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Aviation History
1919
1919 - 1191.PDF
SEPTEMBER 4, 1919 must not be allowed to distort to any extent, and in order to maintain a good joint between the nozzle box and the manifold on either side the manifold must be made quite rigid without being heavy. The cross stays have proved a most satis factory expedient. Limitations and Future Possibilities of Supercharging Of course, one of the first considerations in connection with the adding of any equipment or accessory to an aeroplane is whether or not the increased complication and additional weight which it entails are justified in the advantage gained. The turbo-supercharging system without question adds greatly to the complication of the power-plant of the plane, increases the number of things that can get out of order, and contributes some 200 to 300 lb. to the total weight of the ship. Unless automatically controlled, it also means one more unit for the pilot to handle, when he already has about as many things to control as one human being can possibly take care of. Further, due to the very high operating temperatures of the devices that have so far been designed (the temperatures range anywhere from 1,250 to 1,400 degs. Fahr.), there is an added fire risk where a supercharger is used, which is a factor of no small importance. To minimise the danger from this source, it is impracticable to have the combustible parts of the fuselage—wood and fabric parts—very near the super charger. This would ordinarily mean a wider fuselage than would otherwise be necessary. Another thing that enters into the problem is the matter of the propeller. If the screw is designed to operate efficiently in conjunction with the plane at or near the ground level, this same propeller will not be able to hold the engine down to proper speeds when in the less dense atmosphere at altitudes, if there is some form of supercharging apparatus maintaining the engine's power at approximately the same value as at ground level. It would be impossible to design a fixed-pitch propeller to operate efficiently with the supercharged engine ; therefore, when the supercharger is developed to the point- where it is entirely practical for high altitude planes, we must also have adjustable pitch propellers so that as the height is increased the pitch of the screw can be changed to get the greatest advantage out of the supercharger boost. This; then, gets down to the fact that the supercharging apparatus must not only be charged with the added weight which it adds itself, but it must also be considered as weighing whatever additional the variable pitch propeller figures as compared with the fixed-pitch type. Not only that, but there has not as yet been a successful type of variable pitch air screw developed, which means that this experimental work should be carried on along with the development of the supercharging system itself. The variable pitch propeller spells still more weight, further complication of the aeroplane, and last, but not least, another control lever for the pilot to worry about. Again, the supercharging system must be practically air tight on both the induction side and the exhaust side. The apparatus cannot operate satisfactorily if there is any pressure leakage either where the induction pipe joins the air discharge of the compressor, or where the carburettor or carburettors attach to the intake pipe. At 20,000 ft., as an example, the pressure outside the induction pipe is about 7.5 lbs. per sq. in., whereas within the induction pipe the pressure is about twice this amount. The reason for tight joints is there fore obvious. In order to have as little pressure drop on the exhaust side between the point of engine exhaust and the point where the pressure is utilised to drive the turbine rotor, it is also apparent that there must be no leakage around the joints of the cylinder exhaust passages to the exhaust mani- Int&ke valve Exhaust y to. turbine • ^Xr-<^^ • m Exhaust valve Cvlinde . Carburetor Turbine >, ^discharge Air impeller Combined tur-^ bine &• com pressor shatt Carburetor induction pipe Air compressor - mlet Air dis charge to induction system fold, or at the juncture of the manifold to the turbine nozzle * *' -,^°th oi these conditions of tight joints are difficult of fulfilment under field service conditions, as will readily be appreciated, but by the use of special forms of gaskets and the application of red lead to the flange connections this difficulty has been largely overcome in the experimental work so far done. In making these connections on the exhaust side, it also has to be borne in mind that due to the high pressure and temperatures in maximum operation of the apparatus allowance must be made for expansion of the manifolds and gas casing, for otherwise these parts would soon warp and crack. Therefore, although tight joints are essential, there must also be a certain degree of flexibility to the exhaust assembly. Possibly some form of expansion joint might be devised which would allow for movement, while at the same time maintaining pressure-tight connections between cylinders and exhaust headers, and headers and turbine. Use of a turbo-supercharger—or any supercharging apparatus, for that matter—of necessity means greater petrol consumption. This at first thought might seem to be a serious objection, in that it would mean a lesser flight range for the supercharged plane. However, when it is considered that such a plane would, due to the constant power delivery regardless of the altitude, be capable of greater speed, and hence would cover more distance in a given time than the ordinary plane, the increased fuel consumption would be more than compensated for by the greater distance covered with each gallon of fqel. Tests have shown that a Liberty engine on which a super charger is installed uses anywhere from 10 to 15 per cent. more fuel per horse-power-hour than does a standard Liberty engine, which on the average has a fuel consumption of about o. 504 lb. per horse-power per hour. Although the writer has no data on which to base such an opinion, it seems altogether likely that there would be come added trouble with the exhaust valves of a supercharged engine on account of the extremely high temperature and pressures. This would no doubt mean the.use of special material for these valves, otherwise they would be prone to excessive pitting wearing and burning, which would sooner or later affect the engine's power. Up to a certain point, the fact that the compressed air would be heated above normal would have a tendency to promote efficient vaporisation of the fuel. If the air attained too high a temperature, however, it would be detrimental to the mixture. The criterion of the amount of superheating of the air would be the temperature necessary to complete vaporisation of the fuel. For the reason that in operation the temperature of the air sent to the carburettor by the com pressor is often above that required for complete fuel vaporisa tion, it has been found necessary in Europe to instal some form of air-coohng device between the compressor and the car burettor to bring the air back to the efficient temperature These air radiators have been either of the water-cooling type or merely atmospheric air coolers. The necessity for this auxiliary device has not been proven, however, It may not make enough difference in the practical operation of the supercharger to warrant its installation. It takes anywhere from 5 to 10 per cent, of the engine's delivered horse-power to drive the supercharger. Roughly, the engine without super-compression delivers only 60 per cent, as much power at 15,000 ft. as it does at ground level ; at 20,000 ft., it can furnish but about 50 per cent, as much as at the ground. Therefore, figuring the supercharger means a horse-power recovery of 30 per cent, at 15,000 ft., and a recuperation of 40 per cent, at 20,000 ft. This serves to illustrate that the supercharging device is of increasing txha.ust i 5haft f . connection to engin •^~J Cylinder Air inlet to compresses Air compressor Diagrams showing the principles of supercharging systems, driven respectively by exhaust turbine and engine gearing 1193
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