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Aviation History
1933
1933 - 1234.PDF
FLIGHT. DECEMBER 14, 1933 As it was impossible to obtain the necessarj- high tem peratures under the standard full throttle test conditions, a special breadown test bed was initiated by the Bristol Company, in which the cooling wind speed was reduced to 60 miles an hour, and a baffle board was arranged at the rear of the cylinder in order to obstruct the get-away of the cooling air, the unit being run continuously at full throttle (2,100 r.p.m.). corresponding to a mam engine power output of 600 brake horse-power. Standard fuel to Air Ministry D.T.D. 134 specification, 73 octane (68 octane number by C.F.R. motor method) was used, and the fuel consumption limited to 0.58 pts. / brake horse-power/ hr. throughout. Under these conditions the " Jupiter " type cylinder failed after 60 hours, and, as a duration of at least 100 hours was the standard set to ensure satisfactory life under service conditions, it was evident! that the cylinder design would need careful consideration in order to make the cooling capacity satisfactory for use with ring cowling. In the course of the series of experiments carried out during the evolution of a cylinder capable of coping with these conditions, twelve different types were designed and tested before the present " Pegasus " cylinder was evolved. It was found possible, with the lower wind speeds, to reduce the pitch of the cooling fins by 50 per cent, without loss of cooling efficiency, and, by this means, together with an extension of the finned surface, the total cooling area was increased by 66 per cent. With this cylinder the temperatures obtained, under the original breakdown test conditions, were so low that the test would have failed in its object, and it was considered desirable to increase its severity. The wind speed was, therefore, further reduced to 40 miles per hour, the lowest operating speed obtainable with the plant. Despite the lowering of the wind speed by 30 per cent. below the " Jupiter " type cylinder test conditions, the " Pegasus " cylindeY tests showed a drop of the order of 10 per cent, in the cylinder and exhaust valve temperature. Under these abnormal conditions the 100 hours test was completed with entire satisfaction, the performance being well maintained, and, at the conclusion, the cylinder, valves and seats were in first-class condition ; the unit being retained for further high-duty test running. Since the clearance of the cylinder, it has been run continuously for long periods under heavy duty on the single-cylinder test bench with complete success and without any sign of breakdown or failure. To date, approximately 700 hours have been completed, a final check test made comprising over 300 hours' running at 2,200 revolutions per minute, with a normal climbing speed of 90 miles per hour. Under these conditions 150 hours were run at a brake mean effective pressure corresponding to the engine rating, followed by another 150 hours at 10 per cent, increase in brake mean effective pressure, and a final 20 hours' run at 30 per cent, in excess of the rated brake mean effective pressure. Fig. 8 shows side and plan views of the latest " Pega sus " cylinder as compared with the " Jupiter " cylinder, the former type having shown itself capable of running satisfactorily under low wind speed and full throttle test conditions for such long periods that it is considered satis factory for the life of the engine under all reasonable cowling conditions. Such results have been made possible only by the forged- head design of cylinder, on which, with modern machining methods, the increased cooling area has been obtained by i Fig 9 : Relative air passages with cast and machined fins. close-pitched deep but efficient finning, which it would be impossible to produce by any form of sand or die casting. Fig. 9 illustrates the advantage of machined fins. Whereas it is realised that the all-machined cylinder does not give one the same scope of change in design, it is quite a wrong impression to consider it a more costly job. Actually the Bristol Company are producing their forged heads at a lower cost than the cast ones, and there is no doubt that the forged head offers accurate precision methods for producing consistent air-cooled heads in large quantities, which cannot be approached when the head is in cast form. Turning now to heading (3)—the ring cowling of radial air-cooled engines—I do not propose to attempt to touch on the aerodynamic problems, but only to endeavour to explain a few of the difficulties from the engine-cooling aspect. At the outset it may be desirable to state that the increase in cooling problems caused by ring cowling has been checked under full-scale conditions by the Bristol Company, through the courtesy of the Vickers Company, who loaned their wind tunnel for investigating a " Pegasus " engine in front of a streamlined body with various types of ring cowling. Fig. 10* shows the different aspects of these tests, and, although it was not possible to obtain accurate drag figures with the different types of ring cowling tested out, the flight results were entirely con firmed, the air speed over the cylinders being reduced to 75 per cent, when ring cowling was fitted to the engine, as compared with the figure obtained with a bare engine in front of a streamlined body. The Bristol Company standardise four types of ring cowl ing for military aircraft, as shown in Fig. 11, viz.: Smali- diameter forward exhaust ring and plate ring cowl ; rear exhaust ring combined with plate cowl ; Boulton & Paul polvgonal combined exhaust ring and plate cowl, and Bristol large-diameter front exhaust ring combined with plate cowl. On American installations, long chord N.A.C.A. cowlings are, I understand, supplanting the shorter type on single- row radials, and also on some of the new double-row en gines, with success, and when using gearing and super- * (Not published.—ED.) " \^ Fig. 8 : On left, a " Jupiter " " F "type cylinder, and on the right the cylinder of a " Pegasus." 1266
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