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Bullets and the Wind

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Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/16/2010 at 15:58
Chris Farris View Drop Down

Joined: October/01/2003
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Bullets and the Wind: An explanation of why bullets behave as they do in flight

by William C. Davis, Jr.

The wind has bothered riflemen for as long as there have been rifles. It still bothers them. Smallbore riflemen, bench-rest riflemen, and big-bore riflemen all 'dope and hope' for X's, groups, or V's. Experience can teach us, within limits, how to dope for a given load and range and wind. We can also determine why bullets behave as they do in the wind.

It may be somewhat comforting to know that the great Dr. F. W. Mann, to whom all small arms ballisticians and shooters owe a debt of gratitude, seems never to have understood fully the nature of the problem. He saw that the observed deflection of a bullet by the wind is tremendously large, considering the time that the wind acts on it in flight. He surmised this could not be due to 'drift' in the ordinary sense of the word; that is, the action was not the same as that by which a leaf is carried sideways by the wind as it falls to the ground, or a floating log is carried along by the stream.

Dr. Mann's experiment
Dr. Mann possessed a .32 caliber test weapon of unusually fine accuracy. This rifle fired a bullet of 187 grains weight with a 47-grain charge of blackpowder. He had determined by careful experiment that the total drop of a horizontally fired bullet from this rifle was about 9 7/8 inches over 100 yards, and about 41 inches over 200 yards. He reasoned, quite correctly, that a bullet dropped vertically from 41 inches would fall to the ground in nearly the same time as that required for a bullet from his rifle to reach the 200-yard target. He therefore constructed an apparatus which would drop bullets on a piece of paper in such a way that their deviations from a plumb line could be accurately measured. The deviation from the plumb line would be, then, the actual 'drift' which the wind could produce in a bullet over the corresponding range. Any difference between this figure and the observed deflection on the target was the quantity which Dr. Mann sought to identify and explain.

Results were inadequate
Having prepared his experiment carefully, Dr. Mann first tested the apparatus on a calm day, and the bullets fell exactly on the plumb line, as he expected. He then waited for a day on which the wind velocity was 20 to 24 miles per hour (m.p.h.) and dropped several bullets from each height - 9 7/8 and 41 inches - to simulate firing over ranges of 100 and 200 yards. The greatest 'drift' for the 9 7/8-inch drop was less than .1 inch, and about .3 inch for the 41-inch drop. Dr. Mann does not state the effect of a 24-m.p.h. crosswind on a bullet fired from his rifle over 200 yards, but it was certainly much greater than .3 inch; it can be determined from analysis of his drop data that the figure was probably about 33 or 34 inches. This is more than 100 times the 'drift' he observed by dropping the bullet vertically through the 41 inches to the ground.

To understand this behavior, it is first necessary to have clearly in mind some fundamental ideas about the nature of motion. All motion is relative. If we sit in a railway train alongside another train, we may observe from a window that the other train seems to be moving. A passenger in the other train may have the very same impression about us. Not until we look out the opposite window may we be sure which train is actually moving over the ground.

'Frame of reference'
Motion of an object is judged by changes in the relative positions of that object and certain points which we regard for the moment as fixed. A 'frame of reference' is determined by the points which we regard as fixed, and by means of which we judge the motion of other points or objects.

We are accustomed to establishing our frame of reference on most occasions with objects which are fixed on the ground. Thus, although an object on the equator travels some 25,000 miles around the earth's axis each 24 hours, and everything on earth travels some 600 million miles around the sun each year, we usually say that any object which remains planted on the surface of the earth is 'fixed', and whatever changes position with respect to that object is 'moving'. When air moves over the surface of the earth, we say there is wind.

To comprehend the wind deflection of bullets, it is simplest to change our frame of reference from the stationary earth to the moving air. This is accomplished most easily by setting up an imaginary experiment, in which the rifleman and target are regarded as moving, and the air is regarded as not moving. Suppose we wish to determine the effect of a 20-m.p.h. wind on a bullet fired from a long-range match rifle over a range of 1,000 yards. The day is calm, with no breath of air stirring between the rifleman and target. We might provide two level, parallel tracks, 1,000 yards apart, heading - say - due west, as shown in our diagram (Fig. 1). We might then place the rifleman on a car on the southerly track, and a target on an opposite car on the northerly track, so that the rifleman looks directly northward at the target.

Here is the situation
If we then cause each of the cars to move westward through the still air at 20 m.p.h., exactly together, the rifleman will be in the same situation as when he is firing at a stationary target 1,000 yards away in the presence of a 20-m.p.h. wind from nine o'clock. The target does not change position with respect to the rifleman, being always precisely north of him, and it appears perfectly steady as he aims at its center. His only other impression, as he concentrates on the target, is the breeze in his left ear. We must think about this situation, and satisfy ourselves that conditions are essentially the same as those in which a rifleman might fire over a normal 1,000-yard range, with a 20-m.p.h. wind blowing from nine o'clock.

Here are the conditions
Now to define further the conditions of the problem, let us suppose the rifleman has a .300 Magnum match rifle, from which he is firing the 172-grain M1 bullet with a muzzle velocity of 3,000 feet per second (f.p.s.). To make our units of measure consistent, we will regard the range as 3,000 feet (which is, of course, precisely 1,000 yards) and the speed of the cars as 30 f.p.s. (which is approximately 20 m.

Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/17/2010 at 20:08
Sgt. D View Drop Down
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Joined: February/20/2008
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Any chance of posting fig. 1 thru 5 for reference. My mental images tend to be scued at times. Great info. Thanks!
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: January/17/2010 at 22:46
Dale Clifford View Drop Down
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Optics Jedi Knight

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Vector analysis gives a pretty good discription on a large scale, but points to some stray conclusions. Drift is caused by drag not wind pushing the side of the bullet. It makes the bullet turn into the higher pressure gradient, thus keeping the center of air pressure at the nose. This results in the point of rotating center of mass to align itself with the higher pressure gradient and the base to be angled downward. This slowing causes the linear drift. The above account also neglects spin drift.
Post Options Post Options   Thanks (0) Thanks(0)     Back to Top Direct Link To This Post Posted: March/11/2010 at 04:13
55spartan View Drop Down
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Optics Apprentice

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thanks for this information
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