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How The Area Rule Helps Jets Fly Faster

primary USAF

If you've ever seen a 1960's era fighter, you might think it looks like a wasp. The fuselage bends in at the waist, right as the wings reach their full span.

It's a design principle known as the "area rule," pioneered in the 50's by Richard Whitcomb (who also happens to have invented the winglet), an engineer at the National Advisory Committee for Aeronautics. It's a rule designed to minimize wave drag, which starts to appear as aircraft near Mach 1.

Wave Drag - Pulling You Back At Transonic Speeds

Engineers discovered that as an aircraft approaches Mach 1, the air flowing around the fuselage and wings begins to exceed Mach 1 - this is known as the transonic speed range.

As the supersonic flow develops around your aircraft, so do shock waves - boundaries of pressure where the airflow moves from supersonic to subsonic flow. These shock waves generate an immense amount of drag - and require significant thrust to overcome.

Keeping The Area The Same

In the 1950's Whitcomb discovered wave drag forming around models in a wind tunnel as the flow approached Mach 1. He realized that to minimize the amount of wave drag created by an aircraft, its cross-sectional area needed to change smoothly. When the wings spread out from the fuselage, the total cross-sectional area quickly increased - and so did the wave drag.

The solution? Taper the fuselage in where the wings spread out. That balanced the total cross-sectional area, and minimized wave drag.

One of the first - and most popular - examples of the area rule was the F-102 Delta Dagger. The aircraft was originally designed with a straight fuselage - and performed much worse than expected in the transonic range. - limiting the aircraft to Mach 0.98.

The aircraft was resigned with Whitcomb's area rule (which he had just discovered). By smoothing the cross-sectional area along the length of the aircraft, the new prototype reached Mach 1.22 - with only a slightly more powerful engine.

F-102 USAF
f-106 USAF
b-1b USAF

Not Limited To Tapering

The area rule isn't limited to tapering, and you can see its effects on airliners and high-speed aircraft today. And, while you can still see the "waisting" effect on some of the aircraft, the rule's often applied in less obvious ways.

By placing the engines aft on the Citation X, the design can still apply the area rule without constricting the (already narrow) fuselage.

citation-x Michael Bludworth

By placing the engines forward of the wing (instead of directly underneath) on an airliner, you can distribute the cross sectional area.

787 RS Deakin

And, by adding large, streamlined wheel fairings behind the wing of a Tupolov Tu-95 Bear bomber.

tu-95 Wikipedia

So now you know - all those design features on fast jets aren't just made for looks, they're made for speed.

Aleks Udris

Aleks is a Boldmethod co-founder and technical director. He's worked in safety and operations in the airline industry, and was a flight instructor and course manager for the University of North Dakota. You can reach him at aleks@boldmethod.com.

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