At 23:44 07 August 2008, Mark Drela wrote:
In article , "user" writes:

Nontheless, the rudder balances aileron drag.

To be more precise, the rudder primarily balances the fore/aft
tilting of the lift vectors on the left and right wings,
which is a result of _roll rate_. The aileron drag difference
has a much smaller contribution.
The PDF diagram in this link illustrates the effect:
http://www.rcgroups.com/forums/showp...0&postcount=31


No sure that the above is the complete answer as it covers profile drag
but completely ignores induced (lift dependent) drag.

Consider a glider wing in level flight and assume we have a drag factor of
2 (Newtons, foot pounds,bananas, doesn't matter)
The airlerons to roll and the lift on the down going aileron wing is
doubled over the area influenced by the aileron, the induced drag
increases by 4. If the ailerons are non differential the lift over the
same area on the opposite wing is reduced by half to 1 giving a total
force of 5 trying to induce yaw. On a glider with long wings the leverage
of these forces will produce a significant adverse yaw while the aileron
is applied. I would agree that when the glider starts to roll the upgoing
wing suffers a reduced angle of attack, over it's whole area, reducing
the lift and vice versa for the other wing, these forces tending to
mitigate the adverse yaw caused by lift inbalance between the two wings.