How the ‘Area Rule’ dictates plane design

Date:26 September 2017 Tags:, , , ,

The Whitecomb area rule is a technique for aircraft design used to reduce drag at transonic and supersonic speeds.

By David Grossman

In the mid-to-late 40s, in the thick of WWII, engineers designing fighters were discovering that something unusual would happen when planes reached transonic and supersonic levels of speed. Shock waves would begin to form around the plane. These would create a barrier that would sap up the plane’s energy.

The solution to this problem was the development of what’s called the “Whitcomb area rule”. It dictates the careful design of an airplane’s cross-section to limit the places where shockwaves can build up. Real Engineering looks at how engineers worked around this problem. They created designs that are still used in modern flight.

The area rule explained:

Having independently observed this conundrum, German and American scientists joined forces after the war’s end to codify the design recommendation.

In 1950, Richard Whitcomb, a scientist at the National Advisory Committee for Aeronautics (NACA) Langley Research Center in Hampton, Virginia, was struggling with the issue of sonic booms. He had gotten an 8-foot wind tunnel that allowed for tests to hit 0.95 Mach speed, which was only of limited usefulness.

It wasn’t until the following year, when Dr. Adolf Busemann, a German scientist who like Werner von Braun had been scooped up by the American military after the war, came to deliver a speech on transonic airflow that Whitcomb was able to make headway.

Dr. Busemann delivered a radical message: Bernoulli’s Theorem, a standing rule of aeronautics stating that when airflow was made narrower, the speed of that air would increase, did not apply at transonic or supersonic speeds. This ultimately helped Whitcomb realize that he couldn’t look at the wing and fuselage as two separate parts anymore. The entire cross-sectional area of the design had to be designed with as smooth a curve as possible.

This rule still affects the design of many planes today. An example is the Boeing 747, which has its signature upper deck “hump” placed in such a way as to make the change in cross-section along the planes fuselage as smooth as possible.

Source: Real Engineering
Image credit: Deniz Altindas on Unsplash
From: PM USA