Examples: MultiPlanes

Multiplanes include biplanes such as the Wright 1902 glider shown below. Although the Wright brothers exploited the structural advantages of biplanes, rather than the lower vortex drag for fixed span and lift, their motivation was partly aerodynamic. Based on their own tests and those of Otto Lilienthal, it was apparent that at very low Reynolds numbers (typical of test conditions used by these pioneers) highly cambered, thin sections performed much better than thicker sections, making the cable-braced Lilienthal designs or the Wright biplane concepts especially attractive. Because of the low flight speeds required for Lilienthal's take-offs and landings and for the power plants available to the Wrights, the designs needed to be light and incorporate large wing areas. This requirement was satisfied well with the biplane configuration.

The multiplane concept was taken to extremes by Phillips in 1904. The aircraft shown below with 20 wings would have had a high span efficiency, but the very low Reynolds number of each wing would lead to poor performance. The struts and cables of early biplane designs also led to large parasite drag, so the effects of improved span efficiency were not obvious. Several modern proposals for cantilevered or semi-cantilevered biplanes have emphasized the lower vortex drag of such configurations at the expense of structural efficiency, Reynolds number, and fuel volume.

The induced drag of a multiplane may be lower than that of a monoplane of equal span and total lift because the nonplanar system can influence a larger mass of air, imparting to this air mass a lower average velocity change, and therefore less energy and drag. For a biplane, if the two wings are separated vertically by a very large distance, each wing carries half of the total lift, so the induced drag of each wing is 1/4 that of the single wing. The inviscid drag of the system is then half that of the monoplane.

In addition to the well-known advantages in vortex drag, the favorable interference between two wings of a closely-coupled biplane can be used to improve the section performance. The lower-than-freestream velocity at the trailing edge of the forward wing and the new boundary layer on the downstream wing can be exploited and some of the difficulties with lower Reynolds numbers for the biplane as compared with a monoplane can be alleviated if not turned to advantage. Gains in CLmax, width of laminar drag bucket, and drag divergence Mach number at fixed t/c are possible with good multiple element section design. As an example, a single fully-laminar section (100% laminar flow on upper and lower surfaces) can support a CL of about 0.4. A 2-element wing can be designed with an overall CL of about 0.75. This may help to explain the preference for biplanes in the low Reynolds number world of insects.