Sammy Mason on Stalling & Spinning

Just think about the relative air flow in relation
to all the aero dynamic surfaces. It is then quite
clear. Remember that in a spin there is pitch, roll
and yaw so the raf changes. Think about what happens
when it reaches the aerodynamic limit of the relevant
surface. If it makes it easier just think about the
raf in a straight stall first where no attempt at recovery
is made. What happens? Then add rolling and yawing.

At 22:42 10 February 2004, Chris Ocallaghan wrote:
'During a descending turn, or spiral, in addition to
pitch and
yaw, the airplane will be rolling about the roll axis
in the direction
of the turn. As the airplane rolls, it induces an
upflow of air into
the descending wing. This results in the descending
wing having the
greatest angle of attack. If a stall is encountered,
the airplane
will likely roll into the turn.' pp.40-41.


I'm having some trouble visualizing this.

Is it possible that Sammy has posited a reference frame
that looks
only at AOA, ignoring bank and relative speed across
the span? If I
wanted to keep my model and my math simple, rather
than describing the
turn as a hollowed cylinder with inner- and outer-walls
transcribed by
the wings during descent, I could look solely at AOA,
in which case
the model of a turn would look similar to, if not exactly
like, a slow
rolling motion. Our reference frame has no horizon.
In fact, it is
purely scalar. AOA simply has a range of values across
the wing. If
this is the case, I can see how it would be useful
for a snapshot --
such as just prior to the stall, but confusing when
describing the
dynamics of a turn in its fuller context. This is a
kind of partial
differential: an alternative way of describing a turn,
but only
predicts outcomes based on AOA. Good for analysis in
a narrow band...
Certainly counterproductive if integrated haphazardly
into a more
intuitive three axis model.

So it goes like this maybe. The observed effect of
constant sink rate
and differential airspeed across the span of a turning
airfoil when
described in terms of differential AOA can be likened
to the rolling
motion produced by the ailerons in level flight. The
downward moving
wing, during the rolling motion, exhibits an increasingly
higher AOA
as you go out the span (ignoring that part of the wing
with deflected
aileron) than the rising wing, which shows a descending
value of AOA
with span. Thus, during a descending spiral, if the
airfoil were to
stall, this 'psuedo-rolling moment' could be said to
contribute to the
wing drop typically experiended during a turning stall.

I'm not sure I see how this changes with level flight
or a climb.
After all, if we establish the longitudinal axis as
the basis for
measurement, up or down with respect to the ground
shouldn't matter.

This seems to me a more useful short cut for the engineer
than the
aviator. Just remember, similitude is not exact. But
it is an
interesting concept nonetheless.

Maybe someone could do the math for change in AOA for
a 15M glider
traveling at 100kph and rolling at a rate of 30 degrees
per second,
ignoring the ailerons, of course.

Wow, that was fun. Thanks.

Chris O'C