When Alexander Graham Bell and his fellow Aerial Experiment Association members flew the first powered aircraft ever to do so in Canada, its configuration was that of a biplane. Virtually every other aircraft built at the time, led most notably by the Wright Brothers’ Flyer, had the same wing arrangement. By the 1930's, however, the biplane was virtually obsolete. Why two wings instead of one? It’s an interesting question that had roots in many places, not least in the mind of a French-born railroad engineer whose research profoundly influenced the design of powered aircraft for the first 30 years of their existence.
Octave Chanute built stock yards and bridges. When he retired from that in the 1890's, he started experimenting with airplane designs and applied structural concepts used in bridge building to create light-weight biplane gliders. Figuring that the best way to achieve extra lift without adding excessive weight was to stack multiple wings above each other, Chanute came up with the idea of strut-wire bracing. The Wright Brothers used the concept, and Chanute’s idea ended up sticking around. Early airfoils were thin; the braced biplane arrangement provided structural efficiency that a braced single-wing (monoplane) couldn’t match.
A biplane, in theory, should produce half the induced drag – that is, drag caused by an aircraft’s lifting surfaces – of a monoplane with equal span. Induced drag is proportional to the square of the lift being generated. Assuming a given biplane and monoplane produce the same amount of lift, that lift is evenly split between two wings on the biplane. Thus, each wing should have only one-fourth of the drag of the single-wing aircraft of equal span. Since a biplane has two wings, the total induced drag of the biplane should then total half (i.e. one- fourth plus one-fourth) of that obtained with an equivalently-spanned monoplane. Unfortunately, it doesn’t quite work out that way.
A biplane’s wings, given their proximity to each other, tend to work the same portion of air to generate their lift. While a well-designed biplane can provide a 30 percent reduction in induced drag compared to a monoplane with the same span, doubling the number of wings doesn’t double the amount of lift. To obtain that 30 percent reduction in induced drag, and doing so within the same total wing area of a typical monoplane, the biplane would need to have a wing aspect ratio (i.e. its wing’s span compared to its width, or chord) that is about double that of the monoplane’s aspect ratio.
If a biplane and a monoplane were designed with the same total wing area, with the monoplane having an aspect ratio the same as each of the biplane’s wings, then the monoplane would have a span some 40 percent greater than the biplane. Since high aspect ratio wings have less induced drag, the monoplane in this case would have much less induced drag than the biplane with the same wing area. A biplane, therefore, will only provide a reduction in induced drag if it’s total wing span is less than that for a monoplane.
The reduced wing span of a biplane achieves a couple of objectives. The short span on aerobatic biplanes increases their manoeuverability. Biplanes also fit into tighter spaces in a hangar. In terms of other benefits, their strut and wire bracing provide for a light, but very strong, wing. Generally, for practical reasons, the gap between a biplane’s wings is about equal to the average chord length. A shorter gap will increase the interference between the two wings. The span ratio (the ratio between the upper and lower wings’ lengths) has minimal impact on the performance of the aircraft, though equal-length wings minimize induced drag. Offsetting the wings (called stagger) usually improves visibility skyward from an open cockpit when the upper wing (positive stagger) is forward of the lower wing. A forward lower wing relative to the upper wing (negative stagger) improves visibility in closed cockpit biplanes (like the Beech Staggerwing). Too much stagger either way can weaken the structural benefits of a biplane design. A higher angle of incidence on a biplane’s upper forward wing improves its stall recovery.
Today, biplane configurations are mainly used for aerobatic purposes and crop dusting. It’s best applied as a design feature when low structural weight is more important than aerodynamics, and when low speed is required without a large wing span.
Comments