On 6 Nov 2004 07:37:15 -0800, (Ramapriya) wrote:

Unlike the elevators and rudder that change an aircraft's pitch and
yaw with no other secondary effect, why does the banking of wings by
the use of ailerons not just roll an aircraft but also produces a turn
(yaw)? Logically, one would expect an aircraft to keep going straight
ahead even if the pilot banked the aircraft left or right. Where does
the turning effect come from?

All aircraft are a series of compromises.

The location of empennage with respect to the wings,
the dihedral in the wings, the location of the wings with
respect to the thrust line and center of mass, the
airspeed of the maneuvers, and probably a dozen
other variables all affect what happens when one
control input is given.

When an aircraft equipped with unbiased ailerons is
banked, the downgoing aileron causes more drag
than the upward deflected aileron does. In Piper Cubs,
for example, this means that the high wing in a bank
tends to drag the nose AWAY from the direction of
the intended turn. In a left bank (right wing high,
left wing low), the nose tends to yaw to the right
because of the difference in drag. To make a
coordinated left turn, you need to step on the
left rudder to counteract the adverse yaw while
using the ailerons to bank to the left.

In other aircraft that have differential ailerlon trim,
the aileron will not go as far down as the other
aileron goes up. That solves the problem of
adverse yaw. Now we've got a semi-pure bank,
but the aircraft will tend to lose altitude and head
in the direction of the bank because of the loss
of lift due to the changed angle of attack of the
wings with respect to the relative airflow.

Take an extreme example: if you bank the
plane 90 degrees (knife-edge), the nose will
drop quite rapidly toward the ground because
the wings no longer produce lift against the tug
of gravity. The "lift" produced by the wings will
be toward the canopy and the aircraft will
tend to move in that direction.

In a less severe bank, there still is that component
moving the aircraft toward the canopy. If you
mix in a little elevator to maintain altitude in the
bank, the elevator will help to point the nose in
the direction of the turn without requiring rudder
to coordinate the nose with the bank.

There are other factors as well. In making any
motion from straight-and-level flight, there is a small
component introduced by shifting the plane of the
propellor from its equilibrium. It's a little bit like
moving a spinning wheel from its equilibrium.
The gyroscopic forces will act against the
turn 90 degrees away from the direction of the
turn. Physics profs like to show this force by
having someone stand on a turntable holding
a spinning wheel. By tipping the wheel in one
direction or another, they can make the turntable
spin this way and that.

The classic case of this gyroscopic force was found
in the WWI aircraft that used rotary engines. The
crankcase spun with the propellor (!). With all of that
spinning mass at the nose of the plane, very sharp turns
could be made in one direction, for good or for ill.
Modern planes with lighter propellors may not exhibit
this effect much, if at all.


DISCLAIMER:

I am not an aerodynamicist. I just play a lot with
RC aircraft. I know a great deal about accelerated
stalls and have photographs of the debris fields to
prove it. (

Marty