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Aviation History
1944
1944 - 1488.PDF
7* FLIGHT JULY 2OTH, 1944 Rot ol Cabin Supercharger Qround Level Conditions Main tained at All Heights : Full Air-conditioning for Cabins FOR a considerable time Rotol, Ltd. (of airscrew fame) have interested themselves in auxuliary units, the latest of which is a pressure cabin supercharger. This has been developed to a stage of efficiency which main tains the interior of the aircraft cabin at an air pressure corresponding to the atmosphere at 10,000 feet at any height up to 40,000ft. In addition, the air is warmed so that the cabin interior is maintained at a comfortable tem perature all the way up to the aircraft's ceiling. As a result, passengers and crew need neither oxygen masks nor special clothing. In fact, during some experi mental flights in the stratosphere a little too much heat was provided; the crew removed their coats and sunned themselves through the roof, high above the clouds. The superchargers are attached to, and driven by, the inboard main engines. For safety's bake, two superchargers are provided, each capable of delivering the requisite amount of air. This quantity is determined by the breath ing requirements of the passengers and crew plus that required to produce comfortable ventilation. Control of Airflow Unless the aircraft cabin has been specially designed to be airtight, the various leaks may pass more air than is required for ventilation. In such cases extra large units are required or a lower pressure must suffice. In a completely airtight cabin the flow of air is con trolled by a valve, which permits it to escape at a pre determined rate. For civil aircraft this valve is controlled automatically by an aneroid device. The valve is pro- gressivly closed as the altitude increases. The supercharger is of the displacement type, and has two spindles running inside an aluminium casing and carry ing two rotors which interact upon each other without actually touching each other or the casing. No sliding or rubbing parts can be tolerated in the pumping chamber, because no oil must be permitted in the latter in any cir cumstances. Unless the blower chamber is absolutely free of oil, the air will be contaminated and will make conditions unpleasant in the cabin. This requirement introduces a mechanical problem because the supercharger is attached to and driven by an aircraft engine full of hot oil. More over, the driving gears of the supercharger, although out- Sectional drawing o' the supercharger unit showing the rotors An external view of the complete supercharger unit. side the pumping chamber, are running at high speed and need to be drenched in oil. Early designs included conventional types of oil seal between the driving gears and the blowing chamber. This was satisfactory until the seals began to wear or the crank- case back pressure rose for some reason. Finally, the problem was ingeniously solved by separating the driving gear oil seal from the pumping chamber by an annular -1 space drained and vented to the outside atmosphere. The pumping charger spindle is sealed by a non-rubbing steam- turbine type of labyrinth. The latter is fed with compressed air bled from the supercharger delivery. At the sacrifice of some air, this system ensures that at all times there is '' one way traffic '' of air flowing from the pumping chamber out through the drain. Any oil escaping from the driving mechanism is thus discharged harmlessly over board. This system is so effective that the units have been , run experimentally for long periods with no sealing device on the driving gear side. This resulted in heavy loss of engine oil through the drains, but none whatever gained ingress to the air pumping chamber. Owing to the compression of the air (which increases with altitude), the delivery temperature steadily increases, until at maximum altitude it may exceed the temperature of boiling water. This increase conveniently co incides with the exterior drop in air temperature. However, to discharge the heated air directly into the cabin might cause uneven heating and result in discomfort. It is usual, therefore, to admit the air into the cabin between surfaces of the double-skinned win dows and pilot's windscreen. This system also has the merit of keeping - the windows free from cloudiness or ^ frost. As already mentioned, the air in the cabin after doing its work is discharged
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