Model Forum / Radio Controlled / Air Models / January 2010

I visited the NASA visitors center in Cleveland the other day.  Really
an interesting display.  One display that caught my eye was a small
wind tunnel with an airfoil in it.

The observation chamber was plastic so you could see what was going on
inside.  There was an airfoil mounted on a rotatable rod so you could
adjust angle of attack.  You could also adjust air speed.  I would
guess air speed was zero to about 60 mph depending on how you adjusted
it.  Angle of attack was adjustable to any desired angle.

The air foil had tiny strings attached at various points so you could
visualize the flow patterns.  The foil also had four pressure taps on
the top and four more on the bottom on a chord line in the center of
the span.  These pressure taps were each hooked to a simple manometer
so you could see pressures under different conditions.

The results were exactly as anyone who understands why an airfoil
provides lift would expect.  First, under no conditions was there the
slightest down flow of air off the trailing edge of the foil.  Rather,
at angles of attack where the flow was smooth air flow straight back
off the trailing edge.  At angles of attack approaching a stall the
air departed the foil above the trailing edge and went straight back.

Second the manometers showed pressures at different points on the
chord on both the top and bottom.  On top there was a great pressure
reduction at the leading pressure tap when angles of attack were
positive but below stall.  As you went to the back of the chord the
pressure reductions dramatically dropped until by the last tap there
was very little pressure reduction.  On the bottom the front tap
showed a small pressure reduction and as you went back on the chord
the pressure went rapidly to essentually atmospheric.

This display shows clearly that lift is due to a reduction of pressure
on the top of the airfoil relative to the pressure on the bottom of
the foil.  Further it shows that all the nonsense you have read about
lift being due to downflow behind the airfoil is simply wrong.  There
is no downflow under these particular conditions.

The only thing missing in this demo was what a flat airfoil would have
done.  This is too bad as if it had been included the public could
have seen that airfoil shape has very little to do with lift.  At
reasonable angles of attack a flat airfoil would have given exactly
the same air flows and pressure reductions.

Well, there was not a thing new in this demo that anyone who has
studied the science of lift would learn.  But the model hobby has
historically been so burdened with total nonsense about the topic of
lift that is 100% disproved by this demo that I wish AMA would put
such a demo in their display.   There is nothing like an actual
physical display to bring the point home that lift is a result of
Bernoulli caused by the viscous behavior of air in front of the
leading edge of the wing at subsonic speeds.  Those of us who have
worked in the science of lift have known this for the last 100 years.
But it does seem to be a hard concept for non scientists to grasp for
some reason I fail totally to understand.  It really is quite simple.