Sammy Mason on Stalling & Spinning

Many descriptions have been posted here in several threads on spinning
over the last couple of weeks and I have found all of them to be
fascinating. I've been prompted to think through again my own
understanding of the stall and the spin in gliders.

One description that has been strongly asserted in several threads is
that there is a difference in angle of attack on the two wings on an
aircraft in a descending turn AND that the difference in AOA is due to
the slower horizontal speed of the inside wing as it traverses a
shorter circle than does the outside wing in the descending turn.
The slower speed of the inside wing thus coinsides with a higher AOA
on the inside wing than is experienced by the outside wing. This of
course has implications for which wing may reach stall AOA first.

I am only a pilot and have no training in aerodynamics whatsoever.
For what it's worth (not much probably), I find the above explanation
of the cause of different AOA on the left and right wings of an
aircraft in a descending turn to be easily understandable and very
likely accurate, but incomplete.

Many years ago I read in Sammy Mason's book Stalls, Spins and Safety,
an additional description of the AOA on the wings of an aircraft in
ascending and descending turns. I find that his description is
persuasive and illunimating and contributes to a more complete
understanding of the stall and spin.

Here is a short quote from Sammy Mason's book:

"During a level, coordinated turn, once the bank is established,
the airplane will continue to turn about the yaw axis and pitch upward
about the pitch axis. It will not be rolling about the roll axis.
When a stall is encountered in a level turn, the reaction will
normally be very little different than during a wings-level stall.."
"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.

Sammy Mason goes on to describe the opposite differences in AOA on the
two wings of an aircraft in an ascending turn.

In a descending turn are both wings going down? Of course they are,
relative to an outside frame of reference and assuming the rate of
aircraft descent is greater than the rate of roll (or however one
describes the rate at which wings are going around in a roll). From
the frame of reference of the aircraft are the two wings proceeding in
opposite rotational directions? Sammy Mason's description of the
aircraft rolling about its roll axis in a descending turn describes
just such a difference -- and its contribution to the differing AOA on
the two wings. This suggests that the difference in AOA on the two
wings is not due only to their differences in horizontal speed in
their differing size circles.

Your milage may vary, of course, and each pilot likely benefits from
some image of what is happening to an aircraft in a stall and spin.
In my view, Sammy Mason's descriptions add additional insights for me
to the nature of stalls and spins.

Sammy Mason's flying career is described in the book as having begun
in the 1930's, and included WWII, and then working with C.L."Kelly"
Johnson of the Lockheed "Skunkworks". He was a jet aircraft test
pilot for Lockheed and became Lockheed's authority on stall/spin
testing.

"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

On 10 Feb 2004 14:38:27 -0800, (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.


Try it as a wide barrel roll heading straight down.

Rolling into the turn no?

Mike Borgelt

On 10 Feb 2004 14:38:27 -0800, (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

It is wonderful isn't it! I'm a complete dolt about aerodynamics but
I love flying so much, especially flying gliders, that I just can't
get enough of the whole world of it.

When I try modeling the ascending turn without continually rolling
against the turn as the aircraft ascends the aircraft ends up on its
back! Only by a continual rolling input opposite to the ascending
turn does the aircraft to remain in a constant turn! Just wonderful
stuff and fascinating! Mason's book is a treat, bless his heart.

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

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

Where I'm having trouble is seeing why there is any difference between
sinking, level, and climbing. From the point of view of raf, these
should all be same. I choose to set my longitudinal reference based on
the fuselage, not the horizon. If I do this, the aircraft always has a
sink rate, not relative to the ground, but relative to the projection
of the fuselage centerline at a given time (T=0) and proportional to
the angle of attack -- that is, the rate at which the aircraft "falls"
away from this projection (dT). Since it shouldn't matter which
reference frame I use to make my observations, my confusion arises
with the suggestion that the pseudo-rolling moment reverses beetween
sink and climb.

As for the approach, it remains interesting. To help my understanding,
I've been using a train. Imagine a sensor on a rail that only measures
side force. A train going straight on level ground registers zero side
force. As the rail bends though, the sensor would measure a side force
proportional to the train's acceleration. However, a straight rail
with a side pitch would register a force as well. When viewed this
way, a train rolling on a straight rail with several degrees of side
inclination could be said to be "turning." Of course, it isn't. Unlike
a curved rail, no additional power is needed to maintain speed. (Note
the limitations of my reference frame. I only see side force on the
rail, not total force.) It's not exactly analogous, but it's a step in
the right direction.

Chris OCallaghan skrev den 20 Feb 2004 05:53:14
-0800:

Where I'm having trouble is seeing why there is any difference between
sinking, level, and climbing. From the point of view of raf, these
should all be same. I choose to set my longitudinal reference based on
the fuselage, not the horizon. If I do this, the aircraft always has a
sink rate, not relative to the ground, but relative to the projection
of the fuselage centerline at a given time (T=0) and proportional to
the angle of attack -- that is, the rate at which the aircraft "falls"
away from this projection (dT). Since it shouldn't matter which
reference frame I use to make my observations, my confusion arises
with the suggestion that the pseudo-rolling moment reverses beetween
sink and climb.

When turning, the only rotation is about the vertical (earth-fixed) axis.
Then take it to the extreme case of diving straight down (here defined as
the longitudinal axis vertical). In that situation, all of the rotation
will be around the longitudinal axis of the aircraft - i e roll.

In turning flight with the fuselage level (longitudinal axis horizontal),
all of the rotation will be around the yaw axis of the aircraft.

In all the cases between these two extremes, part of the rotation will be
around both the longitudinal and the yaw axis of the aircraft. The bank
will mean some of it is around the pitch axis as well, which is a problem
to be considered in, among other things, turn rate gyros.

As for the approach, it remains interesting. To help my understanding,
I've been using a train. Imagine a sensor on a rail that only measures
side force. A train going straight on level ground registers zero side
force. As the rail bends though, the sensor would measure a side force
proportional to the train's acceleration. However, a straight rail
with a side pitch would register a force as well. When viewed this
way, a train rolling on a straight rail with several degrees of side
inclination could be said to be "turning." Of course, it isn't. Unlike
a curved rail, no additional power is needed to maintain speed.

The equivalent to a banked railway track would be straight slipping
flight. And in both cases, the normal force on the rail (the lift) will
have to be larger if it is to keep the train/aircraft from accelerating
downwards, and there will have to be a lateral (train/aircraft frame of
reference) force as well, assuming that there are no other forces
perpendicular to the direction of travel than the normal and lateral (to
the wings/rail). Both these forces will add friction/induced drag and
require additional power.

Cheers,
Fred

When turning, the only rotation is about the vertical (earth-fixed) axis.
Then take it to the extreme case of diving straight down (here defined as
the longitudinal axis vertical). In that situation, all of the rotation
will be around the longitudinal axis of the aircraft - i e roll.

Remember, we're talking about Sammy's model. When diving there is no
rotation, about the longitudinal or yaw axis. (There is no aileron
input.) Same thing going straight up. The orientation of the lift
vector (positve values) would rotate the glider about the lateral
axis.


In turning flight with the fuselage level (longitudinal axis horizontal),
all of the rotation will be around the yaw axis of the aircraft.


Again, not according to the model as I understand it. If the aircraft
is sinking, the one wingtip is travelling faster than the other, and
therefore there is a difference in angle of attack, and in model, this
is accounted for as a roll. I'm waiting for the book to show up, but I
suspect that we're all taking this too literally, trying to justify it
in the real world.

As for the train, the only force of importance is the side force on
the rail. There is no friction (or better said, it is unabserved). The
point is that a leaning train looks like a turning train to a sensor
that simply measures force acting in a single direction.

In article , Todd Pattist
writes:

The first time I tried this in my Ventus, I was flying along
sideways with full rudder and level wings and heard a loud
"BANG!" After my heart stopped thumping, I figured out that
one of the gear doors had sucked open into the sideways
airflow. :-)

That happened to me once.Scared the hell out of me.

Another scarey bang was in a Puchacz on the winch when the cover, which is over
the rear seat adjustment bar, could not have been secure and flicked down and
hit the side of the fuselage. This was about six inches from my left ear and
was magnified by the megaphone shape of the hollow fuselage.

A third was in a Bocian, also on a winch when the rear canopy, which slid
backwards on rails, came unlocked, slid back and hit the rear stop. This one
was probably the worst because, a) we were about 100ft and starting to rotate
into full climb, b) in addtion to the "bang" as it hit the stop, there was a
huge, disorientating rush of air, and c) it took a while to work out what had
happened, take over from the pupil, (only his second or third winch take off)
and get the nose down etc.

However, one of the the worst I know of these type of incidents was a pilot at
my club who owned a Carmem and lost the complete canopy at about 4000ft agl. It
departing it caught his skull which bled profusely and a combination of blood
and the wind causing his eyes to run, made it extremely hard for him to see.

Once he worked out what had happened, his big concern was what else the canopy
had hit, tailplane for example. He considered baling out but, having checked
all controls decided to ride it down to the nearest field, successfully I am
pleased to say.

The farmers reaction to this bllod covered figure arriving at his door was, I
understand, unprintable.

Needlesss to say, all of the above (except U/c doors) occurred either as a
result of poor take off checks or mechanical faults.

Barney
UK