cided that a hang glider, even though 
relatively inefficient when compared 
with a sailplane, might conceivably 
provide a suitable model for the con- 
struction of a human-powered aircraft. 
On July 14 the MacCreadys visited 
Kitty Hawk, North Carolina, the site 
of the Wright brothers’ first flights. 
The next day, en route to Williams- 
burg, Virginia, Paul MacCready 
found himself meditating first on bird 
flight, then on hang gliders, and won- 
dering whether there was “some little 
hunk of aviation that hadn’t been 
touched yet, something like a super 
hang glider.” Although not as effi- 
ciently shaped for flying as a well- 
designed sailplane, birds are phenom- 
enal flying machines because of the 
finely tuned control they exercise in- 
stinctively. The hang glider cannot of- 
fer any such advantage over the sail- 
plane, but its method of construction 
differs radically. This was the feature 
MacCready thought to exploit. 
A sailplane makes optimal use of 
rigid wings, at speeds that require a 
streamlined design. The framework of 
a hang glider, in contrast, is anything 
but streamlined, for it is strung with 
external wires that provide support 
through tension. Moreover, the sur- 
face of its wing, or sail, is shaped 
partly by the force of air, making it 
less efficient than a wing with a rigidly 
supported contour. Despite their draw- 
backs, however, these design elements 
of the hang glider permit the use of 
light construction materials, an advan- 
tage that becomes increasingly impor- 
tant the larger a structure is. And 
it was with a large, light structure 
that MacCready believed human-pow- 
ered flight might be practical. 
Ultimately, an airplane is supported 
by the upward pressure of the air on 
the underside of its wing. Allowing 
a little simplification, if two airplanes 
weigh the same amount and have 
wings of quite different sizes, the one 
with a much bigger wing is likely to 
be a “floater,” and the one with a 
very small wing will probably be a 
“sinker.” More precisely, an airplane 
falls slower if each unit of wing area 
has less weight to support and faster 
if it has more. If the sinking speed 
of an airplane is low, it can be kept 
aloft by an engine that supplies power 
at a relatively low rate. In addition, 
the lighter the airplane, the lower the 
power required. In an ideal situation, 
where all the power supplied is used 
to generate lift, the power necessary 
to fly is simply equal to the weight 
of the aircraft multiplied by its sinking 
speed. 
MacCready calculated that a rea- 
sonably efficient hang glider carrying 
an average pilot weighs about 200 
pounds and has a sinking speed of 
approximately 4 feet per second when 
flying in still air. Under these con- 
ditions, the minimum power needed 
to keep the glider aloft would have 
to be supplied at the rate of 4 X 
200 = 800 foot-pounds per second, 
or 1.45 horsepower (one horsepower 
equals 550 foot-pounds per second, or 
746 watts). MacCready reasoned that 
if you could triple all of a hang glider’s 
dimensions and keep its weight about 
the same by using very light structural 
materials and external wire bracing, 
then the sinking speed would be de- 
creased to one-third of its former 
value. That meant that such a giant 
hang glider would fly on only a third 
of the power used by a standard hang 
glider: 1.45/3, or 0.48 horsepower. 
Supplying this amount of power is 
within the range of human capability. 
(An “average fit person” can supply 
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