>wing at a positive angle of attack
>making the distance over the top of the wing greater and
>the distance on the bottom of the wing less between the
>forward and aft stagnation points. This increases the velocity
>and reduces the pressure over the top of the wing and reduces the
>velocity and increases the pressure over the bottom of the wing.
>This is how a wing creates lift.
Using the wing as a frame of reference, the air stream velocity
differences over the top and bottom surfaces will vary much more
than the minor differences in these lengths.
Think of a wing as an air pump, it accelerates air downwards
(mostly by sucking air down from above). I offer "proof" of
this below. As a wing with angle of attack moves through
the air, it deflects/sucks air downwards (corresponds to lift)
and also a little bit forwards(corresponds to drag).
The air's resistance (momentum) to this acceleration creates a
pressure differential. Since the wing is not an engine, there's
no overall increase in energy (actually some loss due to
friction), so Bernoulli's relationship between velocity and
pressure apply:
total energy = pressure energy + kinetic energy.
Lower pressure areas will have faster moving air and vice versa.
"Proof" of wings as air pumps:
Imagine a "closed system" consisting of a sealed 10 pound very
large box containing 90 pounds of air (about 1215 cubic feet).
This system weighs 100 pounds. There is a pressure differential
inside the box; pressure at the top is less than pressure at
the bottom. This is how the air exerts it's "weight" within the
box. The pressure differential times the cross sectional area
of the box will equal the weight of the air, 90 pounds in this
case. Now open up this system, remove 1 pound of air, and place
a 1 pound model into the box and reseal the box (since plane
is denser than air, the overall pressure will be a bit less).
This new closed system still weighs 100 pounds, 10 pounds of
box, 89 pounds of air, and 1 pound of model. At all times,
gravity is always exerting 100 pounds of force on all the
components on this system. Air pressure differential creates
89 pounds of force, and the model resting on the bottom
of the box adds another pound directly onto the bottom of the box.
The model then begins to fly in a level oval pattern within the
box. As long as the center of gravity of this system is not
accelerating vertically, the weight of the system remains
at 100 pounds. Since the model no longer touches the box,
the pressure differential must have increased enough so that
it now creates 90 pounds of force within the box, since this
is the only means for the air and model to exert their weight
upon the interior of the box. (and no, the system doesn't get
lighter, gravity is always pulling down with 100 pounds of
force on this system, and if the system isn't "falling",
it continues to weigh 100 pounds).
Since the model's flying increased the pressure differential
within the box, the models wings must be acting as air pumps
in order to create and maintain this pressure differential.
And in case you were curious, a plane with a flock of
birds in it weighs the same even when the birds are flying,
as long as there is no net overall vertical acceleration.
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