Aerodynamics: bullets, footballs, arrows?

[mrblint]

In layman's terms why does a projectile that spins axially travel "true"?

 

[Bandersnatch]

A spinning mass has got high angular momentum, which takes a lot of force to change. The effect is the same as what makes gyroscopes work.
This means that a spinning bullet is less likely to tumble as it flies through the air, which improves accuracy.

 

[bikengr]

The real issue is, what happens if not spinning? Long objects like sticks, if slightly angled to the wind, will tend to turn broadside. They are unstable going straight because the center of pressure on a tilted bar is well in front of the centerpoint.

If you want a long slender object to go straight, here are three strategies:
1. give it tail feathers, so if it tilts, the center of pressure is behind the cm
2. make it nose heavy, for the same reason
3. spin it about its axis, so that if air pressure tries to turn it broadside, instead it precesses around a skinny cone, still generally pointing in the direction of travel.

These are all done to keep the point forward, and keep drag low and travel path precise.

A ball is not helped by spinning, since tumbling is not created by the air, and tumbling does not increase drag nor spoil aim. But if you have an artillery shell, or rifle bullet, I guess the spinning is helpful or essential to keep it straight