Aeroplane construction

by Sydney Camm

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.

Fig. 117.—Rotary engine mounting, in which engine is supported between two plates.

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.