The sinking speed in a turn = straight line sinking
speed / (cos(theta)^(3/2)) where theta is the bank
angle -- provided that the L/D for a given angle of
attack is the same while circling as when flying
straight. Straight line sinking speed, in turn, is
forward speed / (L/D). Now if you're flying straight
and you decrease the L/D by skidding along in a yaw,
your sinking speed goes up. Similarly, if you fly a
circle in a skid, you sink faster than you would in a
nicely coordinated turn of the same diameter.
Of course, for some planes it might still be true that
if you're going to input only aileron or only rudder,
you could be better off using only rudder -- not
because you want a skidding turn, but because that
particular plane gives a more nearly coordinated turn
with rudder alone than it would with aileron alone;
the skid is less with rudder only than it would be
with aileron only. This depends on dihedral and force
arrangement. However, an ideally coordinated turn
(which for most planes requires inputs from both
rudder and aileron) is best.
> I took a full scale glider ride with a pretty
> experienced competition pilot a while back, and
> noticed that all the thermal turns we made were
> coordinated, with the little yaw string dealie
> straight up the canopy. They also seemed to be at a
> fairly high bank angle. The primary purose for the
> rudder seemed to be to prevent the huge amount of
> adverse yaw, not for flattening out the turn. So
> why is it that r/c gliders do flat thermal turns in
> small thermals when the big guys do coordinated
> turns in big thermals
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