Today, we write a rule for choosing retractable
landing gear. The University of Houston's College
of Engineering presents this series about the
machines that make our civilization run, and the
people whose ingenuity created them.
The first retractable
landing gears showed up on airplanes just after
WW-I. They didn't become a common currency of
flight until the mid-1930s. Walter Vincenti asks
about their evolution:
You're an airplane designer. How do you decide
whether or not to make the landing gear retract?
What questions do you ask?
First you weigh the drag force on wheels and struts
moving through air against the cost and weight of
retraction machinery. Then other issues arise:
maintenance, reliability, wheel storage space,
power requirements. And style! Don't forget style.
Most WW-I planes had fixed wheels on a common axle
held by struts. After the war, we held each wheel
on its own tripod of struts. We also began
experimenting with retractable gear. By 1930 Boeing
and Lockheed had made commercial planes with
retractable wheels. Jack Northrup had also
experimented with retraction, but he loved
streamlining. He cooked up an alternative.
Northrup shaped cowls around vertical struts
holding each wheel. He called those cowls trousers.
The bottoms of the wheels look like dainty little
shoes at the end of bell-bottom trousers. Other
makers followed his lead. Amelia Earhart and Wiley
Post set records with planes whose wheels were
cased in teardrop spats.
Cowls on fixed landing gear worked pretty well. A
cowl might reduce airspeed only four percent. Then
the wing had to be a bit larger to carry the weight
of the cowl. At speeds under 200 mph, a little
clumsiness bought a lot of simplicity and economy.
One big problem with retractable gear was the power
needed to move it. The early ones were
hand-cranked. Next they were driven by heavy
electric motors. Then, in 1937, the O-ring
appeared. With O-rings, simple hydraulic systems
became practical for retracting wheels. The last
airplane to win the Thompson Trophy wearing pants
did it a year later. After that, the rule for
choosing fixed or retractable wheels was set.
Ever since WW-II, the rule has been fixed wheels
for light planes flying less than 200 mph;
retractable gear for fast commercial and combat
planes. Streamlined cowls turned into a useless
middle ground. Northrup was one of the last to
agree. Did he hold out too long? Vincenti's study
suggests something else.
The people who pushed cowled wheels gave us a point
of reference. Northrup's love of streamlining drove
him to take fixed wheels as far as they could go,
and that drove learning. Hanging on to those wild
evocative cowls was a basic step toward leaving
them behind. That is a fact of technological
change.
We really had to fully plumb the exception to
retractable wheels -- the Art Deco wheel cowl -- to
finally prove the rule.
I'm John Lienhard, at the University of Houston,
where we're interested in the way inventive minds
work.
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