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