what the heck is lift?

"private" == private writes:

private A - The aerodynamic resultant reaction of an airfoil
private pulling air downward.

private B - The flight physics teaching concept that an aircraft
private (in unaccelerated flight) must generate a force (lift,
private thrust ,drag) that balances its (apparent) weight.

The problem with restricting your example to unaccelerated flight is
that the resulting definition of lift will almost surely be incorrect,
by not being general. Imagine for example an airplane in a
continuously positive-g loop. Neither definition A or B are valid,
yet lift from the wing always occurs.

On Sat, 10 Sep 2005 20:48:51 -0700, Bob Fry
wrote:

"private" == private writes:

private A - The aerodynamic resultant reaction of an airfoil
private pulling air downward.

private B - The flight physics teaching concept that an aircraft
private (in unaccelerated flight) must generate a force (lift,
private thrust ,drag) that balances its (apparent) weight.

The problem with restricting your example to unaccelerated flight is
that the resulting definition of lift will almost surely be incorrect,
by not being general. Imagine for example an airplane in a
continuously positive-g loop. Neither definition A or B are valid,
yet lift from the wing always occurs.

So what do we call the aerodynamic force on the horizontal tail that
forces the back of the airplane downward to keep the airplane from
diving into the ground? If it were acting upward we'd easily refer to
it as lift, but it acts downward. Is that lift?

Of course this same force is upward when it's on an airplane with a
canard. I guess that then it qualifies as lift.

What about the aerodynamic force on the vertical tail/rudder that
controls yaw? It's acting sideways. And what about the aerodynamic
force created by the propeller, which is a wing after all?

RK Henry

RK Henry wrote:
Bob wrote:

The problem with restricting your example to unaccelerated flight is
that the resulting definition of lift will almost surely be incorrect,
by not being general. Imagine for example an airplane in a
continuously positive-g loop. Neither definition A or B are valid,
yet lift from the wing always occurs.

Correct, the whole lift opposes weight description focuses on a very narrow
case (or set of cases). It is not general at all; in fact, it falls apart
when the airplane turns! (Try explain why stall speed increases when lift
stays the same).

IMHO: Those who think of lift as the 'upward' force(s) have simplified the
problem too much and this sets up a whole host of inconsistencies.


So what do we call the aerodynamic force on the horizontal tail that
forces the back of the airplane downward to keep the airplane from
diving into the ground? If it were acting upward we'd easily refer to
it as lift, but it acts downward. Is that lift?

Yes, it is lift. Perhaps 'we' should have called it "push" instead of
"lift", but then some would have said that is really should be called
"pull". Seriously, just as "stall" is a badly chosen word (since 99% of
the world population think when a plane stalls, its engine has stopped),
"lift" is also badly chosen. Think of it as the "push" or "pull" force.


Of course this same force is upward when it's on an airplane with a
canard. I guess that then it qualifies as lift.

Same thing really - their primary objective is to induce a nose-up pitching
moment to oppose the wing's pitching moment. To answer your quesion, yes,
this is also lift.


What about the aerodynamic force on the vertical tail/rudder that
controls yaw? It's acting sideways.

Lift.


And what about the aerodynamic
force created by the propeller, which is a wing after all?

Lift.

Hilton

"Hilton" wrote in message
k.net...
Correct, the whole lift opposes weight description focuses on a very
narrow
case (or set of cases). It is not general at all; in fact, it falls apart
when the airplane turns!

A turn is not "unaccelerated flight", which was the condition specifically
restricting this entire discussion.

IMHO: Those who think of lift as the 'upward' force(s) have simplified the
problem too much and this sets up a whole host of inconsistencies.

In unaccelerated flight, it is an entirely appropriate simplification for
the introduction of the subject. It is certainly FAR more correct than what
the original poster's instructor claimed.

Pete

"PD" == Peter Duniho writes:
PD "Hilton" wrote in message
IMHO: Those who think of lift as the 'upward' force(s) have
simplified the problem too much and this sets up a whole host
of inconsistencies.

PD In unaccelerated flight, it is an entirely appropriate
PD simplification for the introduction of the subject. It is
PD certainly FAR more correct than what the original poster's
PD instructor claimed.

An unaccelerated flight example is fine for the first introduction to
aerodynamic forces. The problem is if one doesn't move beyond it. It
sounds like the instructor in the OP has done that, never engaging in
any thought experiments at the boundaries of the example to explore
the limits of his knowledge. That, and no high school physics.

Once I taught an aviation class to a couple of Boy Scouts. I started
with the typical airplane in level flight and the 4 forces of flight,
weight, lift, drag, thrust. All well and good, nothing hard about
that! For homework I asked them to consider now a glider: "it's still
has weight, so it must product lift, right? And moving through the
air, it experiences drag, so there must be thrust, right? But from
where? A glider has no engine!"