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Aviation History
1944
1944 - 1588.PDF
120 FLIGHT AUGUST 3RD, 1944 Engine Mountings In this Second Article the Fep&tres of a Number\o/ Mounting engines are Surveyed ~j for In-line |By MAURICE F. ALLWARD • A MUCH wider vauation is ^parent in rl^e design of mountings for in-line jfhan those forV^dials, although certain countries, in particular GermaT have standardised to a great degree. Basically, however, nearly all such mountings consist of two horizontal, bearers, one down each side of the crankcase and parallel to the thrust line. These bearers are strutted back to the main structure, the type of strutting used depending chiefly, as in the case of radials, upon that part of the airframe to which the engine is to be attached. German Mountings The mountings on the Me 109, no, JU87, 88, and th€ He in are so similar as to be practically standard. The main horizontal bearers are electron forgings cantilevered forward of the top rear engine attachment points and supported by single tubular struts with welded-on end fittings. The cylinders are thus more accessible than if a fabricated tubular structure was used owing to the inverted position of the engines. The forging of the main bearers is very economical indeed when great quantities are required. One often wonders why some similar forging has not been developed over here for that war-winning engine, the Merlin. Maybe the answer lies in the position jrf^he attachment points on the engine itself. The Germans have cut down the machining necessary between the forged blank and the finished bearer to a minimum. This consists of drilling out the lightening holes (they are not forged in order to obviate any small cracks that might develop at a later date), the holes for the engine feet, machining the rear attachment fitting and drilling the forked lug for the lower bracing member. On the Me 109, no, and the Ju88, the lower bearer pick-up points and the undercarriage attachment points are combined. It would be interesting to know how^ihe electron bearers stand up to damage by " enemy,.*«e¥ion.'' Once hit, they probably fracture under comparatively small loads, but as the engine itself is most likely unserviceable this is not considered important. When one bearer, which appeared to be similar to the British Specification D.T.D. 259 was tested, the results were illuminating. The Ultimate Tensile Strength was nowhere less than 18 tons per square inch and the minimum elongation over 10 per cent. The lowest proof stresses were found to be near the main bosses. On the MeiogFi each forging weighs about 30 lb., and the total weight of the complete mounting together with the cowling is 146 lb., or 7.1 per cent, of the basic struc ture, or 2.5 per cent, of the all-up weight. On the Me 210 and the n«wer 410, which is the same as the former apart from more powerful en gines, the mountings differ slightly. The main bearers are attached to the bottom of the engine bulkhead, and are supported in this positioj^y tubularjfffe rods picking up with fittings at the to^oT tfie bulkh*fd. The bearers themselves are Tcctron forgings,J0t are built up from two halves of hollow light alloy^Kampings welded together to form rectangular se&tjojirrnembers. The arrangement is very clearly shown in the Flight sketch below. All the mountings described above incorporate anti- vibrational mounting assemblies. The engine bearer fit ting, bolted rigidly to the engine itself, forms a journal. Thick lozenge-shaped rubber pads are forced over this and form a saddle for the bearer. T»he pads are fixed in place,, by a cap held down by a bolt through the centre of^bMfe journal. -& In the Messerschmitt Me 210 A-i the engine bearers are of welded box section and the tubular bracing strut is a tension member. The two bearers are not connected by any cross-bracing, this function being performed by the engine itself..-* One very interesting feature in the design of German mountings is that the two halves are completely inde pendent and are not connected by any struts or cross bracing. Thus all the cross loads are taken by the engine which, in consequence, has to be suitably strengthened The small frontal area of German warplanes is partly responsible for this aspect, as it is imperative the under engine contour line be as close to the engine as possible. This "pushes" the attachment points "up" the engine and thus cross bracing becomes impossible. British Mountings The mounting used on the Hurricane, in common with the rest of the fuselage, is of tubular construction. The LIF' i LIFT THRUST ENGINE UP-LOAD INCREASED DOWN-LOAD DECREASED UP-LOAO WEIGHT DOWN-LOAD OFTEN UTILISED FOR U/C HINGES Diagrams of forces on engine mounting on the ground and in flight.
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