page_title: Course Setting Bomb Sight
section_title: Crosswind bombing
hierarchical_section_title: Course Setting Bomb Sight / Description / Crosswind bombing
context_page_description: The Course Setting Bomb Sight is the canonical vector bombsight, the first practical system for properly accounting for the effects of wind when dropping bombs. It is also widely referred to as the Wimperis sight after its inventor, Harry Wimperis.
  The CSBS was developed for the Royal Naval Air Service in order to attack submarines and ships. It was introduced in 1917, and was such a great advance over earlier designs that it was quickly adopted by the Royal Flying Corps, and the Independent Air Force. It has been called "the most important bomb sight of the war".
  After the war the design found widespread use around the world. A US version of the CSBS was used by Billy Mitchell on his famous attack on the Ostfriesland in 1921. The basic design was adapted by almost all air forces and used well into World War II. It was eventually replaced in British service by more advanced designs like the Mark XIV bomb sight and the Stabilized Automatic Bomb Sight. Other services used vector bombsights throughout the war.
context_section_description: The bombsight problem is the need to determine the exact spot in the air where the bombs should be dropped to hit a target on the ground. Due to the acceleration of gravity, bombs follow a roughly parabolic path, the steepness being defined by the forward speed of the aircraft at the instant of release. The distance the bombs travel between being dropped and hitting the ground is known as the range, it is a function of the speed and the time to fall, the later a function of altitude. The bomber attempts to manoeuvre along a line toward the target and then drop the bombs at the instant they are that distance, the range, from the target. The location at that instant is known as the drop point or point of release.
  Simple trigonometry can calculate the angle that the target would appear when the aircraft was at the drop point. This is known as the range angle or drop angle, and was typically looked up from a set of pre-computed tables or using a simple mechanical calculator. The bombsight is then set to that angle, and the bomb aimer drops the bombs when the target passes through the sights.
  In the presence of a cross-wind, as the aircraft flies forward the wind will push it sideways, away from the drop point. The solution is to calculate the angle that the aircraft should fly in order to cancel out this drift, the difference between the course and the heading. Calculating the proper drift angle is a simple task of basic vector addition, and is commonly carried out on a circular slide rule like the E6B. This is a somewhat time-consuming process. The CSBS solved this problem by reproducing the basic vector math in a mechanical system. The vectors that would normally be drawn by hand were duplicated in a series of screws, gears and sliding components. By dialling in the four inputs, altitude, airspeed, wind speed and wind direction, the mechanism moved the aiming pippers so they directly represented the required heading and range angle for the current airspeed and altitude.
  The wind will also have an effect on the bomb after it leaves the aircraft. As bombs are generally well streamlined and have high density, this effect is much smaller in magnitude than the effects of the wind on the aircraft itself. For instance, consider a bomber at 20,000 feet (6,100 m) altitude dropping a stick of AN-M65 500 lb general-purpose bombs. These bombs will take approximately 37 seconds to reach the ground. In a 25 miles per hour (40 km/h) wind the bomb will move about 1,350 ft (410 m) due to the wind's effect on the aircraft's ground speed. In comparison, the effect of the wind after leaving the aircraft would be only 300 ft (91 m).
caption: In order to approach a target at the tip of the green arrow with the crosswind indicated in blue, the bomber has to turn upwind and point its nose in the direction of the yellow arrow. By flying in that direction, the wind will blow the bomber along the green line.
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