Page:Popular Science Monthly Volume 91.djvu/759

This page needs to be proofread.

Will a Projectile Fly True?

The uniformity in the balance of the shell deter- mines its accuracy and ultimate destructiveness

By Reginald Trautschold, M. E.

���IF a projectile is to strike its mark miles from the muzzle of the gun from which it is fired, more is required than accurate range finding, ac- cura e gaging of the propelling charge and ac- curate aiming. The projectile must be well bal- anced. Otherwise it can not be counted upon to fly true. In other words, the pro- jectile must weigh just so much; its dimensions must not vary; and it must be properly balanced, both about its axis and lengthwise. Accurate tests must therefore be made to discover whether a certain, very slight allowable variation (human fallibility cannot be entirely over- come) has been exceeded.

In France, the shell to be tested for balance about its axis is laid at right angles across two level and parallel steel bars. Since the weight of the projectile can never be evenly distributed, the shell will obviously roll until it comes to rest, heavy side down. • Is it too unbalanced ? To find out, a contrivance known as an "eccen- tricity weight" is clamped to the base of the shell. This eccen- tricity weight, as its name implies, is not evenly balanced about its center; one end is heavier than the other.

���Testing the French 120 m.m. shell for weight and ballistic properties. The man is holding a small weight showing the allowable tolerance in weight

��A shell balanced on the knife edges of the weighing appa- ratus to ascer- tain the point of its length- wise balance

���An eccentricity weight is attached to the shell on the bars. If the

weighted end points downward when it comes to rest the shell is approved

��The exact weight of this fixture is known and also the exact distance from its center of the point at which it would balance if laid across a knife-edge. The fixture is clamped to the base of the shell, perpendic- ularly, with its heavier end up, i. e., opposite the heavier side of the shell.

The shell, with attached eccentricity weight, is rolled along the parallel! bars until the fixture is horizontal, and then released. Should the shell and weight re- main in this position, the surplus weight on the heavier side of the shell exactly balances the offset weight of the fixture. Such a state of equilibrium rarely occurs. The shell usu- ally rocks in one direc- tion or the other until a position of exact bal- ance is found, when it comes to rest. Should the weighted end now point toward the


�� �