The point I'm replying to is:

I'm convinced that in very long wing gliders at
high angles of bank and slow speeds (and ergo light weights too),
the inner wing is significantly slower than the
outer wing, and tacking on some knots is most
efficient (to keep the length of the inner wing nicely above stall)...

I grant you that the AOA is slightly higher for the inner wing due to
the contribution from the sink, but this is negligible. Consider a
45deg bank, 45 knots. The turn radius (at the fuselage) is about 50
meters, so for a 15-m glider the speed of the outer wingtip is about
50knots, and the speed of the inner wingtip is about 40knots. If the
sink rate in this configuration is 1.5 knots, the difference in AOA
for the two wingtips is about 0.4 degrees. You will notice that (for a
good reason!) this is much less than the typical twist of a wing. You
cannot stall the inner wingtip in a steep turn without stalling both
wing roots first! For the same reason, the inner wingtip is NEVER on
the back side of the polar when thermaling. If it was the wing roots
would already be stalled.

To answer the original question - should one speed up when thermalling
with a steep bank - the answer is no. There are too many factors that
come into play - the twist of the wing as a function of position, the
wing profile as a function of position, the drag produced by the
aileron deflection needed to correct for the overbanking tendency as a
function of speed, and so on. In the end, these effects will tend to
cancel each other: if you speed up a little to bring the wing roots to
the front side of the polar you will a) create more drag on the wing
tips and b) need more aileron input to correct for the overbanking
torque and hence create more drag. I suspect that amount by which one
should speed up or slow down to optimize the sink rate in theory will
be much smaller than the speed of the turbulent currents in the
thermal, and thus utterly irrelevant in practice. Your time will be
better spent flying cleanly and in the core of the thermal rather than
trying to nail the speed to within 0.2 knots.


(Chris OCallaghan) wrote in message . com...
You are confusing AOA with sink rate. The sink rate is the same across
the airfoil, but AOA is dependent on sink rate and forward speed, so:

If an airfoil has a forward motion of 10 and sink rate of one, then
its angle of attack can be measured -- about 5.7 degrees. If we then
slowed its forward speed to 9 while maintaining a sink rate of 1, the
angle of attack would be higher: 6.3 degrees.

We agree that the angular speed is the same across the span. We agree
"that the inner wing is flying slower." The sink rate is the same
across the span. As you've stated, this is a given: the wings are
fixed to one another. Since AOA is dependent on both sink rate and
forward speed, then the inside wingtip must have a higher AOA.

Inner wing slower, higher AOA. Outer wing faster, lower AOA. Lift is
dependent on both AOA and speed. So even though the outer wing is at a
lower angle of attack, it is moving through the air more rapidly, and
producing slightly more lift than the inner wing. With resulting
overbanking tendency.

Balance this knowledge against the sailplane's response to a turning
stall. Inner wingtip typically drops first. Why? Because it has a
higher AOA. No aggrevation from the aileron required.