Crossed Controls Stall?

Crossed Controls Stall?

It would be dangerous on an approach to landing.

That was warned about in a reply to my previous thread; by Vaughn, who
only mentioned crossed controls and did not include the word "skid."
But what I'd read in my texts did not seem so concerned about forward
slips to a landing. Forward slips use crossed controls.

But I found a discussion on skids in Kershner's Student Pilot's Flight
Manual. (A good book I've only occasional used as a reference, as it
was aqcuired later than the others.) Specifically his warning was
about skidding the aircraft into a stall (while trying to avoid a
steep bank) while attempting to align the aircraft with the runway
after overshooting the turn to final. Yes, of course, you all know
about that one. Maybe you only think you do.

What he wrote (and this is paraphrased to what I think he wrote) was
that the skidding turn has the wing on the outside of the turn going
faster than the inside wing. This creates more lift outside and less
lift on the inside wing. Using the ailerons to lift the inside wing
increases the airplane's skid, and forces the inside wing into a
higher angle of attack such that it eventually stalls.

Is that your understanding?

Maybe Kershner (and you), know that is not quite right and believe it
is a waste of time to go into other details. I will try anyway.
Maybe I'm wrong. But the details should make sense.

I believe skid/stall danger is not described correctly. The aircraft
does not actually stall into a roll from a skidding turn. It has only
been rolled into a bank (which could be quite steep) in an unusual
manner -- yet it is still flying. That bank would be a surprise and
is not desired close to the ground.

First, as the aircraft is maneuvered into the skid, the outside wing
only _momentarily_ goes faster than the inside, and it does not go so
much faster that the aircraft is uncontrollable.

The increased lift on the outside (forward skid) wing is actually due
to wing dihedral, and it will remain so as long as the aircraft is
skidding. It's angle of attack is increased because the dihedral has
it pointing up into the wind during the skid. There would be fuselage
effects, though the ailerons may be able to counter those.

But the inside wing (behind on the skid) is angled down into the wind
and, by the dihedral, has the airflow pouring over the top of the
wing, losing lift. It's angle of attack is less so there is less lift
-- so it does not stall.

The use of the ailerons to hold up the inside wing only work to lift
it up into a low angle of attack condition. Ailerons are not of
sufficient size to counter the loss of lift over the entire wing. If
the skid is great enough they will be ignored and the aircraft will
roll over. They could, however, pull the nose of the aircraft in a
direction more towards the ground, though not directly towards the
ground. The aircraft is skidding.

The point is that if the aircraft banks over during a skid it is still
flying. Releasing the rudder while pushing the yoke some should
restore the aircraft to a flying condition. Then the ailerons should
be able to level the aircraft, with normal rudder to counter the
troublesome adverse yaw.

However, pulling the nose up after recovery from a skidding rollover
might cause a secondary stall -- an actual stall.

Ultimately a rollover, especially one that is not expected, is still
quite a danger at low altitudes and slow airspeeds. But who really
wants to skid his aircraft that bad? Pilots are certainly not taught
to do so for any purpose. A modest bank, with maybe a little
application of power (or just a drop of the nose) should be sufficient
to align the aircraft with the runway.
--
Michael