Coriolis effect...again...

T

tobnpr

New member
Putting this in the bolt section, I guess because most that shoot long-range use bolt guns and there's not a dedicated section for this stuff...

It's been discussed here before, but I came across this Gunworks vid that put it into a different perspective for me.

https://www.youtube.com/watch?v=jX7dcl_ERNs

I can finally wrap my head around the difference it makes- AS EXPLAINED IN THE VIDEO- in East vs. West shooting- and much less of a difference in North vs. South- I think. Due to the curvature of the earth, the target rotates either up (shooting West, resulting in low impact) or down (shooting East, resulting in high impact).

But I'm still confused about this language about "leaving the surface of the earth". Objects in flight co-rotate with the earth, otherwise we could throw a baseball into the sky and it would come down miles away.

I can't get my mind wrapped around the distinction. Can one of you engineers put this into plain english?
 
You have to think of the point of aim as being a point out in space.

An object in motion tends to stay in motion at the same direction and speed unless it is acted upon by another force. Since the bullet is originally aimed at a point above the earth's surface (think somewhere at a point in space), it is always going to want to travel towards that point while being both pulled down by gravity and slowed by friction.

While facing west, the earthy target travels towards that point in space. While facing east, the earthly target travels away from that point in space.
 
Interesting.

Will it be different amounts depending on the latitude you're shooting at?

He's a bit off the mark about objects leaving the earth when launched. Otherwise, people tossing a ball back and forth in a car, train or plane would have a hard time doing it.
 
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Coriolis effect is pretty much meaningless until you start talking about artillery, ballistic missiles, or jet aircraft

Over the relatively short distances covered by most firearms, it's too small to be measured

http://en.wikipedia.org/wiki/External_ballistics#Gyroscopic_drift_.28Spin_drift.29

For small arms, the Coriolis effect is generally insignificant, but for ballistic projectiles with long flight times, such as extreme long-range rifle projectiles, artillery and intercontinental ballistic missiles, it is a significant factor in calculating the trajectory.
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I've never thought about this before and maybe I'm wrong in my deductions and calculations, but I think "Bart" and "tobnpr" may have it right.

The bullet and the target are co-rotating, and the east-west velocity vector is equal for both the bullet and the target if they're located in the same latitude. However, because the earth's surface velocity is different with latitude, then you should see a coriolis effect when you are shooting north or south. When you move toward the poles the surface velocity slows, and when you move toward the equator, the surface velocity increases. If a shooter is shooting north, then the target is moving more slowly and the bullet should move to the right on the target, and it's just the opposite for shooting south.

The earth's rotation is 1527 ft/sec at the equator. The velocity of rotation varies with the latitude by the following equation: v = cosine (latitude)(1527 ft/sec), where the latitude is in degrees. As an example I chose a 168 grain A max hornady, .308 win load. Since their ballistic table stopped at 500 yards, I used that distance, and estimated a time in flight of .633 seconds. I used a latitude of 38 degrees north, which is about the latitude of St. Louis.

If the shooter is shooting due north and is at exactly 38 degrees latitude, then his east/west velocity vector is (cosine 38.0)(1527)= 1203.292 ft/sec.

If the target is 500 yards straight north (or about 15 seconds latitude) then the velocity of the target in an east west direction is (cosine of 38.0042)(1527) or 1203.223 ft/sec. Therefore the target is moving about .069 ft/sec more slowly than the shooter.

The coriolis shift should then be (.069 Ft/sec)(12 in/Ft)(.633 sec time in flight) = 0.524 inches to the right on the target.
 
Ya kind of lost me at the end there, but the argument is that it is most pronounced due East/West.

If you've ever been out to sea, out of sight of land, you know that you can plainly see the curvature of the earth. If I understand what the narrator is saying, if you picture yourself on a sphere (Earth) shooting West, you and your target remain equidistant, but during bullet flight-let's say 1-1/2 seconds- the point of aim you had(target) is now several inches higher-relatively, than it was at the time the bullet left the bore- because the sphere (Earth) is rotating towards you; "uphill", as it were...

The opposite holds true shooting due East, it's rotating away from you- "downhill".

I think...

It would then follow, as Bart mentioned, that this would be more pronounced the further you get from the equator as the curvature of the Earth (raidus of the sphere) is more pronounced.

I've never heard it explained the way the guy in the video does it, with the E/W vs. N/S component. The closer you are to shooting N/S, the more negligible it becomes.
 
Maybe I'm wrong, but I think you had it right in your first post. Shooting east/west, the target ISN'T moving away from you or toward you. The whole system: shooter, bullet, and target are moving together, at the same rotational velocity.

Think of it as shooting at a target from one end to another on a long, moving train. The target is not moving towards you because you are all moving at the same velocity on the train (with the exception of the bullet, which after firing is moving at the train velocity plus muzzle velocity).

Now assume that you're shooting at a target on another train, which is moving faster than you and parallel to you. In this case you would have to "lead" the target or you would hit to the left because the target is now moving at a different velocity that the shooter. That's why you should see a coriolis effect shooting north/south, but not east and west. The target and bullet have different rotational velocities north/south, but the same rotational velocities east/west.
 
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The target and bullet have different rotational velocities north/south, but the same rotational velocities east/west.
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I'm going to disagree on that one. East and west is where you see the greatest difference in rotational velocity. Shooting west, the path of the bullet and the rotational path of the target are getting closer together due to rotation of the earth. The opposite is true when shooting east.

Remember the first law of physics - an object in motion tends to stay in motion at the same direction and velocity unless acted upon by another force. The forces of gravity and wind resistance are the same shooting east or west but the movement of the target in relation to the bullet changes direction when you shoot one way as opposed to the other.
 
FWIW, in artillery or naval gunfire (20 miles or so) you see the Coriolis effect, as point of aim and point of impact can be 400-500 feet apart. But navies have calculators who can and do account for it and allow very accurate gunnery in spite of it. It has a lot to do with height of trajectory and time of flight, which are both considerable in indirect artillery gunfire. At the distances and trajectories most people shoot, the Coriolis effect is negligible, almost non-existant.
 
Not to belabor this but I went back and watched the video in Tobnpr's link. I think we are all correct here but we are using different ways to describe totally different phenomena with different causes.

The coriolis effect is a north/south phenomenon and is due to the target and shooter moving at different surface velocities. If you're shooting north, the target is moving more slowly and you will hit in "front" of the point of aim, or it will appear that the bullet is deflected to the right. Shooting south, the target is moving faster and you will hit "behind" the point of aim, but since you standing in the opposite direction, it will appear to be a deflection to the right. So, shooting north or south the bullet is deflected to the right in the northern hemisphere. The coriolis effect is a horizontal deflection.

As it turns out, the guy in the u-tube video was also right, but for the wrong reasons. The vertical deflection which he was describing is the eotvos effect. The eotvos effect is the result of the additive and subtractive results of the earth's angular velocity on the muzzle velocity and the resultant change in the centrifugal force on the bullet. Traveling east, the centrifugal force is greater, which lessens the perceived force of gravity and the bullet drops less and hits high. Traveling west, the centrifugal force decreases, which increases the perceived force of gravity and the bullet drops more. It has nothing to do the targets moving up or down, or the bullet leaving the earth as the guy stated.

Running through some numbers, the coriolis effect is usually not very large, but the eotvos effect can be significant at high muzzle velocities and long distances.
 
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Oh, so now we have another term to confuse us poor schmucks. :eek: Thanks for pointing out the difference Hammie. You're right. I always called both phenomena "coriolis effect".
 
Doyle: I think you have it right. I was a USAF pilot and coriolis does have have an effect on flying long distances. Since aircraft flies in a river of air that is relative to the earth the vertical effect is irrelavent in East/West flight. But it would have a difference in any ballistic effect (Aircraft are not ballistic)
 
At 1,000 yards, Coriolis has about a 2" effect when a standard 150gr 30-06 is fired North/South at mid latitudes.
(Now who would shoot 150gr at that distance is another topic altogther) ;)

The Spin drift associated with that shot, however, is almost 6-7X that. :eek:
(...and SD don' care what direction yer shootin' arn's pointin')


.
 
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(...and SD don' care what direction yer shootin' arn's pointin')
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For sure. It's interesting that he (guy in the video) equated it to a 70 fps difference in MV for that 7mm load they were shooting.

We discuss all these "little things" all the time, but I remain dumbfounded by the "night and day"- in my own LR shooting abilities- in scoring minute of angle hits consistently at 600 yards, while the ones at 1000 remain elusive.

I know I'm going to dope the wind incorrectly. I know my MV is going to vary from the computer dope. I know my trigger press isn't going to be precise, and I'm going to pull the shot to some degree... It's a matter of compensating for the "knowns", and doing the best I can on the rest.
 
mehavey, all the reaslitic data I've seen on .30-06 bullet spin drift for 150-gr. ball and 172-gr machine gun bullets at 1000 yards is about 5 to 7 inches. JBM's calculator says it's about the same numbers.

150-gr. bullets have been fired from 30 caliber rifles at 1000-yard targets since the 1960's when the Brits switched from .303 British to 7.62 NATO rounds. They have to use arsenal ammo in their matches. Nowadays, 155's are fired by folks in the USA shooting .308's in long range matches.

I've used the same windage zero on several 30 caliber rifles I've shot from 100 to 1000 yards. Whatever down range spin drift there was seemed insignificant to me. For all I know my 1000 yard windage zero was half a minute left of where spin drift put the bullets and as range got shorter, bullets struck closer to the line of sight ending up a quarter minute to the left at short ranges. It's a horizontal curve much like bullet drop; maybe an inch per hundred yards way far away, very little up close and personal.

It's nye impossible to get windage zeros exactly past 500 yards because of subtle winds that blow bullets sideways. There are, however, people who claim to get them exact and religously use spin drift calculators before firing a shot. All in spite of the fact that the best 5 mph + wind readers out there rarely have their first shot strike within 1 MOA horizontally from point of aim 1000 yards away.
 
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Bart, if you have real field data, I bow to that. Mine comes from QuickLoad's
internal Spin Drift results for 1,000 yards as differentiated to when I turn
Coriolis "ON/OFF".

Like you, my actual windage adjustments are based on the meteorology-du-jour...
since God always laughed at my prayers for a still day and no eddy currents
back in the 70s/80s.

I figured He was making a point. :rolleyes:
 
Bart,

I went to JBM to get a comparison w/ QuickLoad. Plug like numbers in,
this is what JBM gets at 1,000 yards for a 30 Caliber/150grSP/2,950:

350wi02.jpg


Which ball-park the effects QL predicts (ran both a 10 & 12 twist)
Spin drift is the big bear, not Coriolis (but we knew that)

As to weather effects......
dlj1ip.jpg
 
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I just ran the JBM calculator and put in a 1/10 left-hand twist and the scope canted 2 degrees.
 
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OK, Mehavey, what input or whatever did I mess up?

Oh well, never mind, I found it. Read the wrong data output.

I wish JBM used "Wind Drift" instead of "Windage" and had "Spin Drift" in another separate column. That way one could see the effects of moving air and coriolis separately.

SSA, your 2 degree cant comment reminds me of when I calibrated the spirit level on my front sights as to how much off plumb its bubble was with a cant enough to move the bullet 1 MOA in windage at 1000 yards. That way, in team matches when the coach gave me a correction (or I saw it myself through a spotting scope), I could cant the rifle to one side or another and easily make 1/4, 1/2 or a full MOA windage correction without going out of position to turn the windage knob on the rear sight. Coaches were not allowed to do that. One of my 1000-yard loads needed a 1.7 degree cant to move bullet impact 10 inches left or right.
 
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