What is critical Mach number in aviation?

It is the lowest freestream Mach number at which local airflow on the aircraft first reaches Mach 1, usually on the upper wing surface.

How does angle of attack affect the critical Mach number?

Increasing angle of attack lowers the critical Mach number because it accelerates airflow over the wing, reaching sonic speeds sooner.

What are the symptoms of exceeding critical Mach number?

Exceeding MCRIT leads to the formation of shock waves, increased drag, possible buffeting, and reduced control effectiveness.

How does wing design influence critical Mach number?

Thinner and less cambered wings have higher critical Mach numbers, allowing higher speeds before encountering compressibility effects.

What is the difference between Mach number and critical Mach number?

Mach number is the ratio of aircraft speed to local speed of sound, while critical Mach number is the threshold where local sonic flow first appears on the aircraft.

Critical Mach Number

Medium4 min readPrinciple of Flight

Moderately Examined

Why this matters

Knowing the critical Mach number helps pilots avoid sudden increases in drag, loss of lift, and control issues that can compromise safety during high-speed flight. Understanding MCRIT is crucial for managing aircraft performance and preventing aerodynamic surprises in the transonic regime.

The critical Mach number (MCRIT) is the lowest freestream Mach number at which airflow somewhere on the aircraft—usually the upper wing surface—first reaches sonic speed (Mach 1). This point marks the onset of compressibility effects, with local supersonic flow and shock waves forming just above MCRIT. Understanding MCRIT is essential for safe and efficient high-speed flight, as exceeding it leads to dramatic changes in aerodynamic behaviour.

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What is the definition of the critical Mach number (MCRIT) in aviation?

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Explanation

What is Critical Mach Number?

Critical Mach number, often abbreviated as MCRIT, is defined as the freestream Mach number at which the airflow over some part of the aircraft (typically the upper surface of the wing) first reaches Mach 1. Below this speed, all airflow over the aircraft remains subsonic. Just above MCRIT, small regions of supersonic flow and shock waves begin to form.

Causes and Influences

Several factors affect MCRIT:

Wing Shape: Thicker or highly cambered wings accelerate airflow more, causing sonic speeds at lower freestream Mach numbers (lower MCRIT).
Angle of Attack (alpha): Increasing alpha increases local airflow speeds, reducing MCRIT.
Aircraft Weight and Manoeuvres: Higher weight or tighter manoeuvres require more lift, increasing alpha and thus reducing MCRIT.
Centre of Gravity (CG): A forward CG requires higher lift (higher alpha), which also lowers MCRIT.

Mach Number, Shock Waves, and Drag

As Mach number approaches MCRIT, compressibility effects intensify. Once MCRIT is exceeded:

Shock Waves: Appear on the upper wing surface, causing abrupt changes in airflow.
Drag Rise (Drag Divergence): The coefficient of drag (CD) increases sharply with Mach number beyond MCRIT, even at constant angle of attack.
Lift Behaviour: The lift coefficient (CL) may initially increase, but then drops as shock-induced flow separation occurs.
Centre of Pressure (CP): Moves aft as Mach number increases, affecting pitch stability.

CL-CD Graph Effects

On a CL-CD graph, the rapid rise in drag and changes in lift beyond MCRIT are clear. The aircraft's performance envelope is significantly altered, limiting safe and efficient operation.

Critical Mach Number vs. Other Mach Numbers

Below MCRIT: All flow is subsonic; compressibility effects are minor.
At MCRIT: Local sonic flow appears; no supersonic flow yet.
Above MCRIT: Local supersonic regions and shock waves form; drag and control issues escalate.

The essentials

Key Points

Critical Mach number (MCRIT) is when local airflow first reaches Mach 1 on the aircraft.

Exceeding MCRIT leads to shock waves and local supersonic flow.

MCRIT decreases with increased angle of attack, weight, or forward CG.

Thicker or more cambered wings have lower MCRIT.

Beyond MCRIT, drag rises sharply and lift behaviour changes.

The centre of pressure moves aft as Mach number increases past MCRIT.

Watch out

Exam Traps & Typical Mistakes

Confusing sonic (Mach 1) with supersonic flow—MCRIT is about sonic, not supersonic.

Believing shock waves form exactly at MCRIT, when they actually form just above it.

Assuming MCRIT is a fixed value for all aircraft or flight conditions.

Thinking MCRIT is where buffet or maximum temperature occurs—it is about local sonic flow.

Mixing up the effects on lift and drag at and beyond MCRIT.

Test yourself

Example Exam Questions

Question 2Medium

What typically happens when an aircraft exceeds its critical Mach number?

Question 3Medium

Which factor does NOT directly decrease the critical Mach number of a wing?

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