Inaerodynamics, thecritical Mach number (Mcr orM*) of anaircraft is the lowestMach number at which the airflow over some point of the aircraft reaches thespeed of sound, but does not exceed it.[1] At thelower critical Mach number, airflow around the entire aircraft is subsonic. Supersonic aircraft such as theConcorde and combat aircraft also have anupper critical Mach number at which the airflow around the entire aircraft is supersonic.[2]
For an aircraft in flight, the speed of the airflow around the aircraft differs considerably in places from the airspeed of the aircraft; this is due to the airflow having to speed up and slow down as it travels around the aircraft's structure. When the aircraft's airspeed reaches the critical Mach number, the speed of the airflow in some areas near the airframe reaches the speed of sound, even though the aircraft itself has an airspeed lower than Mach 1.0. This creates a weakshock wave. As the aircraft exceeds the critical Mach number, itsdrag coefficient increases suddenly, causingdramatically increased drag,[3] and, in an aircraft not designed fortransonic orsupersonic speeds, changes to the airflow over theflight control surfaces lead to deterioration in control of the aircraft.[3]
In aircraft not designed to fly at or above the critical Mach number, the shock waves that form in the airflow over the wing and tailplane causeMach tuck and may be sufficient tostall the wing, render the control surfaces ineffective, or lead to loss of control of the aircraft. These problematic phenomena appearing at or above the critical Mach number were eventually attributed to thecompressibility of air. Compressibility led to a number of accidents involving high-speed military and experimental aircraft in the 1930s and 1940s.
The challenge of designing an aircraft to remain controllable approaching and reaching the speed of sound was the origin of the concept known as thesound barrier. 1940s-era militarysubsonic aircraft, such as theSupermarine Spitfire,Bf 109,P-51 Mustang,Gloster Meteor,He 162, andP-80, have relatively thick, unswept wings, and are incapable of reaching Mach 1.0 in controlled flight. In 1947,Chuck Yeager flew theBell X-1 (also with an unswept wing, but a much thinner one), reaching Mach 1.06 and beyond, and the sound barrier was finally broken.
Earlytransonic military aircraft, such as theHawker Hunter andF-86 Sabre, were designed to fly satisfactorily even at speeds greater than their critical Mach number. They did not possess sufficient engine thrust to reach Mach 1.0 in level flight, but could do so in a dive and remain controllable. Modernjet airliners with swept wings, such asAirbus andBoeing aircraft, do not cruise at airspeeds faster than their critical Mach numbers and have maximum operating Mach numbers slower than Mach 1.0.
Supersonic aircraft, such asConcorde,Tu-144, theEnglish Electric Lightning,Lockheed F-104,Dassault Mirage III, andMiG 21, are intended to exceed Mach 1.0 in level flight, and are therefore designed with very thin wings. Their critical Mach numbers are higher than those of subsonic and transonic aircraft, but are still less than Mach 1.0.
The actual critical Mach number varies from wing to wing. In general, a thicker wing will have a lower critical Mach number, because a thicker wing deflects the airflow passing around it more than a thinner wing does, and thus accelerates the airflow to a faster speed. For instance, the fairly-thick wing on theP-38 Lightning has a critical Mach number of about .69. The aircraft could occasionally reach this speed in dives, leading to a number of crashes. TheSupermarine Spitfire's much thinner wing gave it a considerably higher critical Mach number (about 0.89).