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Aviation History
1945
1945 - 1623.PDF
T8O FLIGHT AUGUST I6TH, 1945 The Nor den Bomb Sight A System of Instruction Between Autopilot and Bomb Sight : An Automatic Speed and Distance Calculator The semicircular gradu- ated scale beneath the telescope eyepiece is for setting trail lag, and the largest of the three central knobs is for setting falling speed. THE famous Nordeu bomb sight, which gained analmost legendary reputation in the earlier pait ofthe war. was actually invented by Carl L Norden as early as 1928. Operation of the instrument is linked in- extricably with the automatic pilot (officially known as Stabilised Bomb Approach Equipment) which was also developed by Norden in 1935, so that full advantage could be taken of his bomb sight. This equipment is the standard automatic pilot fitted to American aircraft and, 111 this system, the Norden bomb sight is installed directly,on the housing of the azimuth stabilising unit. In common with all foims of automatic pilot, S.BA.E. embodies three basic groups ot equipment comprising gyros, servo motois and follow up system? to render the degree of control surface deflection proportional to the deviation of the aircraft. In the autopilot/bomb sight combination, the azimuth stabilising unit (also called the directional stabiliser) can control both the aircraft and the bomb sight in azimuth simultaneously 01 separately. When the S.B.A.E. and sight are used together the bomb sight knobs are rotated to This mock-up of the bombsight/autopilot installation was constructed toillustrate the relationship and function of the majorcomponents. point the sighi at the target, which action also turns the aircraft in the requisite direction to ensure a collision course at any ratio the Dombardier desires. The flight gyros con- trol the aircraft laterally and longitudinally through the follow-up system by operating ailerons and elevators as required. A banking motor introduces bank into turns by moving the aileron sector in the flight gyro by an amount proportional to rudder movement, thereby actuating the appropriate servo motor to displace the ailerons. The follow up system governs the amount of control surface deflection applied for any aircraft deviation, and returns the control surface to neutral after the correction has been made Bombing Problem Thus, without going into details of the composite mechan- ism, the autopilot is a means whereby the aircraft can be controlled in its flight attitude, and the bomb sight is a governing agent of the automatic pilot. The bombardier looks through his sighting telescope and, having preset on the sight the various evaluations of the proposed bombing conditions, he takes over control of the aircraft. As the two basic elements of the composite system are correlated, when the bombardier turns the appropriate knobs on his bomb sight, the autopilot half of the installation ensures that the aircraft is flown according to the bombardier's re- quirements—in fact, he flies the machine. Factors 01 the bombing problem are: aircraft speed, gravity, air resistance, windage and target motion. Gravity and air resistance factors are obtained from a pre-calculated table from which the bombardier gets his first setting. Since initial ve'j:ity ot the bomb is that of the aircraft, and this velocity is decreased by air re- sistance so that the bomb trajectory is a segment of a parabolic curve, these data cam also be' pre-calculated and supplied to the bombardier in order that he can set the sight to compensate for trail. Windage implies correction for drift which, unfortunately, is complex as the bomb does not fall on the ground track of the aircraft but swerves aside. The sight does net require a special setting for this as the correction is automatically made by a mechanical
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