The area rule is an important idea in aircraft design for flight near the speed of sound. At transonic speeds, air over parts of an airplane can become supersonic even if the airplane as a whole is slightly below Mach 1. This creates shock waves that increase drag sharply, making the aircraft harder to accelerate and less fuel efficient.
The famous coke-bottle fuselage shape helps reduce this drag by narrowing the body near the wings.
Key Facts
- Mach number: M = v / a, where v is aircraft speed and a is the local speed of sound.
- Transonic flight usually occurs around M = 0.8 to M = 1.2.
- Wave drag rises sharply when shock waves form on an aircraft.
- The area rule says total cross-sectional area should change smoothly from nose to tail.
- Total area at a station equals fuselage area plus wing area plus other component areas.
- A pinched fuselage near the wings can reduce wave drag by smoothing the aircraft's area distribution.
Vocabulary
- Area rule
- The area rule is a design principle that reduces transonic drag by making the aircraft's total cross-sectional area vary smoothly along its length.
- Wave drag
- Wave drag is the extra aerodynamic resistance caused by shock waves forming near or above the speed of sound.
- Transonic
- Transonic describes flight conditions in which airflow around the aircraft includes both subsonic and supersonic regions.
- Mach number
- Mach number is the ratio of an object's speed to the local speed of sound.
- Fuselage
- The fuselage is the main body of an aircraft that holds the crew, passengers, cargo, and many internal systems.
Common Mistakes to Avoid
- Thinking the area rule only looks at the fuselage shape is wrong because the wings, engine pods, tail, and fuselage all contribute to the total cross-sectional area.
- Assuming a thinner airplane always has lower transonic drag is wrong because sudden changes in total area can create strong shock waves even if the aircraft is slim.
- Confusing wave drag with ordinary skin friction is wrong because wave drag comes from shock waves and pressure changes, not just air rubbing along the surface.
- Placing the narrow waist anywhere on the fuselage is wrong because the pinch must be coordinated with the added cross-sectional area of the wings and other components.
Practice Questions
- 1 An aircraft flies at 290 m/s where the local speed of sound is 340 m/s. Calculate its Mach number and decide whether it is near the transonic range.
- 2 At one station along a jet, the fuselage cross-sectional area is 18 m² and the wing contribution is 7 m². If a smoother target total area is 21 m², what should the fuselage area be at that station?
- 3 A straight fuselage and a coke-bottle fuselage have the same maximum width, but the coke-bottle version is narrowed near the wings. Explain why the narrowed version can have less drag near Mach 1.