At 14:00 11 July 2005, Mike wrote:
In the July issue of Soaring a letter writer contends
that, during a
steep stationary turn, the outside wing creates less
drag than the
inside and adverse yaw is caused by: 1)The inside wing
flying at a
greater angle of attack than the outside wing, therefore
creating more
drag on the inside wing (refers to lift vector diagrams)
and 2)Down
aileron on the inside wing needed to create equal lift
with the outside
wing while flying at a lower speed. He concludes,
'Of course, the
incresed drag of the lower wing, caused by both 1)
and 2) above, is the
source of adverse yaw.' With all this drag on the inside
wing why
wouldn't the glider yaw to the inside of the turn instead
of the
outside? This is counter to everything I've learned.
What am I (or is
he) missing here?


It's not a very clear explanation to me since 1) and
2) are interrelated.

I think of it as follows: For the saliplane to be in
a steady turn (constant bank angle), the total rolling
moments need to sum to zero. Most of these forces will
come from the two wings and therefore in a steady turn
both wings are producing roughly the same amount of
lift.

For the wings to produce equal lift in circling flight
the inner wing needs to produce a higher lift coefficient
than the outer wing because the outer wing is experiencing
a higher average velocity. This is why you need outside
aileron to counter overbanking. At high lift coefficients
and for tight circles (i.e. big difference in average
wing velocity) the induced drag on the inner wing goes
up more than the parasite drag on the outer wing does
and the nose will tend to yaw to the inside of the
turn.

For instance - on my sailplane flying at 48 knots in
a 45 degree bank the inner wing has roughly 8 percent
lower paraasite drag but 18 percent higher induced
drag -- and induced drag is 60% of total drag. (I think
I did the math right).

As I recall, adverse yaw is defined as the nose going
the other way and is the yaw associated with establishing
a roll rate, not a steady turn.

9B