Fig. 4-A. Deperdussin Monoplane with Monocoque Body. Gordon-Bennett Racer.
Biplanes. Besides the advantages of size, the biplane has a number of other good features. The deep spacing of the upper and lower surfaces permits of a powerful and light system of trussing being placed in the gap, and therefore the biplane can be made stronger (weight for weight) than the monoplane in which no such trussing can be economically applied. The vertical "struts" of the bracing can be clearly seen in the figure. The efficiency of this interplane trussing greatly increases the possible size and capacity of the aeroplane. With monoplanes it is seldom possible to exceed a wing span of 36 feet without running into almost unsurmountable structural difficulties. The weight of the large monoplane also increases is leaps and bounds when this critical span is once exceeded. To maintain an equal degree of strength the monoplane requires very careful attention in regard to the design and construction, and is correspondingly more expensive and difficult to build than the biplane, although the latter has by far the greater number of parts. By suitable arrangements in the location of the biplane surfaces a very fair degree of stability can be obtained, an advantage which is impossible with the monoplane.
Fig. 5. S. P. A. D. Tractor Biplane Speed Scout.
A distinct disadvantage of the two superposed surfaces of the biplane is due to the fact that there is "interference" between the upper and lower wings, and that the lift for equal areas is less than in the case of the monoplane. With a given form of wing, 100 square feet of monoplane surface will lift considerably more than the same area applied in biplane form. The amount of the "drag" for the support of a given load is increased, and with it the amount of power required. The greater the separation or "gap" between the wings, the greater will be the lift, but when the gap is unduly increased to obtain a great lift the length of the interplane bracing is increased to such an extent that the resistance of the bracing will more than overcome the advantages due to the large gap. There is a fixed limit to the gap beyond which it is not practical to go. The bracing has a very material effect on the air resistance, no matter how small the gap.
Fig. 6. Pusher Type Biplane in Which the Propeller Is Placed Behind the Wings.
Triplanes. Of late the triplane has been rapidly increasing in use, and in certain respects has many advantages over either the monoplane or biplane. This type has three superposed surfaces which still further diminishes the size for a given area. The interference between the surfaces is even greater than with the biplane, and hence the lift is less for a given area and aspect ratio. This latter defect is partly, or wholly overcome by the possibility of using long narrow wings, and because of the reduced span there is a corresponding reduction in the bracing resistance. It should be noted at this point that the efficiency of a lifting surface is greatly increased when the ratio of the length to the width is increased, that is, a long, narrow wing will be more efficient than a short, wide shape. The relation of the length to the width is called "aspect ratio," and will be described in more detail in a following chapter.
Fig. 6-A. Farman Type Pusher Biplane.... Note the Propeller At the Rear of Body, and the Position of the Pilot and Passenger.