Newbie Qs on stalls and spins

Reference: "Flight Theory for Pilots", Charles Dole, 4th ed.

stall -- airflow separation of the boundary layer from a lifting surface.
characterized by a loss of life and an increase in drag. two types of
stall of interest tot he non-jet pilot: slow speed and accelerated.

"Blanche" wrote in message
...
Reference: "Flight Theory for Pilots", Charles Dole, 4th ed.

stall -- airflow separation of the boundary layer from a lifting surface.
characterized by a loss of life and an increase in drag. two types of
stall of interest tot he non-jet pilot: slow speed and accelerated.


Proofread you work or you might scare hell out of somebody.

Thank you for your response. A lot of this has to do with semantics and
poorly defined terms on both sides...my comments are in the text:


"Peter Duniho" wrote in message
...
"Bill Denton" wrote in message
...
You CAN'T take off without a stall, if an airfoil only has two states:
flying or stalled.

First of all, there is a continuous regime of "flight" between stalled and
not stalled. It's not binary. But secondly, even if you assume the
airfoil
has just the two states, the rest of your conclusion regarding that is
incorrect...

You are both right and wrong on this one. Obviously, different parts of an
aircraft stall at different speeds. This is why a stall in most light
aircraft is generally benign: the stabilizer continues to fly long after the
wing has stalled, resulting in the pitch-down generally required for stall
recovery. So, as you approach the stall speed for an AIRCRAFT, differrent
parts of the airplane will be stalled, or not flying; other parts will still
be flying.

Obviously, there is a range between the point where an element is producing
zero lift, where it is producing enough lift to "fly" the unit itself at a
consistent altitude, and where it is produing enough lift to fly the
required load at a consistent altitude.

Assume a perfect set of conditons, primarily containing an absolutely
"level" portion of the earth, and consider the following scenarios.

You take an airplane to 1,000 ft AGL, and trim it so it is flying perfectly
straight and level. You then close the throttle slightly, resulting in a
slight descent. Even thought you are still flying straight and level,
eventually you will impact the earth, even though the airplane as a whole
(and probably all of it's component parts) are still "flying".

You then take an airplane to 1,000 ft AGL, and trim it so it is flying
perfectly straight and level, but this time you completely close the
throttle. In a short time, the wings will stop producing enough lift to keep
the airplane in flight, it will pitch down and impact the earth, even though
some of the airplane's component parts may still be flying.

It is this second condition that most people consider to be a stall.

But since my terminology may not be correct, it is obvious that I am neither
an aeronautical engineer, a physisist, or as yet a pilot (none of which I
claim to be), I think it is also evident that I do understand at least the
basic principles involved in the discussion.



If the airfoil is flying you cannot take off, and if it's not flying
it's
stalled.

"If the airfoil is flying you cannot take off". Care to rephrase that?
At
best, I can assume you meant to write "if the airfoil is not flying you
cannot take off". Which would be true (inasumuch as I might assume what
you
mean by "flying"), but not particularly germane. Your second clause, "if
it's not flying it's stalled" seems to get to the heart of your
misunderstanding however.

This is a matter of the original poster's lack of precision, and my own
desire to have a little fun. From the rest of the post, it is obvious that
the poster is referring to a larger element than an airfoil, perhaps a wing
or an entire airplane. Had I written the original post I would have used
airplane.

But please assume an airplane, accept my terminology, and consider the
following:

If a wheel is rolling, you cannot start it rolling.

My statement was: " If the airfoil is flying you cannot take off, and if
it's not flying it's stalled", which would translate as follows: "If the
airplane IS flying it cannot START flying"' the rest reflects the
flying/stalled paradyme, which I readly admit is not absolutely correct.



"Flying" is not a technical aerodynamic term, and in particular you cannot
say that "flying" is the opposite of "stalled". The opposite of "stalled"
is "not stalled".

As has already been pointed out, "stall" simply means that the airfoil's
angle of attack is greater than the critical angle of attack. An airfoil
that has no relative wind has NO angle of attack, and the term "stall" is
meaningless in that context. Once the airfoil has relative wind (e.g. you
start your takeoff roll), you can then look at the angle of attack and
compare it to the critical angle of attack. Looking at the example of a
takeoff roll, the wing's angle of attack remains below (and generally,
WELL
below) the critical angle of attack at all times.

No stall at any point in time during the takeoff roll.

Same thing applies to most landings. As the airplane slows after touching
down, the amount of lift being generated is reduced, but this is
compensated
for by the wheels providing the balance of the required support. At no
point does the wing wind up with a higher angle of attack than the
critical
angle of attack, and thus there is no stall.

(There's a whole bunch of physics involved here that I don't yet know,
so
anyone, please feel free to correct whatever I get wrong.)

We're trying.

You stated: "It's flying as soon as you start moving on the runway".
That
is
not correct.

It IS correct. Well, inasmuch as you've failed to define "flying" for us,
and inasmuch as "flying" has no predefined aerodynamic definition. The
instant there is ANY relative wind, the wing is creating lift (since its
angle of attack is below the critical AOA). That's my definition of
"flying": "creating lift". What's your definition?

Actually, it was the original poster who failed to define "flying" and
specify what was flying. As noted above, I am looking at this in the context
of an entire airplane, which seems to have been the original poster's
intent. In fact, the airplane doesn't need to move on the runway at all;
given a sufficient releative wind, parts of the airplane would begin to fly
without the airplane moving forward at all. And given a relative wind even
slightly higher than the stall speed of the aircraft, it could theoretically
take off and continue to ascend with no forward movement.

I agree with your definition, but it has to be consiered in light of whether
we are discussing a single element or an entire airplane.



It doesn't begin to fly until you develop enough relative wind
to create enough lift to overcome drag.

Lift overcomes gravity. Thrust overcomes drag. In order to lift off from
the ground, you do need enough relative wind to allow the wing to generate
enough lift to overcome the force of gravity. But if by "flying" you
simply
mean "to have lifted off from the ground", then it's especially true that
"flying" is in no way the opposite of "stalled".

You are correct on the lift/thrust thing; please understand that it's been
25 years since I read the Jepp private pilot manual ;-)



If an airplane is only moving at 1
kt. down a runway, it is probably not flying.

Again, you'll have to define "flying". But the wing certainly is
developing
lift, and certainly is NOT stalled.

How 'bout if I throw in a 10 kt tailwind? g



Forward motion of the aircraft is not required. Given a strong enough
headwind, an airplane will readily fly backward; just ask some J3
drivers.

Forward motion through the air mass IS required. Given a strong enough
headwind, an airplane may well depart from the ground, but as soon as it's
no longer tied to the ground, it will slow relative to the airmass and
fall
back to the ground. Probably in a stalled state, even.

I proabably should have qualified that, but remember I'm not writing a
textbook, I think all of us here frequently accept some unstated
assumptions.



And an aircraft will not land until it has reached a "stalled" state.

Simply untrue. Virtually all of my landings involve touching down and
coming to a stop without ever exceeding the wing's critical AOA. I
hesitate
to claim that I've *never* stalled the wing during a landing, but I sure
don't do it intentionally.

And this comes back to the flying/stalled paradym.


BTW: I generally pick up a "nugget" or two from your posts, so your work is
not in vain. I appreaciate it.


Pete

Bill Denton wrote:

But if an airfoil has two states, stalled or flying, how can you land
without a stall?

It doesn't have just two states. It can be moving too slowly to be flying
without being stalled. As has been posted several times, it is not stalled
unless the airflow over the upper surface of the wing has separated from it. As
has also been posted, that will not happen without an excessive angle of attack.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.

"Bill Denton" wrote
You take an airplane to 1,000 ft AGL, and trim it so it is flying
perfectly straight and level. You then close the throttle slightly,
resulting in a slight descent. Even thought you are still flying
straight and level, eventually you will impact the earth, even though
the airplane as a whole (and probably all of it's component parts) are
still "flying".

No Bill..."straight and level" means maintaining heading and altitude

You then take an airplane to 1,000 ft AGL, and trim it so it is flying
perfectly straight and level, but this time you completely close the
throttle. In a short time, the wings will stop producing enough lift
to keep the airplane in flight, it will pitch down and impact the
earth, even though some of the airplane's component parts may still be
flying.

No Bill...in this case, the aircraft will pitch down to maintain the
trimmed airspeed and the airplane WILL NOT stall. Yes, it will descend
until it impacts the ground, but at no time will the aircraft stall.

It is this second condition that most people consider to be a stall.

No Bill...it is called a glide

But since my terminology may not be correct,

Boy...you hit that nail on the head

it is obvious that I am neither an aeronautical engineer, a physisist,
or as yet a pilot

Yes Bill...that is absolutely obvious

I think it is also evident that I do understand at least the basic
principles involved in the discussion.

No Bill... you do not.

Bob Moore
ATP B-707 B-727 L-188
Flight Instructor Airplanes and Instruments (since 1970)
Naval Aviator S-2F P-2V P-3B !958-1967
PanAm (retired)

Bill Denton wrote:

You are both right and wrong on this one. Obviously, different parts of an
aircraft stall at different speeds. This is why a stall in most light
aircraft is generally benign: the stabilizer continues to fly long after the
wing has stalled, resulting in the pitch-down generally required for stall
recovery.

You're correct that the stabilizer may stall at a different point than the wing,
but the stabilizer on a traditional (ie. non-canard) aircraft pushes down on the
tail, not up. If it stalls before the wing, the nose falls because there's
nothing hold the tail down. If the wing stalls first, the nose falls because
there's nothing holding the nose up.

George Patterson
If a man gets into a fight 3,000 miles away from home, he *had* to have
been looking for it.

"Bill Denton" wrote in message
...
You are both right and wrong on this one.

No. I am entirely right, not wrong at all.

Obviously, different parts of an aircraft stall at different speeds.

This is true, but completely irrelevant to my statement.

[...]
You then take an airplane to 1,000 ft AGL, and trim it so it is flying
perfectly straight and level, but this time you completely close the
throttle. In a short time, the wings will stop producing enough lift to
keep
the airplane in flight, it will pitch down and impact the earth, even
though
some of the airplane's component parts may still be flying.

First, your scenario is completely incorrect as to what would happen if you
brought the throttle to idle. The airplane would NOT stall. It would
simply pitch down enough to maintain the trimmed airspeed, and glide to
whatever final destination it was aimed at.

But more importantly, your understanding of a stall is flawed. Even simply
looking at just the wing, and even ignoring such design features as washout,
upon reaching the critical angle of attack lift does not go straight from
maximum available lift to 0. There is a very narrow range of angle of
attack where the lift drops off rapidly, and within this range, there is
just as much lift as was available at angles of attack just below the
critical angle of attack.

For a variety of reasons, it's unlikely that a pilot would ever successfully
negotiate this very narrow range. But it does exist, and a person who
claims that a wing is strictly "not stalled" or "stalled" is simply
demonstrated lack of completely knowledge of what happens during a stall. A
stall is more like a dimmer light switch (with a very short throw) than it
is like a regular "on/off" light switch.

[...] I think it is also evident that I do understand at least the
basic principles involved in the discussion.

Very basic principles. But you still have quite a ways to go. You might do
well to stop telling people they are wrong, at least until you've actually
*mastered* the subject.

But please assume an airplane, accept my terminology, and consider the
following:

If a wheel is rolling, you cannot start it rolling.

My statement was: " If the airfoil is flying you cannot take off, and if
it's not flying it's stalled", which would translate as follows: "If the
airplane IS flying it cannot START flying"' the rest reflects the
flying/stalled paradyme, which I readly admit is not absolutely correct.

So you really did mean to write "if the airfoil is flying you cannot take
off". I've got to say, that's got to be one of the most intentionally
obtuse ways of saying something I've seen in a long time.

In any case, you are simply incorrect to say "if it's not flying it's
stalled". That is a patently false proposition. I have an airplane sitting
in my hangar right now that is not flying (at least, it had better not be!),
but it is not stalled either.

Actually, it was the original poster who failed to define "flying" and
specify what was flying. As noted above, I am looking at this in the
context
of an entire airplane, which seems to have been the original poster's
intent.

Another red herring. It really doesn't matter whether you are talking about
the entire airplane or the wing. But for the record, when one refers to a
"stall", they are generally referring to a stall of the main wing (and by
implication, the entire airplane).

In fact, the airplane doesn't need to move on the runway at all;
given a sufficient releative wind, parts of the airplane would begin to
fly
without the airplane moving forward at all.

I thought your claim was that an airplane that was flying (and thus
presumably parts of an airplane that are flying) cannot take off. Now you
are saying parts of the airplane can fly while still on the ground. Your
statements are inconsistent with each other.

And given a relative wind even
slightly higher than the stall speed of the aircraft, it could
theoretically
take off and continue to ascend with no forward movement.

No, it could not. With a strong enough wind, the airplane might lift off
the ground, but it would immediately begin to slow within the airmass
(accelerating backwards relative to the ground) and descend back to the
ground. There would be no "continue to ascend" about it.

I agree with your definition, but it has to be consiered in light of
whether
we are discussing a single element or an entire airplane.

We are discussing the wing, which is the only thing of interest in this
case. If you want to call that "the entire airplane", that's fine too, but
make no mistake: it's still just the wing.

If an airplane is only moving at 1
kt. down a runway, it is probably not flying.

Again, you'll have to define "flying". But the wing certainly is
developing
lift, and certainly is NOT stalled.

How 'bout if I throw in a 10 kt tailwind? g

If you want to change the discussion in an attempt to make your comments
make sense, feel free. I probably won't have the patience to bother though,
and will simply feel justified in assuming that you aren't really interested
in learning what the facts are.

I recommend you stick with the original ideas, and where you've clearly
meant "1 knot of relative wind" by saying "if an airplane is only moving at
1 knot down a runway", you should avoid complicating things by trying to
reinterpret what you've written.

Simply untrue. Virtually all of my landings involve touching down and
coming to a stop without ever exceeding the wing's critical AOA. I
hesitate
to claim that I've *never* stalled the wing during a landing, but I sure
don't do it intentionally.

And this comes back to the flying/stalled paradym.

The word is "paradigm". And frankly, I have no idea what you mean by "this
comes back to". This thread *should* have stayed with the question of a
stall all along. Only your digressions have prevented that.

The bottom line here is that your statements that "you can't take off
without a stall" and that "an aircraft will not land until it has reached a
stalled state" are both patently false. You can very much both take off and
land without ever stalling the airplane.

Pete

Let's assume that I am completely wrong. Wouldn't we all have been better
served had you provided correct information as opposed to simply pointing
out my error. I can't learn anything from that.

Given your credentials, could you not have provided me, and many others,
some useful information?


"Bob Moore" wrote in message
. 121...
"Bill Denton" wrote
You take an airplane to 1,000 ft AGL, and trim it so it is flying
perfectly straight and level. You then close the throttle slightly,
resulting in a slight descent. Even thought you are still flying
straight and level, eventually you will impact the earth, even though
the airplane as a whole (and probably all of it's component parts) are
still "flying".

No Bill..."straight and level" means maintaining heading and altitude

You then take an airplane to 1,000 ft AGL, and trim it so it is flying
perfectly straight and level, but this time you completely close the
throttle. In a short time, the wings will stop producing enough lift
to keep the airplane in flight, it will pitch down and impact the
earth, even though some of the airplane's component parts may still be
flying.

No Bill...in this case, the aircraft will pitch down to maintain the
trimmed airspeed and the airplane WILL NOT stall. Yes, it will descend
until it impacts the ground, but at no time will the aircraft stall.

It is this second condition that most people consider to be a stall.

No Bill...it is called a glide

But since my terminology may not be correct,

Boy...you hit that nail on the head

it is obvious that I am neither an aeronautical engineer, a physisist,
or as yet a pilot

Yes Bill...that is absolutely obvious

I think it is also evident that I do understand at least the basic
principles involved in the discussion.

No Bill... you do not.

Bob Moore
ATP B-707 B-727 L-188
Flight Instructor Airplanes and Instruments (since 1970)
Naval Aviator S-2F P-2V P-3B !958-1967
PanAm (retired)

On Fri, 19 Nov 2004 at 16:21:11 in message
, Bill Denton
wrote:
No, not a trick question...

You CAN'T take off without a stall, if an airfoil only has two states:
flying or stalled.

If you admit you have only a limited knowledge why start off with a
categorical statement? This indicates that you do not yet understand
what an airfoil stall is. A stall occurs when the angle of attack of an
airfoil exceeds a limit beyond which lift falls rather than increases.
This angle would not normally ever be exceed in a take off. [1]

If the airfoil is flying you cannot take off, and if it's not flying it's
stalled.

I suppose you mean above that if an aircraft is already flying it must
have already taken off! Certain amount of truth in that but your second
phrase is definitely wrong. See [1]

(There's a whole bunch of physics involved here that I don't yet know, so
anyone, please feel free to correct whatever I get wrong.)

Fine but the purchase of an elementary book on aerodynamics would be a
good start. You cannot expect a complete course on elementary
aerodynamics by asking questions which to me, sound as if you are not
really seeking elucidation.

You stated: "It's flying as soon as you start moving on the runway". That is
not correct. It doesn't begin to fly until you develop enough relative wind
to create enough lift to overcome drag. If an airplane is only moving at 1
kt. down a runway, it is probably not flying.


While 'it's flying as soon as you start moving on the runway' is not an
ideal way of putting it the rest of your paragraph suggests that you do
not yet understand the difference between lift and drag. Your third
sentence is another categorical statement which is also wrong.

Lift is a force generated at right angles to the airflow and drag is a
force along the direction of the airflow. Since they act at right angles
to each other lift is not a force that 'overcomes drag'. Your fourth
sentence is correct when you talk about the whole aircraft but not if
you are just talking about an airfoil..

And an aircraft will not land until it has reached a "stalled" state.

Thus misconception of either or both of 'landing and stall' has already
been dealt with in this thread.
--
David CL Francis

Follow on...

My examples failed because I didn't properly think them through, and
consequently did not account for pitch. Here is what I should have written,
with my corrections in all caps...

You take an airplane to 1,000 ft AGL, and trim it so it is flying perfectly
straight and level. You then close the throttle slightly, resulting in a
slight descent. IF PITCH TRIM IS NOT CHANGED, eventually you will impact the
earth, even though the airplane as a whole (and probably all of it's
component parts) are still "flying".

You then take an airplane to 1,000 ft AGL, and trim it so it is flying
perfectly straight and level, but this time you completely close the
throttle WHILE ATTEMPTING TO MAINTAIN THE ORIGINAL PITCH. In a short time,
the wings will stop producing enough lift to keep the airplane in flight, it
will pitch down and impact the earth, even though some of the airplane's
component parts may still be flying.

I believe these scenarios are correct, and I promise to do better next time!
g