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Aviation History
1916
1916 - 0482.PDF
l/liGHT) JDNK 8, 1916. CONSTRUCTIONAL DETAILS—XIV.* WHILE a very great variety of engine mountings, and especially cowlings on tractor machines, are available for illustrating typical examples of general practice, we have found it more difficult to compile a set of engine mount ings and housings of " pusher " machines. From our extensive collection of material it has been impossible to find more than the few examples shown that are repre sentative of current practice, a fact which appears to indicate a lack of application of ingenuity to the mount ing of rotary engines in this type of aeroplane. Whether this is due to the fact that the tractor machine had, before the war, been found to possess greater possibilities as regards performance, and therefore was more highly developed, it is difficult to say, but the fact remains that, as we have said, the problem of mounting and housing an engine on a propeller biplane presents an aspect of sameness which leads one to conclude that there must be some reason why so little has been attempted in this respect; either the stream-lining of an engine is less im portant in a " pusher," or its accomplishment is a matter of greater difficulty than in the tractor. Let us examine the first case and see if there is any reason for supposing that a good streamline casing around the engine of a propeller biplane is of less im portance than in the case of the engine mounted in the nose of a fuselage tractor. From experiments on struts and similar streamline bodies it has been found that truncating the tail does not have a very serious effect on the resistance, until it reaches a certain point, after which further truncation increases the diameter of the " dead " region, and the resistance goes up at a comparatively rapid rate. This would appear to indicate that a rotary engine mounted on the extreme rear of a nacelle would present a flat surface of sufficient area to cause a dead water region of considerable magnitude, which would cause a perceptible increase in the resistance of the nacelle, and it cannot, therefore, be argued, we think, that the stream-lining of the engine is of less importance in this type of machine than in a tractor. Turning now to the question of practical considerations for a clue to the reasons for the apparent lack of attempts at stream lining the engine on a "pusher," it should be realised that whereas the nose, and consequently the cowling, of a fuselage can be kept comparatively blunt without adding very materially to the resistance, the tail of a streamline body should, as we have just pointed out, at least be considerably more pointed to give the best results. It will be easily seen that when the engine is placed in the rear, a cowl entirely enclosing it will necessitate a very wide nacelle, to which it will be difficult to impart a good shape. The other alter native would be to so arrange the mounting that only the outer ends of the cylinders of the rotating engine projected outside the nacelle. In this case the normal surface presented by the cylinders would, of course, tend to offer a very considerable amount of resist ance, but it should be borne in mind that whatever happens the importance of sufficient cooling is far greater than that of saving a pound or two in resistance. Judging from such recent examples of high speed " pushers " as the Pemberton Billing " Sparklet" and the de Havilland scout, both of which have been produced during the war, and illustrations of which must not, there fore, be published at present, the method of having the ends of the cylinders projecting beyond the sides of the nacelle is not detrimental to great velocities. On the other hand, in both of these machines it would appear probable that an even better speed could be obtained by adding to the crankcase a conical aluminium tailpiece mounted on and revolving with the engine, and serving to decrease the diameter of the " dead " region caused by the com paratively flat surfaces of the crankcase, propeller boss and inner ends of the propeller blades. Reverting to the illustrations of mountings and cowl- ings in use before the war, that of the H. Farman will be dealt with first, since this machine has served as the prototype of all " pushers." Owing to the comparatively shallow depth of the H. Farman nacelle and to the placing of the latter down on top of the lower wing it has been necessary, in order to bring the centre of thrust into coincidence with the centre of resistance, to raise the engine slightly above the upper longerons of the nacelle. This is accomplished in the H. Farman by mounting the engine on two pressed steel bearers of the shape shown in the illustration, one of which is placed fairly close against the front cover of the crankcase, while the second is situated further forward and supports the front end of the crank shaft. The engine, it will be seen, is hung cantilever fashion, a method which at one time was, and still is by some designers, thought unsatisfactory, but which is, never theless, very frequently employed without, apparently, any serious disadvantage. Another form of mounting is shown in the sketch of the Nieuport-Dunne biplane, which was exhibited at the last Paris Aero Salon. Here the engine is overhung as in the case of the Farman, but instead of the pressed steel bearers resting on the upper rails only they are attached, in this machine, to both upper and lower rails of the nacelle. In the Caudron propeller seaplane the rotary engine was mounted between two bearers, the shape of which can be seen in the illustration, and the drive, it will be noticed, was through spur reduction gearing. We now come to the only two examples we have been able to discover of engines enclosed in aluminium cowling. One of these is the Avro biplane exhibited at the last Olympia show, and the other the Grahame-White biplane on view at the same time. In the Avro the engine was mounted between double bearers, the front one of which was of the ordinary pressed steel type, while the rear one consisted of a ball-race supported on four tubular exten sions of the nacelle longerons. Air was admitted to the cowl through a scoop on top of the nacelle, and was allowed to escape by way of the open lower portion of the housing. In the Grahame-White " pusher" the engine was mounted on three bearers, one of which was adjustable in a vertical sense, while the second was self-aligning. The drive was through chain and sprockets, and the pro peller mounted on a shaft carried in bearings in the manner shown in the illustration. The lower part of the cowl was left open, and air was admitted to the housing from underneath instead of from above, as was * Previous sets of sketches in this series have appeared as follows : Strut sockets Sep. IOJ Single-skid undercarriages Oct. 20 „ „ ,, 17: Vee type undercarriages Nov. 5 ,1 ,1 ••• ••• •» 24. „ ,, vf ,« iz ,, connections, tic... Oct 1 Wheel undercarriages ... „ 19 Wing spar sections ... „ 8, 1916 Streamline struts, sections „ 15 Engine mountings ... Jan. 6 Doub'e-skid undercarriages ,, Z2: „ ,, ...Apl. 27 Any of these back numbers can be obtained from " FLIGHT" Office, 44, St. Martin's Lane, price 6d. each, post free.
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