Re: Whitcomb area rule (was: Drafting in bicycle races)

[John Denker <jsd@MONMOUTH.COM>]

At 12:42 PM 9/15/99 -0400, Hugh Haskell wrote, regarding Drafting in
bicycle races:
> I don't have the answer to these questions, but I am always happy to throw
> some more noise into the situation.

Don't we have enough noise already?  :-)

> Remember the "coke bottle" shapes on
> somemilitary supersonic jet aircraft?

Yes.  R. T. Whitcomb, "The transonic and supersonic area rule" January 1958.

> I believe that the Air Force B-58 and
> F-104 had that shape as did the Navy F11F.

B-58 hustler, definitely.  I don't know about the others.  The famous case
was the F-106, which is basically an F-102 retrofitted to comply with the
area rule.

> I believe that it was done to
> reduce skin friction at supersonic speeds,

No, it wasn't skin friction.  And in the 1960s, the problem was not with
the supersonic regime -- it was the transonic regime.  The F-102 didn't
have enough power to accelerate through Mach 1.  Above Mach 1 the drag is
lower, but it doesn't matter if you can't get there.  Area-ruling fixed the
problem.

> but I know nothing abut what the principle was.

The full story is outside the charter of this list.  But crudely speaking,
imagine a cylinder moving at transonic speeds.  There will be a shock wave
coming off the front, as the air gets shoved out of the way.  There will be
another shock wave at the tail, as the air suddenly tries to "fill in".
Putting in any unnecessary changes in cross-sectional area will cause
unnecessary additional shocks, which means unnecessary additional drag.
The wasp waist occurs right where the wings are, so that the
cross-sectional area of the wings+fuselage is a constant.

> They don't do it any more and I don't know what the
> trade-off was that made them give it up.

The transonic area rule is less significant these days because engines are
so much more powerful that you can just blast your way through to the
lower-drag supersonic regime.

They still pay attention to the supersonic area rule.  At the same crude
level, imagine the same argument, but now rather than transverse
cross-sectional area, you have to look at "sections" tilted at the Mach
angle.  This is harder to optimize since the Mach angle changes a lot.  And
since they use delta-shaped wings these days, whatever area-ruling they are
doing is less conspicuous than it would be for small-chord mid-fuselage wings.

In any case, I assure you the area rule has nothing whatsoever to do with
drafting in bicycle races.

______________________________________________________________
John S. Denker                                jsd@monmouth.com