Lesson 1.10 Transonic and Supersonic Flight

1. Transonic and supersonic flight speeds are expressed by the ratio of the true airspeed in knots to the speed of sound in knots. This ratio is called the Mach number.
a. This ratio is not fixed because the speed of sound varies with altitude and temperature.
2. At low flight speeds, the study of aerodynamics is greatly simplified by the fact that air may experience relatively small changes in pressure with only negligible changes in density.
a. This airflow is termed incompressible since the air may undergo changes in pressure without apparent changes in density.
b. The study of airflow at high speeds must account for these changes in air density and must consider that the air is compressible and there will be compressibility effects.
c. Local airflow velocities around an aerodynamic shape can be greater than flight speed.
1. Thus, an aircraft can experience compressibility effects at flight speeds well below the speed of sound.
3. Because an aircraft can have both subsonic and supersonic airflows simultaneously, certain regimes have been defined.
a. Subsonic: Mach numbers below 0.75
b. Transonic: Mach numbers from 0.75 to 1.20
c. Supersonic: Mach numbers from 1.20 to 5.00
d. Hypersonic: Mach numbers above 5.00
4. The critical Mach number is the highest flight speed possible without supersonic flow.
a. Accelerating past critical Mach is associated with trim and stability changes, an increase in drag, and a decrease in control surface effectiveness.