20 SA Flyer MagazineAerodynamicsts seldom
earn long obituaries in the
New York Times, but Richard
Whitcomb, who died on
October 13, 2009 at the age
of 88, did.H
E had left a
conspicuous
imprint on the
design of modern
aeroplanes. He
was responsible
for the winglet,
the supercritical
aerofoil – which he designed not on a
computer, as would be done today, but by
repeatedly re-shaping a wind tunnel model
with auto body filler – and the transonic
area rule.
The story of the transonic area rule
involves one of those ‘eureka moments’ that
we have learned to expect in great tales
of invention and discovery. It happened
at the Langley, Virginia headquarters
of the National Advisory Committee for
Aeronautics, which would become NASA a
few years later.
Whitcomb had been ruminating about
the rapid increase in drag aeroplanes
experienced as they approached the
speed of sound. He possessed uncommon
insight into the behaviour of air, a gift that
must have been in part a happy congenital
mixture of spatial visualisation andkinaesthetic empathy, and in part acquired,
as he, like Leonardo da Vinci, for some
reason or other found rivers and streams,
blowing leaves, smoke and clouds worthy
of special attention. But a good instinct for
subsonic aerodynamics does not serve
at supersonic speeds. The supersonic
domain, in the memorable and often useful
phrase a British novelist used about The
Past, “is a foreign country; they do things
differently there.”
The legacy of subsonic thinking
was that each part of an aeroplane is
responsible for its own drag; you could
figure out the contributions of wings, tail,
fuselage, antennas, and so on separately
and then add them up, throwing in a few
percent extra for ‘interference effects’. In
other words, the details mattered because,
after all, each little ‘packet’ of air followed
its own path over the surface of the
aeroplane and didn’t know what was going
on far away. This was such an intuitively
obvious proposition that it evidently went
unquestioned, at least in the United
States, until that day when the tunnelling
myrmidons of Whitcombian thought
fortuitously met in mid-mountain.
I say “in the United States” because it
appears, despite widely accepted accounts
to the contrary, that the area rule was not
really born in 1951 in the brain of Richard
Whitcomb, but in Germany during the
mid-1940s. Like so many of the insights of
German aeronautical engineers working
for Hitler – swept and delta wings, for
example – this one didn’t bear much fruitbefore the war ended. Its originator, an
Austrian-born wind-tunnel designer named
Otto Frenzl, obtained a 1944 patent on the
Flächenregel or ‘bottle rule’, and the theory
was presented to a meeting of the German
Academy of Flight Research in March of
that year. It is difficult to imagine that Adolf
Busemann, a German aerodynamicist who
came to NASA Langley after the war and
who influenced Whitcomb, was not aware
of it, and puzzling that it is Whitcomb, and
not Busemann or Frenzl, with whom we
associate the area rule and the ‘Coke-bottle
fuselage’ today.
A familiar staple of transonic theory was
the Sears-Haack body, a sort of elongated
football characterised by a certain smooth
variation of cross-sectional area from one
end to the other. This was the shape that
generated the least shock-wave resistance,
and it was well understood that the reason
was the uniformly varying distribution of its
volume. What Whitcomb grasped was that
the energy lost to shock-wave generation
by a supersonic body – the so-called wave
drag, which was responsible for the rapid
increase in resistance as an aeroplane
approached the ‘sound barrier’ – was a
function not of the shapes of individual
components but of the volume distribution
of the whole.
He conducted a series of wind tunnel
tests to confirm this idea, and then
presented a modest paper in which he
demonstrated that the wave drag of a body
with a wing was the same as that of a round
wingless body with a midriff bulge whose
volume distribution was identical to that of
the original wing and body combined. Only
in the final pages of his paper did Whitcomb
toss off his punch line: By locally reducing
the diameter of the fuselage, the effects
of excrescences like wings, canopies and
engine nacelles could be nullified and an
aeroplane could be made to behave, so far
as shock wave formation was concerned,
like an ideal Sears-Haack body.
The potential for drag reduction wasleading edge
Peter Garrison
RULES TO FLY BY
LEFT - Application of the area rule resulted in
the F-102A having a pinched fuselage, enabling
it to exceed Mach 1.