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Coriolis effect

 
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  #1  
Old 11-08-2003, 11:49 AM
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Coriolis effect

According to my references, in the Northern hemisphere the lateral or horizontal component is always to the right irrespective of shot direction. Does anyone have a source which would challenge this claim?
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  #2  
Old 11-08-2003, 01:19 PM
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Re: Coriolis effect

Blaine, That is ridiculous. Common sence tells you that that would be impossible, as the entire premise of coriolis effect, deals with and varies with direction.

Now, not that this would ever matter from a small arms ballistics point of view, but, the effect from 100% at a north/south shot to 0% at an east/west shot could be roughly estimated by multipling the resultant vector times the sine of the angle from north/south. Gee, don't forget to add in the gyroscopic stability cause that's gonna throw a wrench in the equasion!

Regardless of the hemisphere, a shot fired tward the equator would impact west of it's intended point of aim, and a shot fired away from the equator would impact to the east.

Shots fired due east or west at the equator will have no resultant effect, and shots fired due east or west at any point away from the equator will exhibit a very small effect, spelled VERY SMALL, because of thier altitude varying. If the altitude is increasing, then, so is the southward component of motion due to the orbit not being geo-syncronous.

Since you have put me through this, my challenge to you is to either refute this, or, take a 210gr Berger 30 Cal bullet shot at it's advertised BC, at 2800fps, at an altitude of sea level, From the equator, due north/west (45deg), an impact at 1000meters exactly on target, with the target at exactly the same sea level altitude, and a nice easy theoretical GS of 1.0 . All indoors with no wind, 70 deg F, 70% Hum, 29.5Bar, and you tell me, how far the impact is affected by coriolis effect as opposed to the same shot fired due south east. Should be an easy one! You only need to calculate 1 of the 2 and multiply the result by 2. Simple!
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  #3  
Old 11-08-2003, 02:43 PM
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Re: Coriolis effect

Hell, I think I want in on this one!

As I understand it... in the Norhtern hemisphere Coriolis deflects to the right and it deflects to the left in the southern hemisphere. I find this easiest to understand as when viewed from a downward view above the North Pole... the Earth spins counter-clockwise BUT when this downward view is from above the South Pole the Earth is spinning clockwise.... the Earth spins in opposite directions when comparing North and South hemispheres.

Deflection from Coriolis as I understand it is due to the projectile traveling at the Earth surface speed at it's flight origin and retaining this induced speed as it travels to another part of the earth that may very well have a different surface speed.


For a quick set of numbers (in a special environment, no drag) and an example.... earth nominally 60 miles per degree at the equator, 360 degrees in a circle.....21,600 miles for one revolution...24 hours per rev = 900 mph surface speed at the equator. Shoot a round north or south with a 900 mph origin speed and have it fly for a minute at 5280 fps (one mile per second). In that time it will have flown 1 degree (60 miles) north. At this distance from the equator the earth (our example earth) is only 21,223 mile around so it's surface speed is 884 mph.... Our projectile has an origin speed of 900 mph and when it arrives on degree north it's at a spot that is moving slower that the origin by 6 mph or 8.5 feet per second, with a flight time of 60 seconds * 8.5 FPS we should have a displacement of about 510 feet to the east of the intended target (an arc to the right in the north hemisphere and an arc to the left in the south hemishpere).
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  #4  
Old 11-08-2003, 03:23 PM
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Re: Coriolis effect

Bzzzz, wrong. Sorry Dave.

Your example assumes that the shot is being takn from the equator. That is in contradiction to the original posters question. What is asked is if the shot is taken from the northern hemisphere, will it always have an effect in one direction regardless of the direction of the shot and the answer is NO it will not.

Stand at a latitude any other than the equator and as per your view, the relationship of east/west right/left vary depending weather you are standing facing north or south. Ignore the common viewpoint of looking down from above the poles, you are standing on the earth. Somewhere. If you are in the N Hemis, facing north, east is to your right. If you are facing south, east is to your left. Now your 2 shots have opposite, but not equal effects.
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  #5  
Old 11-08-2003, 07:00 PM
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Re: Coriolis effect

According to Prof. Pejsa ("Modern Practical Ballistics") and Robert McCoy ("Modern Exterior Ballistics") the following describes lateral deflection resulting from a turning earth (i.e., the Coriolis effect):

Deflection = (K*R^2*sin L)/V where K is a constant, R is the range, L is the latitude and V is the average velocity over the range R.

According to McCoy, all deflections are to the right in the Northern hemisphere and math-wise the formula presented will not change in sign.

Intuitively this is hard to accept. Hence, my post: does anyone have a source to the contrary?
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  #6  
Old 11-08-2003, 07:30 PM
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Re: Coriolis effect

As I understand it... After all, I'm not a rocket scientist [img]images/icons/smile.gif[/img].


Okay, northern hemisphere only. Shots will always be deflected to the right in varying amounts with the exception of a shot EXACTLY due east or west. The amount of the deflection is dependant upon origin and target surface speed and the projectile time of flight. We're in essence shooting at a moving target from a moving position... the target is always moving east and the origin position is always moving east...when the speed of the two positions match there is zero deflection.

Shots fired at intevals of the hours on a clock face... 12:00 being due north toward the geographic pole, 3:00 due east, 6:00 due south and 9:00 due west.

Shot fired due north...greater surface speed at origin than at target position... projectile if deflected to the right (east) as seen by the shooter (due north and south have the greatest deflection for this effect). Repeat at 1:00...same shot angle and resultant distance travel but less eastward (right as viewed by the shooter) deflection because the target location is a nearer the same surface speed.... Repeat at 2:00 position and again lesser eastward deflection (right as viewed by shooter) as the target and origin surface speeds are closer. Shoot at 3:00, same origin and target surface speed so no deflection... note that in theory the target can only be EXACTLY due east. Now we swing to the 4:00 position and shoot toward the south east....the origin surface speed is LESS than the target surface speed so the projectile is deflect to the west of the target (to the right as viewed by the shooter). Swing to 5:00 and again the projectile is deflected to the west (the right as viewed by the shooter) but to more extent than a 4:00 position shot. Shoot at 6:00 position and the effect is the greatest again and the projectile is delected to the west (the right again).

The due east and due west "no deflection" zones are only possible if the origin and target are on exactly the same latitude. A pretty skinny possibility for a shot at a distance of 60 miles as in my previous example.
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  #7  
Old 11-08-2003, 07:33 PM
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Re: Coriolis effect

Blaine

I can't see any other way for it to work... deflection is to the right in varying amounts in the northern hemisphere. Perhaps we should consult the folks that built the naval gunfire program for the British Navy... Heard it was a problem for them during the Falklands fiasco.
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