Comment on "Experimental Determination of Improved Utilizing Biplane Wing Configurations

Abstract

I Ref. 1, Olson and Selberg have presented very interesting data relating to lift and drag of biplane wing combinations. They find that, over a wide range of lift coefficients, there are combinations of biplane gap, stagger, and decalage which give a significantly higher lift to drag ratio than a comparable monoplane. They also find that the biplane combinations they tested had significantly lower maximum lift coefficient than the monoplane. 1) As was pointed out recently in Ref. 2, a lower maximum lift coefficient is not an inevitable result of a biplane configuration, nor have Olson and Selberg said that it is. 2) One wonders how much of the observed improvement in aerodynamic characteristics was due to the biplane arrangement, and how much was due to the specific wingfuselage-Reynolds number combinations tested. A study of Fig. 9 of Ref. 1 reveals that almost all of the improvement in L/D for the biplane was a result of a reduction in zero lift drag. It is hard to see how any change in inviscid flow distribution, as postulated in Ref. 1, could produce a change in drag at zero lift. The efficiency factor, e, measured for both monoplane and biplane was significantly lower than what is normally observed for commercial or general aviation aircraft. This indicates a possibility that viscous effects, perhaps due to wing-fuselage interference, may be playing an important part in the increase of drag as lift is increased. It would be interesting to see what the drag comparison would look like if the monoplane airfoil had occupied the same forward, high wing position relative to the fuselage as the upper wing of the biplane combination, and/or if the comparison had been made at Reynolds numbers more nearly representative of those that are encountered in commmercial and general aviation aircraft. 3) As was pointed out in Ref. 2, there are important points relating to biplane aerodynamics other than lift/drag ratio. One of these is handling characteristics at or near stall. Figure 13 of Ref. 1 shows a sharp increase in pitching moment coefficient for angles of attack above 16°, which is just slightly below the angle of attack for maximum lift coefficient. This indicates that perhaps the trailing wing is stalling first. This could cause a quite unpleasant pitch up near stall in a real airplane. As is pointed out in Ref. 3, and as is shown by the performance of the Grumman Ag-Cat, biplanes can be designed which have quite docile and safe stalling characteristics. 4) It is hoped that the results presented in Ref. 1 will not obscure an advantage of the biplane configuration which used to be quite well known to designers: with proper choice of gap and stagger, the biplane structure can be braced so that the wing weight is much less than for a monoplane. Reference 3 indicates that the biplane wing weight per unit area can be made approximately half that for a comparable cantilever monoplane. This, combined with the shorter wing span of the biplane, makes it possible to design a biplane with handling characteristics that one would normally find only in a monoplane with a much smaller pay load. The high stagger configurations tested in Ref. 1 might be difficult to brace. 5) The authors of Ref. 1 are acheiving their low drags with approximately the same configurations that Nenadovitch, tested as two-dimensional configurations. This would indicate that perhaps the low drags which are being measured are due to some two-dimensional inviscid-flow-boundary-layer interaction. If so, analytical tools exist for investigating this effect. In fact, Ref. 3 describes how the problem might be investigated analytically even for finite span biplane wing systems. In short, Olson and Selberg have experimentally demonstrated a very interesting effect. It could be very worth while to investigate the configurations they have tested further, both analytically and experimentally, to see if more can be learned about the causes behind the effects they have demonstrated.