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Aviation History
1941
1941 - 1275.PDF
FLIGHT, Jun- ' Above is the Boeing 307 Stratoliner "over the weather," while the otherview is the cylindrical pressure cabin under construction. of moisture from the body. And stomach gases expand, their volume . being quadrupled at 34,000ft. This causes hearty eructation and also other effects even more unpleasant though sometimes less audible. Another effect, dangerous if allowed to become too great, is the increase of pressure of the spinal fluid on the brain. For all these reasons, man must be '' super- charged " if he is to function at high altitude. Temperature is very low at strato- spheric altitudes, and it is, in fact, by temperature considerations that the stratosphere is defined. Above 36,000ft. the temperature does not de- crease but remains constant at —56.5 deg. C, more than " half-way to boil- ing point'' below freezing point! Though very low, this temperature can > be dealt with by special heated clothing or by heating the air of the cabin from the waste heat of the engine. Supercharging the Engine- Supercharging is usiversal practice to-day but to super- charge for heights of up to 40,000ft. has many difficulties. The great pressure difference is too large for one super- t charger to handle, and two arranged in series—in other'^>rds, a two-stage supercharger—must be used. When air -fljp compressed its temperature rises because of the work aone on it, and this calls for an intercooler between the two stages. A two-speed drive to the supercharger is also common practice so that the supercharger can be driven at a lower speed at the lower altitudes to prevent waste of power where its full blower pressure cannot be utilised. So two-speed, two-stage superchargers with intercoolers can become a very complicated part of the modern high-altitude aero engine. Another possibility is the exhaust-driven turbo-super- charger. As altifude increases, the pressure difference be- tween the exhaust gases and the atmosphere becomes greater, so the power available to drive the supercharger increases. This is just the effect that has been sought in the two-speed drive. Unfortunately, the exhaust gases are at such high temperature that there are very few metals which will stand up to them, coupled with their terrific eroding velocity and the tremendous revolutions per minute of the turbine. Turbo-superchargers are not general prac- tice yet, but several aeroplanes have been built with them, . and the experimental Lockheed XC-35*was so fitted. Very C-like an Electra, it had two 550 h.p. Wasps. The arrangement of a typical turbo-supercharger is shown in the drawing. It will be seen how the exhaust gas (A) is made to drive a turbine (I) which has two centrifugal For high-altitude research and test the Boeing Company hasthis elaborately equipped chamber. pumps on the same shaft. The largef is the engine blower and the smaller (W) provides the pressure for the passenger cabin. The "blast gate" (E) has the same function as the throttle; if opened it by-passes the exhaust to atmo- sphere and so slows the turbine and lessens the boost pres- sure. Normally it is controlled by the aneroid capsule (G) but this can be over-ridden by the pilot from the cockpit (X). (The other letters in the drawing indicate: B, in- take ; C, exhaust piping ; D, carburettor ; F, intercooler ; H, nozzles ; j, fuel tank ; K, nozzle box ; L, air collector ring ; M, air compressor; N, fuel-pressure regulator; O, hand pump; P, engine-driven fuel pump; Q, flexible drive; R, fuel strainer ; S, fuel selector cock ; T, fuel line to auxiliary tank ; U, fuel-pressure gauge; V, oil-pressure gauge ; Y, oil-pressure connection on engine; Z, intake air-pressure gauge). The turbo-supercharger is coming more and more into favour as engineers grapple with its problems, and will be fitted to an increasing number of engines in the future.
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