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 woul
