Overcoming Yaw

When a non-spherical projectile is fired or launched its longitudinal axis rarely lies along the tangent to its trajectory, so there is a non-zero angle of yaw. As a result of yaw the air acts on the projectile to produce

1. (i) an increased drag compared with the drag at zero yaw,

2. (ii) a deviating force which tends to alter the projectile's trajectory,

3. (iii) an overturning moment which rotates the projectile about its centre of gravity.

The last was discussed in Section 6.7, and may increase or decrease the yaw depending on the design of the projectile. A decrease in yaw is obtained by fitting fins to some projectiles (darts, arrows, bombs and mortar shells), so providing a method of stabilising the projectile and its trajectory. This method of stabilisation has some disadvantages however, the principal one being that a cross-wind tends to interact with the fins and push the projectile well off course.