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,
Bf 109,
P-51 Mustang,
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
transonic military aircraft, such as the
Hawker Hunter and
F-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. 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
Supermarine Spitfire's much thinner wing gave it a considerably higher critical Mach number (about 0.89). ==See also==