The Whitcomb area rule, named after NACA engineer Richard Whitcomb and also called the transonic area rule, is a design procedure used to reduce an aircraft's drag at transonic speeds which occur between about Mach 0.75 and 1.2. For supersonic speeds a different procedure called the supersonic area rule, developed by NACA aerodynamicist Robert Jones, is used.

Transonic is one of the most important speed ranges for commercial and military fixed-wing aircraft today, with transonic acceleration an important performance requirement for combat aircraft and which is improved by reductions in transonic drag.

At high-subsonic flight speeds, the local speed of the airflow can reach the speed of sound where the flow accelerates around the aircraft body and wings. The speed at which this development occurs varies from aircraft to aircraft and is known as the critical Mach number. The resulting shock waves formed at these zones of sonic flow cause a sudden increase in drag, called wave drag. To reduce the number and strength of these shock waves, an aerodynamic shape should change in cross sectional area as smoothly as possible from front to rear.

A superficially related concept is the Sears–Haack body, the shape of which allows minimum wave drag for a given length and a given volume. However, the Sears–Haack body shape is derived starting with the Prandtl–Glauert equation which approximately governs small-disturbance subsonic flows, as well as Ackeret Theory, which closely describes supersonic flow. Both methods lose validity for transonic flows where the area rule applies, due to assumptions made in their derivations. So although the Sears–Haack body shape, being smooth, will have favorable wave drag properties according to the area rule, it is not theoretically optimum.^[3]

The first aircraft where the area rule was consequently implemented was the German bomber testbed Junkers Ju-287 (1944).^[10] Other corresponding German designs were not completed due to the end of the war or even remained in the planning stage.

When the area rule was re-discovered by Whitcomb, it was made available to the U.S. aircraft industry on a secret basis for military programs from 1952^[11] and it was reported in 1957 for civilian programs. ^[12] Convair and Grumman, with Whitcomb's help, used it concurrently to design the Grumman F-11 Tiger and to redesign the Convair F-102.^[13] The Grumman F-11 Tiger was the first of the two aircraft to fly and had been designed using the area rule from the outset.^[14] The Convair F-102 Delta Dagger had to be redesigned as it had been unable to reach Mach 1 although its design speed was Mach 1.2. The expectation that it would reach design speed had been based on optimistic wind-tunnel drag predictions.^[15][16] Modifications which included indenting the fuselage beside the wings and adding more volume to the rear of the aircraft, reduced the transonic drag significantly and the Mach 1.2 design speed was reached. The reason for using the area rule on these fighter aircraft was to reduce the peak value of the drag which occurs at Mach 1 and so enable supersonic speeds with less thrust than would otherwise have been necessary.

In 1957 a modified area rule was available for raising the subsonic cruise speed of transport aircraft by 50 mph.^[12] The cruise speed is limited by the sudden increase in drag which indicates the presence of local supersonic flow on top of the wing. Whitcomb's modified rule reduced the supersonic speed before the shock, which weakened it and reduced the drag associated with it. The Convair 990 had bumps called antishock bodies added to the top surface of the wing with the intent of achieving the required cruise speed. However, the area distribution in the channels formed by the nacelle/pylon/wing surfaces also caused supersonic velocities and was the source of significant drag. An area-rule technique, so-called channel area-ruling, was applied to achieve the required cruise speed.

Designers at Armstrong-Whitworth took the sonic area rule a step further in their proposed M-Wing, in which the wing was first swept forward and then to the rear. This allowed the fuselage to be narrowed in front of the root as well as behind it, leading to a smoother fuselage that remained wider on average than one using a classic swept wing.

The extension behind the flight deck on the Rockwell B-1 Lancer and Boeing 747 was added to improve the cross-sectional area distribution according to the area rule.^[17]

Aircraft designed according to Whitcomb's area rule (such as the F-102 Delta Dagger and the Northrop F-5) looked odd when they first appeared and were sometimes dubbed "flying Coke bottles", but this became an expected part of the appearance of some transonic aircraft. Visually-apparent indications that the area rule has defined the shape of an aircraft are fuselage "waisting" and tip-tank shaping as on the Northrop F-5, and rear fuselage thinning on business jets with rear engines such as the Bombardier Global Express. The rule also requires careful positioning of parts, like the boosters and cargo bay on rockets and the shape and location of the canopy on the F-22 Raptor.

The supersonic area rule was applied, at Mach 2, to the prototype Concorde. The rear fuselage was extended by 3.73m on the production aircraft and reduced wave drag by 1.8%.^[18]

• The F-106 Delta Dart, a development of the F-102 Delta Dagger, shows the "wasp-waisted" shaping due to area rule considerations
• NASA Convair 990 with antishock bodies on the rear of the wings
• Oilflow visualization of flow separation without and with antishock bodies
• Northrop F-5 showing fuselage waisting
• Northrop F-5 showing tip-tank shaping
• Bombardier Global Express showing rear fuselage thinning between engines
• Prototype Concorde before tail was modified using Mach 2 application of area rule
• Production Concorde with area ruled tail

• Anti-shock body
• Sonic boom
• Sound barrier
• Supersonic aerodynamics