Airplanes do not fly directly over the target and release their bombs. The airplane imparts forward airspeed to the bomb, launching it on a trajectory to the target.
Due to forward and upward wind resistance affecting the unpowered bomb, the airplane will always be ahead of the bomb when it strikes the target. The distance the airplane travels during the time it takes the bomb to fall to earth is the whole range (time of fall times groundspeed). The distance the bomb lands behind the plane is the trail. The whole range minus the trail is the actual range.
The vertical bombing altitude forms one side of a right angle triangle. The horizontal actual range forms the other side of a right angle triangle. The hypotenuse then forms the drop angle.
Trail distance and time of fall duration are determined by experimental data and found in tables particular to the airspeed, altitude, and bomb type.
To correct for crosswind, the plane must be angled into the wind to fly a straight course. This angle is called the drift angle. For the bomb path to correct for crosswind, the airplane must not only be angled into the wind, but it must be on a parallel path upwind from the target. The distance upwind is the crosstrail, and the bombsight adjusts for this, because the crosstrail is equal to the trail times the sine of the drift angle. Setting the trail number thus solves for both trail and crosstrail once the drift angle is established.
The sight angle is the visual angle on the target, and as the plane approaches the target, this angle becomes more vertical, shown by the telescope sector indicator. The bombardier locates the target and puts the crosshairs on it. This is done by looking through the telescope which is focused on a mirror. The mirror angle is adjusted by a variable speed motor; the telescope is always vertical and establishes a reference. When the bombardier synchronizes the bombsight on the target, it remains stationary in the crosshairs, and the mirror motor speed yields groundspeed information. With time of fall input, the bombsight solves for the whole range, then subtracts the trail and determines the actual range. Using the rate of closure of the sight angle, it computes the drop angle, as shown by the rate sector indicator as its tangent.
Therefore, to perform a bomb run, the bombardier knows the altitude and airspeed and looks up in tables the time of fall and trail. He inputs time of fall information into the disk speed knob, and adjusts the trail arm to the correct number. After sighting the target with the search knob, he finely adjusts the crosshairs using the displacement knob. He synchronizes the bombsight to keep the crosshairs on the target with the rate knob. He then turns the airplane into the wind using the turn knob, and establishes the drift angle using the drift knob and turn knob together. This is called, “killing the drift”.
In summary, as the plane approaches the target, the bombardier puts the horizontal crosshairs on the target with the displacement knob and keeps them there by adjusting the rate knob. He puts the vertical crosshairs on the target by steering the plane using the turn knob and keeps them there by killing the drift with the drift and turn knobs. The mirror and the telescope sector indicator will move at a speed synchronized onto the target, and when the telescope sector indicator meets the rate sector indicator, the sight angle matches the drop angle and the bombs are released automatically.
Here is a video demonstration of how all of this works: