Aeroplane construction

The mounting of the engine and the general arrangement constitute one of the most important and interesting sections of aeroplane construction, and perhaps a brief outline of the various engines in use will suitably preface a consideration of the mountings of the different types. Although there are signs that certain revolutionary engines may eventually come into use, the types in use on modern aircraft are the stationary air and water-cooled, the radial air and water-cooled, and the air-cooled rotary. The greater variety occurs with the stationary type of engine, which may be sub-divided into those in which the cylinders are arranged vertically in line, and those where the cylinders viewed from the front form a V. Engines typical of the former class are the Beardmore, Green, Mercedes, and Benz, all of which are water-cooled; and of the latter class, Rolls-Royce, Sunbeam, Hispano-Suiza water-cooled, and the Renault and R.A.F. air-cooled. The types of radial engines which have been extensively used are confined to two, these being the air-cooled Anzani and the water-cooled Salmson. There is another radial engine of comparatively recent production; but mention of this while present conditions obtain is not permissible. Rotary engines of note are the Gnome, Le Rhone, and Clerget, all of which are necessarily air-cooled.

Essential Requirements of an Engine Mounting.

The essential features of any mounting are absolute rigidity, accessibility to permit ease of erection and dismounting; and it should also be of a moderately low weight. Moreover, the general arrangement must offer a minimum of head resistance, although in this direction the type of engine used is a determining factor. Rigidity is a paramount consideration, for the slightest tendency to slackness or “play,” under the effect of engine vibration, speedily develops, until either serious stresses are induced in the fore part of the fuselage or the engine loses its correct alignment, with a consequent detrimental effect on the flying qualities of the aeroplane. This, of course, should be provided against in the general design; but it is also a contingency which should be kept in mind during the actual construction of the various components of the complete mounting.

A detail which does not always receive sufficient attention is the provision of adequate bracing against the thrust of the engine. Where the construction is such that the engine-bearers form an integral part of the fuselage structure, there is generally little fault to find, but with some sheet steel mountings, particularly those employed for the rotary type of engine, the only bracing in a fore-and-aft direction is that provided by the flanged edges of the plate, which are usually much too narrow to be of real use. Further, the construction of both the engine mounting and the fore part of the fuselage should be of the necessary strength to ensure that the bearers supporting the engine are always correctly in alignment and dead level. With some methods of construction the weight of the engine and various landing shocks, result after a time in the lowering of the bearers at the front, which means that the angle of thrust is not in its correct position relative to the centre of gravity and the incidence of the wings, this being extremely detrimental to the flying properties of the machine.

Materials.

Wood, on account of its property of absorbing vibration, is particularly well suited for the construction of the engine mounting, and one finds examples of its use in a variety of ways. Perhaps the most common form is that in which a bearer of ash or spruce, channelled out between the fastening-down bolt holes for lightness, is attached to steel brackets which in turn are bolted to the various fuselage members. A development of this method consists of mounting the bearers on either multi-ply formers of wood, or built-up wood brackets stiffened with a three-ply covering on each side, and both of these arrangements are being extensively used. Sheet steel is used for the mountings of various machines, but it does not possess the characteristic of absorbing vibration. In some instances one finds that the engine has been specifically designed to be supported on bearers of the tubular variety, in either steel or duralumin; but here again rigidity is difficult of attainment. Although the use of welding, that is to any extent, is not advisable in the construction of the engine mounting, one finds this process very extensively used for the mountings of some modern machines. In one particular instance, the tubular bearers are supported from the steel tube fuselage by various tubes, the whole structure being welded, and although every joint successfully survived a smash which resulted in a considerable bending and distortion of the fuselage, its use does not engender a sense of security or reliability.

Rotary Engine Mountings.

The mountings associated with the rotary type of engine fall under two categories: those where the motor is supported between two or more plates, and those in which the motor itself is overhung. The method of mounting adopted for the first case is generally the type shown by Fig. 117. The plates are pressed or bent up from sheet steel, and all edges flanged to prevent buckling. The front plate embodies a ball race, through which the propeller shaft runs, while to the rear bearer is bolted the back plate of the engine. This arrangement with minor variations has been extensively used for the different makes of small scouting biplanes engined with the 80 h.p. and 100 h.p. Gnome motors.

Where the weight of the rotary engine used is excessive, as in the case of the 160 h.p. Gnome with 20 cylinders, which is now out of date, a mounting incorporating three bearers is used. The arrangement would be similar to that indicated by Fig. 117, with the addition of an extra bearer for the support of the crank-shaft extension.

Overhung Mounting.

The overhung type of engine mounting which is used for both propeller and tractor aeroplanes, is shown by Fig. 118. In this case the back plate of the motor is bolted to the capping plate, while an extension of the hollow crank shaft is supported by a smaller rear plate. This system has been very widely used, chiefly by reason of its extreme lightness, and the great facility afforded for the operation of dismounting the engine; indeed, it would be difficult to find an arrangement in which the demands of accessibility are so well satisfied. Another form of overhung mounting, which has been used for a radial Anzani motor, is shown by Fig. 119. In this case the four longerons of the fuselage are capped by a single flanged steel plate, to which the engine is attached by long bolts through the crank case. Additional support is provided by light steel tube stays, which are taken from various points on the front of the crank case to the centre section of the upper plane, or other parts of the machine.

A distinctly original type of overhung mounting is shown by Fig. 120 in front and side elevation, this being used on a machine incorporating an all-steel fuselage. The ring to which the back plate of the engine is bolted, is supported from the four corners of the fuselage by steel tubing, while the bearing for the crank-shaft extension is formed by a pyramid of tubes welded to a pressing of sheet steel, to which in turn is bolted a ball-race housing. At each corner the three converging tubes are welded together, and bolted to small angle plates, which are also welded to the framework of the fuselage. It will be seen that the strength of this mounting is entirely dependent upon the welding; but such reliance, in view of the generally uncertain nature of this latter process, is not to be recommended.

A Stationary Engine Mounting.

A mounting used for a 70 h.p. air-cooled Renault, which is designed to be supported by short lengths of steel tube projecting from the crank case, is shown by Fig. 121, this particular arrangement being used on a propeller biplane. The four ash longerons of the nacelle are built up in the form of a box girder, the struts immediately under the engine bearers being reinforced with steel plates. The steel tubes from the crank case embedded in a steel bearing, composed of two semi-circular clips, which are let into the upper longerons, and are prevented from moving sideways by steel collars sweated to the tubes and abutting against the fixing clips. In this case, by the removal of the four fixing clips and the necessary pipe connections, the engine can be lifted bodily out.

Multi-Engine Mountings.

Several versions of the type of machine employing two or more engines, and which, by the way, is regarded as being the type most suitable for the commercial purposes of the future, are existent. The usual arrangement with the twin-engined machine is to support the engines between the planes on either side of the body, the bearers being mounted on a structure of struts, which also serve as interplane supports.

With regard to the flying-boat type of machine, a favourite practice is to mount the motor on the four struts supporting the centre section of the upper plane, which is braced by struts and wires from different points on the hull or body.

Although the modern aircraft engine is of greatly increased power, compared with the engine of the period 1912–1914, one does not find any great difference in the structural features of the mounting employed, and in view of the very diverse arrangements for mounting the same type of engine which now exist, there is need for greater uniformity. With regard to the materials employed, there is a very pronounced trend towards the greater use of wood, which circumstance is certainly at variance with the oft-portended approaching era of steel. As indicated in previous chapters, wood possesses remarkable powers of resistance to sudden shock, which, combined with its quality of absorbing vibration, renders it peculiarly suitable for the structure of the aeroplane, and despite its numerous defects, will undoubtedly continue in use until either the available supplies of suitable timber are exhausted, or until the production of a remarkably light alloy possessing high strength values.