The "area rule," also known as the Whitcomb area rule, is an aerodynamic principle that helps reduce drag and increase the efficiency of supersonic aircraft without the need for additional power. It was developed by the American aerospace engineer Richard Whitcomb in the 1950s.
The area rule is based on the observation that the drag of an aircraft is closely related to the distribution of its cross-sectional area along its length. When an aircraft is flying at transonic or supersonic speeds, shock waves start to form due to the compression of air around the aircraft.
The area rule suggests that the cross-sectional area distribution along the length of the aircraft should be smooth and gradual to minimize the intensity of shock waves and their associated drag. Specifically, the principle states that the rate of change of the total cross-sectional area should be minimized, especially in areas where the flow is accelerated or decelerated.
To implement the area rule, aircraft designers typically incorporate features such as "waist" or "coke bottle" shaping in the fuselage. This involves narrowing the fuselage at the wing location, resulting in a narrower waist section and a smoother transition towards the tail. By doing so, the area rule helps to reduce the wave drag caused by shock waves forming along the fuselage.
Reducing wave drag is crucial because it allows the aircraft to maintain higher speeds with the same amount of engine power. This is important for supersonic flight, where drag can be a significant limiting factor. By optimizing the area distribution, the area rule enables the aircraft to fly at higher speeds more efficiently, without requiring a significant increase in engine power.
It's important to note that while the area rule can help reduce drag and improve supersonic performance, it is not the only factor involved. Other design considerations, such as wing shape, wing sweep, and overall aerodynamic design, also play important roles in achieving high-speed performance in supersonic aircraft.