Aeroplane construction

by Sydney Camm

Fuselage Contours.

In the design of the contour of the fuselage the type of the motor used is the determining influence. With the vertical “in line” engine, it is possible to design a slim narrow body, while a rotary or radial engine necessitates an increase in width, which also means increased air resistance. With the Vee type engine, the popular practice is to allow the tops of the cylinders to project through the cowling, which permits of a narrower body than if the width of the body equalled the overall width of the engine. Where a rotary engine is employed and the mounting is of the overhung type, the width of the fuselage may be reduced by allowing the engine to project over the sides, and the cowling carried on an arrangement of formers and stringers, which gradually merges into the main structure, as in [Fig. 69]. It is apparent that the line of the body and that of the fairing should converge as gradually as possible, as, should this be at all abrupt, there is a distinct possibility that the air flow will take the course indicated in [Fig. 70], resulting in a dead air region and inefficiency.

Fig. 69.—Fuselage outline.

Fig. 70.—Fuselage outline.

It may be taken generally that the wider the body the greater the weight, for the struts have not only to be made longer but also of greater overall section. The practice in this country is to keep the longerons parallel to the centre line on plan, as far as the rear cockpit, tapering from that point to the stern post in a straight or slightly curved line, as [Fig. 71].

Fig. 71.—Fuselage outline.

This simplifies the fittings, the sockets for the centre plane struts are in line, and the different lengths of fuselage struts necessary reduced to a minimum.

Fig. 72.—Fuselage outline.

The plan outline of several German machines is shown diagrammatically by [Fig. 72]. It will be seen that from the nose the body gradually widens out until the maximum width, generally in the vicinity of the front seat, is reached, from where it tapers to the tail. This shape appears to satisfy aero-dynamic requirements more closely than either of the foregoing examples; but in practice the difference is not appreciable, and in any case the reduction of head resistance does not compensate for the additional work.

Fig. 73.—Fuselage outline.

Fig. 74.—Fuselage outline.

In side elevation the general practice, with exceptions, is to arrange the top longerons parallel to the line of thrust, i.e. the axis of the motor, as in [Fig. 73]. This simplifies erection and affords a convenient datum line for truing up.

Fig. 75.—Fuselage outline.

On the German Rumpler and early Albatross biplanes, the upper longerons are curved, as in [Fig. 74], but in the most recent versions of the Albatross they are level with the line of thrust. [Fig. 75] illustrates an arrangement where the top rails, from a point some distance along, slope down to the nose. By this method the body weight is kept as low as possible and the engine and accessories rendered more accessible. Although it is usual to terminate the body in a vertical knife-edge, formed generally by the rudder post, another arrangement, typical of the Morane monoplane, finishes in a horizontal edge. The German Fokker, obviously inspired by the French Morane, and the Albatross DI, are similarly terminated. This system of tapering to a horizontal knife-edge is not considered the best arrangement from a strength point of view, the flat angle of the bracing wires permitting a certain amount of movement, eventually resulting in slackness and loss of alignment.

CHAPTER IX.
FUSELAGE FITTINGS.

The design and type of fitting employed for connecting the longerons, cross and vertical struts of the fuselage, varies greatly, being usually one of the distinctive constructional details of a machine. This position renders uniformity of practice a comparatively unattainable quantity, which, in view of present requirements, and the absolute need of rapidity of output (which must commence as soon as possible after a successful design is produced), can only be considered as regrettable. This diversity of design is mainly the result of the desire for originality of each individual designer, and however commendable from this standpoint, is a position which is almost certain to disappear with the progress of the industry. Take as a hypothesis the case of, say, ten makes of scouting biplanes in use, each with approximately the same arrangement of longerons and struts, and with a similar overall size of fuselage. Each of these machines will incorporate a different fuselage clip, which means that somewhere highly skilled labour is being unnecessarily expended in the making of jigs and press tools, whereas a suitably standardized clip for all scout machines of certain dimensions would involve the making of one set of press and bending tools only for the machines of the one type built. Another aspect, quite as important, is the simplification of the supply of spares. Acceleration of aircraft output, if achieved only through the medium of small part production, is one of the most important contributary factors towards ultimate success in the field.