Where does stick collide?

In tribology stick-slip is the jerky motion two sliding surfaces undergo when static friction exceeds kinetic friction: the surfaces grip and stick, elastic strain builds until it overcomes the static limit, they slip suddenly, then grip again, producing the stutter heard as a squealing brake or a bowed violin string. Rabinowicz's 1951 The nature of the static and kinetic coefficients of friction analysed the gap between static and kinetic friction that drives the cycle, and Bowden and Tabor's The friction and lubrication of solids treats stick-slip as a general friction phenomenon. The referent is an oscillatory frictional instability measured by its amplitude and frequency, unrelated to a flight control or a gymnast's landing beyond the word stick.[1][2]

Unit

The amplitude and frequency of stick-slip oscillation between sliding surfaces

Recovery means

Not applicable

Measured by

Characterised by the difference between static and kinetic friction coefficients and the resulting stick-slip amplitude and period

In flight mechanics the control stick is the pilot's pitch-and-roll control, and an aircraft's longitudinal stability is analysed in two cases named for it: stick-fixed stability, with the elevator held in place, and stick-free stability, with the control allowed to float. Irving's 1966 Stick-fixed and stick-free stability sets out how these two conditions give different neutral points and stability margins. The referent is a stability property of an aircraft defined by whether the control stick is held or released, measured by static margin, and it is a wholly separate construct from frictional stick-slip or a stuck gymnastics landing.[3]

Unit

The static margin (distance from the centre of gravity to the neutral point) with the stick fixed or free

Recovery means

Not applicable

Measured by

Determined from pitching-moment curves and the stick-fixed and stick-free neutral points of the aircraft