Does a RB travel with constant rotation, or accelerate?

[smo]

Uphill or Downhill ? lol

 

[Notchy Bob]

Two other BP shooters I know have different opinions about RB rotation after it leaves the barrel. So, I am going to ask the experts here. A 50 cal, 48" barrel, with 1 revolution in 48" twist is fired using 60 grains of 2F.
When the RB leaves the muzzle it has already rotated 1 complete turn. Here is the question: Does the RB continue rotation of 1 turn every 4', or does it accelerate to a faster rotation?
Flintlocklar

Wow, there are some smart people on this forum!

It seems only logical to me that a ball shot from a barrel with rifling cut at a uniform or consistent rate of twist would not increase its rate of spin after it leaves the muzzle. However, I think all of the responses thus far have assumed the barrel was rifled with a uniform twist. A very few makers now and in the past have offered barrels cut with a gain twist, which has a relatively slow rate of twist at the breech, to get the ball started, but the rate of twist in the rifling gets progressively faster toward the muzzle. So, the ball starts out turning slowly but the rate of rotation increases as it travels toward the muzzle. The old Numrich muzzleloader barrels could be had as either uniform or gain twist, and I think Colerain, Hoyt, and possibly Oregon Barrel Company can make barrels with a gain twist on special order now. Maybe the OP's buddy was thinking of a gain twist barrel.

One thing I don't know is what happens with the spinning roundball after it exits the muzzle of a gain twist barrel. Does it continue to progressively accelerate its spin, even as it loses forward velocity and succumbs to gravity, as the OP's friend suggested, or does it continue spinning at the rate it had when it exited the muzzle? I don't know.

Notchy Bob

 

[Pete G]

It continues spinning at the rate it left the barrel (as if that really matters) .

 

[Larry (Omaha)]

Maybe the OP's buddy was thinking of a gain twist barrel.

Notchy Bob
[/QUOTE]
BP shooter #2 was only expressing opinion about regular twist per my example: So, I am going to ask the experts here. A 50 cal, 48" barrel, with 1 revolution in 48" twist is fired using 60 grains of 2F.
Thanks for your thoughts
Flintlocklar

 

[Larry (Omaha)]

It continues spinning at the rate it left the barrel (as if that really matters) .

If your just shootin and do not care why RBs do what they do, I agree. If one wants to know why a RB does what it does, then physics play a huge role in our shooting.
Thanks for posting
Flintlocklar

 

[BadDaditood]

The moment it leaves the muzzle the ball depreciates, just like driving your new car off the lot...

 

[Griz44Mag]

The moment it leaves the muzzle the ball depreciates, just like driving your new car off the lot...

SPOT ON!
Air resistance will slow the ball (or any projectile) and the same air resistance will slow the spin.
Gravity will pull the ball downward -
The ONLY way a ball will increase it's rotation or speed would be if it was equipped with a second stage booster that ignited while in flight.
You can't cheat the laws of physics.

 

[hawkeye2]

"You can't cheat the laws of physics."

The punishment is quite severe when you try to!

I had excluded gain twist rifling from my posts up to this point as I didn't want to muddy the water. It doesn't matter how the projectile reached its RPM at the muzzle, ie. it can start slow and its rotational speed can be increased as it travels down the bore by progressively faster twist rifling but at the point it exits the muzzle it doesn't matter whether it reached its rate of rotation by progressive or regular rifling. If we assume the gain twist to reach 1 turn in 48" in the last few inches of barrel then the projectile will have the same RPM as if it started in a regular rifled barrel with a rate of 1 turn in 48". Once it's beyond the influence of the barrel it cannot increase its RPM without an external force acting on it (Griz44Mag's booster rocket).

 

[tenngun]

The RPM is entirely independent of the forward velocity once the ball leaves the muzzle. The RPM or angular velocity is established by the rate of twist and the velocity of the ball at the time of firing but has nothing to do with the distance it travels any time after leaving the barrel. If we have a barrel with a twist of 1 turn in 48" (constant twist and not gain twist rifling) and the barrel is 48" long It uses that length to reach the maximum velocity and after the ball exits the muzzle it begins to loose velocity and to drop due to air resistance and gravity. Those two forces however do not oppose the rotation of the ball and in turn slow it. The momentum of its spin (see: spin angular momentum) will continue to keep its angular velocity constant and that momentum must be overcome by something to change the rate of spin. If the RPM were to drop the projectile would loose stability. That would have a considerable effect on an elongated projection and I suspect a lesser effect on a round ball.
View attachment 39203

Ok, let’s think about a ball in space. The ball is shot at 1600 fps relative the gun. It’s making a turn once every four feet.
We are in a vacuum. So one minute after it leave the gun the ball is still going 1600 fps. It will be 96000 feet from our muzzle.
It’s making one turn every four feet of that distance. It will have spin on its axis 24000 time in that minute. So it has an rpm of 24000.
Same gun differs powder charge.
1200 FPS. So now our ball travels 72000 feet in a min. In that minute it turns on its axis 18000 times its RPM is 18000.
The rate of twist remains the same one turn in four feet, but The time factor it takes to go four feet and compleat a revolution is longer. The ball has the same spin rate but lower Rotations per minute
Now we’re in space but we have a giant balloon filled with one atmosphere of pressure gas. So our ball is shot, it slows just like it dose on earth but doesn’t hit any ground.
It’s fired at 1600 fps, but by the time it reaches 300 feet from the muzzle it has slowed to 1100 fps, at two hundred yards 800 fps , and at three hundred yard 350 fps.
Wick and dirty math here since the deceleration related to air pressure isn’t constant but here goes
At the muzzle it’s 1600 fps 2400 rpm, at a hundred yards it’s 1100 fps 16500 rpm. Average speed is 1350 fps average RPM is 20250.
It will have turned on its axis seventy five time in that first hundred yards, and 75 in the next hundred yards, and seventy five in the third hundred yards. But the time it takes to go that hundred yards is longer and longer. At 350 fps it’s still turning on its axis one turn ever four feet, buts it’s taking longer and longer to go four feet. It’s RPM now is 5250.

 

[George]

Conserved spin, no velocity.....



Spence

 

[Zonie]

Ok, let’s think about a ball in space. The ball is shot at 1600 fps relative the gun. It’s making a turn once every four feet.
We are in a vacuum. So one minute after it leave the gun the ball is still going 1600 fps. It will be 96000 feet from our muzzle.
It’s making one turn every four feet of that distance. It will have spin on its axis 24000 time in that minute. So it has an rpm of 24000.
Same gun differs powder charge.
1200 FPS. So now our ball travels 72000 feet in a min. In that minute it turns on its axis 18000 times its RPM is 18000.
The rate of twist remains the same one turn in four feet, but The time factor it takes to go four feet and compleat a revolution is longer. The ball has the same spin rate but lower Rotations per minute
Now we’re in space but we have a giant balloon filled with one atmosphere of pressure gas. So our ball is shot, it slows just like it dose on earth but doesn’t hit any ground.
It’s fired at 1600 fps, but by the time it reaches 300 feet from the muzzle it has slowed to 1100 fps, at two hundred yards 800 fps , and at three hundred yard 350 fps.
Wick and dirty math here since the deceleration related to air pressure isn’t constant but here goes
At the muzzle it’s 1600 fps 2400 rpm, at a hundred yards it’s 1100 fps 16500 rpm. Average speed is 1350 fps average RPM is 20250.
It will have turned on its axis seventy five time in that first hundred yards, and 75 in the next hundred yards, and seventy five in the third hundred yards. But the time it takes to go that hundred yards is longer and longer. At 350 fps it’s still turning on its axis one turn ever four feet, buts it’s taking longer and longer to go four feet. It’s RPM now is 5250.

You was doing good until you got to your "Wick and dirty math".
I don't disagree with the idea that the linear velocity of the ball slowed down but the speed the ball spins would only change I'd guess maybe 50 or 100 RPM due to the wind causing circumferential drag on the forward face of the ball. (The back face of the ball is in a near vacuum because the air can't fill that area fast enough to make contact with the ball.)
In other words, the RPM of the ball does not change just because the velocity of the ball changes.

 

[tenngun]

So are you thinking that as the ball decreases its forward velocity the ball would increase its rate of turn. So the ball that started at one turn in four feet is turning one turn in three feet at a hundred yards?, or one in two in a half feet at two hundred yards?

 

[Zonie]

Yup.

 

[mooman76]

Keeps spinning constant, I seen it on Xmen.

 

[tenngun]

Yup.

Interesting

 

[tenngun]

So if the ball can be stable at 1/30 turn rate, why don’t we build our guns with 1/30 twist?

 

[TNGhost]

You was doing good until you got to your "Wick and dirty math".
I don't disagree with the idea that the linear velocity of the ball slowed down but the speed the ball spins would only change I'd guess maybe 50 or 100 RPM due to the wind causing circumferential drag on the forward face of the ball. (The back face of the ball is in a near vacuum because the air can't fill that area fast enough to make contact with the ball.)
In other words, the RPM of the ball does not change just because the velocity of the ball changes.

You have to also take in to consideration the sides of the ball. Though it is a sphere, as it spins, the presence of the air is going to create friction against that spin, as all of the sphere is not encompassed by vacuum. Now it will take some fancy mathematics to show this drag, as it is a sphere and not a flat sided object, so there will be a graduated effect, but however slight that frictional drag may be, it is there and it will act in slowing the rotation.

 

[Canute Rex]

Going from Zonie's calculation of 350 rotations per second, a 50 caliber ball would have a surface velocity relative to the air of 45.8 feet per second. That is, rotationally, or perpendicular to the direction of travel. Not much compared to 1000-1800 fps forward. (note: 45.8 fps at the "equator" of the ball, going to zero at the "poles.")

Still, with really long range shooting the shooters have to account for lift or drop due to crosswind because of the Magnus effect. If the bullet is moving up or down because of the crosswind then energy is transferring from rotation to lift and the rotation is slowing.

 

[George]

The RPM is entirely independent of the forward velocity once the ball leaves the muzzle.

hawkeye2 got it completely right. Whatever velocity the ball drops to, spin rate remains almost constant. There is such a thing as "spin decay rate", but it's extremely small.

Spence

 

[mooman76]

So if the ball can be stable at 1/30 turn rate, why don’t we build our guns with 1/30 twist?

Not needed plus if you try to push it too hard, the ball strips and no longer follows the rifling. They do have guns with faster twist and they still shoot well with limitations. I have one with 1/20 twist.