Re: Turning radii and spanwise airspeed distribution

[JJ Sinclair]

Good observation, Bill. Most long wingers have found it best to keep the nose
pointed towards the outside of the turn. It helps the climb. Your yaw string
should be telling you to, "Put your foot in the inside hole". Just don't do it.
Does that make sense?
JJ Sinclair

[Robert Ehrlich]

Bill Daniels wrote:
>
> It's early and I'm waiting for the outside temperatures to rise enough so I
> can get back to work installing a new radio in my new (to me) Nimbus 2C. I
> decided to examine the spanwise airspeed distribution on the 20.3 meter wing
> in a thermalling turn.
>
> Using the turn radius calculator at
> http://home.twcny.rr.com/ghernandez/turn_rad.htm I determined that at a 45
> degree bank and 45 Knots the turn radius would be 167.9 feet and 14 seconds
> for each full 360. Cranking the numbers further I get an airspeed
> difference of 12.4 Knots across the wingspan and an inside wingtip IAS of
> only 38.4 Knots. I'm not sure what the stalling speed of the inside wingtip
> airfoil is but that has to be close to it.
>

There is no really such thing as a stalling speed, just a stalling angle
of attack. The stalling speed has a meaning only for the whole aircraft,
depending on his current weight and load factor, it is the speed at which
under these conditions you reach the stalling angle of attack. In the case
you mention however there is a reason for the inner wing for reaching the
stalling angle of attack before the other one, which is specific of gliders:
they are always sinking relatively to the airmass. A consequence of this is
that the vertical component of the relative wind is the same for every part
of the aircraft (assumed to keep its bank angle constant) while the horizontal
component is affected by its rotation, this results in a higher angle of
attack at the inner wing tip. The difference of speed of course also results
in a difference of lift, causing the well known induced roll. This roll
tendency is partially compensated by the difference in angle of attack
mentionned above, provided this difference in angle of attack results
in a significative difference of lift coefficients, i.e. the angle of attack
is sufficiently lower than the stalling angle. This is (one of the) reason(s)
the overbanking tendancy increases so much when you come near the stalling angle.