The third system is typical of the method adopted for the series of German Albatross machines. There are few, if any, examples of its use in this country, although prior to the war a few constructors favoured its use, and one successful monoplane of note was so built. The writer is acquainted with one pioneer designer who very strongly believes in this form of construction, and certain later developments in the use of three-ply confirm this view. The advantages of this form of construction are: (1) quickness of production; (2) great strength in a vertical and horizontal direction; (3) the result of the longeron being shot through would not endanger the structure to the same extent as with a wire-braced system. Against this must be balanced the fact that: (1) it entails a considerable increase in weight; (2) is weak under a torsional strain, such as that produced by the combined actions of elevator and rudder; and (3) cannot be trued up in the event of distortion. Examples of this system in pre-war machines are afforded by the Martinsyde and Blackburn monoplanes, although the framework in both cases was so formed as to constitute a lattice girder. The tail portion of the Martinsyde was lightened by cutting away diamond-shaped pieces from each bay.
The formers of the Albatross are extremely simple. In the fore part they are cut from three-ply, while at the rear they are just simple frames composed of laths, reinforced where the longerons occur by three-ply stiffeners. There are six longerons, the two middle ones being fixed slightly more than halfway up each side, which are really longitudinal stringers to prevent the three-ply buckling between the points of attachment.
The Monocoque Type.
The monocoque system originated in France, several constructors having produced machines incorporating this feature. The most successful machine produced on these lines was the Deperdussin, and many will recall the excellent streamline form of the machine exhibited at the 1913 Aero Show. These bodies were built over formers of various sections, which were removed when the glue joining the different layers had set. The resultant shell, which was about four millimetres thick, was then covered with fabric and varnished. Several factors militate against its extensive adoption as a method. It is rather costly, and does not seem to be suited to rapid production. In addition, the attachment of such members as the chassis, wings, and interplane struts, is more complicated. It should be noted, however, that various modern machines are similarly built. The Borel firm produced a machine with monocoque body, this being composed of three-ply covering on ribs running diagonally the length of the body, and although this is not such a lengthy operation as the Dep. system, it has not survived, unless one considers flying-boat construction as its modern version. A slight variation of the monocoque system is used for the bodies of some modern aeroplanes. The framework consists of very small stringers arranged at various points on light formers cut to the fuselage section. To this structure is applied two thicknesses of three-ply in the form of strips about 3¼ ins. wide, each thickness being disposed diagonally in opposite directions, as shown by [Fig. 68]. This is covered with fabric, the total thickness being no more than 1½ mm., and as this is made up of six layers of wood and one of fabric, the fineness of the ply-wood will be realized.
Fig. 68.—Arrangement of three-ply bands in monocoque fuselage.
It should be noted that the ply-wood strips do not completely encircle the formers, but are jointed at the top and bottom, a light longeron being arranged at these points.
A detail which would appear to be of great utility at the present time is the arrangement wherein the nose of the body containing the engine and accessories is a separate unit, and in the event of engine breakdown can be detached and another substituted.
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.