On Fri, 31 Dec 2004 at 21:23:58 in message
, Peter Duniho
wrote:
I understand my error regarding chord versus relative wind. Still, I'm
boggled by the lack of induced drag in a wind tunnel. If the wing's not
creating lift (0 AOA), I can see how there wouldn't be induced drag. But
this would happen in the real world too. If the wing is creating lift,
shouldn't there be a measurable force parallel to the relative wind? Even
in a wind tunnel?

You can measure lift in a wind tunnel. Why not induced drag?

Peter,

After posting another long article with a correction I am a bit
reluctant to step in again here. However the explanation is fairly
simple.

Induced drag and wing tip vortices are almost one and the same thing.
Very roughly induced drag is proportional to

(Lift coefficient)^2/(aspect ratio)

The higher the aspect ratio then the smaller is the induced drag until
when the Aspect Ratio is infinite it is zero. Airfoil sections always
used to be tested in wing tunnels by taking them right across the tunnel
so that there are no tip effects. A correction has to be made for the
tunnel wall but the effect is to test a two dimensional section. Of
course there is still the parasitic drag component to be measured and
that is the Cd that is quoted for the particular wing section.

You can try to measure induced drag but you need a large tunnel and a 3d
model of the aircraft sufficiently far from the walls so that the flow
is not too distorted.
--
David CL Francis