21 SA Flyer Magazinecolumns
enormous. In a stroke of career luck for
Whitcomb, it happened that just as he was
publishing his idea, Convair was putting the
finishing touches to a delta-wing fighter, the
F-102, that was supposed to go supersonic
in level flight. Because of the unfavourable
volume distribution of its barrel-shaped
body and delta wing, however, it didn’t.
Convair, after running out of excuses for
keeping the F-102 as it was, invested in a
new fuselage with a pinched waist – the
so-called ‘Coke bottle’ shape. The F-102A
exceeded Mach 1 while climbing.
Area ruling was very important during
the ‘50s, because engines were just
barely able to push aeroplanes past the
transonic drag rise. As engines became
more powerful (and supersonic speeds
proved to be, in any case, of little military
importance), the conspicuous area ruling
visible in Vietnam-era aeroplanes ceased. It
continues to be found, however, in features
intended to smooth the volume distribution
of airliners and business jets. If material
cannot be taken away in the vicinity of,
say, a wing, bulges can be added ahead
and behind; it’s not the total volume that
matters so much as the smoothness of
its distribution from nose to tail. Long
wing-fuselage fairings, the 747’s hump,
fat flap-track enclosures – these are all
manifestations of area ruling, which today is
part of every jet designer’s toolkit.
The late John Thorp described his
decidedly subsonic T-18 homebuilt as
embodying a “poor man’s area rule.”
This was not an official rule formulated in
mathematical terms, like the Whitcomb
area rule, but more of a rough guide for
designers. The idea is that you should
stagger the parts of an aeroplane so
that, for example, the canopy reaches its
maximum height before the wing leading
edge; the fuselage reaches its maximum
width around the trailing edge of the wing;
and so on. When items are alongside one
another, like a pod on a pylon below a wing,
you shape them so that they do not form a
narrowing channel.
The reasoning is that airflow speeds
up as it makes its way around a body.
The extra resistance, called interference
drag, of two bodies close to or touching
one another, is due to two causes. One is
that flow that is pushed aside by a bulging
body accelerates, increasing its frictional
drag on nearby surfaces. For example, the
front upper surface of a low wing, where
flow accelerates considerably, causes
added ‘scrubbing drag’ on the adjacent
fuselage side. The other is that when the
pinched, accelerated flow slows down
again, turbulence and separation can
occur if the deceleration is not sufficiently
gradual. Thus, wing root fairings are usually
needed where a fuselage and a wing taper
away from one another, as is often the
case on low-wing aeroplanes with rounded
fuselages. (That is why the Bonanza has
a square bottom.) By the same token,
the intersection of a T or cruciform tail
often requires a ‘bullet’ fairing to prevent
separation on the elevator or rudder. The
fairing would not be needed if the surfaces
were staggered, so that the thickest part of
one coincided with the leading or trailing
edge of the other; but that arrangement
is seldom seen, because it is structurally
inconvenient.
On some aeroplanes we see both the
transonic and the poor man’s area rules
at work. The Mach .92 Citation X is an
example. The long, gradual wing-body
fairing alleviates the sudden cross-section
increase of the wing. The deep pinching
of the fuselage beside the engine nacelles
both maintains overall transonic area ruling
and also provides a constant-width channel
for air to pass through without accelerating.
The big fairing between the fin and
horizontal tail effectively separates them
from one another’s influence.
The Whitcomb and the poor man’s
area rules approach aeroplanes quite
differently. One ignores detail and subjects
all the parts of an aeroplane together to the
single criterion of volume distribution. The
other catalogues components separately
and examines how flow conditions created
by each one affect all the others. The
Whitcomb Rule applies only to the wave
drag that appears when flow over parts of
an aeroplane reaches sonic speed, typically
at Mach .8 or above. The poor man’s rule,
on the other hand, applies to all aeroplanes,
because all, at some time or other, need to
operate efficiently at subsonic speed.jCOMMUNICATE
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