page_title: Bombsight
section_title: Tachometric bombsights
hierarchical_section_title: Bombsight / Tachometric bombsights
caption_attribution_description: English: Bomb Aimer Window and Bomb Sight on SAAF 1721 Avro Shackleton SAAF Museum Swartkop Pretoria
context_page_description: A bombsight is a device used by military aircraft to drop bombs accurately. Bombsights, a feature of combat aircraft since World War I, were first found on purpose-designed bomber aircraft and then moved to fighter-bombers and modern tactical aircraft as those aircraft took up the brunt of the bombing role.
  A bombsight has to estimate the path the bomb will take after release from the aircraft. The two primary forces during its fall are gravity and air drag, which make the path of the bomb through the air roughly parabolic. There are additional factors such as changes in air density and wind that may be considered, but they are concerns only for bombs that spend a significant portion of a minute falling through the air. Those effects can be minimized by reducing the fall time by low-level bombing or by increasing the speed of the bombs. Those effects are combined in the dive bomber.
  However, low-level bombing also increases the danger to the bomber from ground-based defences, and accurate bombing from higher altitudes has always been desired. That has led to a series of increasingly sophisticated bombsight designs, dedicated to high-altitude level bombing.
context_section_description: One of the main problems using vector bombsights was the long straight run needed before dropping the bombs. This was needed so the pilot would have enough time to accurately account for the effects of wind, and get the proper flight angle set up with some level of accuracy. If anything changed during the bomb run, especially if the aircraft had to maneuver in order to avoid defences, everything had to be set up again. Additionally, the introduction of monoplane bombers made the adjustment of the angles more difficult, because they were not able to slip-turn as easily as their earlier biplane counterparts. They suffered from an effect known as "Dutch roll" that made them more difficult to turn and tended to oscillate after levelling. This further reduced the time the bomb aimer had to adjust the path.
  One solution to this later problem had already been used for some time, the use of some sort of gimbal system to keep the bombsight pointed roughly downward during maneuvering or being blown around by wind gusts. Experiments as early as the 1920s had demonstrated that this could roughly double the accuracy of bombing. The US carried out an active program in this area, including Estoppey sights mounted to weighted gimbals and Sperry Gyroscope's experiments with US versions of the CSBS mounted to what would today be called an inertial platform. These same developments led to the introduction of the first useful autopilots, which could be used to directly dial in the required path and have the aircraft fly to that heading with no further input. A variety of bombing systems using one or both of these systems were considered throughout the 1920s and 30s.
  During the same period, a separate line of development was leading to the first reliable mechanical computers. These could be used to replace a complex table of numbers with a carefully shaped cam-like device, and the manual calculation though a series of gears or slip wheels. Originally limited to fairly simple calculations consisting of additions and subtractions, by the 1930s they had progressed to the point where they were being used to solve differential equations. For bombsight use, such a calculator would allow the bomb aimer to dial in the basic aircraft parameters – speed, altitude, direction, and known atmospheric conditions –  and the bomb sight would automatically calculate the proper aim point in a few moments. Some of the traditional inputs, like airspeed and altitude, could even be taken directly from the aircraft instruments, eliminating operational errors.
  Although these developments were well known within the industry, only the US Army Air Corps and US Navy put any concerted effort into development. During the 1920s, the Navy funded development of the Norden bombsight while the Army funded development of the Sperry O-1. Both systems were generally similar; a bomb sight consisting of a small telescope was mounted on a stabilizing platform to keep the sighting head stable. A separate mechanical computer was used to calculate the aim point. The aim point was fed back to the sight, which automatically rotated the telescope to the correct angle to account for drift and aircraft movement, keeping the target still in the view. When the bomb aimer sighted through the telescope, he could see any residual drift and relay this to the pilot, or later, feed that information directly into the autopilot. Simply moving the telescope to keep the target in view had the side effect of fine-tuning the windage calculations continuously, and thereby greatly increasing their accuracy. For a variety of reasons, the Army dropped their interest in the Sperry, and features from the Sperry and Norden bombsights were folded into new models of the Norden. The Norden then equipped almost all US high-level bombers, most notably the B-17 Flying Fortress. In tests, these bombsights were able to generate fantastic accuracy. In practice, however, operational factors seriously upset them, to the point that pinpoint bombing using the Norden was eventually abandoned.
  Although the US put the most eff
caption: Bomb aimer's window and bomb sight in the nose of an Avro Shackleton.
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