| GENE SLOVERS US NAVY PAGES DRIFT OF A PROJECTILE |
| UNDER CONSTRUCTION |
| Drift
of a Projectile The drift of an elongated, rotating projectile may be considered to result from three causes: 1. Gyroscopic action. 2. The Magnus effect. 3. The cushioning effect. It is reasonably certain that the last two causes have only a minor effect as compared with the first. The rifling of the gun barrel causes the projectile to rotate in flight with sufficient rapidity to behave as a gyroscope. This serves to stabilize the flight of the projectile, but it makes the projectile subject to gyroscopic precession. Because of the curvature of the trajectory, air pressure on the underside of the nose of the projectile causes a precession to the right. This shift of the projectile axis to the right increases the air pressure on the left-hand side of the nose, which causes the projectile to precess downward. This train of events continues, causing the axis of the projectile to oscillate about the tangent to the trajectory. Since the greatest pressure is on the underside of the nose, the over-all precession is to the right. The initial tendency of a projectile to maintain the original direction of its axis as it falls away from the axis of the bore causes the air stream to strike the projectile’s lower side. With right-hand spin, the air adhering to the right-hand side of the projectile then opposes the air stream created by the projectile’s flight, and the result is an increase of pressure on the right-hand side. At the same time, there is a rarefaction, and the projectile tends to move to the left, to the side of lesser pressure. This effect is known as the Magnus effect, and is the same phenomenon which causes a golf ball to hook or slice. The Magnus effect can be important on the descending end of the trajectory at extreme elevations. Since the air tends to pile up on the underside of projectile in motion, it forms a cushion. The projectile tends to roll on this cushion because of the friction imposed by it. This movement is to the right in a projectile with right-hand spin, opposing the Magnus effect but adding to the gyroscopic effect. The above text is incorrect where it says the following: 1. Gyroscopic action. The rifling of the gun barrel causes the projectile to rotate in flight with sufficient rapidity to behave as a gyroscope. This serves to stabilize the flight of the projectile, but it makes the projectile subject to gyroscopic precession. Because of the curvature of the trajectory, air pressure on the underside of the nose of the projectile causes a precession to the right. This shift of the projectile axis to the right increases the air pressure on the left-hand side of the nose, which causes the projectile to precess downward. This train of events continues, causing the axis of the projectile to oscillate about the tangent to the trajectory. Since the greatest pressure is on the underside of the nose, the over-all precession is to the right. XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX I am always amused when I see where someone copies the above part of the text about drift of the projectile and then tells me that the projectile precesses to the right as a result and then signs their name and wants me to believe that they dreamed this all up on their own. I believe there is a term of people who copy things and then try to lead you to believe they are the originators. The sad part is because the above is not true it shows some of how much they really know on the subject. The projectile most certainly is or becomes a gyro in flight due to it's rotation and is therefore subject to precession when the apparent wind acts upon the projectile and in particular on the nose cone. However the projectile is rotating counterclockwise as it flys into the oncoming stream of air and the text is correct in saying that air pressure builds up on the underside of the projectile and it's nose cone. Whoever wrote the original text should have remembered that when the gun barrel is vied from the breech it has right hand rifling and when viewing the butt of the projectile in flight it is rotating to the right. However when you go around in front of the projectile and it is coming at you you see that the forward end of the projectile is rotating counter-clockwise and any precessional forces on the bottom of the nose cone would cause the projectile to precess to the left of the line of fire, not to the right as the text says. When pressure or force is applied to a gyro it does not precess away from where the pressure or force is applied. It instead precesses at a point 90 degrees in the direction of rotation. Thus the projectile would precess to the left instead of to the right of the line of fire. Since the air pressure is constantly applied to the bottom of the projectile it would continue to precess to the left and quite rapidly so. The projectile would turn sidewise to the oncoming air and would tumble. This of course does not happen. It only precesses slightly both left and right enough to keep it's nose directly into the oncoming apparent wind. The apparent wind is caused by the forward motion of the projectile. The text being correct where it says the following: Since the air tends to pile up on the underside of projectile in motion, it forms a cushion. The projectile tends to roll on this cushion because of the friction imposed by it. This movement is to the right in a projectile with right-hand spin. The projectile does exactly this and it rolls to the right on the cushion of air. As the projectile moves downrange it builds up speed it moving to the right and the further downrange it goes the faster it moves to the right as it accumulates right hand speed due to the rolling motion on the cushion of air. The cushion of air is caused by the projectile falling back to earth and is uniform along the underside of the projectile. Since the cushion of air is uniform across the bottom to the projectile then the force of the air is also uniform across the bottom of the projectile. Thus air pressure or force at the rear of the projectile is trying to precess the butt of the projectile in the opposite direction to the forces of the air on the nose of the projectile. The resultant forces and pressures are equal and thus cancel each other out and the projectile precess neither right nor left as a result of the cushion of air on it's bottom. That cushion of air by the way is as hard as granite, so the term cushion is slightly misleading. The oncoming air does as I said above cause the projectile to precess slightly left and right to keep it's nose directly into the oncoming air. The projectile is also force to precess downward as it goes downrange and the nose is constantly precessed downward to keep the nose directly into the oncoming apparent air and thus it always strikes nose first at the target. |