FAA Flight Instructor Airplane (FIA)Aerodynamics and Principles of FlightHard
A high-performance jet approaches its critical Mach number (Mcrit), and a local shock wave forms over the wing's upper surface aft of the point of maximum camber. The pilot experiences an uncommanded nose-down pitching moment. Which phenomenon is this?
- ADutch roll, caused by coupling between yaw and roll oscillations at high Mach numbers
- BMach tuck, caused by aft movement of the wing's center of pressure and disrupted airflow over the horizontal stabilizer
- CPhugoid oscillation, caused by an exchange between kinetic and potential energy during a pitch disturbance
- DAdverse yaw, caused by increased induced drag on the down-going aileron at high speed
Show answer & explanationAnswer & explanation
Correct answer: B. Mach tuck, caused by aft movement of the wing's center of pressure and disrupted airflow over the horizontal stabilizer
As airflow over the wing becomes locally supersonic near Mcrit, a shock wave forms and moves aft, shifting the wing's center of pressure aft and altering downwash on the tail. This produces an uncommanded nose-down pitching moment known as Mach tuck, a compressibility effect distinct from Dutch roll, adverse yaw, or phugoid oscillations.
Why the other options are wrong
- A. Wrong — Dutch roll is a lateral-directional oscillation, not a pitching moment
- C. Wrong — phugoid is a long-period pitch/airspeed oscillation, not caused by shock wave formation
- D. Wrong — adverse yaw involves yaw from aileron deflection, unrelated to Mach effects
Mach Tuck
An uncommanded nose-down pitching moment occurring near critical Mach number, caused by shock wave formation shifting the wing's center of pressure aft and disturbing tail airflow.
- Occurs near or above critical Mach number (Mcrit)
- Caused by aft shift of center of pressure due to shock wave
- Often countered by Mach trim systems in high-speed aircraft
Memory trick: Mach tuck: the shock wave shoves the lift center back, tucking the nose down