{sorry, posted without entering proper subject, reposting with text cleaned up}
> > Differential aileron and rudder coupling are used to limit the effects
> > of adverse yaw, NOT a pitching moment.
> Actually, I think what he was implying is that the aileron differntial
> that's used to counter adverse yaw could also PRODUCE an unwanted pitch
> response as well. I'm not sure how that would work, since I would assume
> a proper amount of differential would balance the lift and drag on both
> wings, but I'd be curious to hear about it.
>
Balanced lift is not the issue here, I'm assuming that lift is balanced
(see below).
The issue is: is lift produced during a roll is the same as lift produced
during level flight? If there's more up aileron than down (differential),
then it seems to me that the overall lift from both wings is less when
ailerons are deflected than when they are not. Assuming reduced wing
pitching moment (explained below for those that don't know about this)
from lowered angle of attack has less overall effect than reduced lift
from the wings, a model should pitch down (relative to fuselage) when
it's rolling.
Technical stuff here for anyone interested...
Balanced lift
To initiate a roll, the lift from the wings is different (angular
acceleration). Once a model is in a steady roll, whether or
not the lift of the wings are equal depends on how lift relative to a
wing is defined (reference point issue). Since there is no angular
acceleration (about roll axis) some would consider the lift from the
wings to be "equal", even though each wing's flight path is different,
relative to fuselage path.
Wing pitching moment
Most undercambered airfoils have a tendency to pitch "downwards" depending
on angle of attack and airspeed (as either increases, there's more of
a tendency to pitch down). This can contribute to unexpected reactions
when trailing edges are raised or lowered significantly. If the chord
(front to back) distance on the wing is long enough, then trailing edges
also act a bit like elevons (since they are far enough away from center of
mass to cause this effect), for example, raising trailing edge may cause
a model to pitch up (like my DAW 1-26 2m with spoilerons for example).
Some flying wing airfoils (like on my www.f3x.com Bandit) have the
opposite pitching moment. Because the trailing edge is overcambered
(turns upwards), this airfoil has a tendency to pitch up instead of
down. This means that pitch stability can be adjusted somewhat by
just trimming the elevons, without having to move the center of gravity.
It also makes adjusting the center of gravity more difficult (just
keep moving it back until the model starts to be difficult to control,
[pilot judgment here, glad it's a foamie] then move it forwards a bit).
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