Stalls - Angle of Attack versus Vstall

Gary Drescher wrote:
"Stubby" wrote in message
. ..
Don't forget the stall is caused by the elevator losing lift.

No, that's incorrect. There is such a thing as an elevator stall, but it's
very different from a normal stall. In typical small planes, an elevator
stall does not occur unless there is tail icing, or else a CG that's too far
forward when you apply substantial up elevator.

If you have an elevator stall during a landing flare, the nose snaps
abruptly downward, potentially damaging the nose gear.

That's why the nose drops.

No, not in a normal stall. Rather, the wings produce insufficient lift and
so the plane accelerates downward. The plane weathervanes into the new
relative wind, dropping the nose.


There's more to it than that. The centre of pressure is
well behind the centre of gravity in normal flight, and as the AOA is
increased and the boundary layer begins to break up toward the aft wing
surface, the centre of pressure moves forward somewhat, helping to
raise the nose further. At the stall break, where the airflow over the
wing more or less completely breaks down, the CP moves aft again, the
stab/elevator can't hold the nose up against the suddenly increased
nose-down force, and the nose drops. That's not to say the elevator
stalled; it didn't, and elevators don't stall except under unusual
circumstances such as airframe icing or poor design such as the early
Cardinal's stabilator, where the thing would stall in the flare and
drop the nosewheel hard on the runway, sometimes breaking it. Cessna
added slots to the stab to fix that by preventing stab stall.
A stalled elevator would result in the airplane nosing
completely over onto its back in flight, since the stab/elevator's AOA
would increase as it came up, stalling it further, and control would be
totally lost. You'd never get that airplane certified.

Dan

Peter Duniho wrote:
"Andrew Sarangan" wrote in message
oups.com...
If the normal stall speed is 50 knots, you can make it stall at 100
knots if you pull some positive g's, or you can make it stall at 25
knots if you pull some negative g's. On the same token, you can stall
the airplane at higher or lower airspeeds depending on the aircraft
weight.

Minor nit:

Replace "some negative g's" with "less than 1g". Or "between -1 and +1 g",
if you want to get really particular.

You are right. It is not negative g, but less than plus one g.

Dave Doe wrote:


The stall horn is preset to go off at a specific AOA. It does not give
the pilot any indication of the actual AOA being flown. AOA indicator
is typically used in large transport airplanes and military jets where
the operating envelope is large. For typical GA airplanes, the envelope
is so small that the airspeed indicator is a good indication of AOA
under normal operating conditions.

Isn't the texts on AOA about *critical* AOA? - the rest of it, is surely
the other angle - within the load envelope. The stall warning provides
a fixed measurement of the imminent critical AOA - what else does a
pilot need?

The only minor difference is that the stall warning does not tell you
where the critical AOA is. It just goes off at some predetermined angle
before reaching the critical AOA. However, as you say, a pilot of a GA
airplane does not need to know the exact AOA. This is why we don't have
AOA gauges in small airplanes. In fact, one could argue that you don't
even need the stall warning horn. As long as you are not doing any
high-g maneuvers, the ASI tells you how close you are to stall. The
dilemma is the lengthy discussions about AOA in texts that deal with
small airplanes. This causes all kinds of confusions that is not
helpful for the beginning pilot. You can read the archives from this
newsgroup and find how many people have been confused by this. We teach
them about AOA in the classroom, and then use the airspeed indicator in
the cockpit.

Andrew Sarangan wrote:
Dave Doe wrote:


The stall horn is preset to go off at a specific AOA. It does not give
the pilot any indication of the actual AOA being flown. AOA indicator
is typically used in large transport airplanes and military jets where
the operating envelope is large. For typical GA airplanes, the envelope
is so small that the airspeed indicator is a good indication of AOA
under normal operating conditions.

Isn't the texts on AOA about *critical* AOA? - the rest of it, is surely
the other angle - within the load envelope. The stall warning provides
a fixed measurement of the imminent critical AOA - what else does a
pilot need?

The only minor difference is that the stall warning does not tell you
where the critical AOA is. It just goes off at some predetermined angle
before reaching the critical AOA. However, as you say, a pilot of a GA
airplane does not need to know the exact AOA. This is why we don't have
AOA gauges in small airplanes. In fact, one could argue that you don't
even need the stall warning horn. As long as you are not doing any
high-g maneuvers, the ASI tells you how close you are to stall. The
dilemma is the lengthy discussions about AOA in texts that deal with
small airplanes. This causes all kinds of confusions that is not
helpful for the beginning pilot. You can read the archives from this
newsgroup and find how many people have been confused by this. We teach
them about AOA in the classroom, and then use the airspeed indicator in
the cockpit.

And too many students aren't taught that sudden pullups are
deadly because of the load factor. They'll buzz a friend, or take off
and hold the airplane low until they've got some "safe" airspeed, then
yank back on the elevator to get the G rush or to impress someone. Once
in a while the airplane will snap-roll right into the ground and the
guy never knew what hit him. He thought he was well above the stall
speed.

Dan

Andrew Sarangan schrieb:

dilemma is the lengthy discussions about AOA in texts that deal with
small airplanes. This causes all kinds of confusions that is not
helpful for the beginning pilot.

One of the most difficult things in teaching is to explain things an a
simple, but nonetheless correct way. Probably because this requires a
really profound understanding which most teachers or writers don't have.

Stefan

wrote:
Andrew Sarangan wrote:
Dave Doe wrote:
The stall horn is preset to go off at a specific AOA. It does not give
the pilot any indication of the actual AOA being flown. AOA indicator
is typically used in large transport airplanes and military jets where
the operating envelope is large. For typical GA airplanes, the envelope
is so small that the airspeed indicator is a good indication of AOA
under normal operating conditions.
Isn't the texts on AOA about *critical* AOA? - the rest of it, is surely
the other angle - within the load envelope. The stall warning provides
a fixed measurement of the imminent critical AOA - what else does a
pilot need?
The only minor difference is that the stall warning does not tell you
where the critical AOA is. It just goes off at some predetermined angle
before reaching the critical AOA. However, as you say, a pilot of a GA
airplane does not need to know the exact AOA. This is why we don't have
AOA gauges in small airplanes. In fact, one could argue that you don't
even need the stall warning horn. As long as you are not doing any
high-g maneuvers, the ASI tells you how close you are to stall. The
dilemma is the lengthy discussions about AOA in texts that deal with
small airplanes. This causes all kinds of confusions that is not
helpful for the beginning pilot. You can read the archives from this
newsgroup and find how many people have been confused by this. We teach
them about AOA in the classroom, and then use the airspeed indicator in
the cockpit.

And too many students aren't taught that sudden pullups are
deadly because of the load factor. They'll buzz a friend, or take off
and hold the airplane low until they've got some "safe" airspeed, then
yank back on the elevator to get the G rush or to impress someone. Once
in a while the airplane will snap-roll right into the ground and the
guy never knew what hit him. He thought he was well above the stall
speed.


I have a video taken over the shoulder of a acrobatic pilot. He made
it through about 7/8 of a loop. I'm sure he was going quite fast and
pulling back hard to stay off the ground, but it didn't work.

Stefan wrote:
Andrew Sarangan schrieb:

dilemma is the lengthy discussions about AOA in texts that deal with
small airplanes. This causes all kinds of confusions that is not
helpful for the beginning pilot.

One of the most difficult things in teaching is to explain things an a
simple, but nonetheless correct way. Probably because this requires a
really profound understanding which most teachers or writers don't have.

Stefan

The problem is not the method of teaching, but the relevance of the
material being taught. We can talk at length about AOA, but we cannot
show the student how to measure it or control it in the cockpit. You
can fly the airplane just fine even if you knew nothing about AOA. It
is important for understanding of the aerodynamics of an airfoil, but
it does not help a presolo student fly an airplane. It is a concept
best introduced after mastering the basics of airplane control.

Andrew Sarangan schrieb:

The problem is not the method of teaching, but the relevance of the
material being taught. We can talk at length about AOA, but we cannot
show the student how to measure it or control it in the cockpit. You
can fly the airplane just fine even if you knew nothing about AOA. It
is important for understanding of the aerodynamics of an airfoil, but
it does not help a presolo student fly an airplane. It is a concept
best introduced after mastering the basics of airplane control.

Of course. But it must be taught at a later stage, because a pilot must
understand understand that stall speed depends on weight and g-load.
And, especially with laminar profiles, a lot on surface cleanness. No
scientific details needed, but this dependence must be taught and
understood.

Stefan

"Dave Doe" wrote in message
. nz...
Isn't the texts on AOA about *critical* AOA? - the rest of it, is surely
the other angle - within the load envelope. The stall warning provides
a fixed measurement of the imminent critical AOA - what else does a
pilot need?

Calibrated correctly, an AOA indicator could be useful for a variety of
flight regimes, particularly when it comes to maximizing performance (best
glide, for example).

It's true that the usual student aviation texts don't discuss these things.
But that may be more about the lack of a suitable indicator in the airplane
than anything else.

Pete

"Peter Duniho" wrote in message
...
"Aluckyguess" wrote in message
...
Bottom line the wing needs airspeed to fly. At a certain speed the wing
starts to lift, when it loses this speed, losing lift it stalls.

IMHO, this is a misleading description of stalling, and in fact will lead
to just the confusion the original poster describes.

Specifically, the wing's speed is really not directly related to stalling
at all. As others have explained, the reason a stall speed is published
is that it is true that at a given weight and load factor (eg max gross
and 1g), there is a specific amount of lift required, and there is a
specific speed associated with the angle of attack that can produce that
lift.

The published stalling speed is simply a speed at which the angle of
attack required to achieve the necessary lift at that speed is the same as
the critical (stalling) angle of attack. It is not true that under all
conditions, at that speed, the wing is stalled (or "when it loses this
speed, losing lift it stalls"). The wing loses lift because it stalls,
not the other way around. And the wing will only stall at a given
airspeed if its angle of attack exceeds the critical angle of attack.
This is true of any airspeed, above or below the published stall speed(s).

Stick your hand out the window of your car shape it like a wing at a
certain speed it will lift all by itself and basically be weightless.

However, as long as you keep your "hand wing" angle of attack below the
critical angle of attack, it will generate lift at ANY speed above 0.
There is no "stalling speed" for your hand in that scenario, as your hand
is not required to support itself with lift, and so there is no speed at
which the required angle of attack equals or exceeds the critical angle of
attack. (That is, there's not even a concept of "the required angle of
attack" in that case...your hand will fly along quite happily at any
amount of lift, or even zero lift).

Pete
Try it. It wont lift until you have enough airspeed. At 0 it doesnot lift.
The higher the airspeed the higher angle of attack you can have.