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Aviation History
1939
1939 - 0555.PDF
FEBRUARY 23, 1939 THE AIRCRAFT ENGINEER SUPPLEMENT TO FLIGHT T9S' , opposite sense ; that of the flywheel decreases, while that of the engine increases. Tf we consider the point of maximum efficiency when • — M it is obvious that, when the two masses are revolving at the speed -, half of the energy from the flywheel, which is now stored in the spring, will be released, mtil the speed of the mass m (flywheel) is reduced to zero and that of the mass M (engine) is increased by —, and consequently attains the speed V. Thus the two equal masses m and M will have exchanged their respective speeds and the whole of the original energy will now have been transferred to the engine (M). The spring designed to replace the friction clutch is composed of rubber, this being the material capable of storing the maximum of energy lor a given weight (5oo.kgm. per kg of rubber). Fig. 3. is an explanatory diagram of the flywheel V, attached to its shaft by a circular ring of rubber, T, which adheres both to the shaft and to the outer ring of the STARTER USED WITHOUT IGNITION STARTER USED WITH IGNITION NUMBER OF REVOLUTIONS OF THE CRANKSHAFT flywheel. It is rotated by means of the starting handle M until sufficient speed has been attained, when the dog G on the flywheel shaft is engaged with the dog G1 on the engine. The hub m of the flywheel is immediately stopped, but the flywheel V, continuing to rotate, twists the rubber ring and thus brings to bear on the hub a force which increases from zero to a maximum at which the flywheel and the engine (which has thus been progressively carried round) are revolving at the same speed. At this moment, the rubber is twisted to an angle which in certain models is as large as 220 deg. From this point, the rubber ring has a tendency to unwind, but as a result of the equality of action and reaction, its effect is on the one hand to increase the speed of the engine, and on the other to bring the flywheel to a standstill. when the flywheel finally stops, after the engine is started, all the energy has been transferred from the flywheel 0 the engine, and the efficiency is equal to one, minus • e 'osses due to the static resistance and very small losses ^the rubber. Thus, with a given energy available in the jwheel, and all other things being equal, it is possible tha fih an engine tne inertia of which is four times greater that of an inertia starter fitted with a friction clutch. practice, a large flywheel acting directly on the In ^Pne, as shown in Fig. 3/would be of prohibitive weight, sneer? SnJia11 flvwneel is employed, rotating at very high duct?. geared to the starter dog by means of a re- inerti " f'63*' ^h's Practice is the same in the case of is the rters with dutches. The total energy capacity order to^h' bUt the comPlete starter is much lighter. In °f the fl ° u n tae kest conditions, the moment of inertia engine d muSt be y* times smal,er than that of the fi?wbeelB thC moment of inertia of a heavy slow-speed and if ' as,f°ra on Fig- 3, and w its angular speed; M is the moment of inertia of a small high-speed flywheel, U, such that — = y, then, to give the same u output, the two flywheels must have the same kinetic energy. tnu2 U2 — = n — 2 2 Thus U£ = y2 ft M" If the moment of inertia of the engine M must be equal to m in the case of the ordinary flywheel, it must be equal to fiy2 in the case of a high-speed flywheel. H can therefore be y2 times smaller than m. Fig. 4 shows the actual layout of the starter as applied to aero-engines. The engagement of the dogs G and Gl requires no special precautions, and is extremely smooth. This arises from the fact that the energy applied to the engine commences at zero, and although it increases rapidly, it does so progressively and without shock. Data and Test Results of the Type 14500 Starter Number of revolutions of the starter dog.. 130 r.p.m. Dynamic torque limited to . . . . . . 120 m.k.g. Energy accumulated in the flywheel .. 1,250 kg.m. Time required for starting electrically .. 10 seconds Speed for hand starting .. . . .. 60 r.p.m. Voltage .. .. .. .. .. 24 v. Fig. 5 gives a curve of starting with and without ignition. On an engine of 38 litres cylinder capacity, with a nominal h.p. of about 1,000, we obtain the following results : 1. WITHOUT IGNITION. The engine makes six complete revolutions and reaches its maximum speed at the first revolution at 130 r.p.m. 2. WITH IGNITION. The engine starts cleanly on the first revolution on account of the vigorous induction. 3. The commencement of the rotation of the engine itself, when the flywheel is engaged, takes place without shock and follows an entirely progressive curve, although over a very short period of time. 4. As confirmation of the theory on which the starter is based, the engine is started at the speed of the starter dog (130 r.p.m. for example in the tests shown above), whereas in the ordinary inertia starter, the rotation of the engine is exactly half that of the dog in the case of maximum efficiency, and even less in any other case. 5. The amount of energy to be given to the flywheel in order to obtain a start is substantially decreased, and, therefore, turning the flywheel by hand for starting is much more easily accomplished. In the In-iines Den L AST week Mr. A. H. R. Fedden repeated his notable I.A.E.- K.Ae.S. lecture (reported in Flight of February 16) on sleeve-valve engines for the benefit of the Derby branch of the I.A.E. By thus carrying the war into the camp of a friendly enemy Mr. Fedden found himself faced with a barrage of very pertinent questions from the many Rolls-Royce tech nicians present. Pressure of space in this Special Number unfortunately prevents our dealing with the discussion this week, but next week we shall report it in some detail. Honour for British Flying-boat Designer FOUNDED last year as a memorial to Capt. Edwin Musick, who, with six companions, was lost with the Samoan Clip per in the Pacific during the first commercial flight from the United States to New Zealand, the Musick Memorial has been awarded to Mr. Arthui Gouge, B.Sc, F.R.Ae.S. Mr. Gouge is chief designer of Short Bros., Ltd., and was responsible for the design of the Empire boats. He is a member of the Council of the Royal Aeronautical Society. The Musick Memorial is awarded annually to the group, body or individual who have made the contribution, develop ment or improvement, which, by its practical application, has become most effective in furthering the safety of aircraft with special regard to trans-oceanic aviation. The award is confined to citizens of the British Common wealth of Nations and the United States of America. The trophy, in silver, is valued at ^250.
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