Mach Tuck

What is Mach tuck?

Mach tuck is the nose-down pitching moment an aircraft experiences as it passes its critical Mach number (Mcrit).

Mach tuck is a form of longitudinal instability that occurs because of the center of pressure’s rearward movement behind the center of gravity, which induces the aircraft to pitch down (or the aircraft’s nose to tuck).

What is the purpose of a Mach trimmer?

The purpose of a Mach trimmer is to automatically compensate for Mach tuck (longitudinal instability) above Mcrit.

What is a Mach trimmer, and what is it used for?

A Mach trimmer is a system that artificially corrects for Mach tuck above the aircraft’s Mcrit by sensing the aircraft’s speed and signaling a proportional upward movement of the elevator or variable-incidence stabilizer to maintain the aircraft’s pitch attitude throughout its speed range up to its maximum Mach demonstrated flight diving speed (MDF).

Note: Mach trimmers allow for an aircraft’s normal operating speed range to be above its Mcrit. In the event of a Mach trimmer failure, there is usually an imposed reduced Mach maximum operating speed (MMO) value so that a margin is retained below the Mach speed at which the onset of instability occurs.

What are the effects of compressibility?

Compressibility is the effect of air being compressed onto a surface (at a right angle to the relative airflow), resulting in an increase in density, and thus dynamic pressure rises above its expected value. It is directly associated with high speeds.

There are two main effects of compressibility:

1. Compressibility error on dynamic pressure reading flight instruments; e.g., air speed indicator shows an overread error that is greater the higher the aircraft’s speed.

2. Compressed air is experienced on the leading edge of the wing, which disturbs the pressure pattern on the wing and causes the disturbed air shock-wave/drag effect at the critical Mach number.

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