aerodynamics Aerodynamics () is the study of the motion of atmosphere of Earth, air, particularly when affected by a solid object, such as an airplane wing. It involves topics covered in the field of fluid dynamics and its subfield of gas dynamics, and is an ...

, the critical Mach number (Mcr or M*) of an

aircraft An aircraft ( aircraft) is a vehicle that is able to flight, fly by gaining support from the Atmosphere of Earth, air. It counters the force of gravity by using either Buoyancy, static lift or the Lift (force), dynamic lift of an airfoil, or, i ...

is the lowest Mach number at which the airflow over some point of the aircraft reaches the

speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elasticity (solid mechanics), elastic medium. More simply, the speed of sound is how fast vibrations travel. At , the speed of sound in a ...

, but does not exceed it.Clancy, L.J. ''Aerodynamics'', Section 11.6 At the lower critical Mach number, airflow around the entire aircraft is subsonic. Supersonic aircraft such as the

Concorde Concorde () is a retired Anglo-French supersonic airliner jointly developed and manufactured by Sud Aviation and the British Aircraft Corporation (BAC). Studies started in 1954, and France and the United Kingdom signed a treaty establishin ...

and combat aircraft also have an upper critical Mach number at which the airflow around the entire aircraft is supersonic.

Aircraft flight

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 weak

shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...

. As the aircraft exceeds the critical Mach number, its drag coefficient increases suddenly, causing dramatically increased drag,Clancy, L.J., ''Aerodynamics'', Chapter 11 and, in an aircraft not designed for

transonic Transonic (or transsonic) flow is air flowing around an object at a speed that generates regions of both subsonic and Supersonic speed, supersonic airflow around that object. The exact range of speeds depends on the object's critical Mach numb ...

or supersonic speeds, changes to the airflow over the flight control surfaces lead to deterioration in control of the aircraft. 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 cause Mach tuck and may be sufficient to stall 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 the compressibility 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 the sound barrier. 1940s-era military subsonic aircraft, such as the

Supermarine Spitfire The Supermarine Spitfire is a British single-seat fighter aircraft that was used by the Royal Air Force and other Allies of World War II, Allied countries before, during, and after World War II. It was the only British fighter produced conti ...

, Bf 109,

P-51 Mustang The North American Aviation P-51 Mustang is an American long-range, single-seat fighter aircraft, fighter and fighter-bomber used during World War II and the Korean War, among other conflicts. The Mustang was designed in 1940 by a team headed ...

, Gloster Meteor, He 162, and P-80, have relatively thick, unswept wings, and are incapable of reaching Mach 1.0 in controlled flight. In 1947, Chuck Yeager flew the Bell 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. Early

military aircraft, such as the Hawker Hunter and

F-86 Sabre The North American F-86 Sabre, sometimes called the Sabrejet, is a transonic jet fighter aircraft. Produced by North American Aviation, the Sabre is best known as the United States' first swept-wing fighter that could counter the swept-wing Sov ...

, 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. Modern

jet airliner A jet airliner or jetliner is an airliner powered by jet engines (passenger jet aircraft). Airliners usually have twinjet, two or quadjet, four jet engines; trijet, three-engined designs were popular in the 1970s but are less common today. Air ...

s with swept wings, such as

Airbus Airbus SE ( ; ; ; ) is a Pan-European aerospace corporation. The company's primary business is the design and manufacturing of commercial aircraft but it also has separate Airbus Defence and Space, defence and space and Airbus Helicopters, he ...

and

Boeing The Boeing Company, or simply Boeing (), is an American multinational corporation that designs, manufactures, and sells airplanes, rotorcraft, rockets, satellites, and missiles worldwide. The company also provides leasing and product support s ...

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 as

, Tu-144, the English Electric Lightning, Lockheed F-104, Dassault Mirage III, and MiG 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 the P-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. The

's much thinner wing gave it a considerably higher critical Mach number (about 0.89).

References

* L. J. Clancy (1975) ''Aerodynamics'', Pitman Publishing Limited, London

Notes

{{reflist Aircraft aerodynamics

Aircraft flight

See also

References

Notes