what the heck is lift?

I have always been under the impression that lift is the product of
airspeed and angle of attack, and that lift is the measure of upward
force acting on the plane at a given time. For instance, if you are
doing slow flight, your wings are producing the same amount of life
that you would be if you were cruising, GIVEN that you did not lose or
gain any altitude during the maneuver.

My instructor, which is a very knowledgable guy tried telling me that
lift has nothing to do with airspeed. He said that lift is directly and
soley related to AOA and AOA only. So if you are doing slow flight, you
are producing more life than you are when you're cruising. I overheard
a ATP guy who flies King Air's say that this huge 20 ton military plane
he used to fly would fly approaches at 110 knots, and I heard him say
"It is able to do this because it producing so much lift", which I took
as him defining lift as my instructor does.

So whats the deal here? Are we just thinking of two diffrent concepts?

"buttman" wrote in message
ups.com...
[...]
So whats the deal here? Are we just thinking of two diffrent concepts?

Your instructor is wrong, and should not be instructing.

In straight and level flight, lift equals weight. Unless your weight
changes, lift does not change, regardless of airspeed. What *can* change is
the lift coefficient, which is determined by the angle of attack. But lift
itself remains static.

Pete

buttman wrote:
I have always been under the impression that lift is the product of
airspeed and angle of attack, and that lift is the measure of upward
force acting on the plane at a given time. For instance, if you are
doing slow flight, your wings are producing the same amount of life
that you would be if you were cruising, GIVEN that you did not lose or
gain any altitude during the maneuver.

My instructor, which is a very knowledgable guy tried telling me that
lift has nothing to do with airspeed. He said that lift is directly and
soley related to AOA and AOA only. So if you are doing slow flight, you
are producing more life than you are when you're cruising. I overheard
a ATP guy who flies King Air's say that this huge 20 ton military plane
he used to fly would fly approaches at 110 knots, and I heard him say
"It is able to do this because it producing so much lift", which I took
as him defining lift as my instructor does.

So whats the deal here? Are we just thinking of two diffrent concepts?



Good question,

In its most basic form, the amount of lift is determined by how many air
molecules are being deflected by the lifting surfaces, what angle they
are being deflected at, and how fast they are being deflected. We can
mostly ignore the low pressure over the wing stuff, since that is, a.
relatively minor, and b. is also a product of how many molecules are
flowing over the wing and how fast they are flowing.

Obviously, the faster the wing, or rotor, is moving through the air, the
more molecules it will be encountering and accelerating downward in a
given period of time. It is also obvious that the greater the AOA, the
steeper the angle of deflection and the greater the number of molecules
being deflected.

Therefore lift is a product of the airspeed of the lifting surface and
it's angle of attack.

Drag is another issue altogether.

Lift in a fully developed spin or steady sinking mush is also exactly
the same as in level flight.

--

Roger Long

Lift is created by the differing pressures between the upper and lower
surfaces of the wing coupled together with AOA.

Wings generally tend to have a curved suface. The upper surface has a
greater arc or curvature than the lower surface. As the air flows across the
surfaces of the wing, the upper surface air is forced to move faster than
the lower surface air thus causing a pressure difference between the two
surfaces.
The pressure difference coupled together AOA is what causes lift.
On the question of wether the wing is 'pushed' or 'sucked' into the air, as
far as I am aware the jury is still out on that point. Personally I
subscribe to the view that it's probably a bit of both.

As for air density, that is another point. as you will be aware that air
desity decreases with altitude i.e the higher you go the molecules are less
densly packed which is why any wing has a maximum service ceiling, i.e the
point at which it won't generate any more lift. It's also worth noting that
VNE decreases with altitude.

Glider pilot.

-|-
-----===()===-----

Gone,

Lift is created by the differing pressures between the upper and lower
surfaces of the wing coupled together with AOA.

I'd say it's the other way around: Lift creates the pressure differential.


Wings generally tend to have a curved suface. The upper surface has a
greater arc or curvature than the lower surface. As the air flows across the
surfaces of the wing, the upper surface air is forced to move faster than
the lower surface air thus causing a pressure difference between the two
surfaces.

Forced by what? And how does your "theory" explain inverted flight? I don't
buy it.

BTW, this has been beaten to death in countless aviation newsgroup
discussions. I once thought like you, because I was taught that way. It's
still a bad theory. I suggest googling. Keywords might be: lift, flight,
Bernoulli, Newton.

--
Thomas Borchert (EDDH)

buttman wrote:
He said that lift is directly and
soley related to AOA and AOA only. So if you are doing slow flight, you
are producing more life than you are when you're cruising.

So, if follows that if I stand my airplane on its tail, sitting on the
ground with an airspeed of zero, it's producing the maximum amount of
lift possible.

On Fri, 09 Sep 2005 11:12:36 GMT, "Roger Long"
wrote:

Lift in a fully developed spin or steady sinking mush is also exactly
the same as in level flight.


Hmm. If lift was equal to weight in level flight the forces are
equal, thus no change in height.

How do you reconcile this with a mush or spin? Height is changing,
thus left is less than weight.



Jim

http://www.unconventional-wisdom.org

Gone Flyin'. -----==0==----- wrote:
It's also worth noting that
VNE decreases with altitude.


Actually, it's not. the true airspeed at which VNE occurs is greater as
altitude increases, but the calibrated aurspeed (VNE is defined as a
calibrated airspeed) remains the same regardless of altitude.

Thats what I told him. This guy is an amazing instructor. He's an MEI
and a CFII with a ton of CFI and CFII singoffs. I was just wondering if
we were thinking of two different concepts. My definition of lift being
different than his...