Other firms tried trapezoidal wings, or ones that approximated an ellipse with two or three trapezoidal panels, and found them not noticeably inferior to the elliptical. Republic Aircraft’s founder Alexander de Seversky and his chief designer, Alexander Kartveli, both favored a wing plan shared by the prewar P-35 and the later P-47 Thunderbolt: a straight leading edge with curved tips and semi-elliptical trailing edge, which gave an elliptical area distribution without the manufacturing difficulties of a curved leading edge. The Hawker Sea Fury used an elliptical wing, but somehow didn’t get the love for it that the Spitfire did. Nevertheless, the spell of the ellipse persisted. The battle for perfection was inevitably lost before it began. Real airplanes would have fuselages and engine nacelles and control-surface gaps and underwing radiators and air intakes and guns, all of which made the lift distribution depart from the ideal of a smooth ellipse. The idea was always more theoretical than practical. The Douglas DC-3, which first flew in 1935, had a wing with a slightly swept leading edge (and a straight trailing edge). Wings of moderate sweep were common in the interwar period. The principle identified by Lanchester is that induced drag is at a minimum when the spanwise distribution of lift is elliptical, and there is an equation to prove it. Parasite drag, on the other hand, affects top speed. Induced drag increases as the airplane slows down and the angle at which the wing meets the air increases, so it mainly affects rate of climb and high-G maneuvering. Induced drag is the drag incurred by the mere action of generating lift, as opposed to the drag due to skin friction (parasite drag) and to turbulent eddies left behind as the airplane passes. The purported virtue of the elliptical wing was that, theoretically at least, it would generate the least possible lift-dependent, or induced, drag. Still, the success of one design with a conspicuous characteristic could not help influencing others. “I don’t give a damn whether it’s elliptical or not so long as it covers the guns!” Shenstone reported Mitchell saying. The elliptical wing’s taper began fairly far outboard, and so it enveloped the guns comfortably. 303 machine guns per side required by the Royal Air Force would not fit within a thin wing of straight taper. The shape had been chosen for the Spitfire “early on,” Shenstone said, because Mitchell, having used thick airfoil sections on a lackluster precursor of the Spitfire, wanted thinner ones for the new fighter. Indeed, the virtues of elliptical wings had been articulated by British theorist Frederick Lanchester in 1907. Many years later Beverley Shenstone, a Canadian-born aerodynamicist who had had principal responsibility for the design of the Spitfire wing, emphatically denied the charge, pointing out that the elliptical wing shape had been used in other aircraft, and that its advantages were well known. But Mitchell’s generous appreciation of the carefully streamlined He 70 gave legs to a story that the design of the Spitfire was “copied” or “cribbed from” or “influenced by” the Blitz. The British fighters of the day were superannuated biplanes, so the comparison cannot have been deeply gratifying to Heinkel. “We have been unable to achieve such smooth lines in the aircraft that we entered for the Schneider Trophy Races,” Mitchell wrote to Heinkel, and he went on to report that a Blitz that Rolls-Royce had acquired and fitted with their 810-horsepower Kestrel engine was “appreciably faster than our fighters.” One of its noteworthy features was the elliptical planform of its wing another was the extremely smooth surface finish of its flush-riveted airframe. The Blitz (German for lightning), designed in 1932, was a fast five-seat mailplane. Before the outbreak of World War II, relations between British and German aeronautical engineers were sufficiently cordial for Reginald Mitchell, chief designer of the Supermarine Spitfire, to write to his German counterpart, Ernst Heinkel, congratulating him on his He 70 Blitz.
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