George wrote the following e-mail to me:
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"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."
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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.


Regards

Dave Driscoll wrote in message ...
The answer to this question varies depending upon how fast you want to
go. All bets are off once you start looking a compressible flow and its
effects (roughly sonic velocity). For the speeds that most homebuilts
experience, the rounded shape promotes laminar flow around most of the
shape and hence lower drag for a wide variety of angles of attack. The
problem with the "eye shape" is that the point will start developing
turbulent flow at the point at very low angles of attack. This
turbulent flow will cause a lot of drag. Even gear fairings have angle
of attack issues during some flight regimes, steep climb, slips, etc.
As far as a vertical stab, your control surfaces are the last thing you
want to have behind a turbulent flow transition as they need clean air
to be effective.

Play around with dragging some different shapes at various angles of
attack through a tub of water and you'll see the effect very clearly.

Dave

Jay wrote:

I've been wondering which shape is lower drag, a rounded one or a
sharp one. I can understand why the leading edge of a wing would be
rounded, this allows a larger range of AOA before flow speration and
stall, but what about the vertical stabilizer? Wouldn't this be
better with a sharp leading edge?

When the air hits a blunt leading edge it has to accelerate quickly to
get out of the way. At the very front, the molecules are actually
moving in a direction normal to the direction of travel of the wing.
This has got to cause drag. If the edge were sharp and the air didn't
have to react as quickly to displace, this would seem to cause less
drag.

I know that the tear drop shape is the lowest energy state, but it may
not be the lowest drag shape. I'm thinking that an eye shape may be
better for many profiles.

So I see tear drop shape profiles all over on struts, whell pants,
vertical stabilizer, etc. Looking for explanations other than "Thats
what everybody does, so they couldn't be wrong."

Regards