Backwash Causes Lift?

You wrote

On Oct 2, 9:57 pm, Le Chaud Lapin wrote:
Hi,

Student pilot here, self-teaching using the Jeppensen Private Pilot
Kit after taking ground school.

I read in the book that combustion "creates" energy, which is
technically not true, but I decided to ignore it since the pictures
are sooo pretty.

Now, in Chapter 3, section about airfoils, it actually says:

"In addition to the lowered pressure, a downward-backward flow of air
also is generated from the top surface of the wing. The reaction to
this downwash results in an upward force on the wing which demnstrates
Newtons' third law of motion. This action/reaction principle also is
apparent as the airstream strikes the lwoer surface of the wing when
inclinded at a small angle (the angle of attack) to its direction of
motion. The air is forced downward and therefore causes an upward
reaction resulting in positive lift."

IMHO, the latter part of this paragraph is correct, but the former
part is wrong.

Obviously, any air above the wing can only result in a force downward
on top of the wing. The only force causing the plane to want to move
upward comes from beneath the wing. The effect of any air above the
wing is to cause rarefication above the wing, resulting in lower
pressure, thereby giving the 14.7lbs/in^2 (plus) to do its work. That
"reaction" coming from downward movement of air seems just plain silly
to me.


You would do well to think in terms of differential pressure. If one
can by some means cause the pressure on the upper surface of the wing
to be 1 psi less than the pressure on the lower surface, there would
be an upward force on the wing of the order of 144 pounds per square
foot (my physics training, a million years ago, was in english units).

You might also want to think carefully about airfoil shapes, since
wings can provide lift when flying inverted. Any theory that does not
support inverted flight is obviously flawed.

But the neat thing to do is to hold your hand out of a moving car's
window, and feel the impact pressure on its surfaces as you tilt it in
the airstream. It's not that the hand is being "sucked" up, you don't
feel suction on the top surface, you feel push on the bottom one. Any
theory you develop had better be consistant with those observations.
Someone with more time than I have might like to start with the fact
that air weighs about .08 pounds per square foot near sea level, and
crack some numbers to show how that deflecting that mass can result in
lift even if the lifting surface has some funny shapes.