High Speed Flight: Definitions
Speed of Sound ($a$)
The speed at which small pressure disturbances travel through a medium.
- Depends ONLY on Temperature (in air).
- Formula: $a = 38.94 \sqrt{T}$
- $T$: Absolute temperature in Kelvin.
- As altitude increases (in troposphere), Temperature decreases -> Speed of Sound decreases.
Mach Number ($M$)
The ratio of True Airspeed (TAS) to the Local Speed of Sound ($a$). $ M = \frac{TAS}{a} $
- Constant Mach Climb: As altitude increases, $a$ decreases. To maintain constant $M$ ($TAS/a$), TAS must decrease.
- Constant Mach Descent: As altitude decreases, $a$ increases. To maintain constant $M$, TAS must increase.
Flight Regimes
- Subsonic: Airflow over the entire aircraft is subsonic ($M < 1.0$). Typically $M < 0.75$.
- Transonic: Some airflow is subsonic, some is supersonic. Typically $M 0.75 - M 1.20$.
- This is where shockwaves begin to form.
- Supersonic: Airflow over the entire aircraft is supersonic ($M > 1.20$).
- Hypersonic: $M > 5.0$.
Critical Mach Number ($M_{crit}$)
The free stream Mach number at which the airflow over some point on the aircraft first reaches the speed of sound ($M = 1.0$).
- Flying faster than $M_{crit}$ creates shockwaves.