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

("Margy" wrote)
[snip]
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.


Garden hose + thumb:
"As the speed increases the pressure decreases..." part throws me. As the
flow increase?

C'mon over here and explain it again please. Yes, yes. Of course I'll keep
the hose kinked --- almost in range. hehehe


Montblack

"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!"

Admittedly I did simplify it a bit.

Brian

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. ;-)


Still the same Principle, Your just transfering your lift from the
Fixed wing the Rotating Wing (the Propeller)

Brian

But the upward aerodynamic force they are looking for
to accelerate their craft upward as a result of this lift is called
drag.

Not really.

Yea really. The upward acceleration of the flying glider in a thermal
entry is caused 100 percent by the component of the relative airflow
caused by the thermal. It requires a force to accelerate the glider
upward as a result of the thermal. Lets see what aerodynamic force is
most accurately defined as the aerodynamic force that is in the
direction of the relative airflow that caused it? That's right drag.
Any lift from an upward airflow will be horizontal. That will be why
the increased lift points more horizontal.

It is true that angle of attack goes up causing more lift but as far as
accelerating the glider upward this extra lift is negated by the fact
that the direction of this lift moves farther away from the upward
direction. This additional lift comes with additional drag and so does
the additional wind speed as a result of the thermal. And the direction
of this drag is more in the upward direction as a result of the
thermal. The thermal not only changes the direction of the relative
airflow it increases its speed. A fact that you conveniently left out.
Don't dem thar velocity vectors have magnitudes?

Now hear is another clue. When drag causes the acceleration of an
object the faster that object goes the less drag it generates until it
reaches the speed of the air and generates no drag, like the horizontal
flight of a balloon. This is because the more the object moves with the
wind the less motion between the object and the air. This is why the
flight stabilizes to a steady climb at the original constant speed in
the rising air. When lift causes the acceleration of an object it has
similar dynamics as the flying glider in a thermal entry. If the
glider accelerated upward as a result of the aerodynamic force lift it
would also affect the relative airflow by changing its speed and
direction. You never said any thing about this influence in your
analysis.



Before entering rising air, a glider's wing
sees a relative wind pointed slightly upward. It's pointed
straight back up the angle of the glidepath. The lift
vector is perpendicular to this, so it angles slightly
forward. As the glider enters rising air, the relative
wind turns and now angles more steeply upwards as the
upwardly pointed vector of rising air is added to the
previous vector of relative wind from the glide. If the
glider's attitude is unchanged, the changing relative wind
increases the AOA and the corresponding lift vector, and
that lift vector tilts forward.

The drag vector also increases and tilts upwards. Most of
the additional force that initially accelerates the glider
upwards comes from the increased lift vector. A smaller
component comes from the more upwardly tilted drag vector.

As the flight stabilizes to a steady climb at the original
constant speed in the rising air, the lift and drag vectors
will return to the same magnitudes and angles as before
relative to the ground. The glider will be in the same
attitude relative to the ground. The drag will be the same
(same magnitude, same direction), the lift will be the same
and the vectors of lift and drag will add up to produce a
vertical aerodynamic force that exactly opposes the downward
force of gravity. The glider will continue in unaccelerated
flight and the only difference will be that the glider is
now in a steady unaccelerated climb instead of a steady
unaccelerated descent.


Do not spin this aircraft. If the aircraft does enter a spin it will return to earth without further attention on the part of the aeronaut.

(first handbook issued with the Curtis-Wright flyer)

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.

Uh-huh. So if you use a crane to lift up the nose of a 747 sitting on
the ramp, your instructor believes it will be generating the same
amount of lift as it would at that AOA and 400 knots. Detach the crane
and the 747 will just stay there in a nose up attitude without any
visible means of support. Or maybe your instructor has a poor
understanding of lift.

Lift is actually a mathematical formula: L = 1/2 air density * velocity
squared * area of the wing * coefficient of lift for that wing. Your
instructor should know that; it is on both the commercial and flight
instructor written exams.

You generally can't do much about the air density, but you usually can
change your velocity and the coefficient of lift. The coefficient of
lift for most wings increases with AOA, peaking at the critical AOA and
dropping off sharply at higher AOA after that. Some flaps and other
devices (variable geometry wngs come to mind) can change the area of
the wing and/or its coefficient of lift. Also, "the area of the wing"
is not quite right; it really is a reference area which might have
little to do with the actual wing size. A helicopter, for example, uses
a reference area equal to the entire disk, not just the blades. The
same rule applies to propellers. The reference area on a fixed wing
plane includes the area through the fuselage, as if the wing was all
one piece.

You can use either sq. feet or sq. meters (or, heck, sq. rods if you
want to) for the reference area; it all works out as long as you use
the same type of units all through the calculation, including air
density.