"Rotational Drift"

[gjkershul]

While thinking about the the best Minie conical-perhaps excessively- for a friend's 1863 Sprinfield replica-the Dixie two piece stock model made by Miroku- I went to the N-SSA small arms board and searched for "Springfiled twist".
I got a topic on slow v. fast twist for CW rifle muskets.
The information there was Springfields were 1-72 and Enfields three bands were 1-78. Enfield Musketoons and two band riles were 1-48.
Interestingly, US Ordnance is supposed to have gone with 1-72 because faster twist resulted in "rotational drift" at long ranges-in excess of 100yds.
I think I have experienced rotational drift at 400yds with my Enfield Musketoon. The bullet seems to shoot to the right even in a dead calm.
It is dead on at 100yds.
Does anyone know more about the concept and compensation for rotational drift?

 

[54mountain]

Can't tell you about compensation, as for concept I like to compare it to the side spin a baseball pitcher puts on a curve ball. If you spin a round ball or short heavy conical fast enough the air drag will cause a slight drift (curve) in the direction of the spin. this is why round ball barrels are built with a slow twist. but at extreme distances even a slow spin will drift slightly. JMHO, Hope this helps

 

[renovato]

Under normal circumstances and at typical distances, the "curve ball" effect doesn't apply to projectiles. This is because the rotation is around the path of flight, where a curve ball's rotation is horizontal to it. When the spin is horizontal (or vertical for that matter) to the flight path, one side is hitting more air than the other and creating more drag on that side, which causes the curve that direction.

At longer ranges, the rules change though. A properly stabilized bullet's nose will follow the flight path as it curves. A lot like a properly spiral thrown football. If you overspin a football, it won't follow the path anymore, but will remain nose high as it falls. The same with an overstabilized bullet or ball.

If a bullet (or ball) is falling nose high, it's no longer perfectly aligned with it's flight path so there is more air resistance on the bottom than on the top. This allows a small amount of "curve ball" effect from spin to creep in and can indeed cause some drift. It's a problem with modern guns in 600 and 1000 yard shooting too.

 

[gjkershul]

Thanks for the excellent explanation. :hatsoff:

 

[Zonie]

You asked about "compensation for rotational drift".

I know of no Civil war vintage guns that made an allowance for it however, my 1884 Springfield Trapdoor has a flip up sight that was intentionally canted to the left.
As the shooter raises the sliding sight higher it is automatically moved left so the longer the range the sight is set to, the greater the gun is pointed to the left to make the bullet hit the target.

 

[RedFeather]

Probably due to the Coriolis Effect. In the Northern Hemisphere the earth rotates to the left. Shoot a projectile towards the north and it will appear to drift to the right. Actually, from what I understand, the target is moving left due to rotation. I can remember in Torpedo School studying how the older WWII torpedoes had to have their gyroscopes preset at certain changes in latitude or else the fish would drift to the right of the target. I don't think changes in the rate of spin would provide the same influence on a bullet.

 

[BobKrohn]

Probably due to the Coriolis Effect. In the Northern Hemisphere the earth rotates to the left. Shoot a projectile towards the north and it will appear to drift to the right. Actually, from what I understand, the target is moving left due to rotation. I can remember in Torpedo School studying how the older WWII torpedoes had to have their gyroscopes preset at certain changes in latitude or else the fish would drift to the right of the target. I don't think changes in the rate of spin would provide the same influence on a bullet.

I'm positive this applies to artillery too and is a required correction.
Not an expert so would like to know how powerful this effect is. Interesting.
Wikipedia entry is too complex for me.
Can someone actually soleve the math for rifle type distances?
Links please?

 

[Tumblernotch]

In the book Small Arms 1856, written by Ordnance officers testing the new rifle-musket and altered muskets contains a table showing "mean drift" for the .54, .60 (.58) and .69 cal. rifled barrels firing the Minie style projectile. The distances shown were 200, 500 and 1000 yds. Of course the drift increased with distance, but at 200 yds was minimal with the smaller calibers. This increased quite a bit at the longer ranges. One interesting thing I see is that as the caliber was increased, the drift was greater.

As all the barrels were grooved with a right-hand twist, the drift was always to the right.

It was also noted that the increased twist caused an increase in drift, but at the same time reduced the deviations from shot to shot. How or why is not explained.

 

[renovato]

Rotation of the earth has an effect on artillery because the shell is in flight long enough for the ground to have actually rotated underneath it before it lands. I'm not sure how much effect this would have on a rifle projectile since it is in the air for such a short time. I'm curious if a gun with reverse rifling drifts the other direction? If so, then it would be the "curve ball effect" causing it, if not, it is probably the rotation of the earth. What a fascinating discussion!

 

[Dave K]

Then you may also have to ask, if the effect is dependent on which hemisphere you are in.

I do remember in my youth. I would suffer weekend bouts of rotational drift. But, placing one foot out of bed and on the floor, would usually take care of it. :grin:

 

[BobKrohn]

Earth

:shocked2:
(Please, someone check my math here. I can't believe how big the value is)

25,000 miles movement in 24hr @ Equator West to East (ballpark values)
i.e. if you were shooting due North or South
if you were shooting due East or West no effect other than speed over ground.

BUT, there is an atmosphere which tends to (friction, inertia, etc) move with the surface.
I think that would tend to overcome the rotational effect.

There may be a sideways component to the projectile imparted by the rotation?
I think that's why they launch equatorial orbiting satellites in an Easterly direction.

So, here are raw values per my calculator.
Point on Earth as viewed from Space.

25,000/24hr = 1,041.6667 mi_per_hr
/60min = 17.3611 mi_per_min
/60sec = 0.2894 mi_per_sec

at 45 deg Lat = 0.2894 * cos(45) = 0.2046 mi_per_sec
at 90 deg Lat = 0 (North Pole)

0.2046 mi * 5,280 = 1,080 ft_per_sec sideways (W to E) movement!?

Obviously, other factors are in effect. :hmm:

 

[brubincam]

----------WHAT----------

 

[BobKrohn]

----------WHAT----------

That's what I thought.

As passengers on the Earth's surface we don't really notice it since everything is moving with us so to speak.
It's like we are riding a merry-go-round. Nothing "moves" if we just look at the other riders. Then throw a ball AWAY from the merry-go-round. It's path looks very different depending on if you are a rider or just sitting on a bench watching the merry-go-round.
The rider will see a curved path.
The bench warmer sees a straight path.
Try it at the park. I did it in a science class back in high school. Very odd looking effect.
We are passengers on a merry-go-round spinning at 1,000 mph.
Obviously there are other factors since when we shoot we don't need to use a compass and 45 degrees of Kentucky windage.
I am just looking for a plug-in formula from some super bright guy on the forum to see what the actual values are. (and there are some math wizards here)

 

[BobKrohn]

Found short discussion on Coriolis Effect in rifle shooting.

Nothing to worry about with "most" muzzleloaders. :wink:
Just for extreme bench-resters.
1" at 600 yds.

 

[Dphar]

While thinking about the the best Minie conical-perhaps excessively- for a friend's 1863 Sprinfield replica-the Dixie two piece stock model made by Miroku- I went to the N-SSA small arms board and searched for "Springfiled twist".
I got a topic on slow v. fast twist for CW rifle muskets.
The information there was Springfields were 1-72 and Enfields three bands were 1-78. Enfield Musketoons and two band riles were 1-48.
Interestingly, US Ordnance is supposed to have gone with 1-72 because faster twist resulted in "rotational drift" at long ranges-in excess of 100yds.
I think I have experienced rotational drift at 400yds with my Enfield Musketoon. The bullet seems to shoot to the right even in a dead calm.
It is dead on at 100yds.
Does anyone know more about the concept and compensation for rotational drift?

Yes its a fact not a concept and the sights on many rifles like the late Trapdoor (Buffington sight) and the 1903 (IIRC) have compensation built in so as the sight is elevated it compensates for the drift at the range the sight is set to. The other compensation is "sighter shots".

Dan

 

[max]

From what I've read, snipers such as those using the .50 cal BMG cartridge for extreme long range targets use hand held computers which take the Coriolis effect into consideration along with distance,temperature, air density, and wind conditions. The world record for a long range kill is held by a Canadian soldier in Afghanistan who shot an insurgent at a measured 2430 meters. The first shot missed, but the second one found it's target. It's an interesting story and you can read about it on Google.

 

[Chasam2345]

If the atmosphere were not moving with the earth,wouldn't we experience a 1000mph wind? the bullet is moving through the moving atmosphere as are we and the target. I would not rule out the curveball effect,but earths rotation? Maybe Al Gore has an explaination that millions of fools will buy into so he can make more $$$$$

 

[Dphar]

Found short discussion on Coriolis Effect in rifle shooting.

Nothing to worry about with "most" muzzleloaders. :wink:
Just for extreme bench-resters.
1" at 600 yds.

Rotational drift is more than this.
Wind drift is up or down as well as down wind depending on the side the wind is pushing on. Right twist wind from the right bullet will climb somewhat.
At high departure angles gyroscopic precession can be a factor during or even before the bullets descent to the target.
Dan

 

[rawdog]

Plink is almost entirely correct.

A gyroscope, if pushed on one end of its axis in a direction perpendicular to its axis, will respond by moving in a direction +90deg (angularly in the direction of spin) from the direction in which the force was applied. This is basic to the physics of a gyroscope, and is the reason that a gyroscope precesses (and that bullets precess too, by the way, and if adequately stabilized this "spiraling" will settle down after a few ten yards of travel from the muzzle).

A bullet is gyroscopically stabilized and as Plink says, its axis of rotation (due to rifling spin) is never quite aligned to the trajectory because trajectory is constantly curving downward due to gravity.

Because of this tiny upwards "pitch" of the nose of the bullet compared to its actual direction of travel thru the air, there is an upwards force (aerodynamic lift), but because this is a gyroscopically spinning object the gyroscopic reaction is 90 degree from the upwards force direction and is instead in the direction of the spin (most all barrels spin right-handed by convention, I suppose). For this reason a bullet does NOT fly "point-on" to the target it has a slight rightwards "yaw" (called the "yaw of repose" by ballisticians) which in turn causes a cross-wind lifting force (ie, one pushing sideways) that results in drift to the right.

since this drift is a function of the degree of curvature of the trajectory, it occurs increasingly noticeable at distant ranges where the trajectory curvature is more extreme, and is very small at near ranges where the trajectory is more nearly flat.

Since this drift is a function of gyroscopic rate of spin, slower rifling produces less drift with a given bullet at a given range.

This drift is one of perhaps several reasons why for very long range shooting is is not desirable to give the bullet a lot more stability (twist) than it needs.



YHS,
rawdog

 

[jdstrang]

KanawhaRanger, with his cite of the 1856 test records, gave the best source I know of for Minie/Burton "drift." It is real, as your 400-yard experience with the fast-twist musketoon demonstrated. But at most practical (offhand) ranges, it is largely irrelevant, the unfortunate experience of Gen. John Sedgwick notwithstanding.

But, Hamkiller, The question remains: what IS the best Minie-conical for use in the 40-inch, 1-72 twist, 3-land/3-groove, progressive-depth rifle-musket barrel?

The original, pointed, extruded Minie/Burtons being unavailable, from what I can tell the Lyman Old Style best approximates the original design. I have tried the Lyman New Style, Rapine big minie, semi-wadcutter, Hodgdon and Edington's Wilkinson-Pickett bullets as well, and found nothing better. What did you fellows determine? A world too long under a frigid blanket of white wants to know!