can never be a close fit in the bore, and after the driving-band had emerged from the muzzle it would be followed by some 6 inches of ill-fitting body, with the powder-gases escaping past one side of it, which would unsteady the shell. Accordingly the driving-band is always set as far back as possible, leaving only sufficient metal behind it to afford a grip for the cartridge case. Forward Steadying Band.-The unsteadiness or oscillation of the shell in the bore, due to its imperfect fit, is a serious cause of inaccurate shooting. Attempts have been made to overcome this by fitting a forward band in addition to the driving-band. It is said that the new Austrian field shell has a steadying band of this nature. The subject is beset with difficulties. If an increasing twist is used, the forward band must on no account take the rifling. If the forward band is to be a good fit in the bore, then the bands must be eased at the breech in order to enable the shell to be rammed home. The walls have to be thickened at the shoulder to take the band. In spite of these difficulties, it would seem worth while to try the steadying band in field guns with uniform twist of rifling, in order to obtain increasing accuracy for fire at shielded guns. Drift. The tendency of a projectile fired from a gun rifled with a right-hand twist is to drift or whirl out of the plane of fire to the right. This divergence increases rapidly with the range about as the square thereof. The rough correction of deflection for drift is set forth in paragraph 327 Drill Regulations at 3 mils up to 3,500 yards and 5 mils for longer ranges. The subject of drift is a very difficult and complicated one and we understand few of the laws governing this motion. Although the effect of the resistance of the air tends to keep the shell pointing in the direction of its motion, yet the spin of the shell constantly resists this tendency, and tries to keep the shell parallel to its original direction. The result is a compromise, and the shell travels with its nose cocked in the air, well above the line of the trajectory. Now if we remember that the shell, viewed from behind, is spinning in the direction of the hands of a clock, then it will be evident that its friction against the air resistance, which takes it below the center, must tend to make the shell gradually deviate to the right. And since the spin of the shell diminishes more slowly than the forward velocity, therefore the path of the shell curves more and more to the right. Thus if we suppose that the spin of the shell carries it ten feet to the right every second, then in the first second the shell will travel say 1,500 feet forward and 10 feet sideways, and will have acquired a side velocity, at right angles to the line of fire, of 20 feet per second. During the next second this side velocity will increase to 40 feet per second, during the next to 60 feet, and so on; while all the time the forward velocity will be decreasing. It is quite conceivable that if the range were long enough and the twist sharp enough the shell would end by drifting almost square across the line of fire. A good instance of drift is the behavior of a sliced golf ball. Here we have a projectile roughened so that the effect of the twist makes itself fully felt, a comparatively low velocity, and a sharp spin; and the result is often that the ball pitches nearly as far off the course as it carries from the tee. It must not be supposed that the above is either a full account or a mathematically correct statement of the behavior of a rifled projectile. It merely furnishes a working hypothesis sufficiently near the truth for the purposes of the practical gunner. Persistence of Spin.-It was formerly supposed that the spin of the shell was but little affected by the air-resistance, and that, for flat trajectories, the spin continued almost undiminished to the end of the shell's flight. Recent experiments with mechanical fuses depending for their action on the spin of the shell have caused this view to be modified. It is found that a R. F. field shell loses about 10 per cent. of its spin at 3,000 yards, and about 20 per cent. at 5,000 yards. The reasons for this are as follows: I. Part of the spin is expended in overcoming the surface friction of the shell against the air. It must be remembered that the shell is constantly passing through a wave of air compressed by its own forward motion. As may be seen from spark photographs, this wave of compression extends beyond the shoulders of the shell and a considerable distance down the body. The friction caused by the shell rotating in this compressed air is much greater than it would be in air at the normal pressure. II. Part of the spin is expended in giving the lateral drift to the shell. Suppose a shell drifts two degrees at 4,000 yards, it will have moved 400 feet laterally in 10 seconds, its mean lateral velocity will be 40 fs., and its final lateral velocity 80 fs. If the shell weighs 15 pounds, the energy consumed in giving it a lateral velocity of 80 fs. will be 15 x 6,400 15 x 6,400/64.4' or nearly 1,500 foot pounds. How much of this is at the expense of the forward velocity, and how much at the expense of the spin, it is difficult to say. III. When the shell rotates eccentrically, and is noisy in flight, the spin has to do a considerable amount of work in setting air-waves in motion. Theoretically, therefore, a noisy shell should drift less than a steady one, since it loses its spin earlier. But the flight of a noisy shell is usually so erratic as to render it difficult to test this point. IV. If a shell were fired in vacuo, it would maintain its original angle to the horizontal all the way, and would come down on one edge of its base, since there is nothing to make it change its direction. When it is fired in air, its cylindroconical shape keeps it more or less point first all the way, at least for flat trajectories. Now it requires a considerable effort to change the direction of the axis of the rotating body, and this effort is exerted partly by the forward motion of the shell, partly by its spin, reacting on the cushion of compressed air surrounding the shell. It is not therefore surprising to find that a howitzer shell fired at a high elevation and long range has but little spin left at the end of its flight, since if fired at 45 degrees the axis of rotation has been deflected through more than a right angle. |