what the heck is lift?

buttman wrote:

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

Nonsense. Don't listen to what any of these people are saying here.
Lift is produced by the action of millions of tiny gnomes standing on
each other's shoulders. The thing is they're not too strong so you have
to keep moving because each gnome can only hold you up for a brief
instant and pass you on to the next one. Larger wings allow you to
spread the load out over more gnomes, thus creating more "lift." Spins
happen when you move the controls the wrong way and scare the gnomes.
Some people don't accept this because they can't see the gnomes, well,
you can't see air particles either. Even the Ph.D. guys can't explain
it, but any fool can see that planes fly. Gnomes!

-cwk.

Jimbob wrote:
On Fri, 09 Sep 2005 14:29:18 +0200, Thomas Borchert
wrote:


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

Forced by limiting the space through which the fluid must flow. Think
of your garden hose. If you put your thumb over the end and constrict
the space the water flows faster through the opening. As the speed
increases the pressure decreases, air moves from high pressure to low
pressure and the wing of the airplane is in the way of this movement so
it is lifted up with the high pressure air. This also explains wing tip
vortices and why for a given configuration a higher aspect ratio wing
will produce more lift than a lower aspect ration wing.

Inverted flight and equal camber wings use AOA to create the air
pressure differential.

Margy

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.



He is describing the traditional airfoil theory which is correct. It
is the most efficient method as it produces lift with minimal drag.
That's what most people are taught.

There is another mode that is related to the force of the air
impacting on the bottom of the wing at high AOA producing lift as
well. Think of your control surfaces. Your rudder control surface is
symmetric, yet it produces horizontial components of force. IIRC, the
Jeppesen books cover high AOA effects as well.

Inverted flight is accomplished by the second of the two effects.
They have to fly at a higher AOA relative to normal flight to
compensate for the airfoil effect. Some aerobatic planes have
symmetric airfoils for this reason.

As AOA increases, the deflection takes more of a role. At stall, the
deflection is suffcient for the airfoil effect to be interfered with
and ceases. Thus a large component of left is lost. You drop. You
still have some lift, but it is not sufficient to keep you airborne.





Jim

http://www.unconventional-wisdom.org

"Hilton" wrote in message
. net...
Roger Long wrote:
Lift in a fully developed spin or steady sinking mush is also exactly
the same as in level flight.

Not even close!

He's quite close. See Todd' post.

I wrote "straight and level flight" simply because that was the scenario
being discussed in the original post. But any unaccelerated flight means
lift equals weight, and that includes the "fully developed spin" and "steady
sinking mush" Roger described.

Pete

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

Not according to the Jepp Private Pilot's Manual.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.

buttman wrote:

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.

No. From the Jeppesen Sanderson "Private Pilot Manual" -- "Lift can be increased
in two ways; by increasing the forward speed of the airplane or by increasing
the angle of attack."

And elsewhere -- "When an aircraft is in straight and level flight, .... lift
equals weight ..."

So, you have no more lift when you are cruising level than if you're in level
slow flight.

George Patterson
Give a person a fish and you feed him for a day; teach a person to
use the Internet and he won't bother you for weeks.

"George Patterson" wrote in message
news:_8kUe.739$626.593@trndny08...
Roger Long wrote:
Lift in a fully developed spin or steady sinking mush is also exactly the
same as in level flight.

Not according to the Jepp Private Pilot's Manual.

Are you relying on the part of that manual that you quoted elsewhere?

You'll note that the quote you provided does not include the word "only".
It's incorrect to infer from the statement that lift equals weight in
straight and level flight, that when not straight and level lift does not
equal weight.

The statement you quoted is not inconsistent with Roger's post.

Pete

Forced by limiting the space through which the fluid must flow. Think
of your garden hose. If you put your thumb over the end and constrict
the space the water flows faster through the opening. As the speed
increases the pressure decreases, air moves from high pressure to low
pressure and the wing of the airplane is in the way of this movement so
it is lifted up with the high pressure air. This also explains wing tip
vortices and why for a given configuration a higher aspect ratio wing
will produce more lift than a lower aspect ration wing.

PV=nRT

On Fri, 9 Sep 2005 10:51:23 -0700, "Peter Duniho"
wrote in
::

But any unaccelerated flight means
lift equals weight, and that includes the "fully developed spin" and "steady
sinking mush"

Isn't there acceleration in a sinking mush? If the aircraft is
descending, does lift equal weight?

Peter wrote:
Hilton wrote:
Roger Long wrote:
Lift in a fully developed spin or steady sinking mush is also exactly
the same as in level flight.

Not even close!

He's quite close. See Todd' post.

I wrote "straight and level flight" simply because that was the scenario
being discussed in the original post. But any unaccelerated flight means
lift equals weight, and that includes the "fully developed spin" and
"steady
sinking mush" Roger described.

Todd's reply to this clearly shows why Roger's statement is wrong. A large
percentage of the upward force in a spin is drag. The extreme case is a
parachutist coming straight down in one of those old round parachutes. In
this case, the 'aircraft' has zero lift and DRAG == WEIGHT.

Lift, drag, and thrust can be pointed in any direction; the only constant is
weight which always points towards the center of the earth.

Hilton

On Fri, 09 Sep 2005 10:59:00 -0400, T o d d P a t t i s t
wrote:

"Brian" wrote:

Generally speaking lift = AOA * Airspeed.

Not quite. Generally speaking lift is proportional to AOA *
(Airspeed squared).

Except when accerlating or decelerating up or down (i.e Beginning or
ending a climb or descent) the lift = weight of the airplane.

This is close and is often a reasonable approximation, so
I'm not really disagreeing, just expanding. However, lift
is actually defined as a force perpendicular to the flight
path, so in climbs, some weight is supported by thrust, and
in descents, some weight is supported by drag. Lift is
slightly reduced in both cases.

In a somewhat more extreme example, when I pull my 400 hp Sukhoi into
a nearly vertical attitude, the rate of climb decreases to essentially
zero, i.e., the airplane hovers. In this case, the wings are
providing essentially no lift and the airplane is being supported by
almost totally by thrust. Actually, you should imagine Sean Tucker
doing this as I don't do it all that well. ;-)

Klein