High Speed Flight: Definitions

081-02-01Pablo Asensio Martínez2026-04-011 min

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

  1. Subsonic: Airflow over the entire aircraft is subsonic ($M < 1.0$). Typically $M < 0.75$.
  2. Transonic: Some airflow is subsonic, some is supersonic. Typically $M 0.75 - M 1.20$.
    • This is where shockwaves begin to form.
  3. Supersonic: Airflow over the entire aircraft is supersonic ($M > 1.20$).
  4. 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.