Aeroplane Construction and Operation

by John B. Rathbun

In a series of tests made by A. Tcherschersky, the backward stagger as in Fig. 5 gave about 15 per cent greater lift than the orthogonal biplane, or about 4 per cent less lift than a monoplane surface of the same area. The stagger in this experiment was about 0.33C. In default of more accurate information, it would seem that backward stagger would give better results than forward stagger, since the air swept down by the upper surface would pass further to the rear of the lower plane and hence would not so greatly affect the vacuum on the upper surface of the lower wing. This would, however, destroy the view of the pilot to a greater extent than any of the other arrangements.

Stagger always introduces structural difficulties, makes the wings difficult to assemble, and the wires are of varying lengths. A simple orthogonal cell is more compact and better from a manufacturing standpoint, as it simplifies the fittings, and to a slight extent decreases the weight. When combined with sweep back, the complication is particularly in evidence. It is pleasing to note the prevalence of orthogonal cells on modern battle-planes.

Influence of Camber. The amount of air swept down by the upper wing is largely determined by the curvature of the under surface of the upper wing. By decreasing, or flattening out the curvature of this surface, the velocity is increased in a horizontal direction and reduced in a vertical direction, so that the lower wing is less affected. The upper surface of the upper wing is not influenced by interference. It should be noted at this point that air in striking a convex surface is increased in horizontal speed while the reverse is true of the lower concave surface. If the under surface of the upper wing were made convex, the down trend of the air would be still further reduced, and the loss on the lower wing reduced in proportion.

Increasing the camber on the upper surface of the lower wing increases its horizontal velocity and hence affects the upper wing to a less extent, but as the upper wing loss is comparatively slight, the camber increase below is not of great consequence. This has only been tried in one machine to the writer's knowledge, one of the Standard seaplanes, in which the upper wing was an R.A.F.-6 and the lower wing was a deeply cambered U.S.A.-2 section. The lower surface of the R.A.F.-6 is comparatively flat.

Effects of Decalage. When the upper wing incidence is increased in regard to that of the lower wing, or is given decalage, the stability is increased with a slight increase in the power or drag. This angle shown by (d) in Figs. 7 and 8, must be accompanied by stagger to obtain stability, the angle (d) ranging from 1° to 4°. With a decalage of 2.5°, and a stagger of half the chord, a high degree of stability is attained with a loss in the lift-drag of from 4 to 6 percent. The lift and the range of the stalling angle are both increased, the former by about 3 percent, while the latter is nearly double. By increasing the decalage to 4°, the lift-drag is still 4 percent less than with the orthogonal cell, but the range of the stalling angle is nearly tripled. The 4° decalage is very stable and is suitable for training machines or for amateurs. In either case, the stagger-decalage system is usually better than sweep back, reflex curves or negative wing tips.

Without regard to the stability, and only with the idea of a greater L/D in mind, it has been usual in several European machines to adopt a "negative" decalage; that is, to increase the angle of the lower wing in regard to the upper chord. With the top chord horizontal, a negative decalage of 4° would make the incidence of the lower wing equal to 4°. This has not been generally found advantageous in model tests, but in full size machines there is a considerable increase in the L/D ratio. The greater incidence of the lower wing also improves the lift of this surface and thus requires less surface for obtaining the same total lift, especially when top wing is staggered forward. Incidence of top wing of Nieuport = 1°-30'. Lower wing is set at 3°.

Varying Incidence. With several types of European speed scouts, and in the case of the old Handley-Page monoplane, the angle of incidence is reduced from the center of the wing to the tip. Thus in one speed scout, the incidence at the body is 4°, and 2° at the tips. A decrease in angle toward the tips has much the same effect as an increase in aspect ratio; that is, it decreases the lateral flow and end leakage. It also has an effect in aiding the lateral stability because there is less lift at the tips, and hence they are less affected by side gusts. "Washed out" incidence is an aid to longitudinal stability, as the center of pressure at the tips is moved further back than at the center of the wing, and therefore the C. P. is distributed over a longer distance fore and aft than it would be with a uniform angle of incidence.

In driving the propeller, the motor tends to turn the body in a direction opposite to that of the propeller rotation, and if no other provision is made this must be overcome by means of the ailerons. The "Motor torque" on small span machines is particularly difficult to overcome in this way, owing to the short lever arm length of the ailerons. To practically overcome the torque, without excessively loading the ailerons, it is usually the practice to set the lower left wing tip at a greater angle than the lower right wing. The greater angle at the left gives a lift that opposes the turning moment of the motor. This compensation can never be complete, for the motor torque varies with the motor output, hence an average angle is selected so that the incidence will cover the usual horizontal flight speeds.

Triplane Arrangement. When a biplane exceeds a certain weight the area required for a given landing speed makes it desirable to increase the number of lifting surfaces to more than two, if the span and stress are to be kept down within reasonable limits. Thus the biplane has its limits as well as the monoplane, and in the biplane this limit is generally reached when the span approaches 80 feet. In addition to the increased weight due to spans of over 80 feet, there are other troubles in regard to the space required for housing, and awkwardness in maneuvering. On the smaller and faster aeroplanes, the triplane arrangement permits of space condensation, and also allows of larger aspect ratios than with the biplane. The greater depth of the triplane structure makes the interplane bracing even more effective than in the case of the biplane. For equal spans there is less bracing exposed to the wind, and the weight of the wing spars and ribs can be considerably reduced. The shorter ribs of the triplane alone contribute in no small degree to the saving in weight.

Considering the wings alone, without reference to the head resistance of the bracing, etc., there is a greater loss of lift and L/D when three tiers of wings are superposed than with a biplane. In experiments by Dr. Hunsaker upon R.A.F.-6 and Curtiss wing sections, it was found that at about 4°, that the triplane required about 6 percent more power than the corresponding biplane. At this angle, the L/D for the triplane was 12.8, against the ratio of 13.8 for the biplane. The gap-chord ratio in each case was maintained at 1.2. Both the R.A.F.-6 and the Curtiss wings gave results of the same general character, and there was not a great deal of difference in the numerical values. At very high angles, 12° to 16°, the lift of the biplane and triplane only differed by about 2 percent, but at very small angles such as are used at normal flight speeds, the reduction of lift in the triplane was very marked.