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Whitcomb area rule

The Whitcomb area rule is a technique to reduce an aircraft's drag at transonic speeds.

Even at high subsonic speeds, local supersonic flow can develop in areas where the flow accelerates around the aircraft body and wings due to the Bernoulli effect. The resulting shock waves formed at these points of supersonic flow radiate away a considerable amount of power, which is seen to the aircraft as a sudden and very powerful form of drag, called wave drag. In order to reduce the number and power of these shock waves, the body should change in shape as slowly as possible. This leads to a "perfect" aerodynamic shape known as the Sears-Haack body, rougly shaped like a cigar but pointed at both ends.

Making an aircraft with such a shape is not all that easy. It is important to note that the entire aircraft must have such a shape, not just the fuselage itself. This means that areas where the wings and tail attach to the fuselage need to be accounted for in considering the overall cross section. Making the fuselage narrow considerably at these areas was suggested by Richard Whitcomb after an extensive series of tests at NASA's (then still NACA) Langley Research Center[?].

The area rule was emidiately applied to a number of current development efforts. One of the most famous was Whitcomb's personal work on the re-design of the F-102 Delta Dagger, which was desmonstrating performance considerably worse than expected. By indenting the fuselage beside the wings, and (paradoxically) adding more volume to the rear of the plane, transonic drag was considerably reduced and the original Mach 1.2 design speeds were reached. Another example is the Soviet-era bomber, the Tu-95 Bear[?], which adds long nacelles to the rear of the engines in order to increase the cross-section behind the wing.

Aircraft designed according to Whitcomb's area rule looked odd at the time they were first tested, and were dubbed "flying Coke bottles," but the area rule is indubitably effective and came to be an expected part of the appearance of any transonic vehicle. Later designs started with the area rule in mind, and came to look much more pleasing. Although the rule still applies, the visible fuselage "waisting" is no longer common – the same effect is now achieved by careful positioning of aircraft components.

It was later re-discovered that a German WWII design, the Kuchemann Coke Bottle included a fuselage shaping basically identical to the area rule. However in this case Dietrich Kuchemann was attempting to solve another problem entirely, the problem of span-wise flow on a swept wing. It appears that he was unaware of the drag benefits of the design. Another design at Messerschmitt appears to have been built specifically for drag reduction, but their complex double-boom design was never built even to the extent of a model.

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