In the reception area at burt rutan's Mojave, California, skunk works, Scaled Composites, there sits on a corner table a small black tripod with a cup-shaped receptacle on top. Built by composites engineer Stan Stawski, who works at Scaled, it supported two tons before failing. It weighs less than four ounces. Getting the most strength for the least weight is the real crux of aeronautical engineering. In airplanes, structure is a necessary evil. The same could be said of fuel, and the two evils come together in airplanes intended to fly extremely long distances. Three factors control the range of an airplane: propulsive efficiency, aerodynamic efficiency and fuel fraction. Once a powerplant has been decided upon, propulsive efficiency is known. It comes with the engine, or the engine-propeller combination. Aerodynamic efficiency, expressed as lift-drag ratio, is principally (assuming a streamlined shape) a matter of maximizing wingspan and minimizing wetted area; that's the aerodynamicists' business. Fuel fraction is the structural engineers' contribution to the equation. It's the portion of the takeoff weight that is fuel. Most general aviation airplanes carry less than 20 percent of their total weight in fuel; long-haul airliners may carry 35 percent. To increase the fuel fraction from, say, 35 percent to 70 percent is not twice, but four times, or possibly eight times, as difficult as to increase it from 20 percent to 40 percent.
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