"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...

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.

"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?

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".

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.

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.

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.

Pete