On Sat, 08 May 2004 18:44:30 -0700, 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

I looked for some wind tunnel data in my reference books, but I couldn't
find anything conclusive. Hoerner's Fluid Dynamic Drag as a bit of stuff
on page 3-17, but it doesn't really properly answer your question.

You need to understand that the air doesn't come straight at the blunt
leading edge, and then suddenly have to make a 90 deg turn. There is a
high pressure area ahead of the leading edge, and it acts to start
deflecting the air well before it reaches the surface. So the air makes a
smooth transition around the object. You can see this effect in action if
you are ever driving at night in heavy snow, with low density fluffy snow
flakes. The snow flakes provide a good visual clue as to what the air is
doing as the car approaches. You can observe the snow flakes rising quite
a few feet ahead of the windshield, so they make a smooth transition over
it.

Also, we can't ignore the problems of airflow separation. That vertical
stabilizer needs to work properly in a sideslip too. So we need that
rounded leading edge. Pretty much every part on an aircraft will see
quite a variety of airflow angles if we consider all the different flight
conditions. So whatever shape is used needs to be quite tolerant of
variations in airflow angle.

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Ottawa, Canada
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