Advancing our knowledge: Vectored-thrust experiments in flex-wing and rigid-wing hang gliders

August 6, 2007 edition
Steve Seibel
steve at aeroexperiments.org
www.aeroexperiments.org

 

To shed light on some of the basic aerodynamic characteristics of flex-wing and rigid-wing hang gliders, I'm interested in examining the roll torques that arise when a hang glider moves through the air in a sideways manner (i.e., during a sideslip.) I've explored this by attaching a controllable rudder to a flex-wing hang glider, as described elsewhere on the aeroexperiments website. These explorations shed some interesting light on the effects of anhedral at various airspeeds and VG settings.

 

Another way to explore this subject is to use a motor to drive a flex-wing or rigid-wing hang glider through the air in a slightly sideways manner, and to note which direction the glider tends to roll (bank) and turn at various airspeeds and at various VG settings. Some other pilots have already explored, to some extent, the effects of driving a glider sideways through the air with a motor. In this video, RC is twisting the thrust line by sliding his feet back and forth (right and left). In the video, when the pilot slides his feet to the right and/or head to the left, the glider banks and turns to the left, and when the pilot slides his feet to the left and/or head to the right, the glider banks and turns to the right. Note that the pilot is being careful not to actually shift his weight (including the weight of the motor) to either the left or the right. In other words, the pilot is not making any significant weight-shift control inputs. He is simply twisting his body (plus the motor) from side to side, while letting the CG of the pilot/motor combination remain on the glider centerline. To accomplish this, he is exerting only a yawing (twisting) force on the bar, and is not exerting any significant net sideways force on the bar. To avoid accidentally or unconsciously exerting a significant sideways force on the control bar, he is only touching the bar with one hand, while the other hand dangles freely. Of course, flying with one hand in this manner is only possible while the glider is being allowed to fly at the power-on trim airspeed.�

 

I don't (yet) own a motor harness, but I'd be interested in hearing about other pilot's experiences with twisting the thrust line to the side in the manner shown in the video. I'd also like to see RC's experiment expanded to include tests in rigid-wing hang gliders as well as in flex-wing hang gliders, and to include tests at airspeeds (bar positions) well above the power-on trim airspeed, if possible, and in flex-wing hang gliders with VG's, to include tests at the power-on trim airspeed with the VG fully on as well as with the VG fully off. I'm also interested looking closely at the case where the thrustline is twisted to the side and the glider is then forced to fly in a straight line, rather than being allowed to turn, because this special case will potentially shed a lot of light on the fundamental dynamics at play. �

 

To facilitate easy replies, I've asked a series of questions in the form of a poll on the flphg yahoo-group website. Any replies to any of the poll questions would be much appreciated (with the caveat that the answers should be based only on actual in-flight experimentation, not enthusiastic guessing!) Even more valuable would be a personal email to me (steve at aeroexperiments dot org) containing your answers to any of the questions below that you've been able to explore, plus other information such as glider type, pilot name, etc.�

 

Please feel free to contact me for more information if any of this seems unclear or confusing! �

 

Here are the questions I'm interested in (essentially the same questions as appear on the flphg yahoo-group poll, with a few improvements and expansions.):�

 

--Note: all questions refer to flight under power with a prone powered harness in combination with a flex-wing or rigid-wing hang glider. Please make certain that there is no chance of the prop striking the keel or wing of your glider before experimenting with twisting the thrust line to the side, especially while pulling in the bar for extra airspeed!�

 

1) In my flex-wing hang glider, while flying in a prone position, letting the glider fly at the power-on trim airspeed, in order to turn to the RIGHT while under power WITHOUT using weight-shift, I find that I can accomplish this by:
a) sliding my feet to the right and/or head to the left
b) sliding my feet to the left and/or head to the right, as illustrated in the video
c) sliding feet and/or head to the side while leaving weight of pilot/motor combination on glider centerline has no noticeable turning effect
d) my prone motor harness arrangement doesn't permit me to twist the thrust line (slide feet and/or head to the side as illustrated in the video) without also shifting the weight of the pilot/motor combination away from the glider centerline, even at the power-on trim speed�

 

2) In my rigid-wing hang glider, while flying in a prone position, letting the glider fly at the power-on trim airspeed, in order to turn to the RIGHT while under power WITHOUT using weight-shift/spoilerons/ailerons, I find that I can accomplish this by:
a) sliding my feet to the right and/or head to the left.
b) sliding my feet to the left and/or head to the right, as illustrated in the video.
c) sliding feet and/or head to the side without activating the spoilerons/ailerons has no noticeable turning effect
d) not physically possible to twist the thrust line (slide feet and/or head to the side as illustrated in the video) without activating spoilerons/ailerons, even at power-on trim speed�

 

3) This question is ONLY for pilots who have found that in their FLEX-WING hang glider, while flying in a prone position, letting the glider fly at the power-on trim airspeed, they can turn RIGHT while under power WITHOUT any weight-shift by sliding their feet to the LEFT and/or head to the right, as illustrated in the video. Which of the following do you find to be true:
a) This turning effect is stronger with the VG loose
b) This turning effect is stronger with the VG tight
c) My glider has no VG
d) Not feasible to adjust VG and/or fly with VG tight while under power
e) Can't detect a relationship between this turning effect and the VG setting�

 

4) This question is ONLY for pilots who have found that in their FLEX-WING hang glider, while flying in a prone position, letting the glider fly at the power-on trim airspeed, they can turn RIGHT while under power WITHOUT any weight-shift by sliding their feet to the LEFT and/or head to the right, as illustrated in the video. Which of the following do you find to be true:
a) This turning effect is strongest at airspeeds near the power-on trim airspeed, ie at low airspeeds
b) This turning effect is strongest at airspeeds well above the power-on trim airspeed, ie at high airspeeds
c) When I pull the bar in for speed, I find that it's not physically possible to twist the thrust line (slide feet and/or head to the side as illustrated in the video), without also shifting the weight of the pilot/motor combination away from the glider centerline
d) Can't detect a relationship between this turning effect and the airspeed
e) While flying under power, I find that my arms aren't long enough to make the glider fly substantially faster than the power-on trim airspeed �

 

5) This question is ONLY for pilots who have found that in their FLEX-WING hang glider, while flying in a prone position, letting the glider fly at the power-on trim airspeed, they can turn RIGHT while under power WITHOUT any weight-shift by sliding their feet to the LEFT and/or head to the right, as illustrated in the video. Do you find the following statement to be true: To force the glider to fly in a straight line when my feet are pointing to the LEFT and/or my head is pointing toward the right, I need to keep my total weight (including combined weight of pilot+motor) shifted noticeably to the LEFT side of the glider centerline
a) Yes, by experimenting in flight I find the statement to be true
b) Though the statement seems logical, I do not find it to be true in actual practice�

 

6) This question is ONLY for pilots who have found that in their RIGID-WING hang glider, while flying in a prone position, letting the glider fly at the power-on trim airspeed, they can turn RIGHT while under power WITHOUT any weight-shift/spoilerons/ailerons by sliding their feet to the RIGHT and/or head to the left, which is OPPOSITE to the turning effect illustrated in the video. Which of the following do you find to be true:
a) This turning effect is strongest at airspeeds near the power-on trim airspeed, ie at low airspeeds
b) This turning effect is strongest at airspeeds well above the power-on trim airspeed, ie at high airspeeds
c) When I pull the bar in for speed, I find that it's not physically possible to twist the thrust line (slide feet and/or head to the side as illustrated in the video) without activating spoilerons/ailerons
d) Can't detect a relationship between this turning effect and the airspeed
e) While flying under power, I find that my arms aren't long enough to make the glider fly substantially faster than the power-on trim airspeed �

 

7) Please feel free to make any other comments that seem relevant. If the criteria for answering questions 3, 4, 5, or 6 did NOT apply to your situation, I'd still be interested in hearing more about the dynamics you observed when you experimented with changing the VG setting or airspeed.�

 

8) What kind of glider are you flying? And for flex-wing gliders, with your motor harness, where does your hook-in weight fall in relation to the recommended hook-in weight envelope for your glider? (This isn't critical, just give your best estimate.)�

 

9) Have you (or do you plan to) answer(ed) these same questions by participating in the on-line poll on the on the flphg yahoo-group? (Doubling up is fine, just note it here). �

 

Thanks for your participation! �

 

My interest in these dynamics was originally sparked by two excellent articles by Richard Cobbs: �

 

"Thoughts on Handling Under Power"�

 

"Wrestling With the Beast: working with, instead of against, a powered harness for easier handling" (Adobe document)�

 

See also: �

 

More powered harness links from RC

 

 

Some added thoughts (Summer 2007):

RC's analysis (as described in the above links) goes something like this: an asymmetric thrust line will drive the glider sideways through the air. The glider's inherent yaw stability will yaw the nose back into line with the direction of the flight path. This yawing motion will increase the airspeed of the outboard wingtip. This will make the glider roll, which will lead to a turn away from the direction that the pilot's feet are pointing.

This didn't quite make sense to me. My initial analysis went something like this: an asymmetric thrust line will drive the glider sideways through the air. If the glider has effective anhedral, the "upwind" wing will tend to drop, which will lead to a turn away from the direction that the pilot's feet are pointing. If the glider has effective dihedral, the "upwind" wing will tend to rise, which will lead to a turn in the same direction as the pilot's feet are pointing.

However here is a new analysis that I now think is more accurate: the displaced thrust line does not create a yaw torque and therefore cannot make the glider fly sideways through the air. The sideways component of the thrust line can be viewed as a centripetal force that creates a slow wings-level ("flat") turn away from the direction that the pilot's feet are pointing. The aircraft's inherent yaw stability will tend to keep the nose of the aircraft aligned with the relative wind, so the aircraft's heading will change as the flight path curves. This yawing motion will increase the airspeed of the outboard wingtip. This will increase the lift on the outboard wingtip, which will make the outboard wingtip rise, and this change in bank angle will augment the rate of turn away from the direction that the pilot's feet are pointing. This will be true regardless of whether the glider has effective anhedral or effective dihedral. However a glider with effective anhedral will show much more tendency to roll, in response to the initial curvature in the flight path and the initial difference in airspeed between the two wingtips, than a glider with effective dihedral. For example, for a given sideways component in the thrust line, a glider with dihedral might come to equilibrium at a shallow bank angle, and a glider with anhedral might eventually reach a fairly steep bank angle. This is a reflection of the way that anhedral contributes to spiral instability and dihedral contributes to spiral stability.

In any turn, the outboard wingtip experiences more drag than the inboard wingtip, which makes the glider adopt a slipping attitude (so that the nose points slightly toward the outside or high side of the turn in relation to the actual direction of the flight path and relative wind). The sideways airflow over the wing created by this slipping attitude creates a stabilizing roll torque (toward wings-level) if the aircraft has effective dihedral, and creates a destabilizing roll torque (toward a steeper bank) if the aircraft has anhedral. Therefore aircraft with anhedral are naturally more spirally unstable than aircraft with dihedral. Therefore twisting the thrustline will create a stronger roll torque and higher turn rate on an aircraft with effecive anhedral than on an aircraft with effective dihedral. The direction of the roll torque will be the same in both cases, because the roll torque arises from the difference in airspeed between the wingtips as the flight path curves. The aircraft is not being driven sideways through the air by the twisted thrust line. Instead, the only sideslip present is that which is created by the difference in airspeed between the two wingtips as the flight path curves.

This analysis is not yet supported by actual observations: I've not yet heard from anyone who was able to explore these dynamics on a rigid-wing glider (with dihedral.) My suspicion is that so little force on the control bar is required to activate the spoilerons of a rigid-wing glider, that it would be very difficult for a pilot to twist the thrust line of a powered harness to one side without activating the spoilerons.