T o d d P a t t i s t wrote: Once you are on the ground, with your front
and back wheels

rolling (or skid/wheel, etc.) the angle of attack remains

unchanged. Stalling is a function of the AOA, not your

airspeed. Stall is only indirectly related to airspeed when

your aircraft weight is supported by lift. On the ground

your weight's supported by your wheel(s), so you can go very

slowly and not be stalled. (Unquote).

I'm open to correction, but I think Todd is only right in the
circumstances that the glider stays level laterally.

As soon as a wing starts to drop, and the other to rise, the dropping
wing has a higher AoA and the rising wing a lower one. On a nil wind
day, with the glider having reached say 10 knots during the take-off run
(faster than the wingtip runner can run), a tip dropping at even 1 knot
has an extra AoA of 6 degrees - may well be enough to stall it with
neutral aileron. Putting that aileron down (to try to lift the tip) just
increases the AoA even more into its stall - hence, ineffective.
Negative flaps, if ailerons move in sympathy with them, can be enough to
unstall that downgoing tip.

By the way, what Todd says about non-linear CL curve makes entire sense
to me, and Duane's point about geometry, and others' comments. I just
think all effects are in play, at least on some gliders.

Now, why does opening the airbrake (usually called spoilers in the USA
for some unknown reason) help on some gliders? And did it ever help on
gliders that really did have spoilers (UK-speak for things that spoil
lift but don't add much drag, unlike airbrakes [in UK-speak], which do
both in spades)?

Chris N.

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