Does a RB travel with constant rotation, or accelerate?

[TNGhost]

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.

Hadn't even thought of that angle, but conservation of energy would apply(?) and Bernoulli may even have something to say about this aspect.

 

[appalichian hunter]

Wow you guys are really into this, all I look for is the target usually a deer and I am able to punch a hole through his boiler maker, ensuring a short trailing job and humane kill. Guess I am old school.

 

[Carbon 6]

Ya, I miss attending the monthly Mensa meeting.

 

[appalichian hunter]

Know what ya mean, looks like I best catch up on the meetings or find a rocket scientist that likes to run around in the woods. Could be some interesting discussions around the fire at night.

 

[hawkeye2]

"Could be some interesting discussions around the fire at night."

For sure, add a little adult beverage and we could solve world hunger and find a cure for covid-19 too.

 

[Griz44Mag]

Wow you guys are really into this, all I look for is the target usually a deer and I am able to punch a hole through his boiler maker, ensuring a short trailing job and humane kill. Guess I am old school.

HAHA - this bunch would argue about which way a bird will fly off a fence post - and provide supporting science to back it up!!!

 

[Griz44Mag]

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?

Nope - you are confusing the rate of spin (RPM) with the distance per turn (RPF). It's still spinning (close to) the same RPM, but traveling slower.....

 

[George]

Know what ya mean, looks like I best catch up on the meetings or find a rocket scientist that likes to run around in the woods. Could be some interesting discussions around the fire at night.

And it would all be HC/PC. The physical laws which allow us to understand the trajectories of our bullets were published by Isaac Newton in 1687, and the first practical way to measure the velocity of our bullets, the ballistic pendulum, was invented by Benjamin Robins in 1742.

Spence

 

[tenngun]

Nope - you are confusing the rate of spin (RPM) with the distance per turn (RPF). It's still spinning (close to) the same RPM, but traveling slower.....

Certainly what Spence and Zoni are saying. Still trying to work it out in my head.

 

[Springypond]

A very interesting and educational conversation! Math and physics not being my forte , I have the right to remain silent-except to say thanks! Old dogs can learn!

 

[Billy Boy]

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

Nothing accelerates after it leaves the muzzle except drop.

 

[30coupe]

I can't believe I read all three pages of this! My head is spinning faster than it was anyway

Since some of you seem to enjoy physics so much, I'll pose a second problem for you. Since a miniball would have greater side surface area than a round ball, would friction from the atmosphere cause its spin to erode faster than that of a ball?

Have fun folks.

 

[Smokepole55]

On the ballistics table on the back of a Winchester cartridge box it seems to indicate that a .45 colt will leave the muzzle at 750 FPS with 312 pounds of energy. At five yards it’s traveling 747 FPS with 310 pounds of energy and at 25 yard it’s going 735 FPS with 300 ft pounds. This indicates the projectile slows down as it leaves the muzzle With the same rule applying to a muzzle loader.

 

[Carbon 6]

I can't believe I read all three pages of this! My head is spinning faster than it was anyway

Since some of you seem to enjoy physics so much, I'll pose a second problem for you. Since a miniball would have greater side surface area than a round ball, would friction from the atmosphere cause its spin to erode faster than that of a ball?

Have fun folks.

That's an easy one. Yes it can degrade quicker, but not for the reason you proposed.

 

[hawkeye2]

I can't believe I read all three pages of this! My head is spinning faster than it was anyway

Since some of you seem to enjoy physics so much, I'll pose a second problem for you. Since a miniball would have greater side surface area than a round ball, would friction from the atmosphere cause its spin to erode faster than that of a ball?

Have fun folks.

I expect it might, theory says it should if you consider air resistance first. The first issue is that we have no idea how much a round ball slows its spin and no way to measure that so we can't set a standard to compare to. Logic or common sense (??, I don't know which) tells us that stabilization should decrease as the spin slows and that would affect group size but so many other things do that too that I doubt it would tell us anything. My thoughts are that the projectile would have to shed a significant amount of spin before accuracy would be affected and that would be beyond the range where we could expect any true accuracy. BTW a mini possesses a much larger angular momentum to resist a change in angular velocity than a round ball so it's possible that its spin stays more consistent. There are formulas for angular velocity and angular momentum that might enable one to calculate the differences but I have equipment out in the shed to maintain/repair, trees to come down and grass to mow so I will never get to that exercise. BTW the distance a projectile travels, it's velocity in that direction or any change in that do not enter into those equations.

A disclaimer: In my posts I have not discussed the influence of wind, humidity, air density, earth's rotation or a number of other things. Think of a bullet fired down a 300 yard long tunnel. I have approached this in a basic way so as not to confuse the issue. If anyone is interested in external ballistics and the other things that affect them I suggest they look into the discussions on the flight of those pointy things that travel several times the speed of our rounds and their affect over several hundred yards. The BP cartridge shooters in our group know where I'm coming from there.

 

[TNGhost]

I can't believe I read all three pages of this! My head is spinning faster than it was anyway

Since some of you seem to enjoy physics so much, I'll pose a second problem for you. Since a miniball would have greater side surface area than a round ball, would friction from the atmosphere cause its spin to erode faster than that of a ball?

Have fun folks.

Yes, you are correct,...given everything else, like the density of the air (and therefore friction), the weight of the projectile and the rate of twist, etc. were all equal.

It is a simple matter of surface contact area multiplied by the friction coefficient.

 

[Zonie]

IMO, the Minie'ball has a larger surface area on the sides if compared with a roundball of the same diameter but I don't know if it's rotational velocity would slow down faster.

The Minie' ball has a much larger amount of mass than the ball does and the energy in this increased mass is far greater than the ball.
That might make its rotational velocity slow down less than the same sized ball would.

In any case, the changes in the speed either one of them rotates is very slight and for the most part, it can be overlooked when it comes to the original post's question.

 

[tenngun]

So, gyroscopes are used in rockets and compasses to keep direction. They resist turning so as a rocket or torpedo starts to drift off corse the gyroscope is turned, it resist and pulls the rudder or jet in to a different direction.
Like a ball in flight most tings are only in flight a short interval of time. But the gyroscope doesn’t spin with some sort of perpetual motion. Energy has to be fed in to maintain its rpm.
A top doesn’t maintain its speed. Reduce the friction it’s going to last longer but we live in a world of friction.

 

[Carbon 6]

A ball can rotate off axis with respect to it direction of travel and not change it's aerodynamic properties. You can't do that with Minnie ball. As a projectile slows down after leaving the muzzle it eventually becomes unstable. When an elongated projectile becomes unstable things go awry quickly. A round ball may have lesser flight characteristics, but is has greater stability. This is why you can stabilize a round ball with a very slow twist rifling. Very little rotation. It's also why smoothbores work.

 

[shorthair]

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

Approximately 3.3% per second of flight according to the spin decay formula. So in the first second a ball from a1/60 barrel will have slowed down to about 1/62 . Or probably 0 since it may well have run into the ground.