Crossed Controls Stall?

On Thu, 21 Jul 2011 10:31:18 -0400, "vaughn"
wrote:

"Mike Rhodes" wrote in message
news
By simple geometry the forward wing of a dihedraled pair in a skid
will have the steepest angle of attack, and the greatest amount of
lift, and therefore the one most likely to stall. (Not the wing
inside inside of the skid.) Aileron use will not change that --
except they force the aircraft to remain in a skid when operated
"crossed controlled."

Aileron use affects the effective angle of attack of a wing. One way to turn
mere wing drop (caused by a stall or near-stall) into a real spin is to use
aileron to try to raise the stalled wing. This is one of thse disturbing cases
where a pilot's instinctive reaction can kill!

So the adverse yaw on a wing on the verge of a stall will pull the
aircraft into a dangerous spin, when it would have been just a stall.

The slow, draggy aileron will yaw the aircraft like a rudder. Wing
dihedral will cause the opposite wing to rise into the yaw. Still,
full ailerons may be applied to counter the roll.

But the aircraft rolls over surprisingly in the direction opposite of
the applied aileron. This is because of wing dihedral (and inherent
aircraft stability); not because of wing stall, or because the aileron
changes the angle of attack of that wing to cause it to stall. The
aileron is acting like a rudder to yaw the aircraft. Dihedral
stability then rolls the aircraft -- oppositely.

But since the aircraft has rolled then the elevator no longer has
gravity (or the inertia of a bank) to push against, so angle of attack
rises rapidly. Then the wings stall. The aircraft is in an aerobatic
attitude. The tail feathers will ensure the nose will then point
down.


The problem with uncoordinated controls and the stall is that, in a
coordinated bank, the elevator pushes 'up' against the COG properly.
But when the turn is not coordinated (when the aircraft just rolls
over) then the elevator lift is no longer restrained by the load of
the COG, and can raise the nose of the aircraft rapidly such that the
wings then stall.
Slips and skids, however, do not generally cause the plane to roll.
In those conditions elevator control of the COG is not usually in
doubt.

When an aircraft is slow then a lot of elevator is applied to hold the
nose up. Other than the flare, this condition is most likely to occur
when doing stalls.
It is could also occur during the turn when a pilot has overshot
final from base. That is the danger point on final. And the roll
into a spin probably came while using coordinated controls. The
aircraft just stalled in the turn.
It does not typically happen during a level approach while using
crossed controls to skid the aircraft to align with the runway.
(Which students may instinctively do to fine-tune their approach,
which instructors may instinctively hate. It seems a matter of
prejudice.)
--
Michael