On Sun, 28 Jun 2015 06:18:29 -0700, Andy Blackburn wrote:

On Saturday, June 27, 2015 at 7:07:58 AM UTC-7, JJ Sinclair wrote:
Time for another JJ story. We were finishing at Ephrata at 50 feet and
140 knots in our ASH-25 (with 2 meter wing extensions). As I crossed
the finish line, I pulled up and rolled left,. At about 45 degree left
bank, I applied right stick to stop the left roll............IT KEPT
ROLLING LEFT...............I then applied full right stick and rudder
and the left roll stopped, but by then we were in a 75 degree left
turn. The nose was up, so as airspeed slowed, the ship slowly responder
to right stick commands.
After landing, I talked to the guy running the finish gate who was
looking right down our wing as we flew through the gate...........He
said it was the scariest thing he had ever seen! The wing tip leading
edge, twisted down and doug-in, resulting in the un-commanded left
turn, the rest of the wing looked like a sign wave. The wing extension
had been somewhat blessed by Schleicher who had us install some 12# of
lead in the outboard leading edge. I believe it was to prevent flutter,
but probably aided the twist problem. Any aerodynamic experts explain
why the lead was placed in the leading edge? Didn't it help twist the
wing leading edge down?

Not too long after that, I sold the ship and retired to the peace and
tranquility of the finish cylinder in sports class.
JJ

Happened to my Dad doing Max Q pull-ups while flight testing an F-86.
The aileron acted like a trim tab for the entire wing. Not fun at
transonic speeds.

I don't have a precise answer for the lead in the leading edge.
Aeroelastics are complex, but presumably they want the leading edge to
twist downward under loading so that there is a restoring force from
whatever input perturbs the wing. Make sense that this could increase
the resonant frequency and delay the onset speed for flutter and/or add
some damping effect, but obviously it had a second consequence for you.

It also helps delay flutter onset if the wing CG is in front of the
torsional axis. Adding lead in the LE or putting weights in front of the
LE on stingers are both ways of moving the wing's CG forward. I heard
that years ago from a fellow who worked at the Royal Aircraft
Establishment at Farnborough and have seen the stingers used to sort out
flutter problems on models.


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