Aeroplane Construction and Operation

by John B. Rathbun

The drag was not greatly different below 12°, but at 16° the drag-coefficient is less than that of either the biplane or monoplane, and for machines flying at low speeds, or heavily loaded, this decrease is of great advantage since it relieves the motor at a time when power is particularly required. At this point it should be noted that at high angles, the L/D generally is better for multiplanes in an almost direct proportion to the number of surfaces. In this experiment, the lift-drag ratios for a monoplane, biplane, and triplane were respectively 4.5, 5.6, and 6.5. The drop in lift after the point of maximum lift, or the stalling angle, is not as rapid as in the case of the biplane or monoplane, and hence there is less danger of stalling the triplane. With the same area, and loading, the landing speed of the biplane and triplane will be about the same.

The following tables give the lift, and lift-drag ratios as determined in these experiments, the factors being in terms of the monoplane values of an R.A.F.-6 wing. Thus to obtain triplane values, multiply the given monoplane values by that number opposite the required angle of incidence. Aspect ratio = 6.

Curtiss Triplane Speed Scout. Note the Great Aspect Ratio of the Wings, and the Relatively Great Gap-Chord. Ratio. Only One Set of Struts Are Used in a Single Row, Hence the Head Resistance Is at a Minimum. The Span Is 25'-0" and the Chord 2'-0", Giving an Aspect Ratio of 12.5.

Thus, if the monoplane lift value for the R.A.F.-6 wing at 4° is Ky = 0.001.45, then the triplane value will be 0.00145 + 0.757 = 0.001097 as given in the table. The monoplane lift-coefficient of any other wing section can be handled in the same way with fair accuracy. To obtain the corrected lift-drag ratio for any wing section, multiply the lift-drag of the monoplane wing by the factor in the above table corresponding to the incidence of the monoplane test wing.

The Italian Caproni Triplane of the Heavy Lift or Bombing Type. Motors Are Installed in Each of the Three Bodies, Tractor Propellers Being Used in the Two Long Outer Bodies, While a Pusher Screw Is Used at the Rear of the Central Passenger Body. The Enormous Size of This Triplane Can Be Seen by Comparing it with the Caproni Monoplane Shown at the Right. Courtesy "Flying."

The upper wing gives the greatest percentage of lift, and the middle wing the least, since the latter suffers from interference on both sides. It has been found that the sum of the top and bottom wings of a triplane group gives the same lift as the two wings of a biplane under equal conditions. It was also found that the lift-coefficients and lift-drag of the upper plane alone was very nearly equal to the lift of the combined effects of all three wings, and at all angles. Calling the lift of the middle wing 1.00 (4°), the lift of the upper wing will be 1.91 and the lower wing 1.64. Calling the L/D of the middle wing 1.00 (4°), the relative life-drag will be L/D = 2.59 for the upper wing and 1.69 for the lower. With the middle wing still assumed at unity, the lift of the top plane is at 1.49 at 16°, and the lower wing 1.20. The liftdrag at 16 degrees will be respectively 1.00, 1.22, and 1.117 for middle top and bottom. At 0°, the upper wing will carry 2.68, the middle 1.00, and the bottom 1.82. At 0°, the lift-drag of the top is 3.63, the middle 1.00, and bottom 2.30. These relative figures are only useful in comparing the loading when computing the strength of the structural parts. See "Aviation and Aeronautical Engineering" Nov. 1, 1916.

Overhanging Wing Tips. In many American machines, and in some European machines, such as the Farman, the upper wing is given a much greater span than the lower. Of late, the tendency has been to make the wings of equal, span and fully 90 per cent of the modern machines will be found to be arranged in this way. While the overhanging tips may slightly increase the efficiency of the biplane by reducing interference at the ends, it makes the span unduly long and difficult to brace at the end. The added end bracing due to the overhang probably offsets any aerodynamic advantage to be obtained, although I have no accurate data on this point. Compactness is certainly not a feature. It is said that ailerons are more effective when mounted on the upper overhang, and this may be so, but I note that the area is about the same in any case. With overhanging tips, the ailerons are generally placed on upper wings, only while with equal or nearly equal spans, they are placed top and bottom. The overhanging section and the ailerons form a single detachable unit as a general rule. With nearly equal spans, the upper and lower ailerons are generally interconnected with a small strut in such a way that they act together.