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Old Oct 09, 2012, 10:04 AM
kcaldwel is offline
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naive realism
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Joined Jan 2007
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The problem I found with the existing stabilizers is that they are all inertial units. That works fine for power airplanes that fly considerably above stall almost all the time, have power to counteract sink, and you are not interested in "reading" the airplane motion and attitude changes to look for lift and sink.

The inertial stabilizers try to keep the airplane level relative to the earth, which in a sailplane masks the lift/sink changes.

I have flown a conventional sailplane with quite large negative static margins successfully, and even thermalled it. But the stall was absolutely vicious with the CG behind the aircraft neutral point. The stabilizer applies full up at the stall break in an effort to keep the airplane level. This made an airplane with pretty mellow stall characteristics into a spinning top.

A conventional tailed airplane has much more pitch damping than a flying wing, particularly a plank. This makes it easier to electronically stabilize and I've even managed to fly it with unstable CG positions with the stabilizer off for a time. A plank will have a very short pitch doubling period. Recovering from a stall/spin might be very difficult with an inertial stabilizer. I haven't tried it in a plank yet.

I think a stabilization system for a sailplane needs to stabilize on a commanded angle of attack, rather than just an inertial reference frame. That would allow the sailplane to react to lift and sink more conventionally, and might allow tuning of the stall characteristics to something more benign. Both the unstable plank projects I referenced above used AoA sensors. Mark Drela proposed using pressure sensors on the top and bottom of the wing LE, rather than the vane sensor, which would avoid having to get an AoA vane out in "clean" air which limits the design of a flying wing somewhat.

I have been thinking about attempting to reprogram the DIYDrone Arduino autopilot to work with an AoA sensor.

Kevin
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