Aeroplane Construction and Operation

by John B. Rathbun

Biplane Characteristics. From an aerodynamic standpoint, the monoplane wing is more efficient than the superposed wings of the biplane type, since the proximity of the two surfaces in the latter causes a decided loss in the total lift. Other practical advantages, however, offset the losses due to the superposed surfaces, and hence the total efficiency of the complete biplane may be even greater than that of the monoplane. For the same area the structural parts of the biplane are lighter, and this advantage increases rapidly with the size of the machine so that when a span of 36 feet is exceeded, any other arrangement than that of the biplane or triplane becomes almost a practical impossibility. A biplane is easier and cheaper to make than a monoplane, since the wing bracing of the former can be arranged to better advantage, the load-bearing members can be simpler, and the safety factor made higher for an equal weight. By suitable adjustments between the wings of a biplane, it is possible to obtain a very high degree of inherent longitudinal stability without incurring much loss in efficiency, an arrangement that is of course impossible with a single monoplane surface. By "staggering," the view of the pilot is increased, and the generally smaller size of the machine permits of better maneuvering qualities for a given load.

Interference. Due to "interference," or to the choking of the air stream between the upper and lower surfaces, the lift of both wings is reduced, with the drag remaining about the same as with a single surface. This, of course, reduces the total lift-drag ratio at all except certain angles. The relative lift-drag ratios of the monoplane and biplane depend to some extent upon the form of the wing. Interference causes a loss on the opposing faces of the wings, the lift being reduced on the top surface of the lower wing, and on the bottom surface of the top wing. Since the upper surface of the lower wing is under suction, and therefore produces the greater proportion of lift, it is natural that the lower wing lift should be reduced to a greater extent than in the upper wing, since it is only the lower surface of the latter that is affected. At normal flight angles the upper wing carries about 55 per cent of the total load. At zero degrees incidence, the upper wing carries as high as 62 per cent of the total load, while at 12 degrees this may be reduced to 54 per cent.

Gap-Chord Ratio. Calling the distance between the upper and lower wings the "gap," it may be said that the ratio of the gap to the wing chord greatly influences the lift. This ratio is called the "gap-chord ratio," and may vary from 0.8 to 1.0 in small machines or 1.0 to 1.2 in slow, heavy aeroplanes. With the drag remaining practically constant, the lift-drag is of course affected by a change in the gap-chord ratio, this quantity being diminished at small gap ratios. Compared with a monoplane, the lift of a biplane is about 0.77 when the gap is 0.8 of the chord, and about 0.89 of the monoplane value when the gap-chord ratio is increased to 1.6. In this range the lift-drag approximates 0.82 and 0.89, respectively. The center of pressure movement is not greatly changed with any gap-chord ratio, and to all practical purposes remains the same as with the monoplane. It should be understood that these remarks apply only to the "Orthogonal" biplane arrangement in which the wings are vertically over one another.

While biplane efficiency is increased by having a large gap-chord ratio (wing efficiency alone), the total efficiency of the aeroplane is not always increased by a large gap, principally because of the great head resistance due to the longer struts and interplane bracing. At high speeds the longer bracing members often more than offset the gain due to wing efficiency, and as a result the gap of high speed scouts will generally be found in the neighborhood of 0.8 the chord. With slow, heavy machines, where lift is of great importance, and where slow speed does not affect the structural resistance to so great an extent, the gap-chord ratio will range from 1.0 to 1.2.

In making the above comparisons between monoplanes and biplanes, equal aspect ratios have been assumed for both types, but in actual practice the aspect ratio of biplanes is always greater than with monoplanes, and as a result the biplane loss is usually less than indicated above. When correction has been made for the aspect ratio, the disparity in the monoplane and biplane values of Ky and L/D is not as great as commonly supposed. "Biplane reduction factors," or the factors used in reducing monoplane values to those of the biplane, depend to a great extent upon the wing section as well as upon the gap, and for exact values of the factors we should have the tests report of the wings in biplane form. Lacking this information, we can adopt the values obtained by the N. P. L. for an old type of wing in order to get approximate results. To obtain the biplane values, multiply the monoplane values obtained by the wind tunnel test by the factors found under the required gap-chord ratio. These factors apply to an aspect ratio of 6.

Gap-Chord Ratio.	0.8	1.0	1.2	1.6
Ky Reduction Factor	0.77	0.82	0.86	0.89
L/D Reduction Factor	0.82	0.84	0.85	0.89

Dr. Hunsaker conducted experiments at the Massachusetts Institute of Technology on biplane and triplane combinations, and the results were reported in "Aviation and Aeronautical Engineering," Nov. 1, 1916. The R.A.F.-6 section was used with a gap-chord ratio of 1.2. The biplane portions of the experiments are as follows, the actual Ky and L/D values and reduction factors being arranged according to the angle of incidence:

It will be noted that there is steady improvement in the lift factor with an increase in the angle from 2° up (except at 8°), and that the same holds true with the L/D factor. That is, the biplane values become nearly monoplane values at high angles, and in the case of the L/D ratio the biplane actually is 24 per cent greater than the monoplane value at an angle of 16°. The lift coefficient Ky above, is not far from the corresponding Ky, for gap-chord ratio = 1.2 in the first table. The maximum biplane value of L/D occurs at the same point as in the monoplane wing, that is, at 4°. The fact that the lift-drag is so high at 16° is very favorable, since the biplane would be less likely to stall when flying slowly, and with a big demand on the engine. The range of angles at the stalling angle is much greater than with the monoplane wing, and the lift does not fall off so rapidly after the maximum is reached.