16 SA FlyerPeter Garrison - Defence of Bruguet
Some time ago, I regularly
received letters from a fellow
whom I shall call Tom, first name
Doubting. I used to answer
him, but after he accused me
of lying about my homebuilt’s
performance, and the entire
Voyager air and ground crew of
lying about the 1989 non-stop
un-refuelled flight around the
world, I became irritated and
stopped replying. The incoming
correspondence eventually
ceased. I suspect that DT, who
struck me as an old codger, has
gone to his reward.D
T insisted that the
Voyager flight was a
hoax. Now, there are
people who believe
that the Apollo trips to
the moon actually took
place on a Hollywood
sound stage, andothers who think that no Jew was ever harmed
by a Nazi. Scepticism is good, and on the
whole there ought to be a lot more of it. But I
think massive hoaxes involving large numbers
of conspirators are a priori unlikely, because
people’s desire to deceive is exceeded only by
their desire to blab.
DT was not stupid or ignorant. He had
a good grasp of science, mathematics and
English grammar, and his doubts about
Voyager were based in part on his own
analyses, with help from a multitude of errors
and inconsistencies in published accounts
of the flight. Undoubtedly there were many
such errors, but they are more reasonably
attributable to poor reporting than to a vast
right-and-left-wing conspiracy.
He had no patience at all with a stapleof aeronautical science called the Breguet
Range Equation. Louis Breguet was a French
mathematician and aeroplane designer who
reduced range prediction to a very simple
formula. Only four variables are involved:
three efficiencies and the ratio of the take-off
to the landing weight. The efficiencies are
the aerodynamic efficiency of the airframe,expressed as its Lift-to-Drag (L/D, pronounced
‘elloverdee’) ratio; the propeller efficiency,
which is the ratio of the useful work the
propeller does to the work the engine has to
do to drive it; and the specific fuel consumption
of the engine, which is the amount of fuel that
it uses to do a certain amount of work driving
the propeller. (The equation takes a slightly
different form for jets, since no propeller
efficiency is involved.)
It happens that propeller efficiency and
specific fuel consumption don’t stray far
from .85 and .45 respectively: 85 percent
of the work put into the propeller comes out
as thrust, and the engine burns .45 pounds
of fuel per horsepower per hour. To an
approximation, therefore, since the propeller
part is in the numerator and the engine part
is in the denominator, these two boil down
to a constant of about 1.9. If you replace
your gasoline engine with a diesel whose
specific fuel consumption is 15 percent lower,
while leaving everything else the same, your
range will increase by nearly 18 percent
(1.0/0.85). Incidentally, diesel engines are
not only inherently more efficient because of
their higher compression ratio, but diesel fuel
is also denser than gasoline, and so more
horsepower-hours can fit in existing tanks;
thus, diesels have a double advantage with
respect to range.
The L/D ratio of an aeroplane is a
measure of its aerodynamic cleanness.
Sailplanes with extremely long wingspans
claim L/Ds of 60 or more, but the cleanest
aeroplanes of conventional proportions, jet
airliners for instance, have L/Ds around 18
or 20. For general aviation aeroplanes with
reciprocating engines, the range is from seven
to 15 or so, with gradual improvement in
recent years due to increasing wingspans.
Wingspan is important because it is the
principal influence on induced drag – the
drag that results when a wing produces lift.
At any aeroplane’s most efficient speed, half
of its drag is induced and half is ‘parasite’,
which is to say skin friction, cooling, leakage,LEADING EDGE
M. BREGUET
IN DEFENCE OF
The higher a wing's aspect ratio, the lower its most efficient speed and therefore
the less its total drag. Hence Voyager employed sailplane-type wings.