A Great Figure for Transonic Flight

Area rule was counterintuitive, but effective.

In the early 1950s, Convair—now General Dynamics—built a prototype of a very high-performance jet fighter for the U.S. Air Force. It promised to bring supersonic speed, but the prototype XF-102 had one problem: it couldn’t break the sound barrier. The reason was because of the shape of the airframe, and the solution involved the application of the theory that seems counter intuitive by the standards of non-compressible flow dynamics: area rule.

Based on World War II German research, NACA engineer Richard Whitcomb developed it into “area rule” and the waspwaisted look of supersonic aircraft fuselages of the 50s and 60s were a direct result. Area rule is still effective today, and Boom Supersonic’s new Overture airliner is an elegant, modern example.

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Episode Transcript:

I recently reported on the progress of one of the more interesting of the aerospace startups, Boom Supersonic. The Colorado-based company is aiming to create a 21st-century Concorde, an aircraft similar in capacity and form as the original supersonic airliner, but with far greater efficiency and profitability for airlines.

The renderings show a truly beautiful airplane, as Concorde was. An overhead look at the plan form, however, shows a wasp-waisted fuselage, something that keen aviation observers have seen before in military aircraft such as the Northrop T-38 and the Republic F-105 Thunderchief.

There’s a reason for this.

Back in the 1950s, when jet engines were anemic, supersonic flight was very difficult to achieve. In fact, one manufacturer, Convair, made a prototype of a jet fighter designed to be supersonic, w hich simply couldn’t get through the sound barrier. This problem was solved when the company consulted with a NACA engineer named Richard Whitcomb, who, along with some earlier researchers, determined that there was a proportionality between transonic wave drag and the cross-sectional area of aircraft, largely independent of the overall aircraft shape.

Where the wings meet the fuselage, of course, the cross-section area increases considerably. Reducing this cross-sectional area, by narrowing the fuselage locally, reduces drag in the transonic regime—so much so, that with added afterbodies on the tail, it allowed the Convair F-102 jet fighter to fly supersonically.

For higher Mach numbers, a supersonic area rule is applied, similarly requiring careful shaping of aircraft fuselages and engine nacelles. For the Boom Supersonic Overture airliner, a key design feature is “super cruise,” supersonic flight without the use of afterburners. This puts a premium on aerodynamic efficiency, since the airframe can’t simply blast through the transonic regime with a surplus of engine thrust.

The complex curvatures required are very expensive to produce in a metal monocoque airframe, but Overture is made of composites, and there is little to prevent a composite structure from existing in almost any desired shape, and at low cost.

In the 1950s, that Marilyn Monroe look allowed early jets to break the sound barrier. In the 21st century, Boom Supersonic is using it to get a big, 80-seat airliner above Mach one with dry, medium bypass ratio turbo fans. This could make the dream of Concorde’s designers—regular supersonic passenger flight—a reality.

Written by

James Anderton

Jim Anderton is the Director of Content for ENGINEERING.com. Mr. Anderton was formerly editor of Canadian Metalworking Magazine and has contributed to a wide range of print and on-line publications, including Design Engineering, Canadian Plastics, Service Station and Garage Management, Autovision, and the National Post. He also brings prior industry experience in quality and part design for a Tier One automotive supplier.