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The yaw of repose

Yaw of repose formula

Abbreviations

cM a
Overturning moment coefficient derivative coefficient
dp
Yaw of repose vector

More abbreviations

Explanation

The repose angle of yaw (or yaw of repose, also called equilibrium yaw) is the angle, by which the momentary axis of precession deviates from the direction of flight (see figureGo to figure). As soon as the transient yaw induced at the muzzle has been damped out for a stable bullet, the yaw angle d equals the yaw of repose.

The magnitude of the yaw of repose angle is typically only fractions of a degree close to the muzzle, but may take considerable values close to the summit especially for high-elevation angles.

The occurrence of the yaw of repose is responsible for the side drift of spin-stabilized projectiles even in the absence of wind. The spin-dependent side drift is also called derivation.

It can be shown that for right-hand twist, the yaw of repose lies to the right of the trajectory. Thus the bullet nose rosettes with an average off-set to the right, leading to a side drift to the right.

The above formula for the yaw of repose vector is an approximation for stable bullet flight.

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