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Aviation History
1944
1944 - 1533.PDF
JULY 27TH, 1944 FLIGHT Engine Mountings 91 Some Main Features Encountered in the Some of the Types in Wse Throughout H sign, and a Brief Survey of rorld on Military Aircraft MAURICE F. M s aiwftyftj OUNTING the power unit has ilTljii UMI the major structural problems of tbfe an designer. Right from the beginning jot tl air- / craft industry the design draughtsmen have dope tk€ir best to harness the engine—which is by far the heaviest single item of the airframe—to the main structure with the lightest possible, yet adequately strong, structure—the engine mounting. To the layman a mounting without the engine may seem rather massive, but with the engine in position it appears alarmingly fragile. This is not surprising since the members from which the mounting is made are often as thin as a walking stick, and even to-day, when dealing with engines of 2,000 h.p. and more, are rarely lacker t" A a man's wrist. It is evident from the general non-standardisation that the ideal mounting has yet to be found. A large number of different types of mountings are in use to-day, some of them being developed by a particular company, or country. There are three main reasons for the differences found: the engine itself, the type of structure to which it is attached, and the position of the engine in relation to that structure. Engines may be divided into classes when designing mountings: ^ A. Radial, e.g., Hercules<*Wasp, and the BMW8oia. B. In-line, e.g., Sabre, Allison, and the DB 60m. At one time it seemed as if the radial engine was to be ousted, despite its easier maintenance, by the in-line engine with its lower drag and higher power, especially for fighter aircraft. This trend has been reversed, however, with the appearance of the Fw 190, the Thunderbolt, and the new Japanese Nakajima Navy SKH fighter with its 1,050 h.p. Mitsubishi Kinsei radial engine and an estimated top speed 'of 387 m.p.h. Even to-day, while Great Britain has the 2,000 h.p. Napier Sabre in-line engine, America has no in-line engine approaching the Twin Wasp, or Germany one to compare with the 2,000 h.p. BMW 802 18-cylinder radial engine. One hears, too, about a new British radial " better than the Sabre," which the Germans have named as tbe Bristol Centaurus. Stressing ^ The greatest loads will be experienced when the aircraft is turning sharply in flight with engine on, and these may be divided into : , 1. Loads due to gravity. 2. Airscrew thrust and torque. 3. Gyroscopic forces. The latter is now only important when a large and heavy airscrew is used, but it attained considerable importance when rotary engines were much used. A different set of loads will be experienced when gliding and landing with the engine off, the weight of the engine itself exerting the principal load. A third set will be when the engine is run up on the ground and thus exerts maximum thrust and torque. The direction of the loading due to torque depends upon the sense in which the airscrew rotates. It is thus evident tha^WT loads can be great in all directions. The top members of a mounting will be in tension, while the lower ones will usually be in compression, due to the weight of the engine. In flight the thrust of the engine will increase the stress in the upper members and decrease that in the lower ones. Clever designs try to make the number I of members in tension as great as possible, since relatively such members are much stronger than those under com pression, there being no fear of failing by bending. In stressing, both welded and mechanically jointed mountings AWARD irded as being pin-jointed, and thus the centre lines of all members of each joint, if produced, have to meet at a single nodal point. If this assumption were not used, stressing such parts wojjfcf be a great deal more difficult. In both methods of construction the factor of fixity may be increased comparatively easily, but only in a few special cases is this allowed for. In a mechanically jointed mount ing, the bearing strength of bolts and plates at the joints is usually assumed to be only half of their normal strength. When attached to the wings, the problem of stressing the mounting in particular, and the wing in general, becomes very involved and is more complicated than the stressing of the fuselage. The weight of the engine and its overhang puts down-loads on the front spar and up-loads on the rear spar. In flight this loading reduces the stress in the front spar and adds likeajjge to the air load on the rear spat. The thrust of the engine, the torque and gust conditions only complicate the matter. In fact, without detail stress ing of the structure it is impossible to know what the total effect will be. In order to give the mounting the widest possible base, thus reducing the stress in the members, the ENGINE INTEGRATING STRUCTURE IN COMPRESSION COMPRESSION REDUCED WEIGHT Stresses set up in a radial engine mounting. (Top) Engine (Bottom) With engine " on " 1 off " condition.
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