A Level 1 AOA clarification

"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

"Hilton" wrote in message
link.net...
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


The April, 1973 (yeah, that's a while ago) issue of FLYING had a great
article about Safe Flight's AOA indicator. Peter Garrison described it as
"phenomenally useful", after flying a Beech Sierra equipped with one.

Stall/spin is a leading cause of death among GA pilots and
passengers.

Caused by the pilot not paying attention. Will having another
instrument that he's not paying attention to really help?

Best glide (potential emergency situation) is determined by AOA.

A few knots either way isn't going to make much difference. Plus, how
often is maximum glide range critical in an engine out situation? How
closely is the pilot really maintaining one airspeed (or AOA) during
an emergency?

Put an AOA sensor on GA planes with a hand that smacks the pilot on
the head

Some studies I've seen have shown that pilots are often oblivious to
warning horns and lights, though stick shakers are effective.

My prediction: put an AOA indicator on every airplane in the fleet
and you won't see much change in the accident rate due to stall spin.

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.

"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.

Ramapriya wrote:

If my understanding is correct, an aircraft stalls beyond that AOA
which, when increased any further, produces no (further) lift. If

A _wing_ (not an aircraft) is considered stalled whenever the
AOA is greater than the AOA of maximum lift. Beyond that point an
increase in AOA produces less lift, not more. There is still _some_
lift being produced, all the way up to 90 degrees AOA.

(If this was what you meant with "no (further) lift" then you
have the general idea right)

correct, would it be logical to infer that an aircraft's stalling AOA:

The stalling AOA of a _wing_, not of an aircraft. When an aircraft
stalls the AOA will of course be somewhere close to the stalling AOA
of the wing, but they are still different concepts.

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)

The stalling AOA of the _wing_ is generally constant, typically
around 15 degrees, and does not depend on any of those factors.

The _speed_ at which the _aircraft_ stalls does depend on factors
like weight, but the stall will still happen at an AOA somewhere
close to the stalling AOA of the wing.
CV

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

I agree that AOA is a nice instrument to have, but I am not convinced if
that is going to reduce the number of stall spin accidents. Most stall spin
accidents despite all stall indications, such as low airspeed, buffet and
descent rate. Having another instrument on the panel is not going to change
the situation.



"Hilton" wrote in news:5_Azd.10136$9j5.3520
@newsread3.news.pas.earthlink.net:

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

"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