Supersonic flight occurs when an aircraft moves faster than the speed of sound in the surrounding air. At these speeds, pressure disturbances cannot move ahead of the aircraft, so they pile up into sharp shock waves. Engineers must design the aircraft shape, wings, inlets, and materials to handle sudden changes in pressure, temperature, and drag.
Understanding shock waves helps explain why supersonic jets look different from slower airplanes and why they produce sonic booms.
Key Facts
- Mach number is M = v / c, where v is aircraft speed and c is the local speed of sound.
- Supersonic flight occurs when M > 1, and transonic flight occurs near M = 1.
- For an ideal gas, the speed of sound is c = sqrt(gamma R T).
- The Mach cone half-angle is given by sin(theta) = 1 / M for M > 1.
- Shock waves cause abrupt increases in pressure, temperature, and density while reducing flow speed.
- Wave drag increases strongly near and above Mach 1, so thin swept wings reduce the strength of shocks.
Vocabulary
- Mach number
- Mach number is the ratio of an object's speed to the local speed of sound.
- Shock wave
- A shock wave is a thin region where air pressure, density, temperature, and velocity change suddenly.
- Sonic boom
- A sonic boom is the loud pressure pulse heard when shock waves from a supersonic aircraft pass an observer.
- Wave drag
- Wave drag is the extra aerodynamic drag caused by shock waves forming around an aircraft at high speed.
- Swept wing
- A swept wing is angled backward to reduce the effective airflow speed perpendicular to the leading edge and delay strong shock formation.
Common Mistakes to Avoid
- Using one fixed value for the speed of sound is wrong because sound speed depends on air temperature and altitude.
- Thinking the sonic boom happens only when the aircraft breaks the sound barrier is wrong because a supersonic aircraft continuously generates shock waves along its path.
- Drawing shock waves as sound waves spreading equally in all directions is wrong because supersonic motion creates a Mach cone behind the aircraft.
- Assuming thicker wings are always stronger and better is wrong for supersonic design because thick wings create stronger shocks and more wave drag.
Practice Questions
- 1 A jet travels at 680 m/s where the local speed of sound is 340 m/s. Calculate its Mach number and state whether it is subsonic, transonic, or supersonic.
- 2 An aircraft flies at Mach 2.0. Use sin(theta) = 1 / M to find the Mach cone half-angle theta to the nearest degree.
- 3 At a high altitude, the speed of sound is 295 m/s. How fast must an aircraft fly to reach Mach 1.6?
- 4 Explain why a thin swept wing is preferred for a supersonic jet, using the ideas of shock strength, effective airflow direction, and wave drag.