|This article's factual accuracy is disputed. (March 2010)|
Mach tuck is an aerodynamic effect whereby the nose of an aircraft tends to downward pitch as the airflow around the wing reaches supersonic speeds; the aircraft will first experience this effect at significantly below Mach 1.
Mach tuck results from an aerodynamic stall wherein airflow over the upper surface of a cambered wing exceeds Mach 1.0 and thereby forms a shock wave where it returns to subsonic speed; a boundary layer separation therefore forms aft of the shock wave and spoils the lift behind it. The image to the right illustrates this concept.
The camber and sweep of a wing can induce Mach tuck. The shock wave will form at the wing root if it is more cambered than the wing tip. When and as a swept wing gradually stalls from root to tip, its lift moves aft, upward rotating the tail about the aircraft's center of mass; aircraft without enough elevator authority to stay level can thereby enter a steep, sometimes unrecoverable dive. Until the aircraft is supersonic, the faster top shock wave can reduce the authority of the elevator and horizontal stabilizers.
The shock wave can spoil the lift of the wing by engulfing it.
All supersonic aircraft experience mach tuck. Recovery from mach tuck is sometimes impossible in subsonic aircraft; however, as an aircraft descends into lower, warmer, denser air, control authority may return because drag and the speed of sound increase and control authority.
Aircraft that are supersonic for long periods often compensate for Mach tuck by moving fuel between tanks in the fuselage to change the position of the centre of mass and thereby minimize the amount of required trim and keeps the changing location of the center of pressure within acceptable limits.
Supersonic and subsonic aircraft often have an all-moving tailplane (a stabilator) rather than separate elevator control surfaces. This design avoids the shock wave making the control surfaces pitch downwards.
The fastest World War II fighters were the first aircraft to experience Mach tuck. Their wings were not designed to counter mach tuck because research on supersonic airfoils was just beginning; the shock wave would engulf the entire wing. The P-38 was the first 400 mph fighter, and it suffered more than the usual teething troubles. It had a thick, high-lift wing for quick climbing and holding much fuel. It also had three fuselages: the central weapon and pilot nacelle or gondola and the twin booms, which contained engines and turbochargers. It also was a very dense fighter for its day, and it quickly accelerated to terminal velocity in a dive. Bernoulli's effect very strongly worked on the thick wing and was even more pronounced where air was pushed away by and compressed between the nacelle and booms. Mach tuck would occur when the aircraft attained Mach 0.68; the air flow over the wing roots would become transonic, and the wing therefore would lose lift, thereby aftward moving the normal loading of the tail's horizontal control surfaces, leaving the elevator unloaded, bringing the nose further down in a Mach tuck. Lockheed engineers eventually added a small 'speed bump' flap beneath the wing; the pilot would engage it upon diving. The flap so changed the center of pressure distribution that the wing would not lose its lift.
- Pilot’s Handbook of Aeronautical Knowledge. U.S. Government Printing Office, Washington D.C.: U.S. Federal Aviation Administration. 2003. pp. 3–37 to 3–38. FAA-8083-25.
- Airplane Flying Handbook. U.S. Government Printing Office, Washington D.C.: U.S. Federal Aviation Administration. 2004. pp. 15–7 to 15–8. FAA-8083-3A.
- Transonic Aircraft Design
- Bodie, Warren M. The Lockheed P-38 Lightning: The Definitive Story of Lockheed's P-38 Fighter. Hayesville, North Carolina: Widewing Publications, 2001, 1991. ISBN 0-9629359-5-6.
This article incorporates public domain material from the United States Government document "Airplane Flying Handbook".
This article incorporates public domain material from the United States Government document "Pilot's Handbook of Aeronautical Knowledge".