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Old February 13th 18, 09:20 PM posted to rec.aviation.soaring
Bob Kuykendall
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Default New Glider Dream Elon Musk Flow

On Tuesday, February 13, 2018 at 2:59:37 AM UTC-8, krasw wrote:
I think most flaps and ailerons are made of 100% kevlar, though not sure why. Maybe it's lighter than carbon and still strong enough?


It's all about making a control surface that is limber enough in bending so it can deflect up and down while hinged to a bent wing, while still having enough torsional stiffness to transmit motion while resisting flutter.

Think about an early ASW20 at 2g, with the wings bent up about 1.5m. Now imagine deflecting the flaperons up on the left side and down on the right side (left stick). Because the wings and the flaperon hinge path are curved, the left flaperon trailing edge is being shortened as if the surface is being bent aft at the tip. On the right side, the trailing edge is being stretched as if the surface is being bent forward at the tip.

With the flaperons made of carbon, the result would be a control surface so stiff in bending that at any appreciable load factor the stick would spring to the center and resist flaperon deflection. This could be quite dangerous, because for example it would resist the leveling of wings while in a spiral dive.

Waibel's team at Schleicher appears to have tried several construction methods for the ASW20 flaperons before settling on sandwich construction with skins made with 2oz Kevlar on each side of Rohacell core foam. They also refined the design of the trailing edge joint so that it required a minimum of bonding paste, which helps keep the bending stiffness to a minimum. This was just one of a huge series of detail refinements required to go from a design to a functional product.

If you come to one of our Akaflieg sessions (next one is probably in late March) I'll show you all of the complexities that go into making effective flaperons.

--Bob K.
Bob Kuykendall
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