Drift Sight

The Drift Sight was a bombsight developed by Harry Wimperis in 1916 for the Royal Naval Air Service (RNAS). It used a simple mechanical device to measure the wind speed from the air, and used that measurement to calculate the wind's effects on the trajectory of the bombs. The Drift Sight eliminated the need for a stopwatch to perform this calculation, as on earlier devices, and greatly eased the bomb aimer's workload.

The Drift Sight was quickly introduced into RNAS service and quickly thereafter by the Royal Flying Corps (RFC) as well. In British service, Wimperis' Course Setting Bomb Sight (CSBS) started replacing the Drift Sight in 1917, but it remained in widespread use in the US Army Air Service into the 1920s. In US use the Drift Sight is often referred to as the Wimperis sight, but this name is more commonly applied to the CSBS, especially in Commonwealth air forces.

Prior to the introduction of the Drift Sight, bombsights were generally very simple systems of very limited accuracy. The primary pre-World War I device in RNAS service was the "Lever Sight" which had to be held out of the cockpit in one hand by the pilot while flying the aircraft with the other. The Central Flying School bombsight (CFS) replaced this in 1915, but was difficult to install in the cockpit. The CFS was in turn replaced by the Equal Distance Sight (EDS) designed by in 1916 by Warrant Officer Scarff, better known for the Scarff ring. The EDS allowed the bomb run parameters to be entered once and then left the pilot free to fly the plane.

Adjusting for the effects of wind was a difficult process. None of these sights had a way to calculate "drift", the sideways motion of the bombs due to wind. This meant the aircraft had to attack their targets along the wind line. Even in this direction, the wind would cause the bombs to fall long or short. To correct for this, on an extended run the bomb aimer would measure their speed over the ground using a stopwatch, calculate the wind speed, and then adjust their aim point based on that speed and their current altitude. This solution was difficult, time consuming, and prone to error.

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