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Old Sep 01, 2014, 02:23 PM
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The Willamette Valley, Oregon
Joined Dec 2008
1,235 Posts
Weight-shift control-- rc vs full-scale

I just found this thread. Very neat. Amazing workmanship.

It's interesting to give some thought about a fundamental difference between RC servo-controlled weight shift and full-scale weight shift.

In full-scale, when you apply thrust to the power harness or trike car, it swings forward through the control frame. The pilot sees the bar position change, but he can keep on exerting the same constant force (or zero force if flying at trim) through his arms. The wing should keep right on flying at the same constant angle-of-attack. The thrust effectively acts at the hang point. The weight of the trike car or pilot+power harness also effectively acts at the hang point, at least in the case where the pilot is exerting zero force and letting the wing fly at trim, and really in any other case where we say the pilot is exerting a constant force on the bar rather than keeping the bar in a fixed position relative to his body.

When flying with servo controls, the pilot or the trike car isn't free to swing forward in this manner. The servos don't exert a constant force, rather they hold a constant position. The thrust line acts at its actual location, not at the hang point. With the thrust line below the CG of the whole system, you tend to get a much higher-angle-of-attack power-on than power-off.

(I haven't read the OP's threads thoroughly enough to understand what he is doing with "springs" but I suspect it's roughly true to say that the servos are commanding a given position of the trike car or pilot/ powered harness, not exerting a constant force?)

Related but smaller effects apply to roll trim-- the freely hanging pilot's weight acts at the hang point and doesn't make a "pendulum effect". To exert a roll torque on the glider, he has to exert a sideways force with his muscles. When the pilot or trike car is held in place with servos, this effectively lowers the CG of the whole system, creating a stabilizing "pendulum effect", at least in the case where the wing has enough billow to expose enough cross-sectional area to the flow to generate a significant amount of sideforce in a sideslip. That sideforce, acting high above the CG of the whole system, creates a stabilizing roll torque. That roll torque wouldn't exist in the full-scale situation unless the pilot were using his muscles to keep himself exactly centered on the bar. The pilot has to apply the roll torque with his muscles, it doesn't happen automatically. And it's worth noting that in a modern wing, without the huge sail billow of the early Rogallos, even in situations where there's a lot of sideslip, the pilot isn't forced strongly off-center on the control bar. I.e. the "pendulum effect" is modest as far as the roll axis is concerned, even in the case where the pilot uses his muscles to keep his body exactly centered on the control bar.

Just some random thoughts...

Steve
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Old Sep 01, 2014, 02:25 PM
Registered User
The Willamette Valley, Oregon
Joined Dec 2008
1,235 Posts
Vectored thrust?

Mark, thanks again for sharing your powered hang glider with us---

A couple of questions--

* Is the "keel" free to shift left and right relative to the "crossbar" as in a full-scale hang glider or trike wing, or are they tightly connected together where they cross?

* How did that thrust vectoring idea work out? Was the basic set-up rigid enough to keep the pilot from simply twisting around the hang point? Did the glider turn as you hoped?

These wings have a lot of anhedral. Some the anhedral comes not from the airframe geometry but from the billowed shape of the wing. You can see the resulting anhedral geometry very clearly in the first photo on post 1 of this thread-- we see the top side of the right wing and the bottom side of the left wing-- an anhedral geometry.

So-- twisting the motor to the right yaws nose to right but this could interact with anhedral to generate a left roll torque, leading to a left bank and turn. Maybe even when flying slowly (high angle-of-attack). And definitely when flying fast (low angle-of-attack).

Did you find this to be true?

I put a rudder on a full-scale hang glider and got those results. I'd be most interested to hear what you found when you tried the thrust-vectoring idea.

There is video on line of a pilot using thrust-vectoring (twisting his whole body) to turn a full-scale powered-harness hang glider at low airspeed but in that case one wonders if the thrust, acting through the hang strap, wasn't simply pulling the keel to the side relative to the crossbar, which would deform the sail in a way that would make the glider roll in the desired direction. Plus, in that case the thrustline was effectively applied to the hang point, not further back, and that really shouldn't put any yaw torque on the glider at all.

A Rogallo-style wing like this http://www.aeroexperiments.org/image...kysailor1x.jpg is a different deal-- the extreme delta shape makes the wing respond to yaw/slip in the direction you'd expect. Also the extreme taper means that the outboard portions that have anhedral due to sail billow only make up a very small part of the total wing area. By the way this glider always wanted to come back to wings-level on it's own no matter how steep you managed to get it banked up-- a very good plane for a beginner to learn RC on. Totally different from the instability you described at steep bank angles.

Curious to hear how the thrust vectoring worked out for you.... this is a topic that has been at the back of my mind for many years now.

Steve
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