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Name: Steven
Status: student
Grade: 9-12
Country: United Kingdom
Date: Fall 2011


Question:
As planes have got so large, the wings have become huge. Why can they not go back to biplanes for the same surface area but only half as wide?



Replies:
Hi Steven,

In the early days of aviation, biplanes were a very common design since they could be made very strong using the materials and methods of the time (wooden frames with fabric covering). The two wings would form a strong truss structure with diagonal strut wire or bar bracing. It was not as easy to create a strong wing if there was only one (a monoplane in other words), but even so there were early monoplanes as well. For example, there was the Bleriot XI which crossed the English Channel in 1909. It used quite a bit of wire bracing above and below the wing. While the all the struts and wires did strengthen early airplanes, the downside though was a lot of aerodynamic drag just due to the struts and wires alone.

You do not see biplanes in any commercial aviation anymore because they are not as aerodynamically efficient as a monoplane. Even though modern day materials (aluminum alloys and polymer composites) would allow building a biplane without any external bracing that would create drag, the design itself is not as aerodynamically efficient. This is because the top wing interferes with the lift production of the bottom wing. A wing produces lift by disturbing the air below and above the wing to a significant distance (a couple wingspans or so, morose above). So stacking the wings interferes with the lift, and a biplane does not have twice the lift as if it were a monoplane with one of the wings removed. Another reason biplanes are not as efficient is wing tip vortices. Shorter wings tend to produce stronger wing tip vortices which are a source of drag. And biplanes have 4 wing tips while a monoplane has only 2. So to sum up, it is not just surface area that matters here. Aerodynamic drag is also a function of the way that surface area is distributed, i.e., what shape are the wings.

Aerodynamic engineers have found that the most efficient airplanes they can build will have only one wing with a high aspect ratio (ratio of length to width). This is what you find nowadays on airliners which have to be built for the utmost efficiency. But in general aviation you will still see biplanes being flown all the time. They can be highly maneuverable due to their short wing span (e.g., a Pitts Special) and plus they are just cool anyway :) Regards,

John C. Strong






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