The Course Setting Bomb Sight (CSBS) 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 (RNAS) 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".
The design found widespread use around the world after the war. A US version, the Mark III, was used by Billy Mitchell on his famous attack on the Ostfriesland in 1921. Similar designs were adopted 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. The US and Germany introduced advanced designs as well. Most other forces continued using vector bombsights throughout the war.
History
Early bombsights Prior to the introduction of the CSBS, bombsights were generally very simple systems of limited accuracy suitable only for low-level use. The primary pre-war device in RNAS service was the Lever Sight, which the pilot had to hold out of the cockpit in one hand while flying the aircraft with the other. The Central Flying School Sight 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 in 1916 by F. W. Scarff, better known for the development of the Scarff ring. The EDS allowed the bomb-run parameters to be entered once and then left the pilot free to fly the plane.
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 directly along the wind line. Even in this direction, the wind would cause the bombs to fall long or short. To correct for this, the bomb aimer would first measure their speed over the ground using a stopwatch. They would next look up the time it would take the bombs to reach the ground from their current altitude using a pre-computed table. Then, using both values, they would look up the proper angle for the sights, the so-called range angle, and set the sights to that angle. This solution was far from practical, and prone to error.
In 1916, Harry Wimperis started the design of a new bombsight, working in collaboration with Scarff. This new Drift Sight included a simple system that greatly eased the measurement of the wind. By observing their movement over the ground, the aircraft would first determine the direction of the wind. The aircraft would then turn to fly at right angles to this wind direction so that the wind was pushing the aircraft sideways. Observing the sideways drift of the aircraft by comparing the motion of objects on the ground to a metal rod along the side of the bombsight, the drift could be seen. Using a knob, the rod was angled away from the side of the aircraft until objects could be seen moving directly along the line of the rod. A gear in the knob that adjusted the rod angle also drove the sights fore or aft, moving them to account for the wind speed. This eliminated the need for a stopwatch to measure the ground speed. This technique automated the adjustment of the range angle to account for wind, but the Drift Sight was still useful only for bomb runs along the wind line.
Course Setting Bomb Sight
When an aircraft flies in the presence of wind, its flight path over the ground is a function of the aircraft's airspeed, heading, and the speed and direction of the wind. These are combined using basic vector addition to return the course made good or track. These calculations are a basic part of air navigation and dead reckoning, taught to all aviators. Wimperis was an expert on the topic, and would later write a well-known book about it.
To aid in the necessary calculations, it was common to use a simple mechanical calculator that combined a slide-rule like calculator on one side with a vector calculator on the other. The best-known modern example is the E6B, which remains a basic part of every pilot and navigator's toolkit. Using the vector calculator and basic measurements, one can easily calculate the winds aloft and then the course made good. These calculations are identical to those needed to properly account for the effects of winds on the bombing approach. The problem was that these calculations were complex, time-consuming and error prone.
Wimperis decided to attack the calculation problem by incorporating a similar vector calculator directly into the bombsight, combining it with a drift measure similar to the one from the earlier Drift Sight. Like the Drift sight, simply taking a measure of the wind using the sight itself provided all of the unknown variables needed to completely calculate the bombing approach. Unlike the Drift Sight, the new design not only calculated the effect on the distance the bombs travelled, but also indicated the proper direction to fly to approach the target so the aircraft reached it with no residual sideways motion – thereby cancelling any drift no matter the direction of approach.
His new Course Setting Bomb Sight featured a large compass at the rear that could be used for making general calculations of wind speed or solving navigational problems. In most cases these could be ignored; the bomb aimer dialled in the wind direction on the compass, then wind speed, airspeed and altitude on different knobs. Through internal mechanisms, these adjustments carried out all of the calculations needed to set the approach and range angle. Through these calculations the CSBS allowed bombing from any direction, freeing the aircraft from the wind line for the first time.
Production and use
In testing in December 1917 at the Scilly Isles air station, in eight bomb runs the CSBS scored two direct hits, and near-misses on all six other runs. Production quickly followed, and by 1918 about 720 had been produced. The Royal Flying Corps (RFC) started using the Mark I sight as soon as supplies were available, and by April 1918 were also fully converted to this type.
For his work on the CSBS and the Drift Sight Wimperis was awarded £2,100 by the Royal Commission on Awards to Inventors.
In the post-war era, work on new bombsights was seriously curtailed, and little new development had taken place by 1930. Several minor variations of the CSBS had been introduced during this period to adapt to higher speeds, higher or lower altitudes, and new types of bombs. These also included a separate adjustment for trail, the deceleration of the bomb due to drag. At low speeds and altitudes the time between drop and impact was too short for the bombs to reach terminal velocity so the trajectory of the bombs was roughly parabolic. At higher altitudes or forward speeds the bombs would reach terminal long before impact, which had the effect of making the last portion of the flight path more vertical. The trail adjustment, set by dialling in the measured terminal velocity for the bombs being dropped, used a cam to move the height bar forward away from the vertical, reducing the range angle and thereby reducing the range to account for this effect.
Many thousands of CSBSs were sold around the world, and many other sights were developed from the basic idea. In the mid-1930s, the basic CSBS concept was largely universal for production bombsights.
Mk. VII and IX
During its development prior to the opening of World War II, the CSBS added several new features. A simple modification found on pre-war models was the Auxiliary Drift Bar attachment. This consisted of a single drift wire in a C-shaped clamp that could be moved along the main drift wires, and rotated in relation to them. Previously, the bomb aimer would use the main drift bar as a tool to measure wind speed, but it was found that the bomb aimers would forget to reset it to the proper angle for bombing when things got busy. These same measurements could be made with the Auxiliary Bar, leaving the main drift bar in the proper position.
Later versions used by RAF Coastal Command and the Royal Navy also included a further adjustment, the Fourth Vector, for attacking moving targets. This was primarily intended for use against ships and submarines. This was a fairly complex system of rotating rings and sliders that allowed the bomb aimer to dial in the relative course of the target and its estimated speed. This moved the backsight directly fore and aft, and turning the heading dial adjusted how much the speed dial moved the backsight. As the resulting mechanism was fairly large and complex, the sights were also available with the Fourth Vector removed, denoted with a *, as in the Mk. IX A*.
Mk. X
Prior to the war a major redesign of the CSBS was underway. The new Mk. X replaced the vertical slider used for altitude adjustment with a horizontally-moving backsight at the top of the device, and the entire foresight and drift wire area was made considerably smaller. The calculator and wind drift settings, formerly mounted on top and in front of the large compass at the rear of the earlier models, was moved to the left side of the device and changed in form to make it smaller as well. The compass, no longer containing pointers and dials, was replaced with a smaller unit. The result was a version of the CSBS that was much smaller than earlier versions.Compare images of the MK. IX mounted to the Supermarine Walrus, and the Mk. X on its stabilizer.
About 5,000 of the new Mk. X were built and awaiting fitting to aircraft at the opening stages of the war. After the disastrous raid on Wilhelmshaven in 1939, the RAF was forced to abandon daylight attacks and move to night bombing. The Mk. X proved to have very poor visibility at night, and it would be difficult to modify it to correct this problem. The Mk. X had to be abandoned, and Mk. VII's and Mk. IX's hurriedly re-fit to aircraft. Thus the older versions of the CSBS soldiered on long after they were due to be replaced, and remained the primary British bombsights into 1942. The Mk. VII was widely found on slower aircraft and training schools, while the Mk. IX was used in higher speed aircraft.
Mk. XI Another problem with all of the existing CSBS designs was that it could only be read correctly with the aircraft absolutely level. This was true especially during the run-up to the drop point, when the sight was used to correct the direction of flight through the use of the drift wires. The biplane bombers the CSBS had been developed for had the ability to slip-turn using the rudder only, which made it simple for the pilot to adjust their heading without affecting the aim too much. Modern monoplanes were subject to an effect known as dutch roll that makes them oscillate for a time after turning to a new heading. During this time the drift wires were difficult to use, so the entire process of correcting the flight path was greatly extended.
In the aftermath of the Wilhelmshaven raid on 3 September 1939, it was found that the lengthy setup and bomb run demanded by the CSBS made its aircraft extremely vulnerable to fighters and anti-aircraft artillery. At a pre-arranged meeting on 22 December 1939, Air Chief Marshal Sir Edgar Ludlow-Hewitt made a request for a new bombsight that did not require such a long run into the target, and which would allow the aircraft to manoeuvre throughout the bomb run.