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Aviation History
1916
1916 - 0008.PDF
(fUGHf JANUARY 6, 1916. CONSTRUCTIONAL DETAILS.—XII. N AS MUCH as the engine forms such a vital part of an aeroplane, the various methods employed for mounting and housing it are of sufficient importance to merit inclusion in this series of constructional details. In order to facilitate reference, we have attempted to classify the different types according to their characteristics, and as the air cooled rotary engine is, or at least was until comparatively re cently, in a vast majority, this type will be dealt with first. Sub-dividing the various forms of mounting of a rotary engine according. to where they are placed in the machine, there are two forms of mounting for tractors, one in which the engine is supported on two bearers but is overhung, i.e., there is no support in front of the crankcase, and one having one bearer in front and one behind the engine. When the engine is placed in the rear, it is almost invariably overhung. When overhung in the nose of a machine, the bearers usually take the form of pressed steel frames, of which the front bearer forms a capping plate over the nose of the fuselage, and takes most of the weight of the engine, while the rear one supports the rear end of the induction pipe, and is, as a rule, provided with means for alignment. In order to lighten the engine frames these have frequently openings cut in them, the edges of which are bent over to form webs, which help to stiffen the frame against bending strains. In some cases the engine plates are not pressed out of a single sheet of steel, but are built up of parts that are riveted or bolted together. For instance, the front bearer of the Morane-Saulnier monoplane is quite an elaborate structure built up in this fashion, the result being very strong, and at the same time light. Placed right out in front as it is on a tractor machine, where it receives, it is true, the maximum cooling, but where on the other hand it offers a great amount of head resistance, the problem of reducing the head resistance and at the same time admitting enough air to keep the rotary engine at its maximum temperature is one of some considerable difficulty. The result must inevitably be somewhat of a compromise, and some highly ingenious forms of housing have been designed with this object in view. In its simplest form the cowl consists of a sheet metal shield, usually aluminium, placed over the top of the engine and only sufficiently large to prevent too great an amount of oil from being blown back in the pilot's face. In the sketches of the Dyott and Grahame-White cowls will be found two examples of this simple form of shield, while on another page is shown the cowl of the old Handley-Page monoplane, which is a strip of aluminium surrounding the upper half of the engine, but being quite open in front. While this type of shield is reasonably effective in collecting the oil thrown out by the engine, it leaves the nose of the fuselage and the crankcase exposed to the wind pressure, and in order to reduce the bead resistance presented by these parts more elaborate cowls have been designed. Although the Nieuport machines are as a rule fitted with double bearings for the engine, this firm has occasionally turned out machines with the engine over hung, as, for instance, the seaplane exhibited at the last Olympia Aero Show. The cowl, it will be seen from the accompanying sketch, surrounds the upper half of the engine and turns down over part of the upper cylinders. A somewhat similar housing is employed on the Caudron biplane when fitted with a rotary engine. In the Morane- Saulnier monoplane the cowl itself is similar to the two last mentioned, but it is fitted at the sides with wings extending back along the sides of the fuselage, the object of which is to catch the oil that would otherwise be blown back along the sides of the body. When the body of the machine is narrower than the overall diameter of the engine, which is frequently the case, the manner in which the cowling merges into the flat sides of the body is of considerable importance, and one which might with advantage be investigated more extensively. In the Bristol biplane that was exhibited at the last Paris Aero Salon, the cowl enclosed the upper half of the engine, leaving all the lower cylinders exposed- The rounded sides of the cowl were made to merge very gradually into the flat sides of the body by fitting the latter with aluminium covering in front, which was at first of the same curvature as the cowl but flattened out towards the rear until it met the flat sides of the body somewhere near the pilot's seat. The Vickers scout shown at Olympia had a cowl which also covered in the upper portion of the engine, but in shape it was somewhat different to the others, being slightly more pointed and therefore possibly offering a little less resistance. On the biplane built by the Eastbourne Aviation Co. the cowl almost totally enclosed the engine with the exception of a narrow opening at the bottom. Which of the forms of engine housing described and illustrated above is the most efficient from the point of view of low head resistance and sufficient cooling is difficult to say, but modern tendency would appear to be towards the simpler form in which only part of the engine is covered in. That there are designers who believe that a more effective form is possible is shown by the remaining of our sketches, which illustrate various ways of totally enclosing the engine. When the little fast Bristol scouting biplane was first introduced it had a cowl of the enclosed upper half type. This was in later models altered to one surrounding the engine entirely on the sides, but leaving the central portion round the propeller shaft open. It was found, we believe, that the exhaust gases had a tendency to affect the cowl for a space of several inches, through the arc, in fact, through which the exhaust valve was open. As the cowl is considerably greater in diameter than the body of the machine, it projects some distance below the bottom of the fuselage, thus allowing the exhaust gases to escape. An aluminium covering to the sides of the body gradually carries the curve of the cowl into the flat sides of the body. From the speed attained by the Bristol scouts, it would appear that from the point of view of head resistance this type of cowl is highly satis factory. Yet a different type of housing will be seen in the sketch of the Clement-Bayard. Here the main cowl covers the engine in entirely, but air is admitted through an opening in front, which is again partly closed by a hemispherical nose piece in the manner shown. The Deperdussin racing monoplane of Gordon-Bennett fame had a very large and rather pointed nose piece revolving with the propeller, while the engine was enclosed in a casing provided with holes for the egress of exhaust gases. The last of our sketches shows the housing of the engine on the Bristol biplane. Air is admitted through the louvres in the nose piece.
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