To understand the principles underlying the-theory and use of a bomb sight, one should be familiar with the way in which a bomb falls when dropped from an airplane. Because of the horizontal speed imparted to it by the moving plane, the bomb, instead of dropping vertically to the earth, moves forward with the plane in falling. If it were not retarded by the air resistance, the bomb would be directly under the plane when it hit the earth, but because of the air resistance the bomb is always behind the plane when it strikes. This is shown in figure 47.

The distance FS which represents the distance from the,front of impact of the bomb to the vertical line drawn from the plane at moment of impact is called "trail." The angle FRS, denoted by (b) is called the " trail angle." Now let the speed of the airplane relative to the ground be denoted by s, and let the time required for the bomb to fall be denoted by t. Then

Therefore:

PR = OS = st OF = OS - FS = st - FS

The angle OPF denoted by a is calted the "aiming angle." It is given by the equation:

tan a = OF/OP

Let the altitude OP be denoted by h and substitute from equation (1) for OF.

Then: Since:

tan a = OF/h = st - FS/h = s (t/h) - tan b FS/h = tan b

Then Equation::

tan a = s - (t/h) - tan b

is fundamental in the designing of all bomb sights.

Let v denote the speed of the airplane through the air, and w the speed of the wind relative to the earth, then obviously,
flying directly with the wind, we have:

s = v + w

and directly against the wind:

s = v - w

For all bombs of the same size, shape and weight, the time of fall depends on the altitude and the air speed v, but varies so little with v that it is commonly assumed to depend on h alone. The angle b depends on the air speed v and the altitude h, but is entirely independent of the ground speed. But b varies so little for the ordinary air speed and altitudes encountered in practice that it is sometimes assumed to be constant.

It is plain that the " aiming angle " (a) depends on only three quantities, h, v, and s, for in equation (3) t and b depend on h and v. In actual bombing the altitude is determined by an altimeter, and air speed by an air speed indicator, while the ground speed must be determined by some device incorporated in the sight.

The fundamental requirements of a bomb sight are, therefore:

To be so designed that when the three fundamental quantities h, v, and s are known, the sights may at once be set at the proper "aiming angle" a.

The actual operation of the difterent types of bomb sight, together with the method of installation. on aircraft, is described in detail in the handbooks issued by the Ordnance Department.

In any case at the time of release the machine must be flying in a vertical plane passing through the target, and except in the case of the synchronizinz sight, must be flying on an even keel. Bombing is practically always done when flying either up or down wind.

As has already been stated the aiming angle depends on the altitude, air speed, and ground. speed, the first two quantities being determined by auxiliary instruments. In the synchronizing sight the ground speed is automatically adjusted for by varying the speed of the motor.



MARK I - A     BOMB SIGHT

The Mark I-A bomb sight is modeled after the British Wimperis high-altitude sight.

It is an accurately sight at high altitudes and necessitates no stop watch or timing device. It corrects for height, speed of machine, and drift, and is adapted for all American bombing airplanes.