A Level 1 AOA clarification

Ramapriya wrote:

Andrew Sarangan wrote:

a. is dependent on its airspeed, and is independent of its weight
and
weight distribution, and

No, the stall AOA is independent of both airspeed and weight.

Too confusing

I'll try to simplify it a bit. An angle of attack is the angle at which the wing
"attacks" the air. If the air is relatively stable and you raise the nose, you
have just increased the angle of attack. Lower the nose, the angle decreases.

Ok so far?

Now. The stall angle of attack is the angle at which the airflow over the wing
won't follow the curve of the wing anymore. The wing is tilted up too steeply
relative to the airflow. If I undrestand him correctly, Andrew is stating that
the angle of attack at which this occurs is the same regardless of airspeed. I
believe he is incorrect in this - definitely my aircraft will stall at a much
lower angle of attack at 50 mph than at 60 mph, and I've never been brave enough
to get the nose high enough to stall it at higher airspeeds.

Now, there *is* a misconception that stall airspeeds are constant, and this is
not true. The way the truth is usually phrased is "an airplane can stall at any
speed." You can exceed the stall angle of attack while flying perfectly level at
a pretty fair speed if you fly into a wind that is blowing up a steep slope.
There are also "high-speed" stalls caused by attempting to maneuver too rapidly
at high speed.

The true airspeed at which a stall occurs also increases with density altitude.
This is not usually a factor for light aircraft, since the indicated airspeed
for a given true airspeed decreases at the same rate. In other words, if your
plane has a pitot tube and stalls at 60 knots indicated, it will stall in that
configuration at that speed at any altitude it can reach. If the aircraft is
equipped with an indicator that reports true airspeed, however, stall speed is
not constant.

I do not know whether or not the stall angle of attack changes with weight, but
the stall airspeed in any configuration increases as weight increases. Paul's
points on the effects of loading and the downforce produced by the tail surfaces
are also good.

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

"G.R. Patterson III" wrote in message
...
I'll try to simplify it a bit. An angle of attack is the angle at which
the wing
"attacks" the air. If the air is relatively stable and you raise the nose,
you
have just increased the angle of attack. Lower the nose, the angle
decreases.

To elaborate a bit: Ramapriya's assertion that "the angle of the wings can't
be varied" is incorrect. The angle of the wings can be and is varied, by
using the elevator control to adjust the pitch attitude of the aircraft, and
thus of the wings.

This is what George means by "raise the nose".

[...] If I undrestand him correctly, Andrew is stating that
the angle of attack at which this occurs is the same regardless of
airspeed. I
believe he is incorrect in this - definitely my aircraft will stall at a
much
lower angle of attack at 50 mph than at 60 mph

You understand Andrew correctly, but not stalling.

Since you mention stalling at two different airspeeds, let's look at those
as examples. Let's assume that at the lower airspeed, you are stall in
unaccelerated flight. There are two ways to stall the airplane at a higher
airspeed then: one is to pull hard on the yoke to increase loading and pitch
attitude to stall before the airplane slows further; the other is to have
the flaps out at the slower airspeed, but not the higher.

In the first case, the pitch attitude appears higher, but the angle of
attack is the same. The airplane, because of the higher pitch angle, is
accelerating upward, which changes the direction of the relative wind
somewhat downward, making a given angle of attack occur at a higher pitch
angle.

In the second case, the pitch attitude appears higher, but the angle of
attack is the same (sound familiar? ). When the flaps are extended, the
effective chord of the wing changes, essentially pitching the wing upward
and increasing angle of attack. This increases the angle of incidence of
the wing (the angle between the wing chord and the fuselage), causing a
given angle of attack to occur at a lower pitch angle, compared to a
no-flaps stall (at a higher airspeed).

The flaps might also change the stalling angle of attack subtly, but a) most
of the perceived change in angle of attack comes from the change in
effective angle of incidence, and b) the change in AOA in that case is due
to the change in shape of the wing, not the change in airspeed.

[...]
Now, there *is* a misconception that stall airspeeds are constant, and
this is
not true. The way the truth is usually phrased is "an airplane can stall
at any
speed."

You forgot the other half of that: an airplane can stall at any attitude.
Pilots often mistake pitch angle relative to the ground for angle of attack.
In level, 1-G flight this is the case. But you can exceed the critical
angle of attack with the nose pointed down (pulling out from a high-speed
dive for example), and you can have the nose pointed quite high (during a
climb in a high performance airplane, especially at lower weights), without
exceeding the critical angle of attack.

[...]
I do not know whether or not the stall angle of attack changes with
weight, but
the stall airspeed in any configuration increases as weight increases.

Weight does not affect the stalling angle of attack.

Pete

Peter Duniho wrote:
"G.R. Patterson III" wrote in message
...

To elaborate a bit: Ramapriya's assertion that "the angle of the
wings can't
be varied" is incorrect. The angle of the wings can be and is
varied, by
using the elevator control to adjust the pitch attitude of the
aircraft, and
thus of the wings.

My bad. What I intended saying was that the wings on their own can't be
tilted about, barring use of flaps; they're after all rigid structures.
Cheers,

Ramapriya

"Ramapriya" wrote in message
oups.com...
My bad. What I intended saying was that the wings on their own can't be
tilted about, barring use of flaps; they're after all rigid structures.

Even without the use of flaps, you can change the angle of the wings.
That's what the elevator control does.

Peter Duniho wrote:

You understand Andrew correctly, but not stalling.

Well, yes I do, but not that late at night or with that much "Christmas cheer",
obviously.

Rama, in my post, I forgot that at a higher airspeed, the plane is likely to be
climbing, therefore the relative wind will be coming from above. You will reach
the same angle of attack at a steeper pitch angle at higher airspeeds.

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

G.R. Patterson III wrote:
Rama, in my post, I forgot that at a higher airspeed, the plane is likely
to be
climbing, therefore the relative wind will be coming from above. You will
reach
the same angle of attack at a steeper pitch angle at higher airspeeds.

What?

Hilton

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

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.

Andrew is stating that the angle of attack at which this occurs is
the same regardless of airspeed. I believe he is incorrect in this

Andrew is 100% correct on this.

my aircraft will stall at a much lower angle of attack at 50 mph
than at 60 mph, and I've never been brave enough

That sentence doesn't make any sense at all. You may be confusing
pitch angle with angle of attack.

The only means you have of determining your AOA is your airspeed
indicator. At 50 mph, you are at a higher AOA than at 60 mph.

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

Same AOA = Same airspeed.