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Aviation History
1945
1945 - 1626.PDF
AUGUST I6TH, 1945 FLIGHT ISI THE NORDEN BOMB SICHT linkage between the trailmechanism and direction control. Target motion has a similar effect to that of wind of the same velocity in the opposite direction. In actual practice it makes no difference whether the wind or the target is in motion, nor does a combination of wind and target motion re- quire additional setting. After settings for trail lag and falling time are made, the only critical factors are those of resultant "closing" speed and direction of the aircraft relative to the garget. These elements are irre- spective of the fact that the air- craft itself is capable of both linear and angular movement in three dimensions. However, this difficulty is taken care of" by the electrically driven gyros which govern the stabilised line of sight. The essential criterion of bombing is, naturally, the de- termination of the correct point at which to release the bombs, and the Norden bomb sight is basically an automatic speed- ar.d-distance calculator which resolves the critical factors in the form of an angle. As the bomb is released, its momentum moves it forward, gravity pulls it down, and air resistance retards both actions. The first calculation is the length of time required for the bomb to reach the target; then the sight determines the horizontal speed relative to the target and multiplies that speed by the time of fall. The quotient is the horizonal distance, without retardation, which the bomb would travel during the time of fall. After subtracting the trail lag from that distance, the bdmb sight produces a range angle which is formed by the intersection between the bombardier's line of sight to the target and a vertical line from the aircraft to the ground at the exact instant of release. This evokes a riglit-angle triangle formed by the vertical line from the aircraft to the ground, the ground line from that point to the target, with the hypotenuse formed by the line of sight from the bomb sight to target. In the bomb AIRCRAFT HEADING TRACK Aft view of sight. Leftof the eyepiece is the gyroscope window inwhich bubble levels indicate precession cor-rected by the knob above. The scale belowand left of the window is for pre-setting drift. sight itself there is a tinyreplica of this triangle which is created by thesame vertical line be- neath the aircraft andthe same hypotenuse line of sight, but is boundedon the bottom by a hori- zontal wire. The ratioof the sides of these two triangles is, ot course, equal. In the timing type of bomb sight the target is"tracked" by moving a cross-wire back along this horizontal wire for a number of seconds(against an index) corresponding to the estimated time it will take the bomb to fall. This crosswire is kept in line between the bombardier's eye and the target, whilst at the same time a secondcross-wire is moving forward from the vertical point directly beneath the bombardier's eye.The speed of the first cross-wire corresponds to ground speed and, of course, the distance whichit moves along the "line of sight" wire corres- ponds to the distance on the ground over whichthe aircraft travels during the time of tracking. If the rear wire travels forward at the same speedthat the front wire moves rearward, and if the tracking time is equal to the time of fall, then the rear wirewill move ahead of the vertical a distance corresponding to the actual distance which the aircraft will travel duringthe falling time of the bomb. However, this will not give the correct range until tiailhas been subtracted. The rearmost wire is initially set to the rear ot the vertical by an amount corresponding to thetrail distance. It thus starts forward with a handicap so that it will travel beyond the vertical for a distance corre-sponding to the correct range. With all these settings made, the bombardier's lice of sight through the rear cross-wireis the line of sight, which should pass through the target at the instant of release. By virtue of the autopilot syn-chronisation, the bombardier directs the aircraft on the run-up to the target and, when the rear cross-wire comes inline, the bombs are automatically and instantaneously dropped. The line of sight is actually the axis of the sighting telescope, which is swung by a variable-speed drive so that the telescope automatically keeps on the target. Thedrive mechanism sets up within itself a release line of sight angle corresponding to that comprised by the rear cross-wire as previously described. The lateral cross-wire is kept on the target as the telescope is driven, and, when the telescope reaches the correct angle, an electrical contact is made which actuates the bomb releases. j^iagram of relation between aircraft and target throughout bombing operation, indicating linear and angular drift values. SEAFIRE XV OFF THE SECRET LISTS OME details of the Supermarine Seafire XV, which is thefastest naval interceptor fighter built in Britain, were released last week-end by the Admiralty. Fitted with a 1,890 h.p. Merlin engine, it has a top speedin the neighbourhood of 400 m.p.h., a ceiling of more than 35,000ft., and a faster rate of climb than any other British-built naval fighter. Armament consists of two 20 mm. cannon and four 0.303m.machine guns, and one 500 lb. bomb can also be carried. Pro vision is also made for the use of rocket-assisted take-off wheuoperational circumstances demand it. Flying weight of the Supermarine Seafire XV with full radioand other equipment, including deck-arrester hook, is round about 8,000 lb. Dimensions are: Span 36ft. ioin.; length31ft. ioin.; wing area, 242 sq. ft.
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