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Re: Physics of Flight



At 12:48 PM 8/17/99 -0400, Michael Edmiston wrote:

I am just looking for an overall picture of why the air is deflected
downward, and why the overall airfoil shape is important.

Cool. Let's concentrate on that.

I have never
accepted the picture that the airfoil shape provides greater path
length above than below, hence forcing the air to go faster over the
top. Apparently my refusal to accept that picture was correct.

Yes, good for you.

I have
always assumed that the goal was to direct air downward, and that the
airfoil design allows us to do a better job of that.

Yes. But figuring out which bit of the airfoil contributes to which bit of
lift is quite tricky.

The goal for me
has always been to explain why the proper airfoil design does that.


Concerning boundary layers:
Sometimes we might be saying different things because John is looking
at the boundary layer

Actually, I try to avoid looking at the boundary layer. To understand the
basic lift-producing process, you can ignore the boundary layer. You can
treat it as a thin layer of "lubricant" that makes an insignificant change
in the shape of the airfoil, while allowing the air to slide past without
sticking (i.e. without the local velocity going to zero).

I assume it is
very important in terms of analysis and comparisons of wings to try to
understand as much as possible about boundary layers. And since
changes in the boundary layer affect the motion of the air further out,
we need to be quite concerned about what is happening in that boundary
layer. But the typical streamline photos/drawing we see are pictures
of gross air movement, and, as John says, we cannot see what is
happening in the boundary layer in those pictures.

But I need the picture of the gross air movement because that is the
picture that shows sufficient mass of air directed downward to give us
the needed lift. There isn't enough air mass in the boundary layer to
give us the needed lift. So the boundary layer is important in the
sense what happens there eventually determines what happens further
out. But the air velocities and directions further out are what
eventually have to be correct to give the needed lift.

Thus, when I talk about visible turbulence above and behind the wing in
wind tunnel photos of stalled wings, and when John says I am talking
about situations well beyond the "critical angle," I don't see any
disagreement. Yes, I am talking about a grossly stalled wing. I'm way
past a little separation of the boundary layer to the point that
streamlines above and behind the wing are all screwed up from the nice
laminar-appearing streamlines we saw when the wing was working the way
it is supposed to work.

More on stalls.
The reason I use a gross picture of a stall is because I assume the
gross picture is eventually what happened when a plane crashed because
of a stall. If John is correct that nothing drastic happens at the
critical angle of attack (and I assume he is correct) then I presume
all is not lost when a plane slightly exceeds the critical angle. If
the pilot makes the correct maneuvers, recovery ought to be possible.
But if the pilot makes the wrong maneuvers, then the situation gets
worse and worse and a gross stall can occur, including major separation
with turbulence above and behind the wing (including far out from the
boundary layer) with major loss of lift and major loss of air speed.
At that point I assume the pilot needs both talent and altitude if
s/he is going to recover.

I am not sure my original picture was incorrect, although I am sure I
wasn't always using the correct words. Because of the tendency for a
fluid to follow the surface of an object moving through it, a wing
traveling in horizontal flight with positive angle of attack will
direct the air downward. This is true both for air passing below the
wing and air passing above the wing. Careless design of the wing shape
can limit the angle of attack and air speed for which this remains
true. The limitation occurs because the airflow over the top of the
wing separates from the wing surface, hence the original statement that
the air follows the surface of the wing has failed, hence the air going
over the top of the wing is no longer directed downward, hence we lose
lift.

On the other hand, careful choice of wing shape can allow greater angle
of attack and greater air speed before separation occurs.

I readily admit that I do not understand all the intricacies of shape
and surface design, including vortex inducers, spoilers, or other
techniques that might be used to help prevent separation. I do not
understand how some turbulence in the boundary layer actually helps
prevent separation, but I can accept the statements from those who have
studied this that some localized turbulence can do that.