Mark Mallari of Manila, the Philippines, writes in with a question on supersonic flight: “Whatever happened to the Whitcomb area rule that revolutionized the design of supersonic aircraft? A supersonic aircraft’s fuselage was supposed to narrow down at the widest point of the wings, in order to maintain a constant overall cross-sectional area for less drag.
“Early supersonics such as the wasp-waisted F-106 and B-58 Hustler are exemplars of this, but modern fighters such as the Eurofighter Typhoon seem to have no need for area ruling in their design,” he writes.
Mallari points out that the rule’s namesake, aerospace engineer Richard Whitcomb, who discovered it in 1952 while working at the National Advisory Committee for Aeronautics, won the Collier Trophy “so it must not be something insignificant!”
The answer is, the area rule only mattered to designers of early fighters, who struggled with underpowered jet engines. In the 1950s, Convair and Grumman were concerned with the phenomenon known as transonic drag rise, in which the air “bunches up” around the fuselage at the wings, causing drag, which decreases aircraft performance. One way to smooth out the bunching was to “pinch” the shape of the fuselage so the air could flow more evenly around the wings.
“Without area ruling, the wing acts like a speed bump. The air just doesn’t have a place to go,” says Cam Martin, external affairs officer at NASA’s Dryden Flight Research Center in California.
Convair used the area rule in designing the F-102 Delta Dagger, the F-106 Delta Dart, and the B-58, while Grumman incorporated it into its F9F/F-11F Tiger.
With the advent of more powerful jet engines, “drag rise become less of a factor because you have enough power to bulldoze through it,” says Martin. That’s why designers of the Typhoon or the Lockheed Martin F-35 Lightning II don’t give a moment’s thought to drag rise; engines on those aircraft have plenty of brute force to overcome it.
In the NASA volume From Engineering Science to Big Science, Lane Wallace writes that Whitcomb’s discovery “was just an idea. It may have been developed at a NACA laboratory, but it was not up to NACA to apply it. In order for the innovation to have any impact at all, industry had to agree to use it, which is not always as simple a process as it might seem. …The advantages offered by the innovation were the same; the costs of implementing it differed.”
The area rule, says Martin, is simply “a tool or a technique,” one that has little relevance to today’s military jets.