Lower Drag: Rounded or Sharp?

On 8 May 2004, Jay wrote:

I've been wondering which shape is lower drag, a rounded one or a
sharp one.

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.

George Graham
RX-7 Powered Graham-EZ, N4449E
Homepage http://bfn.org/~ca266

George A. Graham wrote:
On 8 May 2004, Jay wrote:


I've been wondering which shape is lower drag, a rounded one or a
sharp one.


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.

George Graham
RX-7 Powered Graham-EZ, N4449E
Homepage http://bfn.org/~ca266


I'd be more concerned about flow seperation at high angle of attacks
being induced by the sharp leading edge. Typically, stabilizer surfaces
have very low aspect ratios to insure that the airflow remains stable
across the stabilizers even when the main wing is fully stalled. Having
a sharp leading edge may cause problems with early flow seperation. This
might cause stability problems or even possibly some flutter. I don't
have any hard data to prove this, but I don't think you're going to gain
enough drag reduction to justify risking the potential problems.

----- quote:
However, after wind tunnel testing, Wilbur found that he obtained the
lowest "head resistance" or drag, with a shape that was rectangular in
section, but with 1/4-round chamfered corners. This doesn't seem to
make sense considering what we now use, but that's what he found, and
that's what's on all of the later Wright machines.
----------

Welcome to the wierd world of low reynold's numbers aerodynamics.

"L. Darte" wrote in message ...

I'd be more concerned about flow seperation at high angle of attacks
being induced by the sharp leading edge. Typically, stabilizer surfaces
have very low aspect ratios to insure that the airflow remains stable
across the stabilizers even when the main wing is fully stalled. Having
a sharp leading edge may cause problems with early flow seperation. This
might cause stability problems or even possibly some flutter. I don't
have any hard data to prove this, but I don't think you're going to gain
enough drag reduction to justify risking the potential problems.

I think you're right. Probably better leave control surfaces well
enough along, but go after the other "protruberences": Antennas,
struts, landing gear, etc. Places that you don't care if they produce
drag in a side slip but clean up nicely in cruise.

(Jay) wrote:


I think you're right. Probably better leave control surfaces well
enough along, but go after the other "protruberences": Antennas,
struts, landing gear, etc. Places that you don't care if they produce
drag in a side slip but clean up nicely in cruise.

You better think that through carefully, and test. E.g., think of the
pitching moment if your wheel pants suddenly got a lot draggier in a
slip.
--
Alex
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