Lower Drag: Rounded or Sharp?

On Sun, 09 May 2004 12:51:48 -0700, Jay wrote:

George wrote the following e-mail to me:
----------------------------------------------------- "The most
instructive seminar I've attended was one by John Ronce at Oshkosh a few
years ago. He began by drawing a three foot long airfoil
shape, then drew a one-quarter inch dot (the profile of a cable), he said
that each had the same drag. Very hard to believe.

He also said that air will create its own nose profile, against a blunt
or flat shape, that is why wings can have a round front for better angle-
of-attack tolerance.

The drag is caused more by turbulent flow trailing the part.

I left the talk dumbfounded, and still am."
---------------------------------------------------------------------------

So the vibe I'm getting is that in general, the tear drop shape is lower
drag with the eye shape being better for a very narrow range of AOA
depending on the "sharpness" of the leading edge.

I guess you could optimize for one particular AOA, say cruise, at the
expense of all other regimes. So different parts or the plane would have
different shapes depending on whether it was a flight surface, horizontal,
vertical, in prop blast, etc.



Don't forget that you may have a handling problem if you have a large
surface were the airflow becomes separated as you change angle of attack.
I.e. control surfaces may lose their effectiveness if they are in the wake
of the separation, or the control forces may become very strange. For
example, it is possible that a vertical stab with a sharp leading edge
could cause the rudder to go all the way to the stop all by itself if you
get some sideslip going. You might need a lot of force to get it back off
the stop again. That could get pretty interesting.

So, unless you are talking about small surfaces (like antennae), then you
should be pretty cautious about going for sharp leading edges. You can
probably get away with it on an antenna, as the separated area would be
small.