what the heck is lift?

Dudley Henriques wrote:


"Hilton" wrote in message
ink.net...
Peter Duniho wrote:
Hilton wrote:
Todd's reply to this clearly shows why Roger's statement is wrong.

No, it doesn't. See my reply to Todd and Stefan's reply here to
understand
what we are all talking about.

You wrote "Had his definition of lift been correct, he would have been
exactly correct." Ummm, OK. But lift is well-defined and it is not
defined as the force that opposes weight. So, you can redefine whatever
you
want, doesn't make it right.

Hilton


I've always STARTED an explanation of lift by presenting it initially as
the aerodynamic force that opposes the relative wind, NOT the force that
opposes gravity or weight. (That comes later :-)))
Dudley Henriques

Lift opposes the Relative Wind?

How does lift (and I assume you are talking wing lift here since you mention
gravity/weight) *oppose* the relative wind?

What do you mean when you use the word "oppose"?

Or were you speaking of prop lift?

--
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"gregg" wrote in message
...
Dudley Henriques wrote:


"Hilton" wrote in message
ink.net...
Peter Duniho wrote:
Hilton wrote:
Todd's reply to this clearly shows why Roger's statement is wrong.

No, it doesn't. See my reply to Todd and Stefan's reply here to
understand
what we are all talking about.

You wrote "Had his definition of lift been correct, he would have been
exactly correct." Ummm, OK. But lift is well-defined and it is not
defined as the force that opposes weight. So, you can redefine whatever
you
want, doesn't make it right.

Hilton


I've always STARTED an explanation of lift by presenting it initially as
the aerodynamic force that opposes the relative wind, NOT the force that
opposes gravity or weight. (That comes later :-)))
Dudley Henriques

Lift opposes the Relative Wind?

This should read "Lift is the component of aerodynamic force perpendicular
to the relative wind.", and not "opposes". My error in presentation.
DH

"Dudley Henriques" dhenriques@noware .net wrote in message
hlink.net...
snip
This should read "Lift is the component of aerodynamic force perpendicular
to the relative wind.", and not "opposes". My error in presentation.
DH

Hello Dudley,

Nicely stated,

Are we not really looking at two different concepts of lift here?

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

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

ISTM that A is the description of the dynamics of motion through a fluid
and B is the description of the dynamics of motion of a mass. The fact
that the mass is moving through a fluid makes it a complex problem that is
straining our definitions, and perhaps is more of a problem of conceptual
semantics.


In regards to "not AGAIN", here in Canada we have an ongoing debate on
Quebec nationalism/referendum that we wags refer to as the "neverendum".

regards,

"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