Biplane Trussing.
Fig. 59.—Biplane Truss.
The most common form of biplane truss is shown by the diagram ([Fig. 59]), sometimes, as in the case of various pusher types, or those for long-distance work where a large wing area is necessary, extended to three bays each side, which probably explains the partiality of German designers for multiplicity of interplane struts, as, prior to the outbreak of the war, the majority of German machines were designed and built entirely for long distance and duration flying. By this means light wing loading, which entails large wing area, was possible without prohibitive weight, for by the addition of a pair of struts to the two-bay type, a lighter wing spar for the same strength is possible. In this type of truss the bays adjacent to the fuselage are varied in width, in order more easily to apportion the stresses, which are greater at the centre of the wing structure. A modification of [Fig. 59] is indicated by [Fig. 60], which illustrates diagrammatically the arrangement of the Maurice Farman biplane, the improvement consisting of the method of strengthening the interplane struts. The outer strut is braced with a small king-post, and from this a wire is taken through each side of the strut. On this machine the struts are of the light, hollow-spar type, and this arrangement must therefore materially reduce their tendency to buckling.
Fig. 60.—Farman wing structure.
Another version of this system is that in which the top plane is of greater span than the bottom, the extension thus formed being stayed with lift and counter-lift wiring, or by means of a strut acting in tension and compression.
Single Strut Systems.
The almost universal arrangement for the small single-seater scout is the single bay, and from this method the progress of design has inclined towards the elimination of as many struts and wires as possible, which has its culminant in the type of truss embodying one strut and one pair of wires, lift and counter-lift, each side of the body. Quite a number of machines have incorporated the single strut assembly, the earliest perhaps being the Brequet, and one also remembers a small Avro scout, the strut in this case being built up with spars and stringers, covered with fabric. The single-lift truss is particularly suited to multiplane construction, where the chord of the wings is narrow, and the bending moment, due to the movement of the centre of pressure, is correspondingly reduced. A disadvantage exists with this form of truss similar to that experienced with the wireless monoplane truss, i.e. the difficulty of maintaining the correct incidence from root to tip. However, some extraordinary machines of recent construction embodying this feature, stand up to active service demands, so that this defect can be of no great moment. A minor detail consists in the circumstance of, for example, a lift wire coming adrift or perhaps being shot away. With the single-lift truss total collapse would ensue, but it is conceivable that the ordinary double-lift truss offers more chances of escape.
