A Level 1 AOA clarification

"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

"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:

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

By reducing the AOA actually, which happens as a consequence of
increasing airspeed. But see below also.

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,

And it is interesting how that actually happens. The vertical component
of thrust takes a bit of the load off the wings which helps reduce the
AOA and keep it under the limit of the stall. Part of the weight is
in fact hanging by the propeller, like a helicopter.
CV

"CV" wrote in message
...
By reducing the AOA actually, which happens as a consequence of
increasing airspeed. But see below also.

No. Increased airspeed happens as a result of reduced angle of attack, not
the other way around. Airspeed has no direct effect on AOA, though it does
have indirect effects (since changes in airspeed affect what AOA you need
for a given performance goal, whether that's turning, climbing, descending,
or whatever).

And it is interesting how that actually happens. The vertical component
of thrust takes a bit of the load off the wings which helps reduce the
AOA and keep it under the limit of the stall. Part of the weight is
in fact hanging by the propeller, like a helicopter.

Thrust does contribute, yes. But the primary reason for requiring
additional power is that, while the wing is capable of generating the
necessary thrust at a lower airspeed, higher angle of attack (all the way up
to the stalling AOA of course), the higher angle of attack results in higher
drag, requiring higher thrust.

Pete

Peter Duniho wrote:

"CV" wrote in message
...

By reducing the AOA actually, which happens as a consequence of
increasing airspeed. But see below also.

No. Increased airspeed happens as a result of reduced angle of attack, not
the other way around.

Be that as it may, flying faster allows us to use a smaller AOA,
which is what prevents the stall.

We can stall at any speed, and at any attitude, but it always
happens at the same (or very close to the same) AOA.

CV

On Sun, 26 Dec 2004 at 21:58:05 in message
, Peter Duniho
wrote:
Thrust does contribute, yes. But the primary reason for requiring
additional power is that, while the wing is capable of generating the
necessary thrust at a lower airspeed, higher angle of attack (all the way up
to the stalling AOA of course), the higher angle of attack results in higher
drag, requiring higher thrust.

I think Peter that an aircraft will climb if trimmed to the same angle
of attack that it was using in level flight. It does this as long as
the lift is slightly less and the speed drops to produce _less_ drag
and lift, leaving more engine power and thrust to climb.

When climbing extra work must be done against gravity. That extra work
can come from increasing power or from reducing speed and therefore
drag.

Nitpicking point: wings do not create thrust! :-) You meant lift of
course.
--
David CL Francis

"David CL Francis" wrote in message
...
I think Peter that an aircraft will climb if trimmed to the same angle of
attack that it was using in level flight.

Well, ignoring for a moment that I never meant to suggest anything about
what happens if you simply increase power without changing anything else
when just above stall speed... (my comments were simply about what
additional power *allows*...not what it *causes*)

You can't make that generalization. Changes in power affect elevator
authority (affecting trim), as well as necessary rudder input (changing
drag). It is entirely possible that when just above stall speed, an
increase in power will result in an increase in angle of attack, an increase
in drag, or both.

What you can say is that if the pilot maintains the same angle of attack,
but increases power, then the airplane will climb (I don't believe that
added drag from rudder will ever be MORE than the added thrust, but I could
be wrong about that). But that's not really what I was talking about.

It does this as long as the lift is slightly less and the speed drops to
produce _less_ drag and lift, leaving more engine power and thrust to
climb.

At an airspeed just above stall, a reduction in speed results in MORE drag.
There is a reduction in parasitic drag, but there is a greater increase in
induced drag, with a net increase in total drag (and that's ignoring drag
caused by the rudder and any other control surfaces that require a change in
position).

When climbing extra work must be done against gravity. That extra work can
come from increasing power or from reducing speed and therefore drag.

The extra work comes ONLY from a net surplus of power. A reduction in speed
is only guaranteed to produce a net increase in power available if the new
airspeed is higher than Vbg. It can sometimes also produce a net increase,
if the old airspeed was sufficiently higher than Vbg, and the new airspeed
is close enough to Vbg, even if less than, but you need to know more about
the old and new airspeeds in that case to say for sure what happens. More
importantly, a reduction in speed is guaranteed to produce a net decrease of
power available if the OLD airspeed is lower than Vbg (as it is when just
above stall speed).

Nitpicking point: wings do not create thrust! :-) You meant lift of
course.

Yes, of course. Thank you.

Pete

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 in
ups.com:

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?

I see where you are getting the misconceptions from. You are thinking of
the takeoff and landing as the start and end of flight. Just because an
aircraft is on the ground does not mean it is stalled. Instead, picture
an aircraft in mid flight. Then imagine what happens if you increase the
angle of attack. The airflow over the wings will start to break up. This
is the start of stall.This point is only related to the angle at which
the airstream strikes the wing.

Think of the AOA as the difference between the angle where the aircraft
is pointing and where it is going.




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.

True, some of the larger aircraft and military jets have an AOA
indicator. Most small aircraft do not have an AOA indicator. There is a
good reason for this. In a large aircraft, the weight can vary
substantially over its flight envelope. This will result in a large
variation in stall speed. In a small aircraft, the stall speed variation
is rather small, and a single stall speed can be used safely.





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


No, you need to take a couple of flying lessons.

Andrew Sarangan wrote:
True, some of the larger aircraft and military jets have an AOA
indicator. Most small aircraft do not have an AOA indicator. There is a
good reason for this. In a large aircraft, the weight can vary
substantially over its flight envelope. This will result in a large
variation in stall speed. In a small aircraft, the stall speed variation
is rather small, and a single stall speed can be used safely.

IMHO, there is no good reason for not having an AOA indicator on GA
aircraft. Stall/spin is a leading cause of death among GA pilots and
passengers. Best glide (potential emergency situation) is determined by
AOA. Put an AOA sensor on GA planes with a hand that smacks the pilot on
the head when the AOA approaches the critical AOA and a lot fewer people
will die while having fun on the weekends.

Hilton