Transonic flight is the speed range where an aircraft is close to the speed of sound, typically about Mach 0.8 to Mach 1.2. It matters because the airflow around the aircraft is no longer all subsonic or all supersonic. Different parts of the same aircraft can experience different flow speeds at the same time.
This mixed-speed zone strongly affects lift, drag, stability, and pilot handling.
Key Facts
- Mach number is M = v / a, where v is aircraft speed and a is the local speed of sound.
- The transonic range is approximately M = 0.8 to M = 1.2, depending on aircraft shape and altitude.
- Local supersonic flow can occur over wings or the fuselage even when the aircraft Mach number is less than 1.
- A shock wave forms when supersonic airflow slows abruptly to subsonic speed, causing a jump in pressure and temperature.
- Dynamic pressure is q = 1/2 rho v^2, and it increases aerodynamic loads as speed increases.
- Critical Mach number is the freestream Mach number at which the first local airflow point reaches Mach 1.
Vocabulary
- Mach number
- Mach number is the ratio of an object’s speed to the local speed of sound.
- Transonic flight
- Transonic flight is flight near Mach 1 where subsonic and supersonic airflow can exist on different parts of the aircraft.
- Shock wave
- A shock wave is a thin region where supersonic airflow changes suddenly, producing sharp increases in pressure, temperature, and density.
- Critical Mach number
- Critical Mach number is the aircraft Mach number at which the first point of local airflow reaches the speed of sound.
- Buffet
- Buffet is shaking or vibration caused by disturbed airflow, often from shock waves and flow separation in transonic flight.
Common Mistakes to Avoid
- Assuming the whole aircraft is either subsonic or supersonic, which is wrong because transonic flight can have both flow types at the same time around different regions.
- Treating Mach number as a fixed speed in meters per second, which is wrong because the speed of sound changes with temperature and altitude.
- Ignoring local airflow acceleration over curved surfaces, which is wrong because air can exceed Mach 1 over the wing even when the aircraft itself is below Mach 1.
- Thinking shock waves only occur after the aircraft reaches Mach 1, which is wrong because local supersonic pockets can create shocks before the freestream Mach number equals 1.
Practice Questions
- 1 An aircraft flies at 270 m/s where the local speed of sound is 340 m/s. Calculate its Mach number and state whether it is likely subsonic, transonic, or supersonic.
- 2 At an altitude where the speed of sound is 295 m/s, a jet flies at Mach 0.92. What is its speed in m/s?
- 3 A jet at Mach 0.85 has local supersonic airflow over the upper wing and a shock wave near the rear of that region. Explain how this can happen and why it may cause buffet.