High-Speed Flight

Mach number is the ratio of an aircraft’s true airspeed to the local speed of sound: M = TAS/a. Because the local speed of sound varies primarily with static air temperature, a given Mach number does not correspond to one fixed true airspeed.

• Mach expresses speed relative to the local propagation speed of pressure disturbances.
• At a constant Mach number, true airspeed changes as local temperature changes.
• High-altitude cruise limits and performance are commonly referenced to Mach rather than indicated airspeed.

The speed of sound is the local rate at which small pressure disturbances propagate through a medium. In air it is determined primarily by absolute static temperature and may be expressed as a = √(γRT) for an ideal gas.

• The speed of sound decreases as static air temperature decreases.
• Pressure and density changes alone do not determine it when temperature is specified.
• Mach indications depend on the local speed of sound, not a universal sea-level value.

Critical Mach number is the lowest free-stream Mach number at which the airflow first reaches Mach 1 at any point on the aircraft. It is affected by wing geometry, airfoil shape, lift coefficient, angle of attack, configuration, and surface condition.

• Local airflow over a wing can become sonic while the aircraft remains subsonic.
• Reaching critical Mach begins significant transonic effects but does not by itself mean the aircraft is at its operating Mach limit.
• Higher lift demand generally accelerates local flow and can reduce the Mach margin to compressibility effects.

Compressibility effects are aerodynamic changes caused by significant variations in air density as pressure disturbances interact with high-speed airflow. They become important as local Mach number increases and include altered pressure distribution, drag divergence, shock formation, separation, buffet, and pitching-moment changes.

• Air can no longer be treated as effectively incompressible when local Mach becomes sufficiently high.
• Transonic compressibility can produce rapid drag rise and changes in lift and control effectiveness.
• The severity depends on Mach, angle of attack, load factor, configuration, and aircraft design.

A shock wave is a very thin compression region across which airflow undergoes abrupt increases in static pressure, temperature, and density with a decrease in Mach number and total pressure. In transonic flight, shocks commonly form where locally supersonic flow decelerates toward or through Mach 1.

• Shock waves can exist on an aircraft whose free-stream Mach number is below 1.
• A shock creates wave drag and an irreversible loss of total pressure.
• The adverse pressure rise across a shock can cause boundary-layer separation and buffet.

Mach buffet is airframe vibration caused primarily by unsteady shock-wave motion and shock-induced boundary-layer separation at high Mach number. It is the high-speed buffet boundary and must be distinguished from low-speed buffet caused by excessive angle of attack.

• Mach buffet generally intensifies as Mach, lift coefficient, or load factor increases.
• At high altitude, low-speed and high-speed buffet boundaries may converge.
• Recovery requires reducing the condition driving the buffet while avoiding abrupt loading.

Mach tuck is a high-speed nose-down pitching tendency caused by compressibility-related changes in pressure distribution, typically including aft movement of the wing’s center of pressure and possible changes in downwash or tail effectiveness. Aircraft design features and flight-control systems may counter or compensate for it.

• Mach tuck is a pitching-moment change, not simply an increase in airspeed.
• Aft movement of the aerodynamic loading on the wing contributes to the nose-down tendency.
• Uncorrected pitch-down can promote further acceleration and intensify compressibility effects.

A high-speed stall is a loss of lift or major degradation of aerodynamic performance associated with excessive angle of attack and shock-induced flow separation at high Mach number. It can occur near the upper buffet boundary even though indicated airspeed is well above the conventional low-speed stall indication.

• A wing stalls because of excessive effective angle of attack and separation, not because of one specific airspeed.
• At high Mach, shocks can promote separation and reduce the angle-of-attack margin.
• High altitude can leave a narrow maneuvering margin between low-speed and high-speed buffet boundaries.