A Level 1 AOA clarification

If my understanding is correct, an aircraft stalls beyond that AOA
which, when increased any further, produces no (further) lift. If
correct, would it be logical to infer that an aircraft's stalling AOA:

a. is dependent on its airspeed, and is independent of its weight and
weight distribution, and
b. varies, for a given airspeed, with the air density (altitude)

I know these are way elementary, but please help out.
Cheers,

Ramapriya

In a previous article, "Ramapriya" said:
If my understanding is correct, an aircraft stalls beyond that AOA
which, when increased any further, produces no (further) lift. If
correct, would it be logical to infer that an aircraft's stalling AOA:

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

Close. The AOA required to maintain altitude at a given airspeed varies
with the aircraft's weight, so a more heavily loaded aircraft will reach
the stalling AOA at a lower speed than a more lightly loaded one.

Also, the tail produces a downforce which is used to maintain stability in
the plane. If you load the CG further forward, the downforce needs to be
greater to counter balance it fore and aft, so you need more upforce from
the wings as well to put the up and down forces in balance, so that
increases the AOA required to maintain lift.

There are good diagrams of this in any decent aviation text book.

--
Paul Tomblin http://xcski.com/blogs/pt/
"GNU is not Linux - Linux has a kernel that boots" - Chris Thompson

"Ramapriya" wrote in news:1104029481.765015.34960
@c13g2000cwb.googlegroups.com:

If my understanding is correct, an aircraft stalls beyond that AOA
which, when increased any further, produces no (further) lift. If
correct, would it be logical to infer that an aircraft's stalling AOA:

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.


b. varies, for a given airspeed, with the air density (altitude)


No the stall AOA does not vary with density.


The stall AOA is determined by the shape of the wing. It is independent of
weight and airspeed. However, the airspeed vs AOA relationship depends on a
variety of factors, such as weight and density. This is why stall speed is
somewhat a misleading quantity. AOA would be a better quantity.
Unfortunately there is no direct way to measure the AOA in most aircraft,
so we use the airspeed as an indirect indication of the AOA.

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

Getting back to basics, wings produce lift only when wind hits them,
i.e. when the aircraft starts moving. This keeps increasing until the
airspeed is adequate enough to produce a total lift that can levitate
the aircraft. Since the angle of the wings can't be varied, ignoring
flaps momentarily, I can't see how the stall AOA can be independent of
airspeed. What then is 'stall speed' of an airplane?

If stalling AOA is reached, adding engine power before the plane goes
into a stall will prevent the stall by increasing airspeed, right?

b. varies, for a given airspeed, with the air density (altitude)

No the stall AOA does not vary with density.

The stall AOA is determined by the shape of the wing. It is
independent of
weight and airspeed. However, the airspeed vs AOA relationship
depends on a
variety of factors, such as weight and density. This is why stall
speed is
somewhat a misleading quantity. AOA would be a better quantity.
Unfortunately there is no direct way to measure the AOA in most
aircraft,
so we use the airspeed as an indirect indication of the AOA.

Don't know much yet about this but I'm sure I saw the AOA indicated in
an A320 cockpit recently. I thought the pitch itself indicated AOA but
when the captain showed me the actual AOA reading, it varied by a wee
from the aircraft's pitch. He had to punch some buttons into the flight
computer to get the AOA reading.

Need to read up John Denker's book and the FAA material a lotttt more,
I guess :\

Ramapriya

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

Getting back to basics, wings produce lift only when wind hits them,
i.e. when the aircraft starts moving. This keeps increasing until the
airspeed is adequate enough to produce a total lift that can levitate
the aircraft. Since the angle of the wings can't be varied, ignoring
flaps momentarily, I can't see how the stall AOA can be independent of
airspeed. What then is 'stall speed' of an airplane?

If stalling AOA is reached, adding engine power before the plane goes
into a stall will prevent the stall by increasing airspeed, right?

b. varies, for a given airspeed, with the air density (altitude)

No the stall AOA does not vary with density.

The stall AOA is determined by the shape of the wing. It is
independent of
weight and airspeed. However, the airspeed vs AOA relationship
depends on a
variety of factors, such as weight and density. This is why stall
speed is
somewhat a misleading quantity. AOA would be a better quantity.
Unfortunately there is no direct way to measure the AOA in most
aircraft,
so we use the airspeed as an indirect indication of the AOA.

Don't know much yet about this but I'm sure I saw the AOA indicated in
an A320 cockpit recently. I thought the pitch itself indicated AOA but
when the captain showed me the actual AOA reading, it varied by a wee
from the aircraft's pitch. He had to punch some buttons into the flight
computer to get the AOA reading.

Need to read up John Denker's book and the FAA material a lotttt more,
I guess :\

Ramapriya

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.

"Ramapriya" wrote in message
ups.com...

Depending on what kind of plane you are flying, you may get good use out of
your shoulder harness if you add power just before or during a stall. While
you probably could delay the stall by adding power, it will eventually
happen if you do not lower your attitude. In most cases, that is.
Don't know much yet about this but I'm sure I saw the AOA indicated in
an A320 cockpit recently. I thought the pitch itself indicated AOA but
when the captain showed me the actual AOA reading, it varied by a wee
from the aircraft's pitch. He had to punch some buttons into the flight
computer to get the AOA reading.

That is because the aoa depends on the relative wind. The relative really
does not have much to do with where the ground is, or what your attitude is.



Need to read up John Denker's book and the FAA material a lotttt more,
I guess :\

Ramapriya

"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

"Ramapriya" wrote in message
ups.com...
Getting back to basics, wings produce lift only when wind hits them,
i.e. when the aircraft starts moving. This keeps increasing until the
airspeed is adequate enough to produce a total lift that can levitate
the aircraft. Since the angle of the wings can't be varied,

See my reply to George. The angle of the wings CAN be varied, and doing so
is essential to the art of flying.

ignoring
flaps momentarily, I can't see how the stall AOA can be independent of
airspeed. What then is 'stall speed' of an airplane?

The stall speed of an airplane is the airspeed at which the airplane will
stall, assuming straight and level unaccelerated flight. Any published
stall speed is actually specific to a certain weight (most popular stall
speeds to know are for maximum weight), and for a specific configuration
(for example, gear and flap extension both can change stall
speed...especially flaps).

If stalling AOA is reached, adding engine power before the plane goes
into a stall will prevent the stall by increasing airspeed, right?

Sort of. By the time you are down to stall speed, what additional engine
power actually does is to allow you to fly at *lower* airspeeds. However,
yes...commonly when one is near stalling and doesn't want to be, increasing
engine power is one part of the recovery. If not combined with a reduction
in pitch attitude, all that more power will do (assuming everything else is
held constant) is to cause the airplane to climb.

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