Mach Tuck - Causes of Mach Tuck

Causes of Mach Tuck

Mach tuck is dependent upon the dynamics of lift.

Mach tuck is the result of an aerodynamic stall due to an over-speed condition, rather than the more common stalls resulting from boundary layer separation due to insufficient airspeed, increased angle of attack, excessive load factors, or a combination of those causes. As the aircraft's wing approaches its critical Mach number, the aircraft is traveling below Mach 1.0. However, the accelerated airflow over the upper surface of the cambered wing exceeds Mach 1.0 and a shock wave is created at the point on the wing where the accelerated airflow returns from supersonic to subsonic airflow. While the air ahead of the shock wave is in laminar flow, a boundary layer separation is created aft of the shock wave, and that section of the wing fails to produce lift. The image to the right illustrates this concept.

In most aircrafts susceptible to Mach tuck, the camber at the wing root, the section of the wing closest to the fuselage, is more pronounced than that of the wing tip. This design ensures that in a standard stall the root will stall before the tips. This allows the pilot to recognize the stall while still maintaining control of the ailerons to enhance stall recovery. However, this also means that when an airfoil exceeds its critical Mach number, the shock wave, and resulting stall condition, will begin to form at the root.

A second design element that leads to Mach tuck is that many aircraft which will approach the speed of sound are designed with swept wings. The center of pressure of a wing is an imaginary point where the summation of all lifting forces across the wing's surface can be resolved into a single lift vector. When the wing root stalls, the center of pressure of the (reduced) lift being generated by the wing is shifted towards the wing tip. With a swept wing, this also means that the center of pressure travels aft (because it is traveling out from the wing root and therefore backwards as the wing sweeps). When the center of pressure moves aft, its movement rearwards compared to the unmoving center of mass of the aircraft will generate a force which will act to depress the nose of the aircraft; this nose down pitching moment is “Mach tuck."

As the wing becomes more affected by the shock wave the center of pressure will continue to travel aft, thereby causing a significantly higher nose-down force and requiring a nose-up input or trim to maintain level flight. Although Mach tuck develops gradually, if it is allowed to progress significantly, the center of pressure can move so far rearward that there is no longer enough elevator authority available to counteract it, and the airplane enters a steep, sometimes unrecoverable dive.

In addition, until the aircraft goes supersonic, as the shock wave goes towards the rear, because of the faster flow there the top shockwave will impinge upon the horizontal stabilizer and elevator control surfaces further back than the lower shockwave; this can greatly exacerbate the nose down tendencies. The horizontal stabilizer at the tail of the aircraft generates a downward force, so loss of effective horizontal stabilizer area will reduce this downward force, so the tail will pitch up and the nose will pitch down. If the shock wave affects the elevators, it may reduce their effectiveness, making it impossible for the pilot to alter the aircraft's pitch.

Finally, there is a related condition that can exacerbate Mach tuck. If enough of the wing surface becomes engulfed in the shock wave, the wing will not produce enough lift to support the aircraft, and a standard stall will occur. This often fatal combination of overspeed and aerodynamic stall can most easily be avoided by not allowing the effects of Mach tuck to develop beyond its incipient stage. This is best accomplished by retarding the throttle, extending speed brakes, and if possible, extending the landing gear. Any actions, which would increase aerodynamic drag and thus reduce airspeed below critical Mach, will prevent further aggravation of the condition.