because the vario’s digital
current-glide-ratio feature is averaged over a slightly longer time interval
and is therefore much less “twitchy” in rough air. Since the vario’s digital current-glide-ratio display is only
available when the vario does not recognize that the attached GPS is navigating
toward a waypoint, I always make a point of using waypoint names that are NOT
coded in a way that the vario can recognize (3 letters followed by 3 numbers). This ensures that none of the vario features
that relate to gliding toward a specific goal become active. Again, the main reason I do this is to
ensure that the vario’s digital current-glide-ratio display remains visible
whenever the glider is descending, and is not replaced by the digital
glide-ratio-to-target display.
Another reason that I normally
avoid using “properly” coded waypoints that the vario can recognize is that I
value being able to see the digital time-averaged climb rate during
climbs. If the digital display window has been configured to display glide ratio information, then whenever the GPS is navigating
toward a “properly” coded waypoint that the vario can recognize, the digital
glide-ratio-to-target value occupies the space that would otherwise be used
for the digital time-averaged climb rate.
I do keep an eye on the wind speed
and direction display that the vario generates after several steady
circles—even when the airspeed probe is not present—though I do find that when
I’m circling, I can make a much more accurate estimate of the wind direction by
simply glancing at the drift in the “breadcrumb” trail on the map screen of my
GPS.
I normally don’t pay much
attention to the upper segmented bar display for several reasons. One, whenever a GPS is connected to the
vario, the upper segmented bar shows a polar-derived expected glide ratio
(taking winds into account), and I don’t have a lot of confidence in the
accuracy of the polar that I’ve entered into the vario. Two, I often fly without an airspeed probe,
and for the purposes of the upper segmented bar display and the approach
altimeter, this causes the vario to assume that I am always flying at the
still-air-best-glide-ratio airspeed, at least when I am descending. (See later sections of this article for the
notes on how the upper segmented bar display works during a climb.) This is often not the case, which can cause
the upper segmented bar display to be quite inaccurate. Three, whenever I am actually in a gliding
descent, the polar-derived expected glide ratio depicted by the upper segmented
bar display is generally less interesting to me than my actual, real-time
current glide ratio, which I can read in digital form both on the vario screen and on the attached GPS.
When the GPS that is attached to
the vario is not navigating toward a waypoint that is coded in a way that the
vario can recognize (3 numbers followed by 3 digits), the lower segmented bar
display simply shows battery strength, and the approach altimeter is not
available. Therefore these are two more
features that I don’t normally pay attention to.
I’m not in the habit of using the
vario’s McCready speed-to-fly features, in part because I often fly without an
airspeed probe and in part because I don’t have a lot of confidence in the
accuracy of the polar that I’ve entered into the vario.
Apart from the digital
current-glide-ratio readout and the wind speed and direction readout, one
reason I fly with a GPS-compatible vario is that I like to be able to save a
detailed barograph trace that includes a groundspeed trace as well as an
altitude trace and a vertical speed trace.
If there is any wind, a groundspeed trace recorded at the highest
resolution (1 point per second) very clearly shows during what portions of the
flight the glider is circling, and during what portions of the flight the
glider is flying in a straight line: the groundspeed trace for the circling
portions of the flight has a characteristic zig-zag shape. The amplitude of the zig-zags reveals the
windspeed. I find this to be
interesting—for more on this, see the related article on this website entitled
“Commentary on the mathematics of circles in wind”. The
groundspeed trace is only recorded when an airspeed probe is not attached to
the variometer, and this is one reason that I often fly without an airspeed
probe.
The philosophy described in this section is well suited to any situation where a pilot has not entered an accurate polar into the vario, or is not flying with an accurately calibrated airspeed probe. For the actual vario settings that
I use to configure the vario as described in this section, see Part Two ("One Pilot's Preferred Settings").
The original reason I decided to configure the vario in this manner is that I was flying with an Garmin Etrex Vista GPS that only had room for 2 numerical display fields on the map screen. By displaying the glide-ratio-to-target on the GPS and displaying the digital current-glide-ratio on the vario, I left myself with one free numerical display field on the GPS map screen that I could use for some other parameter of interest, like heading or groundspeed. Now that I'm flying with a Garmin GPSmap 76S GPS that I configure with 6 different small numerical display fields on the map screen, including both the current-glide-ratio and the glide-ratio-to-target, there's no longer such a need to display the digital current-glide-ratio on the vario, and it might make more sense to reconfigure the vario's digital display window to show the time-averaged vertical speed in climbs and the "netto" value in descents, rather than showing a digital glide ratio readout. In this case there would no longer be any reason to avoid using "properly" coded waypoints that the vario can recognize (3 letters followed by 3 numbers). If Garmin ever comes out with a software upgrade for the Etrex Vista, GPSmap 76S/CS/CSx, etc that changes the current-glide-ratio-display so that it is slightly more damped, and therefore slightly more useable in rough air, I'll probably reconfigure the vario in this manner, but at present I do like to be able to see the digital-current-glide-ratio figure on the vario as well as on my GPS.
PART 5: NOTES ON DESIGN
PHILOSOPHY AND USER PHILOSOPHY: AN OVERVIEW OF THE GLIDE-RATIO-RELATED FUNCTIONS OF THE IQ COMP GPS VARIOMETER
In this section we'll explore the benefits and limitations of the various glide-ratio-related functions of the IQ Comp GPS vario. A recurring theme here
will be the limited number of parameters that can be displayed at any one time,
and the way that this forces some choices upon the vario user. We'll assume that the user has set the digital window to display glide-ratio-related information, and we'll compare the advantages of using the vario in "goto" mode, where the digital glide-ratio-to-target is displayed and the approach altimeter is active, with the advantages of using the vario in the "non-goto" mode, where the digital current-glide-ratio is displayed and the approach altimeter is absent. Pilots who have already decided to set the vario's digital window to show only time-averaged vertical speed and/or netto values, rather than glide-ratio-related information, may want to skip down to the last four paragraphs of this section, which give an overview of the approach altimeter and the upper segmented bar display.
In this section, we'll assume that the vario is
connected to a GPS.
The Brauniger IQ Comp GPS vario's McCready audio
and visual speed-to-fly indicators give a pilot immediate guidance on how to
deal with rising or sinking air encountered while gliding to a goal or while
gliding to the next thermal. The actual measured sink rate at any given
moment--after the "total energy" adjustment--plays a key role in the
calculations for the McCready speed-to-fly indicators. The McCready
speed-to-fly indicators only function when an airspeed probe is connected to
the vario, and they only provide meaningful information when an accurate polar
curve has been entered into the vario. The digital current-glide-ratio display
is another feature that is based on the actual measured sink rate at any given
moment (after the total energy adjustment), along with the actual measured
groundspeed.
Other vario functions --in particular the approach
altimeter and the upper segmented bar expected-glide-ratio display--are not
based on the actual measured sink rate at any given moment. Instead, these
functions are based on a polar-derived sink rate value. This sink rate value is
derived directly from the polar that has been entered into the vario, according
to the glider's measured or assumed airspeed. This polar-derived sink rate value
is not affected by updrafts and downdrafts. Therefore the approach altimeter
and the upper segmented bar display give the pilot guidance on what glide ratio
to expect in the existing wind conditions, in air with no vertical motion. The
designers undoubtedly felt that this approach would yield much steadier, more
useable indications when gliding in turbulent conditions. This makes a lot of
sense--when a pilot is on a long, straight glide to a distant goal in thermal
conditions, it is reasonable to assume that patches of lift and sink will
cancel each other out and the resulting glide over the long run will be about
the same as the pilot could achieve in the same wind conditions in air with no
vertical motion. On a long glide in thermal conditions, it wouldn't make sense
to assume that the deteriorated glide ratio resulting from a localized patch of
sink, or the enhanced glide ratio resulting from a localized patch of lift,
would likely extend all the way to the goal. On a long glide to distant goal in
thermal conditions, with good speed-to-fly information coming from the McCready
analog and visual speed-to-fly indicators, there's little need for a pilot to
be presented with a re-calculated current-glide-ratio number every time the
glider hits a patch of lift or sink. The glider's current position in space
relative to the target may be changed by an updraft or a downdraft, and this
will certainly be reflected in the approach altimeter display. But even when
strong lift or sink are present, the approach altimeter and the upper segmented
bar display continue to indicate the glider's expected future performance in
air with no vertical motion. In thermal conditions, this provides a much better
forecast of the glider's future performance over the long run, than does the
actual real-time glide ratio based on the actual, measured sink rate of the
glider at any given moment.
However there are times when a pilot may be very
interested in knowing his actual glide ratio at the current moment, as
influenced by updrafts or downdrafts. For example, imagine that a pilot has
drifted downwind of a ridgeline while working a thermal and needs to penetrate
back upwind to the ridgeline. Imagine also that there are widespread areas of
sink created by the way that the prevailing airflow is curling over the ridge.
In other words, imagine a scenario where the vertical air currents that exist
at any given moment are fairly representative of the conditions that may exist
all the way to the pilot's chosen target or goal. In this case the pilot will
not be particularly interested in the glider's expected performance in air with
no vertical motion. He will not be particularly interested in the approach
altimeter or the upper segmented bar expected-glide-ratio display. He will need
to know how to optimize the glider's actual path through the air, and he will
need to know whether the glider's current glide path, as influenced by the
widespread sink, is likely to clear the ridgeline by a safe margin. If the
pilot is flying with an airspeed probe, then the vario's McCready audio and
visual speed-to-fly indicators will help the pilot optimize his choice of
airspeed. But the McCready indicators do not help the pilot judge his glide
ratio or help the pilot estimate whether he will likely be able to reach a
given target, such as a ridgeline. In conditions of widespread lift or sink
that may extend all the way to the pilot's chosen target, the vario's digital
current-glide ratio and digital glide-ratio-to-target displays are the best way
for a pilot to judge his glide ratio and decide whether he will likely be able
to reach a chosen target. By comparing the digital current-glide-ratio value
with the digital glide-ratio-to-target value, the pilot can see whether the
current, actual glide path--including the effects of any widespread downdrafts
or updrafts that are present--will likely carry the glider to the desired
target with altitude to spare, or will likely fall short of the target. Also, a
consistent downward trend in the digital glide-ratio-to-target display
indicates that the glider is currently following a glide path that will pass
over the target with altitude to spare, and a consistent upward trend in the
digital glide-ratio-to-target display indicates that the glider is currently
following a glide path that will fall short of the target. If the prevailing
conditions include small, strong updrafts and downdrafts as well as more
widespread areas of rising or sinking air, a pilot may want to ignore the
short-term fluctuations of the glide-ratio-to-target display and focus only on
the long-term upward or downward trend.
The vario's digital current-glide-ratio and
digital glide-ratio-to-target displays will also be of special interest
whenever a pilot is flying without an airspeed probe connected to the vario, or
whenever a pilot has doubts about the calibration of the airspeed probe or the
accuracy of the polar curve that has been entered into the vario. In any of
these situations, the digital current-glide-ratio display and the digital
glide-ratio-to-target display may be the best tools available to the pilot for
predicting his glide path, and also for optimizing his choice of speed-to-fly
in updrafts and downdrafts. If the airspeed probe is miscalibrated or the polar
curve is not accurate for the glider that the pilot is currently flying, the
polar-based vario functions such as the approach altimeter, the upper segmented
bar expected-glide-ratio display, and the McCready audio and visual
speed-to-fly indicators will give misleading indications. Except for transient
total-energy effects, the digital current-glide-ratio display will not be
affected by the errors in the polar or the calibration of the airspeed probe.
If the pilot is flying without an airspeed probe, the approach altimeter and
the upper segmented bar display will be based on the assumption that the pilot
will always choose to fly at the still-air best-glide-ratio airspeed, which
will often not be best strategy. The digital current-glide-ratio display is
based on the actual, measured groundspeed and makes no assumptions about the
airspeed that the pilot chooses to use. If no airspeed probe is connected to
the vario, the McCready audio and visual speed-to-fly indicators will be
absent, but the digital current-glide-ratio display will still be present.
Unfortunately, the vario's digital
current-glide-ratio display is twitchy enough that it is somewhat difficult to
use for real-time speed-to-fly guidance unless the air is relatively smooth. In
rough air the display is still of some value--for example it will show the
difference between the glide resulting from flying at trim and the glide
resulting from pulling the bar in to mid-chest--but the display does not seem
to be as smooth and steady and useable as the vario's McCready audio and analog
speed-to-fly indications. I'm not sure why this is. With the appropriate
damping and total energy compensation, it seems that a digital
current-glide-ratio display should be as smooth as and usable as any other
feature that responds to the glider's real-time sink rate, such as the McCready
audio and analog speed-to-fly indications.
Despite these problems, the vario's digital
current-glide-ratio display is significantly smoother, and significantly more
useable in rough air, than the current-glide-ratio display on my
pressure-sensor-equipped Garmin Etrex Vista GPS or GPSmap 76S. This is true
even when the vario's total energy compensation has been set to zero, or when
an airspeed probe is not connected to the vario. Undoubtedly the vario's
digital current-glide-ratio display is averaged over a longer time period than
is the corresponding display on these GPS’s.
The vario's digital glide-ratio-to-target display
is based on the glider's position in space in relation to the target, rather
than on the glider's horizontal and vertical velocities. Therefore it is
intrinsically much more stable than the vario's digital current-glide-ratio
display. The "tenths" digit in the vario's digital
glide-ratio-to-target display is very helpful for detecting gradual trends in
the glide-ratio-to-target value. For example, if the glide-ratio-to-target
value is slowly scrolling from "4.5" to 4.6" to "4.7",
then the glider is following a path that will intersect the ground before
reaching the target, if the current atmospheric conditions--including any
updraft or downdraft that may be present--continue all the way to the target.
The glide-ratio-to-target display on many pressure-sensor equipped GPS's, such
as the Garmin Etrex Vista or Map76S, lacks a "tenths" digit. This
makes it much more difficult for a pilot to detect slow trends in the
glide-ratio-to-target value, especially when the glide-ratio-to-target value is
down in the low single digits, as is often the case in real-life hang gliding
and paragliding scenarios. When the glide-ratio-to-target number is down in the
low single digits, it may take a long time--and a dramatic worsening in the
glider's situation relative to the target--for a glide-ratio-to-target display
to change by a full whole number (e.g. from "4" to "5").
While the vario's digital glide-ratio-to-target
display is very useful for determining whether or not a glider will likely be
able to reach a chosen target, this display is less useful for fine-tuning a
pilot's choice of airspeed-to-fly. To use this display to optimize his choice
of airspeed-to-fly, a pilot would have to be alert for small changes in the
rate at which the glide-ratio-to-target display was scrolling upward or
downward. The vario's digital current-glide-ratio display works better for
optimizing the pilot's choice of airspeed-to-fly, and the vario's audio and
visual McCready speed-to-fly indicators work better yet for this purpose, so
long as an accurately calibrated airspeed probe is connected to the vario and
an accurate polar has been entered into the vario.
In additional to the digital glide-ratio-to-target
display, the vario also has displays a rough analog indication of the
glide-ratio-to-target. This is the lower segmented bar display, which is broken
into segments corresponding to glide ratios of >4, >6, >8, >9,
>10, >11, >12, >14, >16, and >18:1. (The upper segmented bar
expected-glide-ratio display is also broken up into the same intervals). Of
course, it is difficult to detect slow trends in the glide-ratio-to-target
value with this rough analog display.
In the best of all worlds, a vario would
simultaneously display the digital current-glide-ratio and digital
glide-ratio-to-target values, since it is often very desirable to be able to
compare these two numbers in flight. Many of the newer variometers with
internal GPS’s have a large number of numerical display fields that can be
configured by the user, and can therefore be configured to show both the
digital current-glide-ratio and the digital glide-ratio-to-target. Unfortunately, this is never possible on the
Brauniger IQ GPS Comp vario, because these two functions both use the same
display window. If the vario is in
"goto" mode (see below), then only the digital glide-ratio-to-target
value is displayed, and if the vario is not in "goto" mode (see
below), then only the digital current-glide-ratio value is displayed. Even more
unfortunately, the digital time-averaged vertical speed or netto display also
uses the same display window. If the digital glide ratio displays have been
enabled in the "setup" menu, then whenever the vario is in
"goto" mode, then the digital glide-ratio-to-target display replaces
the digital time-averaged vertical speed display. If the digital glide ratio
displays have been enabled in the "setup" menu, then whenever the vario
is not in "goto" mode, then the digital current-glide-ratio display
replaces the digital time-averaged vertical speed display whenever the glider
is descending at a significant rate, and the digital time-averaged vertical
speed display is only visible when the glider is climbing or flying
horizontally or descending at a very low rate. Many pilots who value the
digital time-averaged vertical speed display, as well as the approach
altimeter, may choose not to enable the digital glide ratio display. These are
rather severe limitations. Again, one of the chief advantages of a newer vario
such as the Brauniger Galileo or Compeo is that the digital
current-glide-ratio, digital glide-ratio-to-target, and digital time-averaged
vertical speed or netto values may all be displayed at the same time.
These limitations mean that there are strong
advantages to using the IQ Comp GPS vario in combination with a pressure-sensor
equipped GPS such as the Garmin GPSMap 76S. If the map screen of the GPS is set up to display the
digital glide-ratio-to-target and the digital current-glide-ratio, then both of
these numbers will always be available whether or not the vario is in
"goto" mode (see below), and also when the pilot has switched off the
vario's digital-glide-ratio functions because he prefers to see the digital
time-averaged vertical speed display at all times. However, we've already noted
that the vario's digital current-glide-ratio and glide-ratio-to-target displays
are considerably more practical to use in flight then are the same displays on
a pressure-sensor-equipped GPS such as the Garmin Etrex Vista or GPSMap 76S.
We've used the term "goto mode" several
times now. When we say that the vario is in "goto" mode, we simply
mean that the user has activated the "goto" function on the GPS that
is connected to the vario, using a waypoint name that is coded in a way that
the vario can recognize. The vario can only recognize names that consist of 3
letters followed by 3 numbers. The 3 numbers code for elevation--for example
"FLY011" would be read as an elevation of 1100 feet or 110 meters. If
the GPS is locked on to a waypoint that has a name that the vario cannot
recognize, or is not locked on to any waypoint at all, then the vario will not
enter "goto" mode.
Let's map out in a concise format the functions
that are available when the vario is not in "goto" mode, and the
functions that are available when the vario is in "goto" mode:
1) When the vario is not in "goto" mode:
If the digital glide
ratio functions have been activated in the setup menu, then the digital
current-glide-ratio will be displayed whenever the glider is descending at a
significant rate, and the time-averaged vertical speed will be displayed only
when the glider is climbing, flying level, or descending at a very low rate. The
digital glide-ratio-to-target display will not be available. The lower
segmented bar display will show the battery strength. The upper segmented bar
display shows a rough indication of the expected glide ratio in whatever
direction the glider is currently travelling, using a polar-derived sink rate
that ignores the actual vertical motion in the airmass. The McCready analog and
audio pointers, which help the pilot optimize the glide path in whatever
direction the glider is travelling at the current moment, may be switched on at
any time that an airspeed probe is connected to the vario.
2) When vario is in "goto" mode:
The lower segmented bar
display will show a rough indication of the glide-ratio-to-target. If the
digital glide ratio functions have been activated in the setup menu, then the
digital glide-ratio-to-target display will be present at all times when the
vario is in "goto" mode, and the time-averaged vertical speed display
will not be available. The digital current-glide-ratio is never displayed when
the vario is in "goto" mode. The upper segmented bar display shows a
rough indication of the expected glide ratio in whatever direction the glider
is currently travelling, using a polar-derived sink rate that ignores the
actual vertical motion in the airmass. The McCready analog and audio pointers,
which help the pilot optimize the glide path in whatever direction the glider
is travelling at the current moment, may be switched on at any time that an
airspeed probe is connected to the vario. The approach altimeter is present
whenever the vario is in "goto" mode, and provides information
relating to the glider's expected glide path toward the designated target,
using a polar-derived sink rate that ignores the actual vertical motion in the
airmass.
Ultimately, a pilot's choice about whether or not
to use coded waypoints and allow the vario to enter "goto" mode, and
a pilot's choice about whether or not to enable the digital glide ratio
functions, will be determined by his philosophy about which functions are the
most valuable, in relation to the kind of flying that he plans to do.
If a pilot values the digital current-glide-ratio
display, then he may want to consider using waypoint names that the vario
cannot recognize, so that the vario does not enter "goto" mode. As
noted in the previous section, in my own flying, I often find myself without an
accurate polar curve or without an accurately calibrated airspeed probe. I also
put a high priority on being able to optimize my glide in conditions of widespread
sink, such as when penetrating upwind toward a ridgeline. For all of these
reasons, I value the vario's digital current-glide-ratio display. Therefore I
often use waypoint names that the vario cannot recognize, so that the vario
does not enter "goto" mode, and the digital current-glide-ratio
display is visible whenever the glider is descending. With this set-up, I still
get to see the digital time-averaged vertical speed display whenever the glider
is climbing--this is quite important to me, and this would not be possible if
the vario entered "goto" mode, unless I had disabled the vario's
digital glide ratio functions. I use my pressure-sensor-equipped Garmin Etrex
Vista GPS or GPSmap 76S to monitor the digital glide-ratio-to-target, albeit
without a "tenths" digit. Many sport pilots involved in casual
recreational flying might prefer to use the vario in exactly this way, rather
than using coded waypoint names.
Pilots who value the vario's digital
glide-ratio-to-target display will need to use waypoint names that are coded in
a way that the vario can recognize, so that the vario does enter
"goto" mode. The same is true of pilots who value the approach
altimeter. Pilots using the vario in this way may want to use a
pressure-sensor-equipped GPS to display a digital current-glide-ratio value,
though this display will likely be much twitchier in rough air than the vario's
own digital current-glide-ratio display would be. As noted above, pilots flying
with accurately calibrated airspeed indicators and accurate polar curves,
engaged on long cross-country flights in thermal conditions, will probably
value the approach altimeter a great deal.
Many pilots pay a lot of attention to a digital
time-averaged vertical speed display, especially in climbs. They use this display
to make decisions about whether to stay with a thermal or to go look for a
better one. Pilots who value the digital time-averaged vertical speed display,
and also plan to use the approach altimeter, will need to enter the setup menu
and switch off the vario's digital glide ratio functions entirely. If the
digital glide ratio functions have been switched on, then whenever the vario
enters "goto" mode, the time-averaged vertical speed display will
disappear, even during climbs. If a pilot has switched off the vario's digital
glide ratio functions, he may want to display the digital current-glide-ratio
values on a pressure-sensor-equipped GPS such as the Garmin Etrex Vista or
GPSMap 76S, though as we've noted, these displays will be inferior to the
digital current-glide-ratio and digital glide-ratio-to-target displays on the
vario.
Bear in mind that the vario only enters
"goto" mode when the attached GPS is locked onto a target whose name
consists of 3 letters followed by 3 numbers. In a contest task where a pilot
has been provided with a pre-existing list of waypoints, it is easy to create
new waypoints that are only a few feet offset from the old waypoints, but are
given appropriately coded names, so that the vario's approach altimeter and
other "goto"-related functions can be used.
Even during everyday recreational flying, a pilot
might want to consider creating dual sets of dual waypoints that are located in
practically the same point in space but are named in different ways: if the GPS
is locked onto one waypoint (e.g. "FLY011") the vario will enter
"goto" mode, and if the GPS is locked onto the other waypoint (e.g.
"FLYTARGET") the vario will not enter "goto" mode. In this
way a pilot is free to choose at will whether to use the vario's
"goto"-related functions, or not.
For a more elaborate solution to some of the
vario's limitations, a pilot could use 2 GPS's and a toggle switch. One GPS
would be programmed with waypoint names that the vario can recognize, and the
other GPS would not. The pilot need not bother with entering any waypoints at
all in the second GPS, or can use waypoint names that the vario cannot
recognize. When the toggle switch is one position the vario will be connected
to the first GPS, so the vario will be in "goto" mode if this GPS is
locked onto a target waypoint, and the approach altimeter and the digital
glide-ratio-to-target display will be available. When the toggle switch is in
the other position, the vario will be connected to the second GPS, so the vario
will not be in "goto" mode, and therefore the digital
current-glide-ratio display will be available during glides and the digital
time-averaged vertical speed display will be available during climbs.
When I fly with an airspeed sensor, I route the wire from the sensor through
an on/off toggle switch. Whenever a GPS, but no airspeed probe, is connected to
the vario, a "groundspeed" display becomes visible. So the toggle
switch changes a display on my vario from "airspeed" to
"groundspeed". This also
controls which of the two parameters gets recorded onto the barograph trace: if
the vario is not receiving airspeed data, than the barograph trace will
automatically include groundspeed rather than airspeed. The original motivation for this toggle
switch was to save me from having to dedicate one of the two digital display
windows on my GPS's map screen to groundspeed.
As noted above, with my current GPS I now have plenty of numerical display windows on the moving map screen, but I
still like being able to have the barograph record groundspeed rather than
airspeed. Ideally the barograph would record
both groundspeed and airspeed, but this is apparently not possible with the IQ
Comp GPS vario.
Let's shift our focus now away from our discussion of which
vario features are of greatest interest in various real-life situations, and
make some general observations about the approach altimeter and upper segmented
bar expected-glide-ratio displays. We'll explore these features in much more
detail in the subsequent parts of this article, but an introductory overview
might be helpful.
The approach altimeter and the upper segmented bar
expected-glide-ratio display both show the glider's expected performance with
the current wind conditions, using a polar-derived sink rate that ignores the
actual vertical motion in the atmosphere. When an airspeed probe is connected
to the vario, and the glider is descending, both of these displays are based on
the glider's expected performance at the current, measured airspeed. When the
glider is descending, the glider's recent average climb rate never has any
effect on the approach altimeter or the upper segmented bar display. When an
airspeed probe is connected to the vario and the glider is climbing, both of
these displays are based on the assumption that the pilot will choose to make
the next glide at the optimal airspeed according to McCready theory for best
cross-country racing performance in thermal conditions. This assumed future
gliding airspeed is optimized for the current winds, and also optimized for the
recent average climb rate, which is assumed to be a predictor of future climb
rates. When the glider is climbing rapidly, the approach altimeter and the
upper segmented bar display predict a poorer glide ratio than when the glider
is climbing slowly, because when the thermals are stronger, the pilot should
use a faster gliding airspeed for best cross-country racing performance, as per
McCready theory. If a glider is in a very fast glide and gets temporarily
lifted into a climb as it passes through a strong thermal, the approach
altimeter and the upper segmented bar expected-glide-ratio display may both
change dramatically, as they switch from forecasting a glide path based on the
glider's expected performance at the current measured airspeed, to forecasting
a glide path based on the glider's expected performance at the ideal McCready
cross-country racing airspeed.
The approach altimeter and the upper segmented bar
expected-glide-ratio display are both designed to work, to a limited degree, in
the absence of an airspeed probe. When no airspeed probe is connected to the
vario and the glider is descending, both of these displays are based on the
assumption that the pilot is always choosing to fly at the still-air
best-glide-ratio airspeed for the polar has been entered into the vario.
Gliding at any other airspeed will create errors in the approach altimeter and
the upper segmented bar display, both because the vario will miscalculate the
headwind or tailwind component, and because the vario will use the wrong
polar-derived sink rate value. When the glider is descending, the glider's
recent average climb rate never has any effect on the approach altimeter or the
upper segmented bar display. When no airspeed probe is connected to the vario
and the glider is climbing, the approach altimeter and upper segmented bar
display calculate the apparent headwind or tailwind component based on the
assumption that the pilot is currently choosing to climb at the still-air
best-glide-ratio airspeed for the polar that has been entered into the vario.
Climbing at any other airspeed will create errors in the approach altimeter and
the upper segmented bar display, because the vario will miscalculate the
headwind or tailwind component. For example, if the glider is climbing at the
wings-level, polar-derived, minimum-sink-rate airspeed, the approach altimeter
and the upper segmented bar display will read too pessimistically, because the
vario will think that the glider has a headwind. (However bear in mind that
real-life thermal climbs in strong conditions are usually are conducted at an
airspeed that is well above the wings-level minimum-sink-rate airspeed). When
no airspeed probe is connected to the vario and the glider is climbing, the
approach altimeter and upper segmented bar display forecast the expected
performance in the next glide based on the assumption that the pilot will go on
glide at an airspeed that is based on the still-air best-glide-ratio airspeed,
but modified upward to optimize for the recent average climb rate as per
McCready speed-to-fly theory for best cross-country racing performance in
thermal conditions, though not modified upward or downward to optimize for the
apparent headwind or tailwind component. This is a bit odd because when the
pilot actually goes on glide, with no airspeed probe connected to the vario, he
must fly at the still-air best-glide-ratio airspeed if he wants the approach
altimeter and the upper bar display to work correctly, as noted above. However,
for practical purposes, this adjustment of the forecast glide ratio to
accommodate the recent average climb rate as per McCready theory is barely
noticeable unless the glider is climbing very rapidly. When the glider is
facing a strong headwind and climbing rapidly, with only a GPS connected to the
variometer, the approach altimeter and the upper segmented bar display will
actually predict a better glide ratio than when the glider is facing the same
strong headwind but climbing slowly. This is not normally what one would
expect, according to McCready speed-to-fly theory for cross-country racing in
thermal conditions. This odd behavior is an accidental byproduct of the fact
that the vario is not really optimizing the assumed interthermal gliding
airspeed for the headwind, only for the climb rate. When the glider raises the
assumed interthermal gliding airspeed to optimize for a high climb rate, this
also improves penetration. However, in actual practice, unless the glider is
climbing very rapidly, it will be difficult to for the pilot to detect that the
approach altimeter and the upper segmented bar display are based on anything
other than the assumption that the pilot will choose to fly at the still-air
best-glide-ratio airspeed.
The main difference between the calculations used
for the approach altimeter and the calculations used for the upper segmented
bar expected-glide-ratio display lies in the way that the vario samples the
apparent headwind or tailwind component. For the upper segmented bar display,
the vario continually re-samples the current headwind or tailwind component, so
every time the glider changes heading, the upper segmented bar display changes.
The upper segmented bar display continually rises and falls when the glider
circles in wind. This display always relates to the glide ratio that would be
expected if the glider were to continue along its current ground track at any
given instant. If there is a systematic error in the way that the vario is
calculating the apparent headwind or tailwind component--for example if the
airspeed probe is calibrated to read too high or too low, or if the pilot is
flying without an airspeed probe and is choosing to fly at an airspeed that is
different from the still-air best-glide-ratio-airspeed for the polar that has
been entered into the vario--then it is quite possible for the upper segmented bar
display to act as if a circling glider were experiencing a continual headwind
or tailwind component throughout the entire course each circle. The upper
segmented bar display is always present, so if the pilot has entered an
accurate polar into the vario, he'll have a good polar-derived performance
estimate, unaffected by updrafts and downdrafts, even when the attached GPS is
not locked on to any target waypoint. The approach altimeter is only present
when the attached GPS is locked onto a properly coded target waypoint. For the
approach altimeter, the vario only samples the headwind or tailwind component
when the glider's ground track points within 20 degrees of the target waypoint,
so the approach altimeter always indicates the performance that would be expected
if the glider were to fly directly toward the target waypoint. The approach
altimeter display does not rise and fall as the glider circles in wind. If the
ground track has not pointed within 20 degrees of the target in the last 30
seconds, the approach altimeter begins flashing, and defaults to an assumption
that the headwind or tailwind component is zero.
PART 6: A DETAILED
DESCRIPTION OF VARIO FEATURES, ORGANIZED BY FEATURE
Now we'll give a more detailed description of some of the features of the
Brauniger IQ Comp GPS vario. This section is not meant to replace the standard
Brauniger instruction manual for this vario, however any differences from the
standard manual are intentional.
FEATURE: Entering
"goto" mode
FEATURE: Wind velocity display,
and other wind calculations
FEATURE--Digital display of
vertical speed, current glide ratio, and glide-ratio-to-target
FEATURE: Upper segmented bar
expected-glide-ratio display
FEATURE: Lower segmented bar
glide-ratio-to-target display
FEATURE: Approach altimeter
FEATURE: McCready speed-to
fly audio functions and visual pointers
FEATURE: Entering a polar curve.
FEATURE: Entering "goto" mode
When we say that the vario is in "goto mode", we simply mean that
the pilot has activated the "goto" function on the GPS that is
attached to the vario, and that he has used a GPS waypoint that is named in a
way that the variometer can recognize--i.e. three letters followed by three
numbers, such as "FLY011". When the attached GPS is in
"goto" mode with a waypoint that is named in this way, then the vario
enters "goto" mode. If the attached GPS is locked onto a waypoint
that is not named in this way, then the vario will not be aware of the
waypoint, and will not enter "goto" mode. Functions that are only
available when the vario is in "goto" mode include the approach
altimeter and the vario's analog glide-ratio-to-target and digital
glide-ratio-to-target displays. Functions that are only available when the
vario is not in "goto" mode include the vario's digital
current-glide-ratio display.
When the pilot selects a waypoint name that consists of three letters
followed by three numbers, the vario interprets the three numbers as being a
code for the elevation of the waypoint. For example if the waypoint is called
"FLY011" the vario will interpret the waypoint as being at 1100 feet
or 110 meters, depending on the units that the pilot has selected in the
vario's "setup" menu. The highest possible waypoint altitudes that
the vario can recognize are 99900 feet or 9990 meters. Note that nearly all
GPS's have the capability for the user to enter an elevation when he creates a
waypoint, but this bit of data is not used by the vario in any way--the vario
derives the elevation of the waypoint strictly from the last three digits of
the name of the waypoint.
Bear in mind that the vario only enters
"goto" mode when the attached GPS is locked onto a target whose name
consists of 3 letters followed by 3 numbers. In a contest situation where a pilot
has been provided with a pre-existing list of waypoints, the pilot may wish to create
new waypoints that are only a few feet offset from the old waypoints, but have
appropriately coded names, so that the vario's approach altimeter and
other "goto"-related functions can be used.
FEATURE: Wind velocity display, and other wind
calculations
When a pilot is flying with a GPS connected to the vario, the vario will
flash an estimate of wind direction and speed for a short while whenever the
glider has completed several circles. This wind estimate is only computed to the nearest 30-degree increment. This wind estimate appears in the area
normally used to display the time-of-day or duration-of-flight. This wind
estimate is generally unaffected by choice of airspeed (as long as it stays
roughly constant during the circles) or by calibration errors in the airspeed
probe, and a reasonable wind estimate will appear during circling even if the
airspeed probe is badly miscalibrated or even absent, so long as the pilot is
flying at a roughly constant airspeed throughout the circles. However when
flying without an airspeed probe, I have noticed that the displayed wind
direction is sometimes up to 40 degrees off from the actual wind direction. I
haven't evaluated the accuracy of the wind display when flying with an airspeed
probe.
This wind estimate is for the pilot's information only. It does not appear
to be used for any of the other vario functions such as the approach altimeter
or the upper segmented bar expected-glide-ratio-display or the McCready analog
and audio speed-to-fly indicators. The vario appears to perform a completely
separate calculation of the apparent headwind or tailwind component which is
used for all of these wind-dependent vario features. This calculated headwind
or tailwind component is not displayed to the pilot, nor can it be manually
selected by the pilot as is the case with some other varios. If the pilot is flying
with both a GPS and an airspeed sensor, this headwind or tailwind component is
calculated from the difference between the measured airspeed and the GPS
groundspeed. If the airspeed flywheel sensor is miscalibrated to read too high
or too low, this will produce a marked, airspeed-dependent error in all of the
wind-dependent and airspeed-dependent vario functions. The exact nature of
these errors is rather complex and in some cases will depend upon the shape of
the polar, and whether the glider is climbing or descending. See the detailed
descriptions of each of the displays later in this section to better understand
how they would be affected by erroneous wind estimates. However, the digital
current-glide-ratio display will not be affected by an incorrectly calibrated
airspeed probe, because the digital current-glide-ratio display is derived
directly from the actual measured groundspeed divided by the actual measured
sink rate, except in the one special case where the vario is connected to an
airspeed probe but not to a GPS.
The approach altimeter and the segmented bar expected-glide-ratio display
are designed to function even if the pilot chooses to fly without an airspeed
indicator. If the pilot chooses to fly without an airspeed sensor, the vario
still needs to come up with an estimate of the headwind or tailwind component,
and an airspeed estimate. The vario solves this problem by assuming that the
glider is currently flying at the still-air best-glide-ratio airspeed for
whatever polar the pilot has entered into the vario. The vario calculates an
apparent headwind or tailwind component by looking at the difference between
the GPS groundspeed and the still-air best-glide-ratio airspeed. For example,
if the pilot chooses to fly faster than the still-air best-glide-ratio
airspeed, the vario will assume that the pilot is experiencing a favorable
tailwind, and the approach altimeter and the upper bar expected-glide-ratio
display will give overly optimistic readings. The digital current-glide-ratio
display will read correctly regardless of the pilot's choice of airspeed,
because digital current-glide-ratio display is derived directly from the actual
measured groundspeed divided by the actual measured sink rate, except in the
one special case where the vario is connected to an airspeed probe but not to a
GPS.
Of course, no wind calculations of any kind are performed if no GPS is
connected to the vario.
There is a significant difference between the apparent headwind or tailwind
component used for the upper segmented bar expected-glide-ratio display and the
apparent headwind or tailwind component used for the approach altimeter. The
apparent headwind or tailwind component used in the upper segmented bar display
is constantly updated as the glider changes heading. As the glider circles in
strong wind, the upper segmented bar display will rise and fall according to
the changing apparent tailwind or headwind component. The position of the
glider relative to the target waypoint--if there is a target waypoint--is of no
consequence to the upper segmented bar display. If there is a systematic error
in the way that the vario is calculating the apparent headwind or tailwind
component--for example if the airspeed probe is calibrated to read too high or
too low, or if the pilot is flying without an airspeed probe and is choosing to
fly at an airspeed that is different from the still-air
best-glide-ratio-airspeed for the polar that has been entered into the
vario--then it is quite possible for the upper segmented bar display to act as
if a circling glider were experiencing a continual headwind or tailwind
component throughout the entire course each circle. In contrast, the approach
altimeter only appears when the vario is in "goto" mode, and always
is based on the apparent headwind or tailwind component that the glider would
experience if the glider were to track directly toward the target waypoint.
This apparent headwind/tailwind estimate is only updated when the ground track
of the glider is pointing within 20 degrees of the direction to the target
waypoint. If the glider's ground track has not pointed within 20 degrees of the
target waypoint within the last 30 seconds, the approach altimeter defaults to
an assumption that the wind is zero, and the display begins flashing, to signal
the pilot that the approach altimeter has not been able to create a valid
headwind or tailwind estimate. As soon as the glider's ground track points
toward the target waypoint again, the approach altimeter recalculates the
apparent headwind or tailwind component and the display stops flashing. During
the course of a circle in strong wind (with a turn rate higher than 1
revolution per 30 seconds) the approach altimeter display does not rise and
fall. Whenever the glider's ground track is pointing directly at the target,
the upper segmented bar display and the approach altimeter are both using
similar values for the apparent headwind or tailwind component.
FEATURE: Digital display of vertical speed,
current-glide-ratio, and glide-ratio-to-target ("Upper window")
We'll use the term "upper window" to describe the area near the
top of the display screen that is normally used for a digital display of the
time-averaged vertical speed or netto value. (Brauniger calls this the
"digital vario" window.) This window is also used for the digital
current-glide-ratio and digital glide-ratio-to-target displays.
In setting mode no. 9, if option "0" is selected then this window
displays the digital climb or sink rate, averaged over the time period that was
chosen in setting mode no. 8. If option "1" is selected then this
window displays a "netto" value, averaged over the same time period.
If option "2" is selected then this window displays a
"netto" value when descending and an actual climb rate value when
climbing.
In setting mode no.9, if option "3" is selected, then the vario's
digital glide ratio display will be enabled, and will appear in the upper
window. In this case, the following observations apply:
1) When a GPS is connected to the vario, and the vario is not in
"goto" mode
a) when the glider is descending at a significant
rate
The upper window will display the digital
current-glide-ratio, calculated by dividing the GPS groundspeed by the actual,
measured sink rate as displayed on the analog vario (i.e. after total energy
compensation according the to the selected TEC ratio, if an airspeed probe is
in use). This is the only place on the IQ Comp GPS vario where actual,
real-time glide ratio information is provided--all the other
glide-ratio-related displays on the vario are based on theoretical sink rates
that are derived from the polar, assuming no rise or fall in the airmass,
rather than on the glider's actual measured sink rate. This digital
current-glide-ratio value appears to be averaged over a time constant that is
independent of the time constant that was selected for the digital vertical
speed display in setting mode no. 8. The digital current-glide-ratio value is
averaged over a long enough time constant that it is significantly smoother,
and significantly more useable in rough air, than the current-glide-ratio
display that is available on some pressure-sensor-equipped GPS's such as my
Garmin Etrex Vista. This is true even when the TEC ratio has been set to zero
or the airspeed probe is absent (in which case the sink rate value displayed on
the analog vario and used to calculate the digital current-glide-ratio has no
total energy compensation). Except for transient total-energy effects, the
vario's digital current-glide-ratio display is not affected by the polar that
has been entered into the vario or the by calibration of the airspeed probe and
functions normally even when the airspeed probe is absent.
If the current-glide-ratio is greater than 20:1,
the symbol "--" appears in place of the current-glide-ratio number.
(Obviously this feature was not designed with Swifts or other sailplanes in
mind!)
b) when the glider is descending at a very low
rate, regardless of the groundspeed and glide ratio, or when the glider is
flying horizontally or climbing
The upper window is used for a time-averaged
digital vertical speed display. The netto option is not available.
2) When a GPS is connected to the vario, and the vario is in
"goto" mode
The upper window will be used for a full-time
display of the digital glide-ratio-to-target as long as the vario remains in
"goto" mode. This display includes a 10th's place (e.g.
"4.3") which is useful for detecting slow upward or downward trends
in the glide-ratio-to-target. Neither the digital time-averaged vertical speed
display nor the digital current-glide-ratio display will be visible as long as
the vario remains in "goto" mode.
3) When an airspeed probe is connected to the vario but a GPS is not
a) when the glider is descending at a significant
rate
This is the only situation where the upper window
display is affected by the calibration of the airspeed probe or by whether or
not an airspeed probe is even present, except for transient total energy
effects. The upper window will be used to display a value representing the
measured airspeed divided by the actual, measured sink rate as displayed on the
analog vario (i.e. after total energy compensation according to the selected
TEC ratio).
If the airspeed divided by the sink rate yields a
number that is greater than 20:1, the symbol "--" will appear.
b) when the glider is descending at a very low
rate, regardless of the airspeed, or when the glider is flying horizontally or
climbing
The upper window is used for a time-averaged
digital vertical speed display. The netto option is not available.
FEATURE: Upper segmented bar expected-glide-ratio display
(This is the uppermost of the two parallel segmented bar displays, located
near the bottom of the vario display screen. For brevity we'll call it the
"upper segmented bar display")
1) Whenever a GPS is connected to the vario
The upper segmented bar displays a polar-derived
expected-glide-ratio value for whatever direction the glider is currently
travelling at the current moment. The upper segmented bar display is based on
the assumption that the glider will not encounter any lift or sink while
gliding to the next thermal. The glider's actual vertical speed is ignored,
except that when the glider is climbing rapidly, this affects the upper
segmented bar display, because when thermal lift is expected to be strong, the
upper segmented bar display assumes that the pilot will choose to use a faster
interthermal glide airspeed, as per McCready theory for best cross-country
racing performance in thermal conditions. Therefore in most cases the upper
segmented bar display is downgraded when the glider is climbing rapidly. When
the glider is descending, the glider's recent average climb rate never has any
effect on the upper segmented bar display. The upper segmented bar display has
some damping--it does not respond instantly to changes in groundspeed or
airspeed.
The upper segmented bar display is never affected
in any way by whether or not the attached GPS is in "goto" mode with
an appropriately coded target waypoint. Unlike the approach altimeter, the
upper segmented bar display shows the expected glide performance in whatever
direction the glider is currently travelling at the current moment. Unlike the
approach altimeter, when the glider circles in wind, the upper segmented bar
display rises and falls due to the continually changing apparent headwind or
tailwind component, which is constantly being re-calculated as the glider
changes heading. If there is a systematic error in the way that the vario is
calculating the apparent headwind or tailwind component--for example if the
airspeed probe is calibrated to read too high or too low, or if the pilot is
flying without an airspeed probe and is choosing to fly at an airspeed that is
different from the still-air best-glide-ratio-airspeed for the polar that has
been entered into the vario--then it is quite possible for the upper segmented
bar display to act as if a circling glider were experiencing a continual
headwind or tailwind component throughout the entire course each circle.
If the approach altimeter is active--i.e. if the
vario is in "goto" mode--then whenever the glider is flying directly
at the target, the glide ratio used by the approach altimeter is approximately
the same as the glide ratio displayed by the upper segmented bar.
2) When a GPS and airspeed probe are connected to the vario, in descending
or horizontal flight
The upper segmented bar displays an
expected-glide-ratio value for whatever direction the glider is travelling at
the current moment. This expected-glide-ratio value is based on the glider's
current measured airspeed, and on a polar-derived sink rate value corresponding
to the current measured airspeed. The effect of the wind is also considered.
The apparent headwind or tailwind component is calculated from the difference
between the measured airspeed and the GPS groundspeed at any given moment. In
other words the value displayed by the upper bar is equal to the current GPS
groundspeed divided by a polar-derived sink rate value. The glider's actual
sink rate is ignored. In general, increasing the airspeed and groundspeed by
going to a lower angle-of-attack (i.e. by pulling in the bar) will result in a
lower glide ratio reading, due to the increase in the measured airspeed, which
increases the polar-derived sink rate, unless the airspeed flywheel is
miscalibrated to read dramatically too low, in which case speeding up can
enhance the upper bar reading due to the increase in the groundspeed and the
increase in the apparent tailwind, especially if a fairly flat polar curve has
been entered into the vario.
There is a discontinuity in the displayed glide
ratio values at 74 mph airspeed--higher airspeeds produce unreliable (too
large) readings. Note that 74 mph is also the highest value that may be entered
for an airspeed when entering a polar curve into the vario.
3) When a GPS and airspeed probe are connected to the vario, in climbing
flight
Whenever the glider is climbing, the upper
segmented bar display is no longer based on the glider's current measured
airspeed. When the glider is climbing, the upper segmented bar displays an
expected-glide-ratio value based on an optimal interthermal gliding airspeed
for whatever direction the glider is currently travelling. This optimal
interthermal gliding airspeed is derived from the polar that has been entered
into the vario, according to McCready speed-to-fly theory for best
cross-country racing performance in thermal conditions, considering winds, and
considering the recent average climb rate, and assuming that no lift or sink
will be encountered after the glider leaves the current updraft and goes on
glide. The recent average climb rate value used for this calculation is not
displayed to the pilot, nor can it be manually selected by the pilot. The
recent average climb rate value is apparently averaged over a stored database
of the last 60 seconds or so of climbing flight. The higher the recent average
climb rate, the lower the upper bar reading, because when thermals are strong,
the best cross-country performance can be obtained by using a higher airspeed
when gliding to the next thermal. Note that when the glider actually stops
climbing and goes on glide, the average recent climb rate no longer plays any
role in the upper segmented bar display.
There is a discontinuity in the displayed glide
ratio values at 74 mph airspeed--higher airspeeds produce unreliable (too
large) readings.
Since the vario calculates the apparent headwind
or tailwind component by comparing the groundspeed and airspeed measurements,
if the airspeed probe is calibrated to read too low then the vario will
overestimate the headwind or underestimate the tailwind, and the upper
segmented bar display will read too low. Errors caused by miscalibrations of
the airspeed probe will become more pronounced as the glider's actual airspeed
rises.
When both a GPS and an airspeed probe are
connected to the vario, the way the upper segmented bar display works in a
climb is very different from the way the upper segmented bar display works in a
glide. In particular, if the glider is racing along at a very high speed and
gets lifted into a brief climb while passing though strong lift, the upper
segmented bar display will change dramatically. During the glide, the upper
segmented bar display was projecting a future glide path based on the glider's
actual measured airspeed. During the climb, the upper segmented bar display was
projecting a future glide path based on the optimal airspeed-to-fly according
to McCready theory for best cross-country racing performance in thermal conditions.
4) When only a GPS is connected to the vario, in descending or horizontal
flight
The upper segmented bar displays an
expected-glide-ratio value for whatever direction the glider is travelling at
the current moment. This expected-glide-ratio value considers the effects of
winds, but assumes that the pilot is choosing to fly at the still-air
best-glide-ratio airspeed for the polar that has been entered in to the vario.
The sink rate is taken from the polar curve, without any reference to the
glider's actual measured sink rate. The apparent headwind or tailwind component
is calculated from the difference between the GPS groundspeed and the still-air
best-glide-ratio airspeed for the polar that has been entered into the vario.
In other words the value displayed by the upper bar is equal to the current GPS
groundspeed divided by a polar-derived sink rate value that corresponds to the
still-air best-glide-ratio airspeed for the polar curve that has been entered
into the vario. If the pilot flies at any other airspeed, the upper segmented
bar display will not be accurate. For example, if the pilot flies too fast, the
vario will assume that the pilot is still flying at the still-air
best-glide-ratio-airspeed, with a tailwind, and so the upper segmented bar display
will be overly optimistic.
There is no discontinuity in the upper bar display
at groundspeeds above 74 mph.
5) When only a GPS is connected to the vario, in climbing flight
As the glider climbs, the vario continues to
calculate the apparent headwind or tailwind component from the difference
between the GPS groundspeed and the still-air best-glide-ratio airspeed, so
flight at any other airspeed (such as the min. sink rate airspeed) will cause
the vario to use an erroneous wind estimate for the upper segmented bar
display. The upper segmented bar displays a polar-derived expected-glide-ratio
that relates to the performance that the pilot could expect if he were to go on
glide, in whatever direction the glider is travelling at the current moment, at
some presumed interthermal gliding airspeed. This presumed interthermal gliding
airspeed is usually almost the same as the still-air best-glide-ratio airspeed
for the polar that has been entered into the vario, but not exactly. This
presumed interthermal gliding airspeed is not adjusted upward or downward to
optimize the glider's performance according to the apparent headwind or
tailwind component. However this presumed interthermal gliding airspeed is
adjusted upward when recent climb rates have been high, as per McCready theory
for best cross-country racing performance in thermal conditions. In other words
the vario starts with the assumption that the pilot will go on glide at the
still-air best-glide-ratio-airspeed, and then increases this presumed interthermal
glide airspeed when recent thermal climb rates have been high. In light wind
conditions, or when there is a tailwind, this generally means that the upper
bar display is modified to show a poorer expected-glide-ratio whenever the
glider is climbing in strong lift, because the vario assumes that the pilot
will go on glide at an airspeed that is higher than the still-air
best-glide-airspeed, and so the resulting glide ratio will be poorer. When
there is a strong headwind, the upper segmented bar display behaves a bit
peculiarly. In this case, a fast thermal climb will actually increase the
expected-glide-ratio displayed by the upper segmented bar. This is not normally
what one expects, based on McCready theory. The reason for this odd phenomenon:
when the vario adjusts the presumed interthermal gliding speed upward to
optimize for the strong recent climb rate, this also gives better penetration.
This odd feature is an accidental by-product of the fact that the vario is not
optimizing the presumed interthermal gliding airspeed for the apparent wind
conditions, only for the recent thermal climb rate. It's also a bit odd that
the vario doesn't simply use a presumed interthermal gliding airspeed equal to
the still-air best-glide-ratio airspeed for the polar that has been entered
into the vario, since when the pilot stops climbing and actually goes on glide,
the vario will revert to the assumption that the pilot is flying at the
still-air best-glide-ratio airspeed, and flight at any other airspeed will
cause errors in the upper segmented bar display. When the glider stops climbing
and goes on glide, the average recent climb rate no longer plays any role in
the upper segmented bar display.
The recent climb rate value used to drive the
upper bar display is not displayed to the pilot, and is apparently averaged
over a stored database of the last 60 seconds or so of climbing flight
It not essential that pilots fully understand the
behavior of the upper segmented bar display in climbing flight when only a GPS
is connected to the vario. For all practical purposes, when only a GPS is
connected to the vario, the upper segmented bar display behaves noticeably
different in a climb than in a glide only when recent climb rates have been
quite strong, or when a relatively flat polar curve has been entered into the
vario.
6) When only an airspeed probe is connected to the vario
This is the only situation where the upper bar
display is based on the actual, measured sink rate rather than on a
polar-derived sink rate. The upper bar displays the measured airspeed divided
by the actual, measured sink rate as displayed on the analog vario (i.e. after
total energy compensation according to the selected TEC ratio).
This is the same value that is displayed in the
top window under these conditions. These are the only conditions where the
upper bar display and the digital expected-glide-ratio consistently show
similar values.
FEATURE: Lower segmented bar glide-ratio-to-target
display
(This is the lowermost of the two parallel segmented bar displays, located
near the bottom of the vario display screen. For brevity we'll call it the
"lower segmented bar display")
1) When the vario is in "goto" mode
The lower segmented bar displays the
glide-ratio-to-target, i.e. the horizontal distance to the target divided by
the glider's elevation over the target. This is the glide ratio that the glider
must achieve if it is to reach to the target. The display is blank if the
glide-ratio-to-target is <4. If the vario's digital glide ratio function has
been enabled, then the glide-ratio-to-target will also appear in digital form
in the "upper window" display area.
2) When the vario is not in "goto" mode
The lower segmented bar displays battery strength.
FEATURE: Approach altimeter
The approach altimeter is only available when the vario is in
"goto" mode, i.e. when the vario is attached to a GPS that is locked
on to an appropriately coded waypoint.
The approach altimeter display is based both upon the glider's physical
position in space with respect to the target, and upon a polar-derived model
for the glider's expected performance on the remaining portion of the glide.
The approach altimeter digital readout displays the expected arrival height
at goal. The range of possible readings is from "-999" to
"999". A reading of "999" represents 9990 feet, or 999
meters, depending upon the units that the pilot has selected in the setup menu.
The approach altimeter has a "safety margin" feature; for
simplicity here we'll assume that the pilot has selected a safety margin of
zero in the setup menu.
Unlike the upper segmented bar expected-glide-ratio display, the approach
altimeter relates to the performance that would be expected if the glider were
to follow a ground track that would take it directly toward the target
waypoint. Unlike the upper segmented bar display, when the glider circles in
wind, the approach altimeter display remains steady and does not rise and fall
as the headwind or tailwind component changes. The approach altimeter only
updates its estimate of the apparent headwind or tailwind component when the
glider's ground track is pointing within 20 degrees of the target waypoint. If
the glider's ground track has not pointed within 20 degrees of the target
waypoint in the last 30 seconds, the approach altimeter begins flashing, which
signifies that it has switched to a default assumption of zero wind. The
approach altimeter reading may change dramatically at this point. As soon as
the glider's ground track points within 20 degrees of the target waypoint, the
approach altimeter will again sample the apparent headwind or tailwind
component.
The approach altimeter is based both on the glider's current position in
space with respect to the target waypoint, and on the forecasted gliding
performance that the vario expects the glider to achieve as it travels toward
the target waypoint. The forecasted gliding performance matches the glide ratio
shown on the upper segmented bar expected-glide-ratio display, at least during
those moments that the ground track of the glider is pointing directly at the
target. To forecast the performance that the glider will achieve as it glides
toward the target waypoint, the approach altimeter uses a polar-derived sink
rate. The glider's actual vertical speed is ignored, except that when the
glider is climbing rapidly, this affects the approach altimeter display,
because when thermal lift is expected to be strong, the approach altimeter
assumes that the pilot will choose to use a faster interthermal glide airspeed,
as per McCready theory for best cross-country racing performance in thermal
conditions. Therefore in most cases the approach altimeter reading is
downgraded when the glider is climbing rapidly. When the glider is descending,
the glider's recent average climb rate never has any effect on the approach
altimeter display.
The approach altimeter has some damping--it does not respond instantly to
changes in groundspeed or airspeed.
Since the approach altimeter is based in part on the glider's position in
space relative to the target waypoint, a patch of strong sink will produce a
downward trend in the approach altimeter as the glider loses altitude. This
downward trend is only due to the way that the sink is affecting the glider's
current position in space. Even when the glider is in strong sink, the
forecasted gliding performance that the vario expects the glider to achieve
during the remaining portion of the glide to the goal is still based on the
assumption that the remaining portion of the glide will occur in air with no
vertical motion. Obviously this assumption is not always realistic, as we
explored in Part 2. If the approach altimeter reading is positive, but is
steadily trending downward, pilots need to be aware that may not be able to
reach the target, at least at the current airspeed, if they are in an area of
widespread sink.
The details of the way the approach altimeter functions depend on whether or
not an airspeed probe is connected to the vario, and upon whether the glider is
descending, climbing, or flying horizontally:
1) When a GPS and airspeed probe are connected to the vario, in descending
flight
The approach altimeter display is based on the
measured airspeed, the apparent headwind or tailwind component, and a
polar-derived sink rate value corresponding to the measured airspeed. The
apparent headwind or tailwind is only updated when the glider's ground track is
pointing within 20 degrees of the target waypoint, and is calculated from the
difference between the GPS groundspeed and the measured airspeed. In general,
increasing the airspeed and groundspeed by going to a lower angle-of-attack
(i.e. by pulling in the bar) will result in a lower approach altimeter reading,
due to the increase in the polar-derived sink rate, unless the airspeed flywheel
is miscalibrated to read dramatically too low, in which case speeding up can
enhance the approach altimeter reading due to the increase in the groundspeed
and the increase in the apparent tailwind, especially if a fairly flat polar
curve has been entered into the vario.
There is a discontinuity in approach altimeter
display at 74 mph airspeed--higher airspeeds produce unreliable readings. Note
that 74 mph is also the highest value that may be entered for an airspeed when
entering a polar curve into the vario.
An icon appears if the pilot should be able to
make the target by flying at the airspeed that will yield the flattest glide
ratio given the existing apparent headwind or tailwind component, assuming that
the atmosphere will have no vertical motion while the glider is gliding toward
the target.
This icon will not appear if the approach
altimeter display is flashing, i.e. if the glider's ground track has not
pointed within 20 degrees of the target in the last 30 seconds.
2) When a GPS and airspeed probe are connected to the vario, in horizontal
flight
In horizontal flight, the approach altimeter
behaves just as it does in descending flight, except for one very strange
quirk: when the analog vario display needle is pointing exactly at zero, showing
neither a climb nor a descent, the approach altimeter assumes that the winds
are zero. This may be a software bug. This is quirk is difficult to observe in
actual flight when both a GPS and an airspeed probe are attached to the vario,
because even without this quick, there would be usually be big jump in the
approach altimeter reading whenever the glider transitioned from a glide to a
climb or vs. vs..
Even though the approach altimeter is assuming the
winds to be zero, the numerical display does not flash, unless the glider's
ground track has not pointed within 20 degrees of the target waypoint within
the last 30 seconds.
3) When a GPS and airspeed probe are connected to the vario, in climbing
flight
When the glider is climbing, the approach
altimeter display is no longer based on the glider's current measured airspeed.
When the glider is climbing, the approach altimeter is based on the performance
that would be expected if the glider were to begin gliding toward the target
waypoint at the optimal cross-country racing airspeed, derived from the polar,
according to McCready theory, considering apparent winds and recent climb
rates, and assuming that no lift or sink will be encountered after the current
climb ends. The recent climb rate value used to drive the approach altimeter is
not displayed to the pilot, nor can it be manually selected by the pilot. The
recent average climb rate value is apparently averaged over a stored database
of the last 60 seconds or so of climbing flight. The higher the recent average
climb rate, the lower the approach altimeter reading, because when thermals are
strong, the best cross-country performance can be obtained by going on glide at
a high airspeed. Note that when the glider actually stops climbing and goes on
glide, the average recent climb rate no longer plays any role in the approach
altimeter display.
There is a discontinuity in the approach altimeter
display at 74 mph airspeed--higher airspeeds produce unreliable readings.
Since the vario calculates the apparent headwind
or tailwind component by comparing the groundspeed and airspeed measurements,
if the airspeed probe is calibrated to read too low then the vario will
overestimate the headwind or underestimate the tailwind, and the approach
altimeter display will read too low. Errors caused by miscalibrations of the
airspeed probe will become more pronounced as the glider's actual airspeed
rises.
An icon flashes whenever the pilot should be able
to make the target by flying at the airspeed that will yield the flattest glide
ratio given the existing apparent headwind or tailwind component, assuming that
the glider will encounter no updrafts or downdrafts while on glide. The icon
becomes steadily illuminated whenever the pilot should be able to make the
target by flying at the optimal airspeed for cross-country racing as per
McCready theory, given the existing headwind or tailwind component and recent
average climb rate, and assuming that the glider will encounter no updrafts or
downdrafts while on glide. The icon changes from flashing to steadily
illuminated at the same time as the approach altimeter changes from a negative
reading to zero. When the icon is flashing rather than steadily lit, the glider
can make the target by flying at the airspeed that will yield that flattest
possible glide ratio given the existing headwind or tailwind component, but the
glider would make the best use of the thermal, and reach the target faster, if
it were to continue climbing for a while and then go on glide at a faster
airspeed. If the recent climb rates have been quite low, the icon will not
flash; it will appear in its steadily illuminated form at the same time as the
approach altimeter changes from a negative number to zero.
The icon does not appear when the approach
altimeter numerical display is flashing, i.e. when the glider's ground track
has not pointed within 20 degrees of the target in the last 30 seconds.
When both a GPS and an airspeed probe are
connected to the vario, the way the approach altimeter works in a climb is very
different from the way the approach altimeter works in a glide. In particular,
if the glider is racing along at a very high speed and gets lifted into a brief
climb while passing though strong lift, the approach altimeter display will
change dramatically. During the glide, the approach altimeter was projecting a
future glide path based on the glider's actual measured airspeed. During the
climb, the approach altimeter was projecting a future glide path based on the
optimal airspeed-to-fly according to McCready theory for best cross-country
racing performance in thermal conditions.
4) When only a GPS is connected to the vario, in descending flight
The approach altimeter display assumes that the
pilot will fly toward the target at the still-air best-glide-ratio-airspeed for
the polar that has been entered into the vario. The glider's actual sink rate
is ignored. The expected glide performance is adjusted for the apparent
headwind or tailwind component, which is calculated from the difference between
the GPS groundspeed and the still-air best-glide-ratio airspeed. The apparent
headwind or tailwind component is only updated when the glider's ground track
is pointing within 20 degrees of the target waypoint. If the pilot flies faster
or slower than the still-air best-glide-ratio airspeed for the polar that has
been entered into the vario, the approach altimeter display will not be
accurate. For example, if the pilot flies too fast, the vario will assume that
the pilot is flying at the still-air best-glide-ratio airspeed, with a
tailwind, and so the approach altimeter will give an overly optimistic reading.
There is no discontinuity in the approach
altimeter display at groundspeeds above 74 mph.
An icon remains steadily illuminated whenever the
pilot should be able to make the target by flying at the still-air best-glide
speed. This means that the icon is illuminated whenever the approach altimeter
reading is zero or positive.
The icon does not appear when the approach altimeter
numerical display is flashing, i.e. when the glider's ground track has not
pointed within 20 degrees of the target in the last 30 seconds.
5) When only a GPS is connected to the vario, in horizontal flight
In horizontal flight, the approach altimeter
behaves just as it does in descending flight, except for one very strange
quirk: when the analog vario display needle is pointing exactly at zero,
showing neither a climb nor a descent, the approach altimeter assumes that the
winds are zero. This may be a software bug. This produces some interesting
jumps in the approach altimeter display whenever a bit of turbulence brings the
analog vario display needle through the zero mark, especially in windy
conditions. These fluctuations are much more noticeable when only a GPS is
connected to the vario, than when an airspeed probe is also connected to the
vario. The reason for this: when only a GPS is connected to the vario, the way
the approach altimeter works in a climb is very similar to the way the approach
altimeter works in a glide, so the anomaly that occurs when the analog vario
pointer is exactly on "zero" is very noticeable.
Even though the approach altimeter is assuming the
winds to be zero, the numerical display does not flash, unless the glider's ground
track has not pointed within 20 degrees of the target waypoint within the last
30 seconds.
6) When only a GPS is connected to the vario, in climbing flight
As the glider climbs, the vario continues to
calculate the apparent headwind or tailwind component from the difference
between the GPS groundspeed and the still-air best-glide-ratio airspeed, so
flight at any other airspeed (such as the min sink airspeed) will cause the
vario to use an erroneous wind estimate for the approach altimeter display. Again,
for the purposes of the approach altimeter, the apparent winds are only updated
when the glider's ground track points within 20 degrees of the target waypoint.
The approach altimeter is based on a polar-derived expected-glide-ratio value
that relates to the performance that the pilot could expect if he were to go on
glide, toward the target waypoint, at some presumed interthermal gliding
airspeed. This presumed interthermal gliding airspeed is based on the still-air
best-glide-ratio airspeed, but is adjusted upward to optimize the glider's
performance when recent thermal climb rates have been strong, as per McCready
theory for best cross-country racing performance in thermal conditions. This
presumed interthermal gliding airspeed is not adjusted upward or downward to
optimize the glider's performance in an apparent headwind or tailwind. In light
wind conditions, or when there is a tailwind, this generally means that the
approach altimeter display is modified to show a poorer expected-glide-ratio
whenever the glider is climbing in strong lift, because the vario assumes that
the pilot will go on glide at an airspeed that is higher than the still-air
best-glide-airspeed, and so the resulting glide ratio will be poorer. When
there is a strong headwind, the approach altimeter display behaves a bit
peculiarly. In this case, a fast thermal climb will actually increase the
expected-glide-ratio used by the approach altimeter. This is not normally what
one expects, based on McCready theory. The reason for this odd phenomenon: when
the vario adjusts the presumed interthermal gliding speed upward to optimize
for the strong recent climb rate, this also gives better penetration. This odd
feature is an accidental by-product of the fact that the vario is not
optimizing the presumed interthermal gliding airspeed for the apparent wind
conditions, only for the recent thermal climb rate. It's also a bit odd that
the vario doesn't simply use a presumed interthermal gliding airspeed equal to
the still-air best-glide-ratio airspeed for the polar that has been entered
into the vario, since when the pilot stops climbing and actually goes on glide,
the vario will revert to the assumption that the pilot is flying at the
still-air best-glide-ratio airspeed, and flight at any other airspeed will
cause errors in the approach altimeter display. When the glider stops climbing
and goes on glide, the average recent climb rate no longer plays any role in
the approach altimeter display.
The recent climb rate value used to drive the
approach altimeter is not displayed to the pilot, and is apparently averaged
over a stored database of the last 60 seconds or so of climbing flight
An icon flashes whenever the pilot can make the
target waypoint by going on glide at the still-air best-glide-ratio airspeed
for the polar that has been entered into the vario, assuming that the glider
will encounter no updrafts or downdrafts while on glide. The icon then becomes
steadily illuminated whenever the pilot should be able to make the target by
flying at a presumed interthermal gliding airspeed that is based on the
still-air best-glide-ratio airspeed, but adjusted upward to optimize the
glider's performance when recent thermal climb rates have been strong, as per
McCready theory for best cross-country racing performance in thermal
conditions. This is the same presumed interthermal gliding airspeed that is
used by the numerical portion of the approach altimeter display. It is not
adjusted upward or downward to optimize the glider's performance in an apparent
headwind or tailwind. Therefore, the behavior of the icon when the glider is
climbing is a bit complex and depends on the apparent headwind or tailwind
component. In light winds or when there is an apparent tailwind, the icon
changes from flashing to steadily illuminated at the same time as the approach
altimeter changes from a negative reading to zero. But if the glider will face
a strong headwind when it goes on glide, then there will not be any interval in
the climb when the icon is flashing. Instead, it will simply appear in its
steadily illuminated form at the same time as the approach altimeter changes
from a negative number to zero. The reason: with a strong headwind, flying to
the target at the still-air best-glide ratio airspeed will actually take more altitude
than flying to the target at a higher airspeed, which will yield better
penetration. Again, this odd behavior is an accidental by-product of the fact
that the vario is not optimizing the presumed interthermal gliding airspeed for
the apparent wind conditions, only for the recent thermal climb rate. In any
wind conditions, if the recent climb rates have been quite low, the icon will
never be seen to flash; it will appear in its steadily illuminated form at the
same time as the approach altimeter changes from a negative number to zero.
The icon does not appear when the approach
altimeter numerical display is flashing, i.e. when the glider's ground track
has not pointed within 20 degrees of the target in the last 30 seconds.
It not essential that pilots fully understand the
behavior of the approach altimeter icon and numerical display in climbing
flight when only a GPS is connected to the vario. For all practical purposes,
when only a GPS is connected to the vario, the approach altimeter behaves
noticeably different in a climb than in a glide only when recent climb rates
have been quite strong, or when a relatively flat polar curve has been entered
into the vario.
FEATURE: McCready speed-to fly audio functions and
visual pointers
The McCready speed-to-fly pointers cannot be switched on unless an airspeed
probe is connected to the vario. Also, the McCready pointers cannot be switched
on unless the barograph is active. The McCready pointers are switched on with
the same button that is used to adjust the altimeter reading in the downward
direction; the altimeter cannot be adjusted when the barograph is active. The
McCready speed-to-fly pointers function even when the vario is not connected to
a GPS, although in this case the vario cannot take the wind into consideration.
If the McCready acoustics have been switched on, then whenever the McCready
pointers are switched on, there will also be an audio tone that allows the
pilot to use the McCready pointers without looking at the vario. Normally this
tone is replaced by the usual climb acoustics whenever the glider is climbing.
Two different McCready speed-to-fly pointers appear in the analog vario
display window. One of the McCready pointers represents the glider's average
climb rate over the last 10 minutes of climbing. This serves as a predictor of
the expected average climb rate for the near future. (It appears that the vario
also keeps track of the average climb rate during the last 60 seconds or so of
climbing flight; this shorter-term value is used in the calculations that drive
the approach altimeter and the upper segmented bar display during climbing
flight.) The other McCready pointer is called the "active McCready
pointer" and essentially represents the optimum airspeed-to-fly for a
given set of conditions including wind, updrafts or downdrafts, and expected
future thermal climb rate. More precisely, the "active McCready
pointer" points to the expected future thermal climb rate for which the
glider's current airspeed would be exactly optimal for the fastest
cross-country racing performance, considering the polar curve, and considering
the current conditions, including wind, and including any updrafts or
downdrafts that are present at the current moment. When the pilot increases the
glider's airspeed, the "active McCready pointer" moves upward to
point toward a higher rate-of-climb value (or a lower sink rate value), and
when the pilot decreases the glider's airspeed, the "active McCready
pointer" moves downward to point toward a lower rate-of-climb value (or a
higher sink rate value).
It's not the purpose of this article to get into a comprehensive description
of McCready speed-to-fly theory, but here are a few general observations: when
the "active McCready pointer" points to "zero", the glider
is flying at the airspeed that will yield the flattest possible glide path,
given the existing wind conditions, and given any updrafts or downdrafts that
are present. (For now we're assuming that any updrafts encountered on glide are
weak enough that the glider cannot actually climb; McCready theory gets a bit
more complex when dealing with strong updrafts encountered on glide.) If a
pilot is only interested in getting the flattest possible glide path, and is
not interested in cross-country racing, then he should simply fly at whatever
airspeed puts the "active McCready pointer" on zero. This will lead
the pilot to speed up in a headwind, slow down in a tailwind, speed up in sink,
and slow down in weak lift. If the pilot is interested in getting the best cross-country
performance, then he should fly at a higher airspeed, so that the "active
McCready pointer" rises above zero to point to a value that matches the
pilot's expectation of the future average thermal strength for the remainder of
the task. (Unless the pilot expects a change in future thermal strength, he can
simply align the "active McCready pointer" with the
average-past-climb-rate pointer). This strategy will lead the pilot to adjust
his airspeed in tailwinds, headwinds, sink, or lift, exactly as if he were trying
to get the flattest possible glide, except that he will end up using higher
airspeeds, and will end up following glide paths that are steeper than the
flattest possible glide path. The stronger the expected future thermal
strength, the more the resulting airspeeds and glide paths will deviate from
the flattest possible glide path.
The great advantage of this type of display, over older styles of McCready
displays, is that the pilot need not adjust the instrument in flight to enter
the expected future thermal strength. The display continually gives information
about the best speed to fly for the flattest possible glide and also about the
best speed to fly for optimal cross-country racing performance for any
anticipated average future climb rate.
When the "active McCready pointer" points to a negative value, the
pilot is flying slower than the airspeed that would yield the flattest glide
path. There is no benefit to this, unless the glider is actually able to climb.
However the "active McCready" typically does not point to a negative
value when the glider is climbing (see below). Therefore there is never any
benefit to flying with "active McCready pointer" pointing to a
negative value.
If the McCready acoustics have been switched on, then whenever the
"active McCready pointer" points to a negative value, a warning tone
will sound. This tone is very similar to the tone that is used for the sink
alarm. If the McCready acoustics have been switched on, then whenever the
"active McCready pointer" points toward a positive value, a tone will
sound that is modulated in a way that provides an indication of the position of
the "active McCready pointer", so that the pilot can use the McCready
functions without actually looking at the vario.
It is possible to switch off the McCready acoustics and just fly by visual
reference to the McCready pointers. The McCready acoustics, and the sink alarm,
are both toggled on and off by the "descent acoustics" button. When
the McCready pointers have not been switched on, this button toggles the sink
alarm on and off. When the McCready pointers have been switched on, this button
no longer toggles the sink alarm on and off, it only toggles the McCready
acoustics on and off. If the McCready pointers have been switched on but the
McCready acoustics have been toggled to "off", then the sink alarm
will be available, or not, depending on whether the sink alarm was last toggled
to "on" or "off". However, the McCready acoustics have
priority over the sink alarm--the sink alarm will never be heard when the
McCready acoustics have been toggled "on" and the McCready pointers
have been switched on.
The McCready pointers have to be newly switched on at the beginning of each
flight, after turning on the barograph, but the vario remembers whether the sink
alarm has been toggled "on" or "off", and whether the
McCready acoustics have been toggled "on" or "off".
The vario has been designed so that the McCready acoustics are normally not
heard while climbing, since they would interfere with the vario's normal climb
acoustics. (Sometimes the McCready acoustics are heard for about one second
when the glider transitions from a glide to a climb). A short time after the
glider transitions from a climb to a glide, the McCready acoustics become
active. This time period is the "McCready switching time" and may be
selected by the pilot in setting mode number 12. The selected time period
should be long enough so that the McCready acoustics are only heard when a
pilot has left a thermal and gone on glide, and not when a pilot has just
strayed from the thermal core for a few seconds. The default setting is seven
seconds.
Oddly, if the "McCready delay" is set to zero, then the normal
rate-of-climb acoustic is omitted and the pilot will hear the McCready acoustic
even while climbing. It most situations this would be quite annoying.
The behavior of the "active McCready pointer" and the McCready
acoustics during a climb seems strange to me--perhaps they have been designed
for some application that I don't fully understand, or perhaps they do not
really function as the designer intended. During climbing flight, the
"active McCready pointer" nearly always points to a positive value.
If the rate of climb is strong, or if the glider's airspeed is well above the
min. sink speed for the polar that has been entered into the vario, the
"active McCready pointer" is often pegged at the top of the scale.
When the glider's airspeed drops below min. sink and the glider is still
climbing, the "active McCready" pointer vanishes, but the McCready
acoustics continue to give a sound that corresponds to a positive value for the
"active McCready pointer". Normally, when the "active McCready
pointer" points to a positive value, this indicates that the pilot should
fly slower if he wishes to achieve the flattest possible glide path, and that
if future thermals are expected to be about equal in strength to the current
position of the "active McCready pointer", then he is flying at the
optimum airspeed for best cross-country racing performance, given the
conditions that exist at the current moment. That interpretation of a positive
McCready value doesn't fit the behavior of the "active McCready
pointer" and the McCready acoustics during climbing flight, particularly
when the glider's airspeed drops below min. sink. One could imagine several
other possible ways that McCready functions could be designed to operate when a
glider is climbing, but I can't come up with anything that fully matches the
behavior of the McCready indicators on the IQ GPS Comp during climbing flight,
especially at airspeeds below min. sink. Since I haven't been able to decipher
exactly what the "active McCready pointer" signifies during climbing
flight, I only pay attention to this pointer when the glider is descending. Likewise
for the McCready acoustics. Of course, most pilots will choose to set the
"McCready switching time" at some number other than "zero",
so they will not hear the McCready acoustics in climbing flight.
One more little quirk--if the McCready pointers have been switched on, and
the airspeed probe becomes disconnected, the McCready pointers and audio will
immediately vanish, but the "McCready" symbol on the vario will not
vanish until the airspeed indicator is reconnected.
FEATURE: Entering a polar curve
The user can enter a polar curve into the vario by entering 2 pairs of
(airspeed, sink rate) data points. The first of these data points is assumed to
correspond to the glider's minimum-sink-rate airspeed. The second data point
can be any other point on the polar--technically it can even involve an
airspeed that is slower than the minimum-sink rate-airspeed.
It is easy to use the Brauniger PC Graph software to invent a polar curve or
to fit a polar curve to a set of existing data points. The PC Graph software
can take any polar, defined by two pairs of (airspeed, sink rate) data points
as described above, and print out the airspeed that will yield the best L/D
ratio (i.e. the best glide ratio in still air).
There will be some rounding error when taking a pair of (airspeed, sink
rate) data points from the vario and entering it into PC Graph, or vs. vs.,
because the vario and the PC Graph software do not use the same increments.
We've mentioned that when a pilot chooses to fly without an airspeed sensor,
the vario performs wind and glide ratio calculations by assuming that the pilot
is always choosing to fly at the airspeed that that matches the still-air
best-glide-ratio-airspeed for the polar that has been entered into the vario.
Pilots who choose to fly without an airspeed sensor may wish to know the
precise value of the still-air best-glide-ratio airspeed for the polar that has
been entered into the vario. This is easily accomplished by going into setting
mode, checking to see what polar coordinates have been entered into the vario,
and then entering those same polar coordinates into the PC Graph software. The
PC Graph program will immediately calculate a best L/D airspeed, which is also
the still-air best-glide-ratio airspeed. If that airspeed does not seem
satisfactory, it is easy to use the PC Graph software to invent a polar that
has a still-air best-glide-ratio-airspeed occurring at whatever airspeed the
pilot desires, matched to whatever sink rate the pilot feels is realistic for
that airspeed.
As long as the pilot is not flying with an airspeed sensor, the shape of the
rest of the polar curve will not matter and need not represent a realistic
polar for an actual glider. When the pilot is flying with a GPS but no airspeed
sensor, the only point on the polar that has any significance is the still-air
best-glide-ratio airspeed, and the associated sink rate. There is one exception
to this rule: when the glider is climbing without an airspeed indicator, the
approach altimeter and the upper segmented bar expected-glide-ratio displays do
apply McCready theory to increase the expected interthermal glide speed above
and beyond the still-air best-glide-ratio-airspeed whenever the recent climb
rates have been strong, as discussed above. This adjustment is contingent upon
the shape of the polar. This adjustment is not meant to accommodate headwinds
or tailwinds, only to accommodate strong recent thermal climb rates. This
adjustment only occurs when the glider is climbing, not when the glider is
flying horizontally or descending. In actual practice this upward modification
of the assumed interthermal glide speed is rarely noticeable unless the recent
average climb rate has been very high. But if the pilot doesn't like this
upward modification of the expected interthermal glide speed during climbing
flight--after all, when the pilot actually goes on glide, he must remain at the
still-air best-glide-ratio airspeed if he wants the approach altimeter and the
upper bar display to work correctly--then he can use the PC Graph software to
circumvent it. He can invent a polar that has a strong "peak" at some
desired airspeed, and then "drops off" very abruptly to dramatically
higher sink rates at all other airspeeds. With this type of polar, when the
pilot is flying without an airspeed indicator, the vario will always assume
that the pilot will fly at an airspeed that is virtually the same as the
still-air best-glide-ratio airspeed for the polar that has been entered into
the vario, even when the glider is climbing and the recent average climb rate
has been high.
PART 7: A DESCRIPTION OF VARIO
FEATURES, ORGANIZED BY “MODE”
This section contains much of the
information that is given in Part 6, but we’ve now organized the information by
the various “modes” in which the vario may be used. Although this creates some repetition, the goal is to allow
readers to easily skip over all the information that doesn’t pertain to the way
that they plan to use the vario in actual practice.
In relation to the “upper window” display”, we’ll assume that
option 3 (digital glide ratio display) has been chosen in setting mode number
9. See Part 6 for more on how the “upper
window” display behaves when options 0 (digital climb or sink rate), 1 (netto),
or 2 (netto during descent, climb rate during climb) have been chosen in
setting mode number 9.
MODE 1: GPS and airspeed probe are connected to the vario, and the GPS is not in “goto” mode
toward a properly coded waypoint
MODE 2: GPS and airspeed probe are connected to the vario, and the GPS is in “goto” mode toward
a properly coded waypoint
MODE 3: Only a GPS is
connected to the vario, and the GPS is not in “goto” mode toward a properly
coded waypoint
MODE 4: Only a GPS is
connected to the vario, and the GPS is in “goto” mode toward a properly coded
waypoint:
MODE 5: Only an airspeed
probe is connected to the vario
MODE 6: When neither a GPS nor
an airspeed probe are connected to the vario
“MODE 1”: When a GPS and an
airspeed probe are connected to the vario, and the GPS is not in “goto” mode
toward a properly coded waypoint:
“Upper window” display: When the
glider is descending at a significant rate, the “upper window” will display the
digital current-glide-ratio, calculated by dividing the GPS groundspeed by the
actual, measured sink rate as displayed on the analog vario (i.e. after total
energy compensation according the to the selected TEC ratio). This digital
current-glide-ratio value appears to be averaged over a time constant that is
independent of the time constant that was selected for the digital vertical
speed display in setting mode no. 8. The digital current-glide-ratio value is
averaged over a long enough time constant that it is significantly smoother,
and significantly more useable in rough air, than the current-glide-ratio
display that is available on some pressure-sensor-equipped GPS's such as my
Garmin Etrex Vista. This is true even when the TEC ratio has been set to
zero. This is the only place on the IQ
Comp GPS vario where actual, real-time glide ratio information is provided--all
the other glide-ratio-related displays on the vario are based on theoretical
sink rates that are derived from the polar, assuming no rise or fall in the
airmass, rather than on the glider's actual measured sink rate. If the current-glide-ratio is greater than 20:1,
the symbol "--" appears in place of the current-glide-ratio number:
obviously this feature was not designed with Swifts or other sailplanes in
mind!
When the glider is descending at a
very low rate, regardless of the groundspeed and glide ratio, or when the
glider is flying horizontally or climbing, the “upper window” is used for a
time-averaged digital vertical speed display.
“Upper segmented bar” display: In
descending or horizontal flight, the upper segmented bar displays an
expected-glide-ratio value for whatever direction the glider is travelling at
the current moment. This expected-glide-ratio value is based on the glider's
current measured airspeed, and on a polar-derived sink rate value corresponding
to the current measured airspeed. The effect of the wind is also considered.
The apparent headwind or tailwind component is calculated from the difference
between the measured airspeed and the GPS groundspeed at any given moment. In
other words the value displayed by the upper bar is equal to the current GPS
groundspeed divided by a polar-derived sink rate value. The glider's actual
sink rate is ignored. In general,
increasing the airspeed and groundspeed by going to a lower angle-of-attack
(i.e. by pulling in the bar) will result in a lower glide ratio reading, due to
the increase in the measured airspeed, which increases the polar-derived sink
rate, unless the airspeed flywheel is miscalibrated to read dramatically too
low, in which case speeding up can enhance the upper bar reading due to the increase
in the groundspeed and the increase in the apparent tailwind, especially if a
fairly flat polar curve has been entered into the vario.
In climbing flight, the upper
segmented bar display is no longer based on the glider's current measured
airspeed. When the glider is climbing, the upper segmented bar displays an
expected-glide-ratio value based on an optimal interthermal gliding airspeed
for whatever direction the glider is currently travelling. This optimal
interthermal gliding airspeed is derived from the polar that has been entered
into the vario, according to McCready speed-to-fly theory for best
cross-country racing performance in thermal conditions, considering winds, and
considering the recent average climb rate, and assuming that no lift or sink
will be encountered after the glider leaves the current updraft and goes on
glide. The recent average climb rate value used for this calculation is not
displayed to the pilot, nor can it be manually selected by the pilot. The
recent average climb rate value is apparently averaged over a stored database
of the last 60 seconds or so of climbing flight. The higher the recent average climb rate, the lower the upper bar
reading, because when thermals are strong, the best cross-country performance
can be obtained by using a higher airspeed when gliding to the next thermal.
Note that when the glider actually stops climbing and goes on glide, the
average recent climb rate no longer plays any role in the upper segmented bar
display.
Since the vario calculates the
apparent headwind or tailwind component by comparing the groundspeed and
airspeed measurements, if the airspeed probe is calibrated to read too low then
the vario will overestimate the headwind or underestimate the tailwind, and the
upper segmented bar display will read too low. Errors caused by miscalibrations
of the airspeed probe will become more pronounced as the glider's actual
airspeed rises.
When both a GPS and an airspeed
probe are connected to the vario, the way the upper segmented bar display works
in a climb is very different from the way the upper segmented bar display works
in a glide. In particular, if the glider is racing along at a very high speed
and gets lifted into a brief climb while passing though strong lift, the upper
segmented bar display will change dramatically. During the glide, the upper
segmented bar display was projecting a future glide path based on the glider's
actual measured airspeed. During the climb, the upper segmented bar display was
projecting a future glide path based on the optimal airspeed-to-fly according
to McCready theory for best cross-country racing performance in thermal
conditions.
Regardless of whether the flight
path is descending, horizontal, or climbing, whenever both a GPS and an
airspeed probe are connected to the vario, there is a discontinuity in the
displayed glide ratio values at 74 mph airspeed--higher airspeeds produce
unreliable (too large) readings. Note that 74 mph is also the highest value
that may be entered for an airspeed when entering a polar curve into the vario.
“Lower segmented bar” display: The
lower segmented bar displays battery strength.
Approach altimeter: Not available.
“MODE 2”: When a GPS and an
airspeed probe are connected to the vario, and the GPS is in “goto” mode toward
a properly coded waypoint:
“Upper window” display: The upper
window will be used for a full-time display of the digital
glide-ratio-to-target as long as the vario remains in "goto" mode.
This display includes a 10th's place (e.g. "4.3") which is useful for
detecting slow upward or downward trends in the glide-ratio-to-target.
“Upper segmented bar”
display: In descending or horizontal
flight, the upper segmented bar displays an expected-glide-ratio value for
whatever direction the glider is travelling at the current moment. This
expected-glide-ratio value is based on the glider's current measured airspeed,
and on a polar-derived sink rate value corresponding to the current measured
airspeed. The effect of the wind is also considered. The apparent headwind or
tailwind component is calculated from the difference between the measured
airspeed and the GPS groundspeed at any given moment. In other words the value
displayed by the upper bar is equal to the current GPS groundspeed divided by a
polar-derived sink rate value. The glider's actual sink rate is ignored. In general, increasing the airspeed and
groundspeed by going to a lower angle-of-attack (i.e. by pulling in the bar)
will result in a lower glide ratio reading, due to the increase in the measured
airspeed, which increases the polar-derived sink rate, unless the airspeed
flywheel is miscalibrated to read dramatically too low, in which case speeding
up can enhance the upper bar reading due to the increase in the groundspeed and
the increase in the apparent tailwind, especially if a fairly flat polar curve
has been entered into the vario.
In climbing flight, the upper
segmented bar display is no longer based on the glider's current measured
airspeed. When the glider is climbing, the upper segmented bar displays an
expected-glide-ratio value based on an optimal interthermal gliding airspeed
for whatever direction the glider is currently travelling. This optimal
interthermal gliding airspeed is derived from the polar that has been entered
into the vario, according to McCready speed-to-fly theory for best
cross-country racing performance in thermal conditions, considering winds, and
considering the recent average climb rate, and assuming that no lift or sink
will be encountered after the glider leaves the current updraft and goes on
glide. The recent average climb rate value used for this calculation is not
displayed to the pilot, nor can it be manually selected by the pilot. The
recent average climb rate value is apparently averaged over a stored database of
the last 60 seconds or so of climbing flight.
The higher the recent average climb rate, the lower the upper bar
reading, because when thermals are strong, the best cross-country performance
can be obtained by using a higher airspeed when gliding to the next thermal.
Note that when the glider actually stops climbing and goes on glide, the
average recent climb rate no longer plays any role in the upper segmented bar
display.
Since the vario calculates the
apparent headwind or tailwind component by comparing the groundspeed and
airspeed measurements, if the airspeed probe is calibrated to read too low then
the vario will overestimate the headwind or underestimate the tailwind, and the
upper segmented bar display will read too low. Errors caused by miscalibrations
of the airspeed probe will become more pronounced as the glider's actual
airspeed rises.
When both a GPS and an airspeed
probe are connected to the vario, the way the upper segmented bar display works
in a climb is very different from the way the upper segmented bar display works
in a glide. In particular, if the glider is racing along at a very high speed
and gets lifted into a brief climb while passing though strong lift, the upper
segmented bar display will change dramatically. During the glide, the upper
segmented bar display was projecting a future glide path based on the glider's
actual measured airspeed. During the climb, the upper segmented bar display was
projecting a future glide path based on the optimal airspeed-to-fly according
to McCready theory for best cross-country racing performance in thermal
conditions.
Regardless of whether the flight
path is descending, horizontal, or climbing, whenever both a GPS and an
airspeed probe are connected to the vario, there is a discontinuity in the displayed
glide ratio values at 74 mph airspeed--higher airspeeds produce unreliable (too
large) readings. Note that 74 mph is also the highest value that may be entered
for an airspeed when entering a polar curve into the vario.
“Lower segmented bar” display: The lower segmented bar displays the
glide-ratio-to-target, i.e. the horizontal distance to the target divided by
the glider's elevation over the target. This is the glide ratio that the glider
must achieve if it is to reach to the target. The display is blank if the
glide-ratio-to-target is <4. If the vario's digital glide ratio function has
been enabled, then the glide-ratio-to-target will also appear in digital form
in the "upper window" display area, which makes the lower segmented
bar display redundant.
Approach altimeter: Unlike the upper segmented bar expected-glide-ratio display, the
approach altimeter relates to the performance that would be expected if the
glider were to follow a ground track that would take it directly toward the
target waypoint. Unlike the upper segmented bar display, when the glider
circles in wind, the approach altimeter display remains steady and does not
rise and fall as the headwind or tailwind component changes. The approach
altimeter only updates its estimate of the apparent headwind or tailwind
component when the glider's ground track is pointing within 20 degrees of the
target waypoint. If the glider's ground track has not pointed within 20 degrees
of the target waypoint in the last 30 seconds, the approach altimeter begins
flashing, which signifies that it has switched to a default assumption of zero
wind. The approach altimeter reading may change dramatically at this point. As
soon as the glider's ground track points within 20 degrees of the target
waypoint, the approach altimeter will again sample the apparent headwind or
tailwind component, based on the difference between the groundspeed and the
measured airspeed.
The approach altimeter is based both on the
glider's current position in space with respect to the target waypoint, and on
the forecasted gliding performance that the vario expects the glider to achieve
as it travels toward the target waypoint. The forecasted gliding performance
matches the glide ratio shown on the upper segmented bar expected-glide-ratio
display, at least during those moments that the ground track of the glider is
pointing directly at the target. To forecast the performance that the glider
will achieve as it glides toward the target waypoint, the approach altimeter
uses a polar-derived sink rate. The glider's actual vertical speed is ignored,
except that when the glider is climbing rapidly, this affects the approach
altimeter display, because when thermal lift is expected to be strong, the
approach altimeter assumes that the pilot will choose to use a faster
interthermal glide airspeed, as per McCready theory for best cross-country
racing performance in thermal conditions. Therefore in most cases the approach
altimeter reading is downgraded when the glider is climbing rapidly. When the glider
is descending, the glider's recent average climb rate never has any effect on
the approach altimeter display.
Since the approach altimeter is based in part on
the glider's position in space relative to the target waypoint, a patch of
strong sink will produce a downward trend in the approach altimeter as the
glider loses altitude. This downward trend is only due to the way that the sink
is affecting the glider's current position in space. Even when the glider is in
strong sink, the forecasted gliding performance that the vario expects the
glider to achieve during the remaining portion of the glide to the goal is
still based on the assumption that the remaining portion of the glide will
occur in air with no vertical motion. Obviously this assumption is not always
realistic. If the approach altimeter reading is positive, but is steadily
trending downward, pilots need to be aware that may not be able to reach the
target, at least at the current airspeed, if they are in an area of widespread
sink.
In descending flight, the approach altimeter
display is based on the measured airspeed, the apparent headwind or tailwind
component, and a polar-derived sink rate value corresponding to the measured
airspeed. In general, increasing the
airspeed and groundspeed by going to a lower angle-of-attack (i.e. by pulling
in the bar) will result in a lower approach altimeter reading, due to the
increase in the polar-derived sink rate, unless the airspeed flywheel is
miscalibrated to read dramatically too low, in which case speeding up can
enhance the approach altimeter reading due to the increase in the groundspeed
and the increase in the apparent tailwind, especially if a fairly flat polar
curve has been entered into the vario.
In horizontal flight, the approach altimeter behaves
just as it does in descending flight, except for one very strange quirk: when
the analog vario display needle is pointing exactly at zero, showing neither a
climb nor a descent, the approach altimeter assumes that the winds are zero.
This may be a software bug. This is quirk is difficult to observe in actual
flight when both a GPS and an airspeed probe are attached to the vario, because
even without this quick, there would be usually be big jump in the approach
altimeter reading whenever the glider transitioned from a glide to a climb or
vs. vs..
In this situation, even though the approach
altimeter is assuming the winds to be zero, the numerical display does not
flash, unless the glider's ground track has not pointed within 20 degrees of
the target waypoint within the last 30 seconds.
In descending or horizontal flight, an icon
appears if the pilot should be able to make the target by flying at the
airspeed that will yield the flattest glide ratio given the existing apparent
headwind or tailwind component, assuming that the atmosphere will have no
vertical motion while the glider is gliding toward the target. This icon will not appear if the approach
altimeter display is flashing, i.e. if the glider's ground track has not
pointed within 20 degrees of the target in the last 30 seconds.
When the glider is climbing, the approach
altimeter display is no longer based on the glider's current measured airspeed.
When the glider is climbing, the approach altimeter is based on the performance
that would be expected if the glider were to begin gliding toward the target
waypoint at the optimal cross-country racing airspeed, derived from the polar,
according to McCready theory, considering apparent winds and recent climb
rates, and assuming that no lift or sink will be encountered after the current
climb ends. The recent climb rate value used to drive the approach altimeter is
not displayed to the pilot, nor can it be manually selected by the pilot. The
recent average climb rate value is apparently averaged over a stored database
of the last 60 seconds or so of climbing flight. The higher the recent average
climb rate, the lower the approach altimeter reading, because when thermals are
strong, the best cross-country performance can be obtained by going on glide at
a high airspeed. Note that when the glider actually stops climbing and goes on
glide, the average recent climb rate no longer plays any role in the approach
altimeter display.
In climbing flight, an icon flashes whenever the
pilot should be able to make the target by flying at the airspeed that will
yield the flattest glide ratio given the existing apparent headwind or tailwind
component, assuming that the glider will encounter no updrafts or downdrafts
while on glide. The icon becomes steadily illuminated whenever the pilot should
be able to make the target by flying at the optimal airspeed for cross-country
racing as per McCready theory, given the existing headwind or tailwind
component and recent average climb rate, and assuming that the glider will
encounter no updrafts or downdrafts while on glide. The icon changes from
flashing to steadily illuminated at the same time as the approach altimeter
changes from a negative reading to zero. When the icon is flashing rather than
steadily lit, the glider can make the target by flying at the airspeed that
will yield that flattest possible glide ratio given the existing headwind or
tailwind component, but the glider would make the best use of the thermal, and
reach the target faster, if it were to continue climbing for a while and then
go on glide at a faster airspeed. If the recent climb rates have been quite
low, the icon will not flash; it will appear in its steadily illuminated form
at the same time as the approach altimeter changes from a negative number to
zero. The icon does not appear when the
approach altimeter numerical display is flashing, i.e. when the glider's ground
track has not pointed within 20 degrees of the target in the last 30 seconds.
Note that when both a GPS and an airspeed probe
are connected to the vario, the way the approach altimeter works in a climb is
very different from the way the approach altimeter works in a glide. In
particular, if the glider is racing along at a very high speed and gets lifted
into a brief climb while passing though strong lift, the approach altimeter
display will change dramatically. During the glide, the approach altimeter was
projecting a future glide path based on the glider's actual measured airspeed.
During the climb, the approach altimeter was projecting a future glide path
based on the optimal airspeed-to-fly according to McCready theory for best
cross-country racing performance in thermal conditions.
Since the vario calculates the apparent headwind
or tailwind component by comparing the groundspeed and airspeed measurements,
if the airspeed probe is calibrated to read too low then the vario will
overestimate the headwind or underestimate the tailwind, and the approach
altimeter display will read too low. Errors caused by miscalibrations of the
airspeed probe will become more pronounced as the glider's actual airspeed
rises.
When both a GPS and an airspeed probe are
connected to the vario, there is a discontinuity in approach altimeter display
at 74 mph airspeed--higher airspeeds produce unreliable readings. Note that 74
mph is also the highest value that may be entered for an airspeed when entering
a polar curve into the vario.
“MODE 3”: When only a GPS is
connected to the vario, and the GPS is not in “goto” mode toward a properly
coded waypoint:
“Upper window” display: When the
glider is descending at a significant rate, the “upper window” will display the
digital current-glide-ratio, calculated by dividing the GPS groundspeed by the
actual, measured sink rate as displayed on the analog vario. This digital current-glide-ratio
value appears to be averaged over a time constant that is independent of the
time constant that was selected for the digital vertical speed display in
setting mode no. 8. The digital current-glide-ratio value is averaged over a
long enough time constant that it is significantly smoother, and significantly
more useable in rough air, than the current-glide-ratio display that is
available on some pressure-sensor-equipped GPS's such as my Garmin Etrex Vista.
This is true even when there is no TEC compensation because the airspeed probe
is absent. This is the only place on
the IQ Comp GPS vario where actual, real-time glide ratio information is
provided--all the other glide-ratio-related displays on the vario are based on
theoretical sink rates that are derived from the polar, assuming no rise or
fall in the airmass, rather than on the glider's actual measured sink
rate. If the current-glide-ratio is
greater than 20:1, the symbol "--" appears in place of the
current-glide-ratio number. (Obviously this feature was not designed with
Swifts or other sailplanes in mind!)
When the glider is descending at a
very low rate, regardless of the groundspeed and glide ratio, or when the
glider is flying horizontally or climbing, the “upper window” is used for a
time-averaged digital vertical speed display.
No “netto” display is available.
“Upper segmented bar”
display: When the glider is descending
or flying horizontally, the upper segmented bar displays an
expected-glide-ratio value for whatever direction the glider is travelling at
the current moment. This expected-glide-ratio value considers the effects of
winds, but assumes that the pilot is choosing to fly at the still-air
best-glide-ratio airspeed for the polar that has been entered in to the vario. The
sink rate is taken from the polar curve, without any reference to the glider's
actual measured sink rate. The apparent headwind or tailwind component is
calculated from the difference between the GPS groundspeed and the still-air
best-glide-ratio airspeed for the polar that has been entered into the vario.
In other words the value displayed by the upper bar is equal to the current GPS
groundspeed divided by a polar-derived sink rate value that corresponds to the
still-air best-glide-ratio airspeed for the polar curve that has been entered
into the vario. If the pilot flies at any other airspeed, the upper segmented
bar display will not be accurate. For example, if the pilot flies too fast, the
vario will assume that the pilot is still flying at the still-air
best-glide-ratio-airspeed, with a tailwind, and so the upper segmented bar
display will be overly optimistic.
In climbing flight, the vario
continues to calculate the apparent headwind or tailwind component from the
difference between the GPS groundspeed and the still-air best-glide-ratio
airspeed, so flight at any other airspeed (such as the min. sink rate airspeed)
will cause the vario to use an erroneous wind estimate for the upper segmented
bar display. The upper segmented bar displays a polar-derived
expected-glide-ratio that relates to the performance that the pilot could
expect if he were to go on glide, in whatever direction the glider is
travelling at the current moment, at some presumed interthermal gliding
airspeed. This presumed interthermal gliding airspeed is usually almost the
same as the still-air best-glide-ratio airspeed for the polar that has been
entered into the vario, but not exactly. This presumed interthermal gliding
airspeed is not adjusted upward or downward to optimize the glider's
performance according to the apparent headwind or tailwind component. However
this presumed interthermal gliding airspeed is adjusted upward when recent
climb rates have been high, as per McCready theory for best cross-country
racing performance in thermal conditions. In other words the vario starts with
the assumption that the pilot will go on glide at the still-air
best-glide-ratio-airspeed, and then increases this presumed interthermal glide
airspeed when recent thermal climb rates have been high. In light wind
conditions, or when there is a tailwind, this generally means that the upper
bar display is modified to show a poorer expected-glide-ratio whenever the
glider is climbing in strong lift, because the vario assumes that the pilot
will go on glide at an airspeed that is higher than the still-air
best-glide-airspeed, and so the resulting glide ratio will be poorer. When
there is a strong headwind, the upper segmented bar display behaves a bit
peculiarly. In this case, a fast thermal climb will actually increase the
expected-glide-ratio displayed by the upper segmented bar. This is not normally
what one expects, based on McCready theory. The reason for this odd phenomenon:
when the vario adjusts the presumed interthermal gliding speed upward to optimize
for the strong recent climb rate, this also gives better penetration. This odd
feature is an accidental by-product of the fact that the vario is not
optimizing the presumed interthermal gliding airspeed for the apparent wind
conditions, only for the recent thermal climb rate. It's also a bit odd that
the vario doesn't simply use a presumed interthermal gliding airspeed equal to
the still-air best-glide-ratio airspeed for the polar that has been entered
into the vario, since when the pilot stops climbing and actually goes on glide,
the vario will revert to the assumption that the pilot is flying at the
still-air best-glide-ratio airspeed, and flight at any other airspeed will
cause errors in the upper segmented bar display. When the glider stops climbing
and goes on glide, the average recent climb rate no longer plays any role in
the upper segmented bar display.
The recent climb rate value used
to drive the upper bar display is not displayed to the pilot, and is apparently
averaged over a stored database of the last 60 seconds or so of climbing flight
It not essential that pilots fully
understand the behavior of the upper segmented bar display in climbing flight
when only a GPS is connected to the vario. For all practical purposes, when
only a GPS is connected to the vario, the upper segmented bar display behaves
noticeably different in a climb than in a glide only when recent climb rates
have been quite strong, or when a relatively flat polar curve has been entered
into the vario.
“Lower segmented bar” display: The
lower segmented bar displays battery strength.
Approach altimeter: Not available.
“MODE 4”: When only a GPS is
connected to the vario, and the GPS is in “goto” mode toward a properly coded
waypoint:
“Upper window” display: The upper
window will be used for a full-time display of the digital
glide-ratio-to-target as long as the vario remains in "goto" mode.
This display includes a 10th's place (e.g. "4.3") which is useful for
detecting slow upward or downward trends in the glide-ratio-to-target. Neither the digital time-averaged vertical
speed display nor the digital current-glide-ratio display will be visible as
long as the vario remains in "goto" mode.
“Upper segmented bar” display:
When the glider is descending or flying horizontally, the upper segmented bar
displays an expected-glide-ratio value for whatever direction the glider is
travelling at the current moment. This expected-glide-ratio value considers the
effects of winds, but assumes that the pilot is choosing to fly at the still-air
best-glide-ratio airspeed for the polar that has been entered in to the vario.
The sink rate is taken from the polar curve, without any reference to the
glider's actual measured sink rate. The apparent headwind or tailwind component
is calculated from the difference between the GPS groundspeed and the still-air
best-glide-ratio airspeed for the polar that has been entered into the vario.
In other words the value displayed by the upper bar is equal to the current GPS
groundspeed divided by a polar-derived sink rate value that corresponds to the
still-air best-glide-ratio airspeed for the polar curve that has been entered
into the vario. If the pilot flies at any other airspeed, the upper segmented
bar display will not be accurate. For example, if the pilot flies too fast, the
vario will assume that the pilot is still flying at the still-air
best-glide-ratio-airspeed, with a tailwind, and so the upper segmented bar
display will be overly optimistic.
In climbing flight, the vario
continues to calculate the apparent headwind or tailwind component from the
difference between the GPS groundspeed and the still-air best-glide-ratio
airspeed, so flight at any other airspeed (such as the min. sink rate airspeed)
will cause the vario to use an erroneous wind estimate for the upper segmented
bar display. The upper segmented bar displays a polar-derived
expected-glide-ratio that relates to the performance that the pilot could
expect if he were to go on glide, in whatever direction the glider is
travelling at the current moment, at some presumed interthermal gliding
airspeed. This presumed interthermal gliding airspeed is usually almost the
same as the still-air best-glide-ratio airspeed for the polar that has been
entered into the vario, but not exactly. This presumed interthermal gliding
airspeed is not adjusted upward or downward to optimize the glider's
performance according to the apparent headwind or tailwind component. However
this presumed interthermal gliding airspeed is adjusted upward when recent
climb rates have been high, as per McCready theory for best cross-country
racing performance in thermal conditions. In other words the vario starts with
the assumption that the pilot will go on glide at the still-air
best-glide-ratio-airspeed, and then increases this presumed interthermal glide
airspeed when recent thermal climb rates have been high. In light wind
conditions, or when there is a tailwind, this generally means that the upper
bar display is modified to show a poorer expected-glide-ratio whenever the
glider is climbing in strong lift, because the vario assumes that the pilot
will go on glide at an airspeed that is higher than the still-air
best-glide-airspeed, and so the resulting glide ratio will be poorer. When
there is a strong headwind, the upper segmented bar display behaves a bit
peculiarly. In this case, a fast thermal climb will actually increase the
expected-glide-ratio displayed by the upper segmented bar. This is not normally
what one expects, based on McCready theory. The reason for this odd phenomenon:
when the vario adjusts the presumed interthermal gliding speed upward to
optimize for the strong recent climb rate, this also gives better penetration.
This odd feature is an accidental by-product of the fact that the vario is not
optimizing the presumed interthermal gliding airspeed for the apparent wind
conditions, only for the recent thermal climb rate. It's also a bit odd that
the vario doesn't simply use a presumed interthermal gliding airspeed equal to
the still-air best-glide-ratio airspeed for the polar that has been entered
into the vario, since when the pilot stops climbing and actually goes on glide,
the vario will revert to the assumption that the pilot is flying at the
still-air best-glide-ratio airspeed, and flight at any other airspeed will
cause errors in the upper segmented bar display. When the glider stops climbing
and goes on glide, the average recent climb rate no longer plays any role in
the upper segmented bar display.
The recent climb rate value used
to drive the upper bar display is not displayed to the pilot, and is apparently
averaged over a stored database of the last 60 seconds or so of climbing flight
It not essential that pilots fully
understand the behavior of the upper segmented bar display in climbing flight
when only a GPS is connected to the vario. For all practical purposes, when
only a GPS is connected to the vario, the upper segmented bar display behaves
noticeably different in a climb than in a glide only when recent climb rates
have been quite strong, or when a relatively flat polar curve has been entered
into the vario.
“Lower segmented bar” display: The
lower segmented bar displays the glide-ratio-to-target, i.e. the horizontal
distance to the target divided by the glider's elevation over the target. This
is the glide ratio that the glider must achieve if it is to reach to the
target. The display is blank if the glide-ratio-to-target is <4. If the
vario's digital glide ratio function has been enabled, then the
glide-ratio-to-target will also appear in digital form in the "upper
window" display area, which makes the lower segmented bar display
redundant.
Approach altimeter: Unlike the upper segmented bar
expected-glide-ratio display, the approach altimeter relates to the performance
that would be expected if the glider were to follow a ground track that would
take it directly toward the target waypoint. Unlike the upper segmented bar
display, when the glider circles in wind, the approach altimeter display
remains steady and does not rise and fall as the headwind or tailwind component
changes. The approach altimeter only updates its estimate of the apparent
headwind or tailwind component when the glider's ground track is pointing
within 20 degrees of the target waypoint. If the glider's ground track has not
pointed within 20 degrees of the target waypoint in the last 30 seconds, the
approach altimeter begins flashing, which signifies that it has switched to a
default assumption of zero wind. The approach altimeter reading may change
dramatically at this point. As soon as the glider's ground track points within
20 degrees of the target waypoint, the approach altimeter will again sample the
apparent headwind or tailwind component, based on the difference between the
groundspeed and the measured airspeed.
The approach altimeter is based both on the
glider's current position in space with respect to the target waypoint, and on
the forecasted gliding performance that the vario expects the glider to achieve
as it travels toward the target waypoint. The forecasted gliding performance
matches the glide ratio shown on the upper segmented bar expected-glide-ratio
display, at least during those moments that the ground track of the glider is
pointing directly at the target. To forecast the performance that the glider
will achieve as it glides toward the target waypoint, the approach altimeter
uses a polar-derived sink rate. The glider's actual vertical speed is ignored,
except that when the glider is climbing rapidly, this affects the approach
altimeter display, because when thermal lift is expected to be strong, the
approach altimeter assumes that the pilot will choose to use a faster
interthermal glide airspeed, as per McCready theory for best cross-country
racing performance in thermal conditions. Therefore in most cases the approach altimeter
reading is downgraded when the glider is climbing rapidly. When the glider is
descending, the glider's recent average climb rate never has any effect on the
approach altimeter display.
Since the approach altimeter is based in part on
the glider's position in space relative to the target waypoint, a patch of
strong sink will produce a downward trend in the approach altimeter as the
glider loses altitude. This downward trend is only due to the way that the sink
is affecting the glider's current position in space. Even when the glider is in
strong sink, the forecasted gliding performance that the vario expects the
glider to achieve during the remaining portion of the glide to the goal is
still based on the assumption that the remaining portion of the glide will
occur in air with no vertical motion. Obviously this assumption is not always
realistic. If the approach altimeter reading is positive, but is steadily
trending downward, pilots need to be aware that may not be able to reach the
target, at least at the current airspeed, if they are in an area of widespread
sink.
In descending flight, the approach
altimeter display assumes that the pilot will fly toward the target at the
still-air best-glide-ratio-airspeed for the polar that has been entered into the
vario. The glider's actual sink rate is ignored. The expected glide performance
is adjusted for the apparent headwind or tailwind component, which is
calculated from the difference between the GPS groundspeed and the still-air
best-glide-ratio airspeed. If the pilot
flies faster or slower than the still-air best-glide-ratio airspeed for the
polar that has been entered into the vario, the approach altimeter display will
not be accurate. For example, if the pilot flies too fast, the vario will
assume that the pilot is flying at the still-air best-glide-ratio airspeed,
with a tailwind, and so the approach altimeter will give an overly optimistic
reading.
In horizontal flight, the approach
altimeter behaves just as it does in descending flight, except for one very
strange quirk: when the analog vario display needle is pointing exactly at
zero, showing neither a climb nor a descent, the approach altimeter assumes
that the winds are zero. This may be a software bug. This produces some
interesting jumps in the approach altimeter display whenever a bit of
turbulence brings the analog vario display needle through the zero mark,
especially in windy conditions. These fluctuations are much more noticeable
when only a GPS is connected to the vario, than when an airspeed probe is also
connected to the vario. The reason for this: when only a GPS is connected to
the vario, the way the approach altimeter works in a climb is very similar to
the way the approach altimeter works in a glide, so the anomaly that occurs
when the analog vario pointer is exactly on "zero" is very
noticeable.
In this situation, even though the
approach altimeter is assuming the winds to be zero, the numerical display does
not flash, unless the glider's ground track has not pointed within 20 degrees
of the target waypoint within the last 30 seconds.
In descending or horizontal
flight, an icon remains steadily illuminated whenever the pilot should be able
to make the target by flying at the still-air best-glide speed. This means that
the icon is illuminated whenever the approach altimeter reading is zero or
positive. The icon will not appear if the approach altimeter numerical display
is flashing, i.e. if the glider's ground track has not pointed within 20
degrees of the target in the last 30 seconds.
In climbing flight, the vario continues to calculate the
apparent headwind or tailwind component from the difference between the GPS
groundspeed and the still-air best-glide-ratio airspeed, so flight at any other
airspeed (such as the min sink airspeed) will cause the vario to use an
erroneous wind estimate for the approach altimeter display. Again, for the
purposes of the approach altimeter, the apparent winds are only updated when
the glider's ground track points within 20 degrees of the target waypoint. The
approach altimeter is based on a polar-derived expected-glide-ratio value that
relates to the performance that the pilot could expect if he were to go on
glide, toward the target waypoint, at some presumed interthermal gliding
airspeed. This presumed interthermal gliding airspeed is based on the still-air
best-glide-ratio airspeed, but is adjusted upward to optimize the glider's
performance when recent thermal climb rates have been strong, as per McCready
theory for best cross-country racing performance in thermal conditions. This
presumed interthermal gliding airspeed is not adjusted upward or downward to
optimize the glider's performance in an apparent headwind or tailwind. In light
wind conditions, or when there is a tailwind, this generally means that the
approach altimeter display is modified to show a poorer expected-glide-ratio
whenever the glider is climbing in strong lift, because the vario assumes that
the pilot will go on glide at an airspeed that is higher than the still-air
best-glide-airspeed, and so the resulting glide ratio will be poorer. When
there is a strong headwind, the approach altimeter display behaves a bit
peculiarly. In this case, a fast thermal climb will actually increase the
expected-glide-ratio used by the approach altimeter. This is not normally what
one expects, based on McCready theory. The reason for this odd phenomenon: when
the vario adjusts the presumed interthermal gliding speed upward to optimize
for the strong recent climb rate, this also gives better penetration. This odd
feature is an accidental by-product of the fact that the vario is not
optimizing the presumed interthermal gliding airspeed for the apparent wind
conditions, only for the recent thermal climb rate. It's also a bit odd that
the vario doesn't simply use a presumed interthermal gliding airspeed equal to
the still-air best-glide-ratio airspeed for the polar that has been entered
into the vario, since when the pilot stops climbing and actually goes on glide,
the vario will revert to the assumption that the pilot is flying at the
still-air best-glide-ratio airspeed, and flight at any other airspeed will
cause errors in the approach altimeter display. When the glider stops climbing
and goes on glide, the average recent climb rate no longer plays any role in
the approach altimeter display.
The recent climb rate value used
to drive the approach altimeter is not displayed to the pilot, and is
apparently averaged over a stored database of the last 60 seconds or so of
climbing flight
In climbing flight, an icon
flashes whenever the pilot can make the target waypoint by going on glide at
the still-air best-glide-ratio airspeed for the polar that has been entered
into the vario, assuming that the glider will encounter no updrafts or
downdrafts while on glide. The icon then becomes steadily illuminated whenever
the pilot should be able to make the target by flying at a presumed
interthermal gliding airspeed that is based on the still-air best-glide-ratio
airspeed, but adjusted upward to optimize the glider's performance when recent
thermal climb rates have been strong, as per McCready theory for best
cross-country racing performance in thermal conditions. This is the same
presumed interthermal gliding airspeed that is used by the numerical portion of
the approach altimeter display. It is not adjusted upward or downward to
optimize the glider's performance in an apparent headwind or tailwind.
Therefore, the behavior of the icon when the glider is climbing is a bit
complex and depends on the apparent headwind or tailwind component. In light
winds or when there is an apparent tailwind, the icon changes from flashing to
steadily illuminated at the same time as the approach altimeter changes from a
negative reading to zero. But if the glider will face a strong headwind when it
goes on glide, then there will not be any interval in the climb when the icon
is flashing. Instead, it will simply appear in its steadily illuminated form at
the same time as the approach altimeter changes from a negative number to zero.
The reason: with a strong headwind, flying to the target at the still-air
best-glide ratio airspeed will actually take more altitude than flying to the
target at a higher airspeed, which will yield better penetration. Again, this
odd behavior is an accidental by-product of the fact that the vario is not
optimizing the presumed interthermal gliding airspeed for the apparent wind
conditions, only for the recent thermal climb rate. In any wind conditions, if
the recent climb rates have been quite low, the icon will never be seen to
flash; it will appear in its steadily illuminated form at the same time as the
approach altimeter changes from a negative number to zero. The icon does not
appear when the approach altimeter numerical display is flashing, i.e. when the
glider's ground track has not pointed within 20 degrees of the target in the
last 30 seconds.
It not essential that pilots fully
understand the behavior of the approach altimeter icon and numerical display in
climbing flight when only a GPS is connected to the vario. For all practical
purposes, when only a GPS is connected to the vario, the approach altimeter
behaves noticeably different in a climb than in a glide only when recent climb
rates have been quite strong, or when a relatively flat polar curve has been
entered into the vario.
“MODE 5”: When only an airspeed
probe is connected to the vario:
“Upper window” display : When the
glider is descending at a significant rate, the “upper window” will be used to
display a value representing the measured airspeed divided by the actual,
measured sink rate as displayed on the analog vario (i.e. after total energy
compensation according to the selected TEC ratio). This is the only situation
where the “upper window” display is affected by the calibration of the airspeed
probe or by whether or not an airspeed probe is even present, except for
transient total energy effects.
When the glider is descending at a
very low rate, regardless of the airspeed, or when the glider is flying
horizontally or climbing, the “upper window” is used for a time-averaged
digital vertical speed display.
“Upper segmented bar” display:
This is the only situation in which the upper segmented bar display is based on
the actual, measured sink rate rather than on a polar-derived sink rate. In this
“mode” only, the upper bar displays the measured airspeed divided by the
actual, measured sink rate as displayed on the analog vario (i.e. after total
energy compensation according to the selected TEC ratio). This is the only situation where the upper
segmented bar display and the upper window display consistently show similar
values, at least during descending flight.
“Lower segmented bar” display: The
lower segmented bar displays battery strength.
Approach altimeter: Not available.
“MODE 6”: When neither a GPS nor
an airspeed probe are connected to the vario:
“Upper window” display : The
“upper window” is used for a time-averaged digital vertical speed display.
“Upper segmented bar” display:
This display has no function when neither a GPS nor an airspeed probe are
connected to the vario.
“Lower segmented bar” display: The
lower segmented bar displays battery strength.
Approach altimeter: Not available.
PART 8: TIPS ON AVOIDING ACCIDENTALLY OVERWRITING BAROGRAPH DATA
The capacity of the barograph
memory is as follows (memory capacities are slight underestimates in all
cases):
1 second recording interval: 5
hours of memory capacity
5 second recording interval: 25
hours of memory capacity
15 second recording interval: 75
hours of memory capacity
25 second recording interval: 125
hours of memory capacity
I usually record at the “1 second”
interval, and if I find the duration of a flight exceeding 4 hours, I switch to
the “5 second” interval, which will expand the 1 remaining hour into 5 more
hours of recording time. Before the
recording interval can be changed, the barograph must be switched off, so the
flight will be broken up into 2 different barograph traces; this would not be a
suitable strategy for any flight where the user wanted to ensure that a single
barograph trace spanned the entire flight.
Here is one of the oddities of the IQ Comp GPS variometer: during the course of a long
flight, if the barograph traces for previous flights are in the process of
being overwritten, leaving the barograph on and entering the “memo” mode to see
how many past flights still retain their “barograph” icons will NOT give any
indication whatsoever of which barograph traces have already been overwritten
during the course of the present flight.
Not until the barograph is switched off are the “barograph” icons of the
older flights in memory revised to accurately show which flights still retain
their barograph traces. For example,
during one long flight with the barograph on, I entered “memo” mode and saw
that several past long flights still retained their “barograph” icons, which
suggested that several hours of barograph recording time still remained, but
when I landed soon afterwards, I found that every one of these past barograph
traces, plus the current barograph trace, had already been overwritten and were
therefore blank. When the barograph is
running, the “memo” mode does NOT give an accurate indication of how many old
flights still retain their barograph traces.
This brings up another important
point: if the beginning of the present flight’s barograph trace is overwritten,
the entire present flight’s barograph trace will be lost.
When barograph traces are being
overwritten, the oldest existing barograph trace is always overwritten first,
and then the next-oldest, etc. When a
barograph trace is overwritten, the basic flight data (peak altitude, flight
time) etc for that flight is not lost.
My vario now bears the following
markings, which tell me how many additional hours of barograph recording time
remain if I switch from the “1 second” interval to the “5 second” interval
after x hours of recording: For example, during a given flight, if I switch from the “1 second” interval to the “5 second” interval
after 4.5 hours of recording, I'll have room for at least 2.5 more hours of recording before any of the barograph data for that flight will be lost.
4—5
4.5--2.5
5--0
If these times aren’t adequate for
the situation, I switch to the “15” second mode instead of the “5 second”
recording interval, which will yield triple the times listed here.
The basic vario memory, as well as
the barograph trace, records the time from when the vario and/or barograph was
switched on to the time that the vario and/or barograph was switched off, as
long as there was a significant (30 meter) altitude change somewhere in that flight. Unless the vario or barograph is turned on
immediately before launch, and is turned back off immediately after landing,
the recorded times will be significantly longer than the actual flight time. Many pilots aren’t aware of this. Of course, on any flight where the barograph
is used, a pilot can easily reconstruct the actual time-in-flight by using
PC-Graph or some other program to examine the barograph trace.
Here is another quirk of
the IQ Comp GPS vario--if you turn the barograph off in flight, the vario's flight-time counter will stop running, and the
remaining flight time that unfolds after the barograph has been turned off will
fall into a “black hole”, never to be seen again. The flight associated with the newly-ended barograph trace will
continue to be recognized as flight number “zero” until the barograph is
switched back on again, or until the entire vario has been switched off and
then back on, either of which will bump the previous flight up to flight number
“one”, and will allow a new flight number “zero” to begin. In practical terms: if you want to end a
barograph trace but want to continue recording the basic flight data (flight
time, peak altitude, etc), simply turn the whole vario off and then back
on. This will bump the previous flight
(with the barograph trace) up to flight number “one”, and flight number “zero”
will be a new flight, and the flight time counter will start anew. There is no problem associated with turning
the vario off when the barograph is still running—the barograph trace will end
and will be saved. But turning the
barograph off and leaving the vario running means that the subsequent basic
flight data for that particular flight will not be saved.
Here is another thing you need
to know to avoid losing barograph data: if you use the PC graph program, it is
very important that you use the “back up flight log” option from time to
time. Otherwise, you may someday suffer
the loss of all your flight log data.
It is also important that you have some idea of what to do—and what not
to do—if the PC Graph program crashes.
We’ll discuss these points in some detail in Part 10 of this article,
entitled “More on PC Graph version 1.5, with notes on backing up data.” If you
take the time to read through this section, you may save yourself a lot of
aggravation in the future!
PART 9: NOTES ON DOWNLOADING DATA TO PC GRAPH VERSION 1.5
The vario cannot download data to
a computer while the barograph is running.
Turn the barograph off before beginning to download data to a computer.
To understand the discussion that
follows, you should know that flight number “zero” is normally the current
flight, regardless of whether or not the vario has yet experienced the
requisite (30 meter) altitude change that will allow that flight to be
permanently stored in memory. Turning
the vario off, or turning the barograph off and then back on, bumps the latest
flight from number “zero” to number “one” (unless there was no significant
altitude change, in which case the latest flight will simply be deleted), and
starts a new, current, flight number “zero”.
Normally data is downloaded from
the IQ Comp GPS vario to the PC Graph program in a one-flight-at-a-time
fashion. But here’s an odd little
quirk: if you connect the IQ Comp GPS vario to PC Graph and try to download
flight number “zero”, this will download the basic data (max. altitude, flight
duration, etc) for every single flight in the vario’s memory, but none of the
barograph traces will be downloaded along with this basic data. Also, the pilot’s name as entered in the
vario will not be downloaded along with the other basic data, so the default
pilot name that is entered into the PC Graph program will be used instead. Fortunately, if any of these flights are
already in PC Graph—along with their barograph traces and/or downloaded pilot’s
names and/or additional comments entered after downloading—then none of the
data that is already present in PC Graph will be overwritten when you download
flight number “zero”. Still, in the
event that there is still more strangeness associated with flight number “zero”
that I haven’t yet discovered, I recommend that you don’t try to download
flight number “zero”. If flight number
“zero” actually represents a real flight—which would happen if the vario hasn’t
been switched off since the last time you flew—then cycle the barograph switch
through the “on” position and back to the “off” position, or turn the whole
vario off and then back on again.
Either of these actions will bump that last real flight up from number
“zero” to number “one”, and you’ll be able to download it to PC Graph in the
normal fashion.
Here’s a reassuring point: if you
download a flight with an associated barograph trace, and then at some later
date after the barograph trace has been overwritten you download the same
flight again, this will not erase the downloaded barograph trace, nor will this
erase any data that you may have entered by keyboard after the first download.
There is no need to clear flights
out of the vario’s memory after downloading them to PC Graph. They will eventually be overwritten anyway.
One interesting value that does
not get downloaded to PC Graph along with the other basic flight data (max
altitude, flight duration, etc) is the “A3” altitude value, which is the total
sum of all the climbs during the course of a single flight. I make a point of entering this value
manually in the “remarks” section of every flight that I download to PC Graph.
If a flight is not associated with
a barograph trace, then no “maximum speed” value will appear in PC Graph when
that flight is downloaded, even though a maximum speed value will appear on
vario’s “memo” page for that flight. If
a flight is associated with a barograph trace, then a “maximum speed” value
will appear in PC Graph when that flight is downloaded, but it may be slightly
different than the “maximum speed” value that appears on the vario’s “memo”
page for that flight.
When the vario is flown without an
airspeed probe, so that the recorded speed data represents satellite-derived
groundspeed data rather than probe-derived airspeed data, then it sometimes
happens that the recorded “maximum speed” value clearly represents an anomalous
spike in the satellite-derived groundspeed data (e.g. max speeds of well over
100 mph.)
When English units are used, the maximum rate-of-climb value
displayed on the vario’s “memo” page is always divided by 100 and rounded up to
the nearest EVEN 10th. (E.g. a maximum
climb rate of 1001 feet per minute would be expressed as 10.2). The maximum rate of climb values displayed
in PC Graph are expressed to the nearest whole digit (e.g. 1001 feet per minute). Therefore peak rates of climb displayed on
the vario are always higher than the peak rates of climb displayed on PC Graph.
PART 10: MORE ON PC GRAPH VERSION 1.5, WITH NOTES ON BACKING UP DATA
More on PC Graph version 1.5
When a flight is downloaded to PC Graph, even though it
appears at the top of the flight list page on PC Graph, it doesn’t receive an
appropriate (consecutive) serial number until the next time PC Graph is started
up. For example, on the PC Graph flight
list page you might see your old flights numbered 1 through 250, with the
numbers ascending with age and the flights arranged with the newest at the top
of the page and the oldest at the bottom of the page. If you then downloaded 4 more flights, you would see that your 4
newly-downloaded flights were sitting at the top of the page in the appropriate
chronological position, but these new flights would be numbered 254 through
251, descending with age rather than ascending with age. When you restart PC Graph again, the newest
flights, which were numbered 254 through 251, will now be changed to bear the
numbers 1 through 4, ascending with age, and the index numbers of all the other
flights will have been bumped up by 4.
This is of no real consequence.
PC Graph often crashes when the
user is manipulating the cursor lines on the speed, altitude, or vertical speed
traces. In my experience, this type of
crash does not cause the loss of data.
Moving on to more important
matters: PC Graph is somewhat prone to
crashing. I can’t emphasize strongly
enough the importance of frequently using the “backup flight log” function in
PC Graph. I back up my flight logs
every few weeks, or every month at an absolute minimum. Before I began doing this, I did lose
several years worth of flight data due to a crash of the PC Graph program. And in the time since I’ve begun
systematically backing up my flight files, I’ve been able to restore my files
on two different occasions after a crash of PC Graph that would have otherwise
wiped out all my flight data.
PC Graph is designed in such a way
that it creates a file named pcgraph-Pilot Name.FLG and a second file named
pcgraph-Pilot Name-old.FLG. (Of course,
your name is substituted for “Pilot Name”.)
These files aren’t always easy to find—they reside in the same directory
that PC Graph resides in. Back-up files
usually reside in the same place, and have whatever name you gave them when you
created them. Whenever you exit
PC-Graph, the active file (e.g. pcgraph-Pilot Name.FLG ) is automatically
modified to save any changes you made, and the old file (e.g. pcgraph-Pilot
Name-old.FLG) is automatically modified to represent the previous version of
the active file. If you have a problem
with PC Graph and experience a crash, and you haven’t recently backed up your
data, it’s very important that you do not exit PC Graph. It is even more important that you do not
restart PC Graph again after a crash.
If you experience a crash in PC Graph and exit the program, and then
enter the program and exit it a second time, you may have just made the fatal
mistake of ensuring that both the active .FLG file and the old .FLG file have
been over-written with the corrupted data.
Don’t do this, unless you have recently backed up your flight log!
If you experience any kind of a
problem in PC Graph, and you have not recently backed up your flight log, then
before you do anything else, go into you computer’s file directory and find the
active .FLG file and the old .FLG file and make copies of each, with different
names. This way, you should have a good
copy of your flight data even if PC Graph does end up overwriting both the
active .FLG file and the old .FLG file with the corrupted data.
In addition to the .FLG file,
which comprises the entire log of all the flights a given user has a stored in
PC Graph, PC Graph can also create files that represent a single flight. These files have the suffix “.cmp”, and are
created by using the “export flight” option.
With the “import flight” option, the .cmp file for a single flight can
be imported into PC Graph, where it will become part of the active .FLG file.
Here are a few more oddities of the PC Graph program, version 1.5. The "remarks" portion of each flight only saves approximately the first 250 characters of text that the user types in. Any additional text will appear to be entered normally, but then will be missing when that flight is re-visited. The barograph screen of the PC Graph program has function buttons that toggle up and down to switch on and off the altitude, vertical speed, and speed displays. Often the position of these function toggle buttons gets "out of synch" with the actual traces that are displayed on the screen.
PART 11: NOTES ON SOME
DIFFICULTIES IN CALIBRATING THE AIRSPEED PROBE, AND NOTES ON SOME DIFFICULTIES IN INTERFACING WITH THE GARMIN ETREX GPS
My Brauniger Comp IQ GPS variometer is several years old; originally it
would not work with my Garmin Etrex GPS. This problem was solved when I sent
the vario back to the factory in summer 2003 for a software upgrade. The
software upgrade also provided me with the new digital current-glide-ratio and
glide-ratio-to-target displays that can be selected to appear in the area
normally used for the digital time-averaged vertical speed or netto display.
The software upgrade also added the new A3 cumulative altimeter function. I'm not sure whether or not this same software upgrade would also be required to allow an older Brauniger Comp IQ Comp GPS vario to work with other popular GPS's such as the Garmin GPSmap 76S.
I also asked that the
factory to raise the internal calibration of the airspeed sensor, because the
airspeed probe reading seemed extremely low. I had to turn the adjustable
airspeed probe calibration all the way up to the maximum just to get reasonable
values in a free airstream, to say nothing of the additional boost that would
be needed to get reasonable values in the slowed airflow beneath a wing. By
borrowing some other variometers and airspeed probes I confirmed that this was
indeed a problem with the vario, not with the airspeed probe. The factory
adjustment helped but when I leave the adjustable calibration at the neutral
point, I still get in-flight airspeed readings that are about 40% too low. To
get reasonable airspeed readings in flight, I need to have the adjustable
airspeed probe calibration up around 190. The total calibration range is 0 to
255, with 100 being the neutral point. Each step represents a 0.2% adjustment
in the airspeed reading, so that 5 steps represent a 1% adjustment.
APPENDIX 1: About the tests
The ideas presented in this article are based on a series of
experiments where data was taken during carefully
controlled flight conditions. The vario functions were explored primarily
though a series of experiments in a light airplane. By adjusting the airspeed
probe calibration, the shape of the polar curve, the actual airspeed, and the
engine power setting, and by flying in a variety of wind conditions, I was able
to generate a large array of apparent airspeeds, apparent wind conditions, and
apparent updraft and downdraft velocities. I also manipulated the aircraft's
apparent altitude over the target waypoint by changing the vario's altimeter
setting or changing the waypoint's altitude code. In no case were the
aircraft's actual gliding performance or the actual atmospheric conditions ever
an issue--I was always comparing the performance predicted by the vario, with
the performance indicated by the polar curve, given the existing apparent
conditions. I took extensive data at frequent intervals during the tests,
including GPS groundspeed, measured airspeed, apparent height above or below
waypoint, distance to waypoint, the expected-glide-ratio indicated by the upper
segmented bar, and the approach altimeter reading. Afterwards, I calculated the
glide ratio that the approach altimeter was using. This number, and the
expected-glide-ratio displayed by the upper segmented bar, were checked against
the polar curve, to see what assumptions the vario was making. Special
attention was given to checking the vario functions during climbing flight, and
during flight without an airspeed probe.
Some of the tests were carried out while driving in a car. While driving at
a high speed on flat ground, the pressure altitude inside a car can instantly
be increased by about 50' by opening a window, which creates a high climb rate
value for a few seconds on the analog vario display. The reverse happens when
the window is closed. Manipulating the vehicle's internal pressure altitude in
this way is a good way to explore how the vario works in a climb vs. in a
descent. Also, a series of sudden rises and dips in the car's internal pressure
altitude is a good way to "pump up" the magnitude of the vario's
stored "recent climb rate" value. To explore the effect of a high
"recent climb rate" value when the actual driving speed is low, it's
handy to first "pump up" the "recent climb rate" value
while driving at high speed as described above, and then to decelerate and
carry out the tests. (The "recent climb rate" value used by the
approach altimeter and the upper segmented bar display seems to be averaged
over the last 60 seconds or so of climbing flight.) Even at low speeds, opening
or closing a window generates enough of a pressure altitude change to make the
analog vario display indicate a slight climb or a descent for a few seconds.
I found it helpful to experiment with a wide variety of polar curves. For
example, some of the nuances of the various algorithms were easier to detect
when an unrealistically flat polar curve was entered into the vario, and other
details were easier to detect when an unrealistically sharp, peaked polar curve
was entered into the vario.
My tests provided some opportunities to look for errors in the various vario
functions. I wasn't able to pinpoint any systematic errors in any of the vario
functions, other than the handful of oddities that I've already described in
Part 5 (such as the omission of winds from the approach altimeter calculations
whenever the glide path is exactly horizontal). At times there did seem to be
some errors in the approach altimeter and the upper segmented bar
expected-glide-ratio-display, but I wasn't able to isolate any specific cause
of these apparent errors or to predict when they would occur, so I can't
completely rule out the possibility of an error on my part, such as an error in
setting the altimeter or the waypoint altitude code. There were some instances
when the approach altimeter and the upper segmented bar expected-glide-ratio
display seemed to be significantly underestimating the influence of apparent
headwind or tailwind components, but in other instances the effects of very
strong apparent winds seemed to be correctly factored into the calculations.
There were some instances when large positive or negative approach altimeter
readings (several thousands of feet) seemed to be associated with significant
underestimates or overestimates, respectively, of glide performance. In other
words the absolute value of the approach altimeter reading sometimes seemed
smaller than it should have been. In particular, when approaching a waypoint on
a glide path that would take me several thousand feet over the actual waypoint,
the approach altimeter sometimes read many hundreds of feet too low as I neared
the waypoint, underestimating my margin of extra altitude, and read many
hundreds of feet lower than my actual height-over-waypoint at the instant that
I passed over the waypoint. Other times the approach altimeter worked very
well, even when displaying positive or negative readings of many thousands of
feet. (The vario's safety altitude feature was set to zero at all times during
my tests.) Since I've not been able to isolate or reproduce specific errors in
the vario calculations, my best guess is that the approach altimeter and the
upper segmented bar expected-glide-ratio display usually give accurate
indications, apart from the quirks already described in Part 6.
APPENDIX 2: Related articles
See these related articles on the this website:
1) "An
idea for a new McCready pointer"--describes an idea for a new
"glide-to-target" pointer that would work well with the "active
McCready pointer" that currently exists on the Brauniger IQ Comp GPS vario
and other similar varios. This new pointer would help the pilot optimize his
choice of speed-to-fly whenever the pilot is gliding to a known target, in a
way that existing approach altimeters do not.
2) "Notes on the glide ratio functions of some Garmin GPS receivers with pressure sensors, including the GPSmap 76S/CS/CSx, GPSmap 60CS/CSx, and Etrex Vista/Vista C/Cx"--practical
notes on how the quality of satellite reception affects the vertical speed and
current-glide-ratio displays on these pressure-sensor-equipped Garmin GPS
receivers.
3) "Using a GPS
in soaring flight"--general notes, with links to other GPS-related articles on the Aeroexperiments website and elsewhere.
