Newbie Qs on stalls and spins

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