How and why does spinning a bullet make it stable?

If I understand correctly, bullets whose center of gravity is behind their center of pressure are unstable. If the bullet deviates just a little from being parallel to the line between the shooter and the target, that deviation will become self-reinforcing and the bullet will yaw or pitch and acquire a flight path which is impossible to predict before shooting the bullet thereby making it inaccurate. Have I got it right so far?

So, why does having the bullet spin on its roll axis prevent it from spinning on its pitch or yaw axis?

In planes or projectiles, is the point of fins/stabilizer to counter pitch/yaw forces by making it so that when the projectile/plane starts to pitch/yaw, wind resistance opposite the pitching/yawing pushes it back into flight straight?

The spin makes it gyroscopically stable along its length, which is parallel to the path of its flight. It will thus tend to resist deflecting forces and stay on a ‘truer’ path than a non-spinning projectile.

“The spin makes it gyroscopically stable along its length”

Thanks. What is it about spinning that makes something gyroscopically stable along its length?

What’s the difference between being gyroscopically stable and being plain old stable?

You must have seen a top. Don’t spin it and it falls to the floor. Spin it and it can stand upright, the axis of its spin being perpendicular to earth and seeming to defy gravity. And while it is spinning, the spin axis is stable (relatively speaking - precession effects tend to cause some movement in both tops and bullets).

If a round bullet comes out of a musket barrel it’s spin characteristics (which affect its flight like the spin imparted to a baseball does) will be essentially random. If spin is deliberately imparted along the axis of travel of a modern bullet, the bullet tends to stay stable and resist deviation in the same way a top does.

Gyroscopic stabilization is why tops stay upright, bicycles don’t fall over, bullets fly a straighter path and plates stay atop wobbly sticks - but it’s not simple to explain exactly how the forces work.

You’re best looking up “gyroscopes” and gyroscopic stabilization; understanding the latter is key to the question you’re asking. I’m sure one of our more physicky types can explain but they’ll likely end up pointing you to sites with diagrams etc. anyway.

“plain old stable” has different meanings in different contexts, but the general meaning of “stable” is that if the object deviates from its stable position, it somehow induces a force that tends to return it to its stable position. For an airplane, rocket or dart whose nose gets shoved to one side, there are tailfins that apply aerodynamically-induced forces that assure the rear end will always stay at the back; for a ship on the water that rolls to one side or the other, the location of buoyant forces changes when that happens and tends to restore the ship to an upright position.

A bullet that tips on its pitch or yaw axis is being subjected to an aerodynamically-induced torque about its pitch or yaw axis (either that, or a tiny yaw/pitch was induced as it left the barrel). If it’s not aerodynamically stable (sort of like the airplane, rocket or dart), then you need to turn it into a gyroscope in order to stabilize it. When a spinning bullet experiences a torque about its pitch or yaw axis, it exhibits precession: instead of tipping further around its pitch or yaw axis (as we would expect for a non-spinning bullet), it will start to wobble around its path of travel. The rate at which its spin axis wobbles around its axis of travel (and the angular deviation from its axis of travel) decrease with increasing RPM, which means the faster the bullet spins, the less it’s going to wobble, and the more slowly it will wobble. IOW, faster spin = more stable.

Noel Prosequi brings up the interesting case of a spherical projectile (as used to be fired from muskets). With a spherical projectile fired from an unrifled barrel, variations in friction from one side of the barrel to the other can result in an unintentional spin on an axis that is far from (possibly even perpendicular to) the axis of travel. This can induce aerodynamic forces that result in a severely curved flight path; you can read about the Magnus effect to understand why. Imparting a deliberate spin about the ball’s axis of travel prevents a random spin from being imparted by imperfections in the barrel, so your musket won’t throw curveballs.

I don’t know if this this scientifically accurate, but I think of it as by spinning, any bias in the bullet is spread evenly in all directions. Lets say a non-spinning bullet (magically stays on axis; or better yet, imagine a sphere) has a minor defect that would cause it to veer left. If the bullet spins, then that defect pulls the bullet a little left, then a little down, then a little right, then a little up, etc. for a net straight shot.

There are two things going on.

The first is gyroscopic stabilization, as was already mentioned. This is easy to demonstrate by picking up the front end of a bicycle and spinning the tire very quickly. Once the tire is spinning, you’ll find that turning the wheel is more difficult using the handlebars. The spinning wheel wants to keep spinning along the same axis. This is the gyroscopic effect. So a spinning bullet doesn’t want to tilt away from its axis of spin because of that.

The second thing is that any imperfections in the bullet that make it want to tilt in one direction or another won’t cause the bullet to veer off in a single direction. Instead, since the bullet is spinning and the direction of “tilt” is constantly changing, it will make the bullet corkscrew through the air, which will still put it pretty close to the center of the target when it finally reaches it.

bit off topic but bicycles don’t use gyroscopic stabilization, TMS Bicycle, stable without gyros or trail (TMS=two-mass-skate) - YouTube

Bicycles are designed using rules of thumb and trial and error testing to achieve desirable handling. Engineers and physicists can usually agree on what factors influence it, but blood feuds erupt over how much each factor matters.

Gyroscopic stabilization is a factor in bicycle stability, but does not dominate. It contributes virtually nothing at low speeds. Special test bicycles have been built with counter-rotating flywheels to cancel the gyroscopic effect of the wheels, and they are as easily ridden. Razor scooters with minuscule wheels (virtually no gyro effect) stay upright just fine.

Gyroscopic stability IS what keeps rifle bullets from tumbling. Interestingly, a bullet can be too stable. A bullet flies in an arc, and for minimum drag it must stay pointed in the direction of travel. A bullet that is spinning excessively fast tends more strongly to stay pointed the direction it had when it left the barrel. Not only does this increase drag, but it causes a slight drift (to the right for the usual case) due to Magnus effect of the vertical component of air velocity over the bullet.

Many satellites that orbit the Earth also use spin stabilization.

They do, it is just one factor. The guy in the video says that can be eliminated from a bike and it can still be stable, though I don’t see how counter-rotating wheels remove gyroscopic stability. But regular bicycles do benefit from gyroscopic stability, it’s just not the only thing keeping them stable.

The projectile from an AK-47 is deisgned to tumble inflictiing a larger wound than a direct firing bullet.
I don’t know if any current “assault weapons” replicate this.

Let’s just all agree that for any standard bicycle with a rider, gyroscopic stability is much less important for stability than front-wheel trail.

Or any assault bicycle either.

Easily agreed.

:dubious: Got a cite for this? If the bullet is deliberately tumbling through the air, it’s unlikely to hit its target (unless its target is just a few feet away).

I don’t know about the AK, but something like that was done to M16s at one point because the bullets were passing right through people and not providing ‘stopping power’. I don’t think it was tumbling per se, just allowing a certain amount of wobble so the bullet would tear up flesh when it struck.

OK, I looked around online. A number of references (such as this one) talk about the M16 bullet’s tendency to yaw/tumble after impact with a target, but not during flight. Supposedly the tendency for post-impact tumble was enhanced by shaping the bullet so that it is tail-heavy. It’s still spin-stabilized in flight for accuracy,and the spinning provides enough stability to counter aerodynamic forces, but not enough to counter the yaw forces presented when such a bullet strikes flesh: the nose of the bullet gets pushed to one side, which presents a longer lever-arm with respect to the bullet’s center of mass, which results in greater yaw, and so on.

The key assumption is that any fleshy target is almost never going to be struck at a perfect 90-degree angle of impact, so there almost always will be a tendency for the bullet to receive some initial yaw impulse when it hits.

If the bullet actually tumbled in flight, you’d expect to see the round hit its target straight-on only very rarely; most times it would be canted at some angle, or even completely sideways. If you were shooting at paper targets, you’d see a lot of very weirdly-shaped holes, and very few small circular holes. Instead, the opposite is true: you never see oblong holes in a paper target, just nice circles.

:slight_smile: One of my frequent types of replies. A “/sarcasm” text after “thanks” is just itching to bust out.

On bulletin boards I guess, unfortunately, “/sarcasm” is too aggressive, although in face to face a smile could be used with no harm.

No love for footballs here? Also thrown with spin for stable-path reasons.