Mach number – Sonic, subsonic, transonic?

May 10, 2024

Ref FAA PHAK 5-44 to 5-51

Mach number is a kind of true airspeed indication method, it makes relationship with local speed of sound (LSS).

Mach number = True Airspeed / Local Speed of Sound (LSS)

LSS = 39 x sq root of  Temperature (K)

Note: C + 273 = K

 

By FAA book’s definition,  “Sonic” , “Subsonic” , “Transonic” and “Supersonic” can use speed regime to define:

  • Subsonic – Mach number below 0.75  (i.e 500KT)
  • Transonic – Mach number 0.75 -1.2
  • Supersonic – Mach number 1.2 to 5
  • Hypersonic – Mach number over 5

Notes: speed of sound is 343 m/s -> equivalent to 666.7 KT

Mcrit – Mach Crit?

 

Read More –

Concept and challenge question: What’s the LSS at 30000 feet in ISA atmosphere?

  • ISA sea level -> 15C –> C + 273 = K , i.e 15 +273 = 288 K
  • Lapse rate 2 (1.98C) / 1000 feet –> i.e (30000/1000) * 1.98 = 59.4 C lapse
  • 30000 feet temp = 15 – 59.4 = -44.4 C –> C+ 273 = K –> so i.e 228.6 K
  • LSS = 39 * sq rt of Temp(K) = 39 * sq rt 228.6 = 589.7

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