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Home > Flying the Plane > What is ‘Mcrit’ (critical Mach number) with regards to subsonic flight?
What is ‘Mcrit’ (critical Mach number) with regards to subsonic flight?
Flying - Flying the Plane
Thursday, 24 January 2013 15:55

Sonic Boom

Hi Captain Lim,

Could you please explain to me what is Mcrit? I don't really understand it.

Thank you,

Sharifah

(Note: This answer may be a little too technical for most readers to understand unless you are a flying enthusiast )

Hi Sharifah,

‘Mcrit’ in aerodynamics, is in fact the ‘critical Mach Number’ of an aircraft. It is the lowest Mach number at which the airflow over some point on the aircraft reaches the speed of sound. A Mach number of 1.0 indicates an airspeed equals to the speed of sound in the air.

In flight, the airflow around the aircraft is not exactly the same as the airspeed of the aircraft due to the airflow speeding up and slowing down to travel around the aircraft structure. At the critical Mach number, local airflow in some areas near the airframe reaches the speed of sound even though the aircraft’s speed is still lower than Mach 1.0. This creates a weak shock wave.

As such, a pilot needs to understand that should he allow the plane to approach beyond the Mcrit, he may reach the ‘coffin corner’ – an altitude and speed that may cause the plane to crash.

Since the critical Mach number is the maximum speed at which air can travel over the wings without losing lift due to flow separation and shock waves, any further increase in speed will cause the airplane to lose lift and fall off the sky.

In older planes which have relative thick un-swept wings, they have lower Mcrit and any attempt to allow the plane to fly above the Mcrit, shock waves flowing over the wings may cause control problems.

The actual Mcrit varies from wing to wing. In general, a thicker wing will have a lower critical Mach number because a thicker wing accelerates the airflow to a faster speed than a thinner one.

For instance, the fairly thick wing on the P-38 Lightning has a critical Mach number of about 0.69. The aircraft could occasionally reach this speed in dives, leading to a number of crashes. The much thinner wing on the high speed Spitfire resulted in a critical Mach number of about 0.89 for this aircraft.

Modern subsonic planes such as Airbus and Boeing are designed with super critical wings and therefore have higher maximum operating Mach numbers (MMO) and consequently higher Mcrit – hence less likely to encounter control problems. (Mcrit is normally below the MMO)

For more details on this topic, please see here

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Supersonic Flight, Sonic Booms

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Great Description!
Well explained! Thank you
Jude , 19 May, 2013

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