A Level 1 AOA clarification

Frankster wrote:
You will reach the same angle of attack at a steeper
pitch angle at higher airspeeds.

What?

Hilton

He's saying that, by definition, the AOA is the wing's angle to the
*relative* airflow. Pitch is relative only to the ground, and really has
no
bearing on this entire discussion.

Frank,

The sentence above read: "at a higher airspeed, the plane is likely to be
climbing, therefore the relative wind will be coming from above."

I don't understand the first part (higher speed and climbing?) and the
second part is wrong.

Hilton

Hilton wrote:

I don't understand the first part (higher speed and climbing?) and the
second part is wrong.

If I leave the flaps at 0 degrees in my aircraft, bring the power back to
decelerate, and maintain level flight, she will stall at about 53 mph indicated.
The relative wind will be essentially horizontal, since that is the direction in
which the aircraft is actually traveling.

If I leave the flaps at 0 degrees, slow down to 60 mph indicated and raise the
nose enough to stall, the aircraft will be climbing just prior to the stall. The
relative wind will be "coming from above", since that is the direction in which
the aircraft is traveling.

George Patterson
The desire for safety stands against every great and noble enterprise.

The relative wind will be "coming from above", since that is the
direction in which the aircraft is traveling.

The relative wind doesn't ever "come from above" while the aircraft
has a positive angle of attack..by definition. ;-) Nor will the
aircraft stall with the relative wind "essentially horizontal."
Sounds like you think there is a zero angle of attack in that
situation? Cannot be true.

When not pulling a g-load, an aircraft climbs because the *flight
path* is inclined relative to the horizon; the AOA depends on the
chord line angle with the *flight path*.

If your level flight stall speed is 53 and you're stalling at 60,
you're probably achieving an accelerated stall.

The flight testing guys try to decelerate 1 knot per second; oddly,
decelerating at a greater rate produces a *lower* stall speed, which
must be normalized during the data processing. (I'm sure this only
occurs up to a point.)

Greg Esres wrote:
The flight testing guys try to decelerate 1 knot per second; oddly,
decelerating at a greater rate produces a *lower* stall speed,...

Why?

Hilton

Why?

I don't know. None of the books explain it. They just descibe
methods of correcting for it. (Test pilots seem the practical sort,
rather than theoretical.)

My suspicion is that it's due to the "dynamic stall" concept. When an
aircraft is rotated rapidly to a high AOA, it can generate a much
higher lift coefficient than steady state. Apparently it takes some
finite amount of time for the adverse pressure gradients to do their
magic and cause the airflow to separate.

"Greg Esres" wrote in message
...
The relative wind doesn't ever "come from above" while the aircraft
has a positive angle of attack..by definition. ;-) Nor will the
aircraft stall with the relative wind "essentially horizontal."
Sounds like you think there is a zero angle of attack in that
situation? Cannot be true.

I am sure that George means "coming from above relative to the horizon".
Which is a fine statement to make, IMHO. I do find it odd that you were
apparently unable to make this inference, given your next paragraph:

When not pulling a g-load, an aircraft climbs because the *flight
path* is inclined relative to the horizon; the AOA depends on the
chord line angle with the *flight path*.

Yes, the flight path IS inclined relative to the horizon, and this causes
the relative wind to also become inclined relative to the horizon.

If your level flight stall speed is 53 and you're stalling at 60,
you're probably achieving an accelerated stall.

Yes, he certainly is, and he said as much.

Pete

I am sure that George means "coming from above relative to the
horizon". Which is a fine statement to make, IMHO. I do find it odd
that you were apparently unable to make this inference, given your
next paragraph:

That is possibly what he meant, but I think you're trying to interpret
what he said in light of your own understanding. That's a common
mistake that instructors make and it hides the fact that the student
really *doesn't* understand.

Odd words or phrases used to explain something can often give a clue
that the mental model is wrong. This "relative wind coming from
above" sets my detectors going off. Where the relative wind is coming
from relative to the horizon is irrelevant.


[an accelerated stall.] Yes, he certainly is, and he said as
much.

Where?

Greg Esres wrote:

The relative wind will be "coming from above", since that is the
direction in which the aircraft is traveling.

The relative wind doesn't ever "come from above" while the aircraft
has a positive angle of attack..by definition. ;-)

The relative wind always comes from the direction in which the aircraft is
actually traveling. If the aircraft is climbing, the relative wind comes from
above the horizon; ie. it is not horizontal.

Nor will the
aircraft stall with the relative wind "essentially horizontal."

It certainly will if the aircraft is neither climbing nor descending, is not
banked, and the pitch angle exceeds the stall angle of attack.

Sounds like you think there is a zero angle of attack in that
situation?

No, I don't.

George Patterson
The desire for safety stands against every great and noble enterprise.

G.R. Patterson III wrote:

Hilton wrote:

I don't understand the first part (higher speed and climbing?) and the
second part is wrong.


If I leave the flaps at 0 degrees in my aircraft, bring the power back to
decelerate, and maintain level flight, she will stall at about 53 mph indicated.
The relative wind will be essentially horizontal, since that is the direction in
which the aircraft is actually traveling.

If I leave the flaps at 0 degrees, slow down to 60 mph indicated and raise the
nose enough to stall, the aircraft will be climbing just prior to the stall. The
relative wind will be "coming from above", since that is the direction in which
the aircraft is traveling.

George Patterson
The desire for safety stands against every great and noble enterprise.

I think I see a lot of confusion happening in this thread due to
the use of fuzzy and unnecessary concepts like "relative wind",
"pitch angle", "from above" and a couple of others.

Angle of Attack is simply the angle at which the airflow meets
the wing. There is no need to complicate matters by calling the
airflow "relative", especially as some posters seem to be confused
about what is _relative_ to what.

If we must use "relative" then it would be better to say exactly
what we mean "relative to the wing/aircraft" or "in relation to
the wing/aircraft", but as this is the only relation that makes
sense when discussing AOA it shouldn't be necessary to mention
it at all.

And "wind" is positively misleading as it makes you think of
movement of an airmass in relation to the ground. "From above"
is similarly meaningless, unless we specify whether we mean it
in relation to the wing/aircraft or the horizon.

Cheers CV

If we must use "relative" then it would be better to say exactly
what we mean "relative to the wing/aircraft" or "in relation to
the wing/aircraft", but as this is the only relation that makes
sense when discussing AOA it shouldn't be necessary to mention
it at all.

That's why it's almost impossible to discuss a subject meaningfully
with someone unless he has the basic vocabulary down. For the CFIs I
have taught, my first step is have them read an entry-level
aerodynamics book. Much of our discussions after that is making sure
they use the correct words and have a clear idea of what they mean.

Words like "pitch", "angle of attack", and "climb angle" all have
different meanings, but the distinction is so fuzzy in most pilots'
minds that it's no wonder that people get confused.

Now, "relative wind" is a standard aerodynamics term and is as
ordinary to me as the word "chair". Surely all pilots understand what
relative wind is?


Yes, "wind" probably isn't the ideal word to use; vast numbers of
pilots out there still think that the motion of the airmass relative
to the ground affects the aerodynamics of the aircraft, and it's very
difficult to rid them of that notion.