Frank J. Miller; Photo Researchers 
1975, but the structure he visualized 
for his human-powered airplane was 
based on the x-y-z frame of the classic 
Rogallo hang glider. 
The structure can easily be seen 
in the Gossamer Condor. The main 
airframe joint is still in the same cen- 
tral position, with the king post ex- 
tending above it and a bottom post 
replacing the hang glider’s control bar 
below it. The keel, as before, lies along 
the line of flight, but its forward end 
droops, becoming a bowsprit to sup- 
port a stabilizing control surface. The 
most dramatic change is in the cross- 
bar; from a modest brace, about 16 
feet long in a standard 24-foot Ro- 
gallo, it has grown to the 96-foot main 
spar of the Gossamer Condor ’s wing. 
Although the frame members are not 
truly orthogonal — the wings were 
eventually swept back nine degrees 
to adjust the center of gravity — a 
child’s jack is>still evident at the center 
of the Condor ’ s skeleton. 
The decision to use this structure 
for a human-powered plane was an 
inspiration, but by itself a lightweight 
structure is not a guarantee of success. 
Like any object that must conform 
to the laws of nature, an airplane is 
a set of interlocking compromises, 
some of which have a higher priority 
than others. 
MacCready had determined early 
on that a very large wing was im- 
portant to reduce sinking speed. He 
eventually calculated that for a pilot 
to fly his hypothetical human-powered 
airplane any great distance, the sink- 
ing speed would have to be a quarter 
of the value for a hang glider. Al- 
lowing some realistic increased weight 
for the much larger structure, Mac- 
Cready estimated that the projected 
human-powered aircraft would need 
nearly 1,000 square feet of wing area 
to provide an adequately low sinking 
speed. 
Long, narrow wings are more ef- 
ficient in most flight regimes than 
short, wide ones. The area and shape 
requirements predicted a wingspan of 
nearly 100 feet for MacCready’s air- 
plane. Such long wings would require 
an intricate web of external bracing, 
hundreds of feet of exposed, drag-pro- 
ducing wire — a prospect that would 
make a designer of conventional air- 
planes shudder. In order to fly, a plane 
must sustain a balance between the 
paired forces that act on any flying 
object: its lift must be equal to the 
pull of gravity and its thrust must 
be equal to its drag. There is not much 
thrust to spare in a human-powered 
aircraft. 
In aerodynamics, drag is divided 
into two kinds: parasite drag, which 
is caused by the frictional resistance 
of air to things moving through it, 
and induced drag, which is the energy 
price the wing pays for generating lift. 
Since airplanes must generate lift in 
order to fly, designers usually try to 
reduce parasite drag to a minimum 
by streamlining and eliminating fric- 
tion-producing structures. Their aim 
is an aerodynamically “clean” plane, 
which in this case seemed impossible. 
The only option was to fly very slowly, 
so that parasite drag would be low, 
and to depend on the big wing to 
provide enough lift. 
MacCready probably did not realize 
at the time how much his own back- 
ground had conditioned him to accept 
a wire-braced airframe. He attributed 
it to hang glider design, but the de- 
signers of other Kremer Prize con- 
tenders were also aware of hang glid- 
ers. Almost all of them, in different 
parts of the world, chose to build 
68 
