Magnus effect

The Magnus effect is the name given to the physical phenomenon whereby an object's rotation affects its path through a fluid, in particular, air. It is a product of various phenomena including the Bernoulli effect and the formation of boundary layers in the medium around moving objects.

The Magnus effect in external ballistics, also known as 'spin drift'

Another context where the Magnus effect can be found is advanced external ballistics. A spinning bullet in flight is often subject to a sideways wind. In the simple case of horizontal wind, depending on the direction of rotation, the Magnus effect causes an upward or downward force to act on the projectile, affecting its point of impact. Even in a complete calm, with no sideways wind movement at all, a real bullet will still experience a small sideways wind component. This is due to the fact that real bullets have a yaw motion that causes the nose of the bullet to point in a slightly different direction that the bullet is actually travelling in. This means that the bullet is "skidding" sideways at any given moment, and thus experiences a small sideways wind component.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

In the figure, a bullet is viewed from behind and is assumed to have right-handed twist. Additionally, the bullet should have an angle of yaw d, its longitudinal axis should be inclined to the left. Then there is a component of the flowfield velocity vn, perpendicular to the bullet´s axis of symmetry.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~

Due to bullet spin and air molecules adhering to the bullet´s surface, the flowfield in the vicinity of the bullet becomes asymmetric. Air stream velocity and the rotational velocity of the body subtract at point A and add at point B (see above figure). However, according to Bernoulli's rule (see any elementary physics book), this coincides with a pressure difference. A higher pressure at A and a lower pressure at B give rise to a downward directed force, which is called the Magnus force.

~ ~ ~ ~ ~ ~ ~ ~ ~ ~