Area Rules!

by Dr. Marshall Michel
86th Airlift Wing historian

Several previous articles have alluded to the “area rule” in early jet aircraft design, and given its importance in jet aircraft design and, at the risk of committing an aeronautical engineering faux pas, an explanation is in order.

By the end of World War II, the latest piston-engine fighters and the early jets were beginning to have problems as they approached the speed of sound in dives with the associated change in the air flow over the wings, or compressibility. Different aircraft had different symptoms, but basically compressibility led to out-of-control dives, controls too heavy to be moved or various types of potentially destructive control flutter.

Supersonic phenomena was initially poorly understood but by the early 1950s experimental aircraft such as the X-1 and advanced wind tunnels, many streamlined production fighters were capable of exceeding the speed of sound in a dive. It seemed that a fighter which could easily exceed the speed of sound in level flight was an easy next step.

One of the first production aircraft intended to exceed the speed of sound in level flight was a variation of the Convair XF-92 – the Convair YF-102 delta wing interceptor. In August 1951 the Air Force issued a contract for the aircraft, but despite a powerful engine and seemingly sleek design, the wind tunnel tests showed the YF-102 had much greater drag than expected and would never exceed the speed of sound in level flight.

Several prototypes were constructed but, as predicted, they could not reach the speed of sound except in a dive.

Faced with cancellation of the program, Convair was forced to consider using what was known as the Richard Whitcomb Area Rule, which described a phenomenon first noted by German aircraft designers in December 1943 but was made practical by Mr. Whitcomb. The basic idea of the Area Rule was that to make supersonic air flow easily, the aircraft cross section had to increase and decrease as smoothly as possible (as a sudden change of drag would continue over the entire surface for a longer period), with the largest cross section in the middle and narrower at each end.

This seems to be obvious – after all, this is normal design for aircraft fuselages. But what Mr. Whitcomb found was that the cross section in the middle of the fuselage was total cross section, not just fuselage cross section – and the total cross section included the wing cross section. Thus, the fuselage had to narrow where the wing was thickest to provide the smooth cross section needed for supersonic flight. This ‘wasp waist’ was especially necessary with a large delta wing like that of the YF-102.
It was too late in the design process for Convair to build a whole new fuselage, but they were able to narrow the fuselage at the point where the wing was thickest. The Convair designers then added fairings to the extreme aft of the fuselage to increase its area and complete the smooth cross section.

The modified aircraft was called the YF-102A. It made its first flight Dec. 15, 1954, at Edwards Air Force Base, Calif. On its second flight, the YF-102A easily went supersonic in level flight and a week later hit Mach 1.22 in level flight.

Mr. Whitcomb continued his work on advanced aircraft design, including the supercritical wing and the ubiquitous winglets that produce greater fuel efficiency and are seen on many aircraft at Ramstein. In 1954, Mr. Whitcomb was awarded aviation’s highest award, the Collier Trophy, for his work on the Area Rule.