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Old Oct 09, 2012, 11:04 AM
agnotology
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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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Old Oct 09, 2012, 05:45 PM
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All of that sounds very interesting, and I see your points on all of it. I realized later that the stabilizer would make it very difficult to read the air around our planes, as it continuously makes inputs of its own. I just tried my flying wing with a more rearward CG, and although it was (barely) flyable, a stall was disastrous, and in a turn always resulted in a spin. The glide, frankly, was horrendous against my Zone foiled DLG. No comparison.

Anyway, good luck with your work and keep us posted. I still think that wings may offer an advantage as a "fun fly" thermal class, with cheap construction and bungee launches. Midairs are more benign as well.
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Old Oct 10, 2012, 01:34 AM
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Well, thanks for all the intelligent posts. I certainly did not know a quarter as much on flying wings. Very interesting.....

Joseph
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Old Oct 12, 2012, 01:43 AM
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United States, CA, Castro Valley
Joined Apr 2007
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Kevin,
What do you think would be better suited for a DLG: plank or chevron planform? Which design is better suited for each flight regime: launch, speed, cruise, thermal? Ignore actively stabilized wings which I think current F3K rules may look upon dubiously.

Would an oblique wing have any advantage in launch? Or are the advantages only seen when approaching the speed of sound?
Cheers
Paul
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Last edited by RCPC; Oct 12, 2012 at 01:52 AM. Reason: Oblique wing
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