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Aviation History
1935
1935 - 0610.PDF
298 FLIGHT. MARCH 21, x935. whether a height above 8,000-10,000ft. will ever be commonly used unless air tight cabins are introduced. So much for the aspects of high- altitude flying which especially affect the occupants. When we turn to the technical side, some advantages and a good many difficulties are found. It is a fundamental law of aero dynamics that the drag of an aeroplane varies as the square of the speed and inversely as the air density. On the face of it, therefore, one would say that, by going up to great heights, the drag of the aeroplane would be reduced, so that for the same power one could travel very much faster. (Above) In this Farman monoplane, specially built for high-altitude attempts! the pilot flew for a time with his head outside the cockpit ; then, when sufficient height was gained to make risk of collision with other aircraft improbable, he lowered himself in the cockpit and shut an airtight door above his head. (Left) The Vickers " Vespa " biplane, with special supercharged Bristol "Pegasus" engine, on which Mr. C. F. Uwins gained the altitude record for Britain in 1932 with a height of 44,000 ft. The record is now held by a " Pegasus "-engined Caproni biplane with a figure of 47,353 ft. ft!*; Unfortunately, there are many factors which make the attainment of these higher speeds difficult. The first one is related to the engine. A petrol engine, as everybody knows, draws into its cylinders a certain mixture of air and petrol, the oxygen in the air enabling combustion to take place. Owing to the decrease in air density at heights, the weight of air drawn in on each induction stroke of the piston is less than the weight drawn in at ground level, although the volume is the same. To obtain a proper gas mixture it is, therefore, necessary to reduce the quantity of petrol, otherwise the mixture will be too rich. The cutting down of the amount of petrol admitted results at once in a fall in engine power, this fall being approximately proportional to the altitude. At 20,000ft., for example, where the density is roughly one-half the ground level density, the engine power will also be reduced to approximately one-half, and one's hopes of attaining the extra speed are doomed to failure unless means can be found for supplying extra oxygen to the engine. Such means have been found in the supercharger, which is, basically, merely a centrifugal blower, driven by the engine and causing a raised pressure in the induction pipes of the engine. That, one would say, has solved the problem. But, unfortunately, it has brought with it some other difficulties. If the engine was originally designed for giving its full power at ground level when naturally aspirated, it will receive too large a charge of mixture when the supercharger is brought into action at a low height, and the engine may not be able to stand the extra strain. During the earlier stages of the climb, it may be necessary to put the supercharger out of action until a certain height has been reached. Then, when the super charger is brought into use it will enable the engine to maintain its ground level power up to a height which depends upon the degree of supercharging. This height may be 5,000ft. or 15,000ft., according to the design. Whatever the altitude is, the effect is to make that height the equivalent of ground level as far as the engine is concerned, the power beginning to fall off from that altitude onwards, just as it previously did in the unsupercharged engine from ground level onwards. It will be obvious that power is absorbed in driving the supercharger, so that here is an obvious limit to the gam that can be expected. For reaching heights much above 40,000ft. it is no longer practicable to get the necessary boost with one supercharger, and two or more in senes must be used. This again means a considerable dram oil the engine's power, and steps have to be taken to cool the air during its passage from one supercharger to the When heights such as those contemplated are involved, the airscrew problems make themselves felt. An ordinary airscrew with fixed blades has its maximum efficiency^ some particular value of forward speed, rotational spe and diameter. If the airscrew is designed to give g«w efficiency near the ground, at the speeds attained ere, its efficiency will be poor at a great height and at a m^ increased forward speed of the aircraft. On th*L hand, if the airscrew is designed for the altitude COT an ^ its efficiency at low altitudes will be poor, and the will suffer badly. $ Two remedies are obviously possible. Either ^ introduce a gear box, or one can design the air ^ that the pitch angle of its blades can be varied. ^^ alternative is the one which is coming int°ttebfin» use, a number of controllable-pitch airscrew UBC, <X llUlliUCl UX <_<_I1IC11J11U.LI1G-JJ'JI>-1J ,, available. Just as the supercharger restorfallabie.>:i> of the engine at great heights, so the controiia airscrew restores the airscrew efficiency. follows From the fundamental laws of aerodynamics ^ lat. "other thines being equal" as the matn ]v that uiaL, other things being equm »*> —- inverse have it, the speed of an aeroplane sn0uldr]!ln^efs to say,at That is as the square root of the air density, inar • ^^ 20,OOOft. the speed should be increased by aD ^ cent, and at 40,000ft. it should be nearly don only hied. beca"se practice this is not, of course, possible, ""'"'['jisof*' of the power absorbed in driving the engine ^ number of other reasons. A very substan
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