I have been told two things:

1. That the Reynolds number applicable to a glider changes with altitude,

2. That the Reynolds number affects the stall/spin behaviour and recovery.

I can remember an anecdote (I am vague as to who or when) about a K21 at
Aboyne. The glider was at about 20,000 ft., and the crew wanted to get
down for the next pupil. They put it into a spin (which it entered without
difficulty), and then held the spin without moving the controls. It span
down to about 7,000 ft. and then self-recovered without any
change in control position. Heights are QNH.

I don't understand Reynolds numbers, but I know it matters; and it would
seem not only to designers. It might well be that some gliders more easily
spin and are harder to recover at height. I would expect this to apply at
altitudes above about 7,000 ft. QNH, above which height they are almost
certainly not test flown.

W.J. (Bill) Dean (U.K.).
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"Chris Nicholas" wrote in message
...

Reading of these brave souls who have been testing the Puchacz spinning
characteristics with multi-turn spins at high altitudes reminds me of an
anecdotal rumour which reached me about at least one other glider type,
and I think also some power aircraft, which similarly misbehaved until
many turns/it got lower. It made me wonder a few things:

At the heights people here have been writing about - 10 to 17 thousand
feet - what is the true airspeed at which it enters the spin on command
and how does that differ from the lower altitude airspeed used for
certification tests? Bear in mind also one poster's comments that a
glider does not instantly cease forward motion and go instead into
vertical motion with a rotational component - in the absence of infinite
forces, the first is subject to some deceleration taking time and space,
and the second some vertical acceleration taking time and height.

Similarly, what is the true vertical velocity at onset and when stable
in the spin?

What is the ratio of those two velocities compared with the ratio at
test air densities?

Does the rotation rate remain identical, whether at height (lower air
densities) or at lower altitude (higher density)?

Does all that have an effect on true angle of attack?

Could such things account for high altitude spins when fully developed
requiring more turns to recover?

I wonder if the people who conduct these high altitude tests were in a
regime not tested by the maker or the certification test pilots such as
Chris Rollings?

Chris N.