Questions of interest part 3:
Roll torque created by the
sideways airflow component as a hang glider sideslips
August 28 2005 edition
Steve Seibel
steve at aeroexperiments.org
www.aeroexperiments.org
Here are some of the questions that I'm interested in, many
of which I've explored though in-flight experiments. Brief answers are also given--many of these topics are explored
in much more detail elsewhere in the "experiments" and
"theory" sections of this website.
This article was meant to serve as a concise yet comprehensive
introduction to these ideas, and in most cases I've tried to keep the theory
down to the bare minimum that was necessary to avoid ambiguities in meaning.
Your comments, questions, and related in-flight observations
are most welcome!
Note: in this article, unless we state otherwise, we'll always be talking about flex-wing hang gliders. Rigid-wing hang gliders will exhibit very different slip (yaw)-roll coupling characteristics.
Q: When the nose of a flex-wing hang glider is pointing in a slightly
different direction than the glider is actually travelling through the airmass,
does the resulting sideways (slipping) component in the airflow over the glider
create a roll torque? If so, in which
direction?
A: These questions
were explored through experiments with a controllable rudder, and also through
experiments with a small wingtip-mounted drogue chute (don't try this at home
without contacting me first!), that allowed me to make a yaw input on a
flex-wing hang glider and then continue to fly the glider in a linear,
non-turning slip, while noting what roll inputs were required to hold the bank
angle constant. These questions were
also explored by seeing which way the glider naturally wanted to bank after I
deployed the yaw control device and refrained from making any roll inputs. In general all the gliders tended to roll
in the opposite direction as the nose was yawed in relation to the airflow--a
left yaw input, causing the nose to point to the left of the actual direction of the flight path and airflow, and causing a yaw string to deflect to the left, would create a
right roll torque in most cases. For reasons that we'll explore in more detail below, we'll call this relationship a "negative coupling between slip (yaw) and roll". The
results were strongly airspeed-dependent with higher airspeeds (lower angles-of-attack)
correlating to a stronger negative coupling between slip (yaw) and roll,
and low airspeeds (high angles-of-attack) correlating to a weaker negative
coupling between slip (yaw) and roll.
The results were also strongly dependent on VG setting: when the VG was
tight this decreased the negative coupling between slip (yaw) and roll at low
angles-of-attack (high airspeeds), and created a weak positive coupling between
slip (yaw) and roll at high angles-of-attack (low airspeeds).
Overview of several flex-wing hang gliders: Ranking of strength of negative
coupling between slip (yaw) and roll from weakest negative coupling to
strongest negative coupling:
Airborne Blade with VG on*--Wills Wing Spectrum--Wills Wing
Raven--Wills Wing Skyhawk--Airborne Blade with VG off
* The Airborne Blade with VG on showed a mild positive
coupling between slip (yaw) and roll at low airspeeds. Pulling the bar perhaps 6 to 10 inches in
from trim created a neutral coupling, and the bar had to be pulled in rather
far before a strong negative coupling between slip (yaw) and roll
appeared. Other gliders showed a
negative coupling between slip (yaw) and roll at all airspeeds but the coupling
was always much stronger at low angles-of-attack (high airspeeds) than at high
angles-of-attack (low airspeeds).
The reasons for these effects are explored in detail
elsewhere on the Aeroexperiments website; they are caused by the competing
effects of the way that the way that the sideways airflow component interacts
with the anhedral geometry of the wing to create a "negative" roll
torque, and the way that sideways airflow component interacts with the swept
geometry of the wing to create a "positive" roll torque.
Q: Are you making a distinction between the idea of a
"coupling between slip and roll" and a "coupling between yaw and
roll"?
A: The above results are based on primarily on observations,
made while experimenting with the controllable yaw devices described above, of
the roll torque required to maintain a constant bank angle as a hang glider flew
in a straight-line, constant-heading, non-turning slip. They are also based on observations of the
direction that a hang glider tends to roll (starting from an initial
wings-level attitude) when a yaw control device is deployed, but even in these
instances, as the nose of the glider swung to one side in relation to the
actual direction of the flight path and airflow at any given moment, this
"swing" or yawing motion of the glider was so small and so brief that
the sustained action of the sideways airflow on the glider as the yaw control
input was maintained, not the actual yawing motion that occurred when the yaw
control device was initially deployed, seemed to be the dominant factor in
creating the roll torque.
In other words, the above results solely reflect the roll
torque created by the interaction between a sideways (slipping) airflow and the
three-dimensional shape of the glider.
They do not reflect the roll torque created by the difference in
airspeed between the two wingtips that is created as a glider actively yaws to
a new heading with respect to the outside world. That's why I'm using the term "coupling between slip and
roll" rather than simply "coupling between yaw and roll"--the latter term
would need to include the roll torque created by the difference in airspeed
between the two wingtips that is created as a glider actively yaws to a new
heading with respect to the outside world, and I don't want to include this
roll torque when I speak of a "coupling between slip and roll". I'm only including the word yaw in
parenthesis ("negative coupling between slip (yaw) and roll") to
provide a bit more clarity because I know that the word "slip" is
used in many different ways in the world of hang gliding and triking. I want to make it clear that when I use the
word "slip" I'm talking about a true slip in the usual aviation sense
of the word, which refers to a yaw-axis phenomenon--the nose of the aircraft
has been yawed to point to the right or the left of the direction that the
aircraft is actually moving through the air, creating a sideways airflow over
the aircraft--not a pitch-axis phenomenon or a loss of altitude or a downward
curvature of the flight path. Also I
want to make it clear that when I talk about a "negative coupling between
slip (yaw) and roll", I mean that when the nose of the glider is yawed to
the left in relation to the actual direction of the flight path and relative
wind, which creates a sideways (slipping or skidding) airflow component that blows from right to left across the glider, this makes the glider roll away from the direction that the nose is pointing in relation to the airflow (relative wind) and flight path, i.e. this makes the glider roll toward the direction that the airflow is coming from, i.e. this makes the glider roll toward the right. The truth is that the terminology is starting to fail us here because depending on the which direction the glider is banked, if at all, if we use standard aviation terminology (which is fixated on the idea that an aircraft can end up moving in the "wrong" direction in relation to its heading rather than on the idea that the nose can end up pointing in the "wrong" direction in relation to the flight path and relative wind) we could either argue that the glider is "skidding toward the left" or "slipping toward the right", and in the latter case it seems odd to call a right roll torque a "negative coupling between slip and roll." But we really don't care whether the glider is slipping or skidding. We're just trying to say that the nose of the glider is not pointing directly into the airflow (relative wind), and that this is making the glider roll in the opposite direction as the nose is pointing in relation to the airflow (relative wind). The most correct phrase would probably be "a negative coupling between nose-pointing and roll" but we know the readers will all flee if we mention this idea again so we won't belabor this point any further! We'll just stick with the idea of "a negative coupling between slip (yaw) and roll".
Note that the differential wingtip airspeed effect is
present whenever the glider is actively yawing to a new heading with respect to
the outside world, regardless of whether the nose of the glider is actively
yawing to one side with respect to the actual direction of the flight path and
relative wind (i.e. the slip angle is increasing), or the nose of the glider is
maintaining a constant yaw attitude (slip angle) with respect to the actual
direction of the flight path and relative wind, or the nose of the glider is
pointing directly into the relative wind in a turn that is perfectly
"coordinated" in the yaw axis with no sideslip.
Q: Considering the comments in the above two paragraphs, and
assuming we're not dealing with the case of a hang glider flying at low
airspeed with the VG full on (where the coupling between slip and roll is
likely to be positive rather than negative), when a flex-wing hang glider pilot makes a roll
input and the hang glider adverse-yaws, is the cumulative result of the
adverse-yaw motion (including both the roll torque created by the interaction
between the resulting sideways (slipping) airflow and the three-dimensional
shape of the glider, and the roll torque created by the difference in airspeed
between the two wingtips) a helpful roll torque or an unfavorable roll
torque? In other words, when a hang
glider pilot gives a left roll torque and the glider adverse-yaws to the right,
which is the stronger effect: the left roll torque created by the interaction
between the sideways (slipping) airflow and the three-dimensional anhedral
shape of the wing, or the right roll torque created by the fact that the left
wingtip is moving faster than the right wingtip?
A: Based on the experiments I've carried out, it's hard for
me to give a definitive answer on this one, but I have some suspicions.
The fact that most flex-wing hang glider pilots feel that their
gliders handle better--presumably including a higher roll rate or a quicker
roll response when the pilot makes a roll input--without a vertical fin on the
keel than with a vertical fin on the keel strongly suggests that a typical
adverse-yaw motion creates a net helpful roll torque. In other words, when the pilot makes a left roll input and the
glider adverse-yaws to the right, the left roll torque created by the
interaction between the sideways (slipping) airflow and the three-dimensional
shape of the wing is stronger than the right roll torque created by the fact that
the left wingtip is moving faster than the right wingtip.
It's worth keeping in mind that an adverse-yaw motion that
creates a large difference between the way the glider's nose is pointing and
the actual direction of the flight path and relative wind at any given moment
could involve only a small "wrong-way" yawing motion with respect to
the outside world. For example, the
main result of an adverse-yaw torque (say toward the right) could be to simply
tend to keep the glider on its original heading as the flight path began to
curve (to the left) and the direction of the relative wind began to
change. This would still create a large
difference between the direction that the nose of the glider was pointing and the
actual direction of the flight path and relative wind at any given moment.
I've certainly observed that hang gliders do adverse-yaw
with respect to the outside world as well as respect to the actual direction of
the flight path and airflow (relative wind) at any given moment, but often the
deflection of a yaw string is rather large as a pilot rolls a glider into a
turn, while the "wrong-way" swing of the nose with respect to the
outside world seems much smaller.
Considering all these idea together, my suspicion is that in
most cases when a hang glider adverse-yaws in response to a pilot roll input,
the roll torque created by the difference in airspeed between the two wingtips
is much smaller than the roll torque created by the interaction between the
sideways (slipping) airflow and the three-dimensional shape of the wing.
However, at high angles-of-attack where the glider's negative coupling between slip
(yaw) and roll might be very weak, the reverse could be true. A good example of a glider with a very weak
coupling between slip (yaw) and roll would be the Airborne Blade with the VG
full on at airspeeds ranging from near min. sink to perhaps 10 mph above min.
sink--the slip-roll coupling in this region appears to range from weakly
positive to neutral to weakly negative.
In any part of the flight envelope where a glider shows a
neutral coupling between slip (yaw) and roll or a positive coupling between
slip (yaw) and roll, then the net effect of an adverse-yawing motion (including
the roll torque from the differential wingtip airspeed effect) would clearly be
a roll torque that acts in the same direction as the nose is
adverse-yawing. This is an unfavorable
roll torque: a left roll input would create a right adverse-yaw motion and
right roll torque. In this part of the
flight envelope, the glider should respond quicker to a pilot's roll inputs
when a vertical keel fin or tip fins are present to minimize adverse yaw, than
when they are not.
(By the way, in the case of a "conventional"
aircraft with some dihedral, this describes the entire flight envelope, as long
as ailerons rather than spoilerons are being used for roll control.)
After a conversation with a very experienced dune pilot,
I've been giving some thought to the following idea: perhaps the way that dune
pilots use yaw inputs in various situations--for example during a decelerating ground run after a dunetop landing in strong wind--suggests that a strong enough yaw input can indeed create a net roll torque in
the same direction as the yaw input, presumably to the differential tip speed
effect, over a wide range of angles-of-attack.
I don't quite what to make of this idea as many different confounding
variables may be present (wind gradient etc) in the dune environment. Even if this idea is entirely accurate and
even if this effect is unrelated to gradient effects etc, perhaps this situation
involves lower forward airspeeds, and higher yaw rotation rates, than would
normally occur in flight. This would
make the roll torque created by the differential wingtip airspeed effect
relatively more important, and the roll torque created by the sideways
(slipping) airflow around the glider relatively less important, than they would
be when the glider adverse-yawed in normal flight.
One interesting thing I've noticed during ground-handling is
that whichever wing is raised tends to keep rising, be it the upwind wing or
the downwind wing. This is different
from what we see in the case of a sailplane on the ground (with dihedral rather
than anhedral) where the upwind wing always has a strong tendency to rise and
the downwind wing always has a strong tendency to drop. My best explanation of this: in the case
where the glider is sitting with the downwind wing higher in than the upwind
wing, if the aircraft has anhedral the downwind wing will be in the shadow of
the upwind wing, while if the aircraft has dihedral the downwind wing will be
high up in the faster airflow and well out of the shadow of the upwind
wing. (By the way, one of the reasons
the Wrights used anhedral on their 1902 glider and 1903 airplane was that they
felt it minimized the tendency for a sudden wind gust to roll the aircraft in
the downwind direction, particularly in the dune environment.)
My experiments involving very abrupt yaw inputs on a
modified Zagi RC glider with enough anhedral to create a negative coupling
between slip (yaw) and roll failed to yield evidence that the differential
wingtip airspeed effect was ever more important than the interaction between
the sideways (slipping) airflow and the anhedral geometry of the wing. But a Zagi flies at a much higher scale
speed (which we'll define as the inverse of the time required to traverse one
wingspan) than a full-scale hang glider does, so the differential wingtip
airspeed effect would be expected to be less important in the case of the Zagi
than in the case of a full-scale hang glider.
Q: Why on earth should a hang glider pilot care about
theoretical topics like these?
A: A clearer understanding of the complexities of a glider's
"coupling between slip (yaw) and roll" may motivate a pilot approach
certain questions with a more open mind rather than with a formulaic but
erroneous approach, and may lead him to be more perceptive of what is really
going on around him in flight.
Such questions would include:
"Does installing a fixed vertical fin on the keel help
or hinder the handling of glider X, and does the answer noticeably vary
according to the angle-of-attack and VG setting?"
"In this context, and exactly what is meant by
'handling"? The roll response
rate? The need for high-siding or low-siding in constant-banked thermalling
turns?"
"How does making changes in a glider's anhedral
geometry for tuning purposes actually end up affecting the need for high-siding
or low-siding in a constant-banked thermalling turn?"
Q: So how does a fixed vertical fin affect a flex-wing hang glider's handling?
A: As I've noted above, many pilots seem to think that a
fixed vertical fin diminishes the roll response rate of most hang gliders. As suggested by our above discussion of the
helpful roll torque created by a "negative coupling between slip (yaw) and
roll"), this may indeed be true across much of the flight envelope. I'd expect the reverse to be true at low
airspeeds when the VG is tight.
I have to admit that I've not detected that a fixed vertical
fin has any significant effect on the roll handling of my Airborne Blade (VG on
or off) or any of the other hang gliders I've flown.
If a fixed vertical fin substantially reduces the amount of
adverse yaw that occurs in response to a pilot's roll inputs, and if the
sideways airflow (slip) arising from the an adverse-yawing motion creates a
roll torque in the "negative direction" (away from the direction that
the nose is pointing, and toward the direction that the pilot wants to go) that
is stronger than the roll torque in the "positive" direction (toward
the direction that the nose is pointing, and away from the direction that the
pilot wants to go) arising from the difference in airspeed between the two
wingtips created by the adverse-yawing motion, then a fixed vertical
fin certainly should reduce a hang glider's responsiveness in the roll
axis.
In general the main effect I notice when I fly a hang glider
with a fixed vertical fin is that the glider just feels less
"yawy". I generally like
this--I feel I can better sense what the flight path is actually doing when the
nose isn't yawing all around. Also I
like the reduced susceptibility to yaw-roll oscillations at high speeds with
the VG off, and on tow.
If a pilot wants improved handling with the VG full on, he
should be very alert to the theoretical possibility that with certain glider
types, a fixed vertical fin might offer a relatively small handling (roll rate)
penalty with the VG loose, and might offer a large handling (roll rate)
advantage with the VG tight.
Q: Why should a hang glider show a stronger "negative
coupling between slip (yaw) and roll" when the VG is loose than when the
VG is tight? Doesn't tensioning the VG
increase the glider's anhedral?
A: No, tensioning the VG decreases the glider's net
geometric anhedral. The keel is
essentially an arbitrary reference line that has very little to do with the
actual average chord line of the wing as a whole, especially when we consider
the billowed shape of the sail. It is
in no way adequate to define a glider's anhedral as the droop in the leading
edge tubes in relation to the keel tube.
A better--but still inadequate--definition of anhedral would be the
droop in the leading edge tubes in relation to a line parallel to the average
chord line of the wing as a whole, drawn through the center of the wing
root. By this measure, anhedral
actually decreases as the VG is applied, because of the way that this average
chord line tilts in a nose-up, tail-down manner as the VG is applied and the
sail billow is decreased. Photos illustrating
this will be posted on this website in the future. But a more holistic view of anhedral would consider the
three-dimensional shape of the entire wing.
Since the trailing edge of the sail as a whole, as viewed from the rear,
essentially takes the shape of a stretched-out "M", the situation is
somewhat analogous to "gull-winged" aircraft (think of the famous F4U
Corsair, but inverted) where the inboard wing panels have dihedral, but the
outer wing panels--which are further from the CG and generate much more roll
torque in the presence of a sideways airflow than the inboard wing panels
do--have anhedral. Because of their
distance from the CG, the shape of the outboard wing panels are the dominant
factor in determining the aircraft's net "anhedral effect" or
"dihedral effect", in terms of a coupling between slip (yaw) and
roll. (We're intentionally ignoring
sweep here, and by "net anhedral geometry" or "net anhedral
effect" we aren't talking about the combined effects of sweep and
anhedral. We're only talking about the
effects that are directly related to anhedral). Therefore in the case of any aircraft whose wing has a complex
three-dimensional "M" or "W" shape, it is by no means
adequate to quantify anhedral or dihedral simply by measuring the vertical
distance that the wingtips ride above or below some reference line drawn
through the wing root, regardless of whether this reference line is parallel to
the keel or fuselage or to the average chord line of the wing as a whole. In the case of a flex-wing hang glider, when
we tension the VG, we remove much of the sail billow and much of the
"M" shape of the wing (as seen in a rear view of the trailing edge),
which removes much of the net geometric anhedral.
These issues are addressed in more detail elsewhere on this
website.
Q: If tensioning the VG decreases the glider's net geometric
anhedral, why does a glider typically need more high-siding in a steady, constant-banked turn when the VG is
tight?
A: Possibly because tightening the VG reduces washout and
makes the wingtips work "harder" than they normally would, so the
rolling-in torque created by the difference in airspeed between the two
wingtips becomes more important.
Bear in mind that anhedral (or dihedral) does not
automatically create a roll torque away from (or toward) wings-level whenever
an aircraft banks. A rolling-in torque
only arises when a sideways (slipping) airflow component is present, which interacts with the anhedral to create the roll torque. In other words, a rolling-in torque
only arises when the nose of the glider points toward the outside or high side of the turn, in relation to the actual direction of the flight path and relative wind (airflow) at any given moment. During a constant-banked turn, if the glider adopts an orientation
where the keel is nearly parallel to the overall direction of the flight path
and relative wind, then the sideways airflow component will be small and the
resulting rolling-in or rolling-out torque created by anhedral will also be
small. If the nose of a hang glider
points toward the inside rather than the outside of a constant-banked turn
(which I suspect never happens), creating a skidding airflow rather than a slipping airflow, then any anhedral
that was present would create a rolling-out torque rather than a rolling-in
torque.
Strictly speaking the above comments really need to consider
the orientation of each part of the glider with respect to the direction of the
relative wind, not just the direction that the nose is pointing with respect to
the direction of the relative wind. The airflow or relative wind curves to follow the circumference of the turn--this is the "airflow curvature" effect that we discuss in more detail elsewhere on this website. If
the glider as a whole were to point "squarely" into the overall
average direction of the relative wind in some sense--i.e. if the airflow near the near the sail's center of area were to be roughly parallel (tangent) to the keel with no sideways component--then nose of the glider would actually
be pointing slightly toward the outside or high side of the turn in relation
to the direction of the relative wind at the nose. The curving relative wind (airflow) cannot be parallel to the entire keel of the glider. The curving relative wind (airflow) can only be tangent (parallel) to one point along the line defined by the keel of the
glider. (If the nose of the glider is pointing far toward the outside of the high side of the turn, so that the entire glider is experiencing a strong sideslipping airflow, then this tangent point is located far behind the actual end of the keel). If this tangent point is near
the wing's center of area--or perhaps we should say if this tangent point is further aft, near the
center of area of the outboard portions of the wing, where most of the anhedral
is concentrated--then any anhedral that is present should not create much roll
torque in either direction.
Q: So in the real world, how does a flex-wing hang glider orient itself in
relation to the actual direction of the overall relative wind in a constant-banked
turn? Or to put it into "airflow
curvature" terms, what point on the keel of the glider (if any) is the
keel tangent to the curving airflow?
A: From theoretical considerations (especially the increased
airspeed and drag experienced by the outboard, faster-moving wingtip) I would
strongly expect the nose of a circling glider to point far enough toward the
outside or high side of the turn to make the wing a whole experience at least a
slight slipping component in the airflow.
I would not expect the wing as a whole to experience even a slight
skidding component in the airflow.
My actual in-flight observations of yaw stings in a variety
of hang gliders (with VG both on and off) have shown that yaw strings mounted
near the nose of the glider (a few feet in front of the pilot) do blow slightly
toward the outside of a constant-banked turn, illustrating a slight slipping
component in the airflow, as measured at this point. The deflection of the yaw string is slightly stronger in a turn at
low airspeed than in a turn at high airspeed.
However this yaw (slip) angle was small enough that at
present I can't confidently say that it clearly reflects any significant
slipping airflow component over the wing as a whole, when we take the airflow
curvature effect into consideration. I'll
update this section after I do the math to convert the slip angles I saw at the
nose, to the slip angles that would exist further rearward on the aircraft,
taking into account the curvature of the relative wind.
But as a very rough rule of thumb my suspicion is that the
most rearward parts of a circling hang glider--such as the rear of the
keel--might tend to be almost parallel to the curving airflow.
Since much of a flex-wing hang glider's "net geometric anhedral" is created by sail billow and is therefore concentrated in the rearward
(outboard) parts of the wing, this suggests that perhaps only a small amount of
rolling-in torque will be created by the wing's anhedral geometry in a steady, constant-banked turn.
However, if we think in terms of a given amount of "net geometric anhedral" that would create a given amount of roll torque when the entire glider experienced a given (non-curving) sideways airflow component, we realize that the less billow the sail has, and the more the leading edge tubes droop in relation to the keel, the more
evenly the glider's "net geometric anhedral" will be distributed across the whole wing surface, which might tend to increase the amount of rolling-in torque that would arise when the airflow is highly curving. (Note that we're not talking here about simply tensioning the VG, which would definitely decrease the glider's "net geometric anhedral" and would also definitely decrease the amount of roll torque that would arise when the entire glider experienced a given sideways airflow component.) Admittedly we're going through some rather convoluted twists of logic here. The main point we're trying to make is that analyzing the roll torque created by anhedral in a case where the overall angle of sideways airflow is rather small, and the flight path and relative wind are highly curving, is much more complex than analyzing the roll torque created by anhedral in a case where the overall angle of sideways airflow is rather large, and the curvature in the flight path and relative wind is small. In both cases, the distribution of the anhedral across the wingspan will be important because the parts of the wing that are farthest from the CG will act at the greatest moment-arm from the CG, but in the former case we also have to consider the fact that the angle of sideways airflow over the wingtip areas could be quite small.
We're obviously getting into the realm of conjecture here
and the above two or three paragraphs are meant to illustrate some ways of thinking
about the true effects of anhedral, not to make authoritative comments about the direction of the airflow, or the balance of roll torques, in a steady, constant-banked turn on any
particular glider.
Q: Would a controllable rudder be a beneficial addition to
an off-the-shelf modern flex-wing hang glider?
A: Generally, no.
See part 2 for some thoughts on how using a controllable rudder to
eliminate adverse yaw and sideslip might create only a slight increase in the
glider's turn rate (rate of curvature of the flight path), for any given bank
angle. And in some of the comments
above, we've suggested that the net effect of an adverse-yaw motion--including
both the roll torque created by the interaction between the sideways (slipping)
airflow component and the glider's anhedral geometry, and the roll torque
created by the difference in airspeed between the wingtips--is probably a net
helpful roll torque over most of the flight envelope, especially at lower
angles-of-attack (higher airspeeds) and especially with the VG loose. In any part of the flight envelope where
adverse yaw creates a net helpful roll torque, eliminating adverse yaw would
decrease the glider's responsiveness in the roll axis. In addition to countering adverse yaw, a pilot
might be tempted to use a controllable rudder to point the nose of the glider
toward the inside or low side of the turn (this would be a skid) as he
increased the bank angle (or when the bank angle was held constant.) In this case, the interaction between the
sideways (skidding) airflow component and the anhedral geometry of the wing
would produce a "wrong-way" roll torque in the opposite direction
that the pilot was trying to roll the glider (or in the case of a
constant-banked turn, a rolling-out torque).
In my experiments with a controllable rudder, I saw that this effect was
quite strong at low angles-of-attack (high airspeeds) when the VG was off. If a hang glider pilot rigged a moveable
rudder to move automatically in the "normal" direction whenever he
made a weight-shift roll control input, and if the rudder sometimes generated more yaw torque than was needed simply to overcome adverse yaw and keep the nose aligned with the flight path and airflow, so that the rudder actually made the nose skid to point in same direction as the pilot's roll input, then the pilot could be in for quite a surprise
when he explored the low-angle-of-attack, high-airspeed, VG-loose corner of the
flight envelope--the resulting "backwards" roll torques could render the glider uncontrollable.
Q: Would spoilerons be a beneficial addition to an
off-the-shelf modern flex-wing hang glider?
A: Other pilots' experiences with spoilerons on flex-wing hang gliders have been very
good: they've reported that roll handling was very good in all parts of the
flight envelope. They did not
experience problems when the VG was loose and the angle-of-attack was low (i.e.
the airspeed was high), which is where I've found that a modern flex-wing hang
glider exhibits a strong negative coupling between slip (yaw) and roll. (We'll describe these pilots' experiences
with spoilerons in more detail elsewhere on this website.) This suggests that if the yaw torque from
the spoilerons forced these pilots' gliders to skid to any significant degree
while entering a turn, the unfavorable roll torque that this would have created
in some parts of the flight envelope was overpowered by the spoilerons'
lift-killing function.
For more, see these related articles on the Aeroexperiments website:
Photos of hang gliders and models to illustrate how billow contributes to the net geometric anhedral of a swept wing
Questions of interest part 1: Relationship between pitch inputs and sideslips in hang gliders and other aircraft
Questions of interest part 2: Aerodynamic sideforce created by the sideways airflow as a hang glider sideslips
The real purpose of dihedral and anhedral: creating an aerodynamic coupling between yaw and roll
How sweep creates an aerodynamic coupling between yaw and roll
Competing effects of sweep and anhedral at various angles-of-attack
How billow and washout increase the net geometric anhedral of a swept wing--abbreviated version
The effect of VG on anhedral: why the net geometric anhedral decreases as a VG is applied
Causes of adverse yaw in hang gliders and "conventional" aircraft--with notes on slips, skids, yaw strings, slip-skid balls, rudder usage, yaw rotational inertia, "airflow curvature", aerodynamic "damping" in the roll axis, and flex-wing billow shift
