Air Force the Official Service Journal - Air Force Historical Studies ...

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By Col. Donald L. Putt
Materiel Command
FOR more than 25 vears, aeronautical engineers have experimented
with pressurized cabins as the solution to
problems of anoxia and bends encountered at high
altitudes.
The mechanical problem has been to build an airtight
plane that can fly in the substratosphere where the air is
thin and the pressure is low-an aircraft with a leak-proof
cabin that would maintain low-altitude atmospheric pressure
during high-altitude operations.
Although the first tactical aircraft incorporating pressurization
was the B-29, the AAF had launched its research
program on pressurizing airplanes in 1919.
There were two possible solutions to the problem. One
was to force oxygen into the lungs through pressure masks
worn at high altitudes; the other, to pressurize the cabin air.
In 1919. Lt. Harold R. Harris, a test pilot, took off in an
airplane with a sealed tank-like chamber covering the cockpit-the
first test flight of a pressure cabin. Air from a
wind-driven blower mounted on the plane's wing was compressed
in the cockpit. The test failed when the pressure
control mechanism went havwirc. Instruments recorded a
cabin temperature of I SO degrees and an altitude of 9,000
feet below sea lcvcll
Our cuzinccrs were faced with thc problem of designing
the fuselage structure of airplanes to seal perfectly the joints,
doors, windows and instruments. The project liad to wait
because the proper planes wcrcu' t even on the drafting
boards. Fifteen years had passed before the problem was
tackled again. .
In 19,4, \Vilev Post submitted a pressure snIt-a blown
up pair of overalls made air tight \vith a rubberized .par~chute
silk and an aluminum oxygen helmet-for expenmental
test flights to high altitudes. The suit worked, but
became of it: weight and bulk it greatly impaired movement
of airmen.
26
The pressurized interior of the 8.29 is the result
of research and experimentation which began in 1919.
With development of larger planes, more powerful engines
and improved turbo superchargers, AAF engineers
again turned to the pressurized cabin.
The first successful pressurized airplane was the XC-)), a
modified Lockheed Electra airliner. Engines had exhaustdriven
turbo superchargers that maintained an interior pressure
under 10,000 feet when at an operational altitude of
25,000 feet, later increased to 30,000 feet.
Cabin supercharging was accomplished by an auxiliary
impeller operating on the same shaft as the engine supercharger.
Major disadvantage of this system was that the
speed of the cabin supercharger varied with the main engine
output. \\'hcn the engine slowed down, pressurc within the
cabin dropped off. Among other drawbacks, frost fanned
on the inside and ice on thc outside of windows, and airspeed
meters, rate of climb indicators and other instruments
malfunctioned because of instrument case leakage.
These problems were partially solved in the XC-,5 airplane.
In principle, at least, AAF engineers were on the
right track toward a practicable pressurized cabin system.
Boeing built the ideas into the ..30/" transport, fourengined
Stratolincr which brought "upper-level" flight to the
airlines in 19~9-40.
Supercharging the 307's cabin was accomplished hy a
mechanical compressor geared directly to the engine rather
than to the turbo-supercharger. Constant speed 'propellers
insured constant engine rpm so the geared compressor would
permit the cabin to maintain its pressurc during descents.
Thc cabin, however, was only pressurized to 12,000 feet at
20,000 or about 2V2 pounds per squarc inch.
Although the fuselage of the "307" was much largcr in
diameter than XC-3S's, it also utilized the circular crosssection
introduced in the Army's first substratospherc aircraft
as the best, strongest and lightcst shape for pressurized
cham bers. Theoretically, since air under pressure tends to
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