Originally Posted by
hugh flung_dung
.
Also, I'm totally happy with washout giving pitch stability for swept-wing tailless deltas; it's the concept of pendulum stability when the mass is suspended on a pivot that I'm finding difficult to accept.
That's because it's largely rubbish - there is no wing pendular stability, beyond the very small amount due to the hangpoint being below the chordline of the wing. As a pilot you can change this of-course by hanging on hard to the bar - so in effect you can act as a roll damper through your arms introducing a pendular stability term. But, hands-off, it's not there for all reasonable purposes, for the wing - it does help the trike stay the right way down however.
Finally, it's the sequence of events for turning that I don't yet understand. Here's my best attempt: pilot displaces weight to one side, because of the relative masses of trike/wing this causes the wing to slightly bank to the same side, gravity now allows the mass of the trike to move the keel to the same side (compared to the leading edges, permitted by the floating cross-tube), this results in less tension in the sail fabric on the "down" side, so this billows and decreases the effective angle of attack of that half of the wing (the opposite on the "up" wing) and this gives the "servo" system. The luff lines running over the king post amplify this effect.
Is this a correct sequence?
HFD
Roughly, but there's an additional important factor (shown in an otherwise very poor paper by some chaps from Cranfield a couple of years ago, attempting to explain hang-glider stability in
Aeronautical Journal which is that once the wing is tilted it moves the lift vector to one side of the whole-aircraft CG, creating a rolling moment about the CG.
What the relative contributions are of billow shift and movement of the vertical lift vector, I'm not sure anybody really knows. The largely useless Cranfield paper didn't add much to the science.
G