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Aviation History
1940
1940 - 0598.PDF
SUPPLEMENT TOFLIGHT h FEBRUAFV 29, 1940 THE AIRCRAFT ENGINEER compression engine would run better with small gaps, as the higher efficiency reduces the engine inlet temperature. Very small gaps are difficult to maintain under service conditions, owing to the varying thermal expansion of the various parts. Tests were also made on closed impellers, of the Junkers type (Fig. 4), and of the D.V.L. type (Fig. 4), of which more will be said later. Not much was said on the Junkers tests, probably to spare the Junkers people's feelings, but the stationary air wedges between the boxes are stated to cause losses. This type is also obviously bad from the structural point of view, owing to the large central opening ; it is, however, still used in the latest types of Junkers engines. The D.V.L. closed type impeller is the apple of the D.V.L. eyes, and their only original development of any importance. Though an understandable enthusiasm for their own work must be discounted when D.V.L. papers are read, this closed impeller is obviously very DvlPELLEP WITH INLET FAN TOR V WITHOUT - Casino <$ Leads unalterd Effect of auxiliary inlet fan. interesting ; it does not seem to be used yet on standard engines, however. Great importance is given to the absence of axial thrust, more particularly for multi-stage blowers, which the Germans are obviously developing as fast as they can. There are, however, other means of eliminating axial thrust from multi-stage blowers than the closed impeller. leaving for the moment all structural considerations, Fig. 6 gives comparative data between a closed impeller and a web type impeller, the latter having been tested with two different gaps. The advantage of the closed type is obvious, though not great, and it is obtained over the whole field, there being very little deterioration in the shape of the curve. The gain in efficiency is the more important, in that the efficiency is very high and leaves little margin for improvement. It must be noted, however, that the supercharger tested is relatively small (about 2 1b. of air per second). For larger flows the disadvantage of the web type impeller would gradually disappear, as the relative importance of the friction and eddy loss, caused by the air in the impeller passages touching the stationary casing, becomes obviously smaller when the flow increases. It is interesting to note that, while the research pro- gramme of the D.V.L. contemplates as a matter of course altitudes of 50,000 to 65,000 feet, no special effort seems to have been made to study the supercharger needs of the very large engine, all work being apparently done on blowers suitable for engines of about 1,000 h p. The largest supercharger mentioned in the German publications is not a D.V.L. development, but a type that the Bramo people were trying to develop ; even then the flow was only about 3 lb. per second. Inlet Fans Another most interesting series of tests was made on auxiliary inlet fans. To avoid an excessive inlet shock, it is common practice to twist the inlet side of the impeller vanes, which are then no longer radial at the inlet (this can be seen in Fig. 4). This causes structural and manu- facturing difficulties, and the whole impeller must be changed if the supercharger is to be adapted to different inlet conditions. By using an auxiliary inlet fan (Fig. 4, right) in conjunction with an impeller with purely radial vanes, the impeller proper (which can be either of the web type or of the closed type) can be made stronger and is easier to manufacture, while the best inlet shape to suit varying conditions can be easily adjusted by changing the inlet fan. This type of construction is used, of course, by Rolls-Royce. Incidentally, the Rolls supercharger seems to be greatly admired by the D.V.L. people, who consider it, and their own type, the bright exceptions in a rather mediocre field. An extreme example of the application of an inlet fan is given by the very interesting Planiol axial-radial blower, developed in France, which has given very good test results. Fig. 7 shows comparative tests on a purely radial impeller, probably of the closed D.V.L. type, with and without the auxiliary fan, all the rest being unchanged. Beside the efficiency TJAD, the quality factor qkD and the reaction degree are also given. It must be noted that the curves are given in terms not of the usual inlet volume flow Vj, but of the relative volume flow Vj/Vgx, where VST is the rate of flow at which the entrance of the air into the impeller is effected without shock. The gain is con- siderable ; the large increase in reaction degree shows that the inlet fan improves mainly the impeller. Other D.V.L. tests have shown the importance of a clear, straight inlet lead, or of an inlet spiral volute ; these, however, can improve the effect of an inlet fan, but not take its place. (To be concluded next month.) EXHIBITION AT SCIENCE MUSEUM A PLEASANT oasis for the traveller thirsty after knowledgein the desert of closed museums is the exniliicion of Aircraft in Peace and War at the Science Museum, South Kensington. The museum models have been arraaged int<"> a compact collection illustrating the various periods through which flying has passed, and we are reminded forcibly that while "there is abundant evidence of man's genius in the achievement and development of flight, there has not always been a corresponding enlightenment in its application." The Historic Period, 1903-1914, of course, starts oil with the Wright model of 1903, and is followed by other representa- tive ones. Then comes the period of intensive rnil'tary development during the Great War of 1914-18, and what name more familiar than the Sopwith Pup? The FE 2D, with its open tail of two braced booms, looks strange in these days, a veritable forest of struts aud wires. The Junkers Di mono- plane is conspicuous tor the period, being an all-metal low- wing design. The 1919-1939 period of civil and military development is represented by these types, many of which have just passed out of use, and a separate case of models ushers in the 1940 period, perhaps the Flying Forties? Blen- heims, Wellingtons, Hurricanes, Spitfires—they are all there, though the Whitley is a notable absentee. Engines, too, are shown. A .Rolls-Royce Merlin X, two Napiers and a Pegasus XVIII look impressive, and there is a sectioned moving model of a Siddeley Tiger VI. A Taurus sleeve-valve engine and a de Havilland constant-speed airscrew make the collection a very complete one. Ministry of Informa- tion photographs—of very familiar subjects—occupy a corner and The Aeroplane contributes some excellent ones of British, French and German aircraft of types engaged in settling the present difference of opinion.
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