Bombsight Calibration

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2 years 2 months ago - 2 years 1 month ago #17324 by Westcoast
Since our B-26 has no Norden or Sperry bombsight, the crack SPA engineering development team came up with the Westcoast bombsight (patent pend.), which depends on geometry and the measured ballistic properties of the M117 bomb to determine the proper drop angle, or alternatively, drop time for your bombs.
The core concept behind this approach is to determine the exact point at which a target is at the same distance from a point directly below the aircraft as the altitude (AGL) of the aircraft.  This is the "45 deg. aim point" on the "M117 Bomb Ballistics" chart and this aim point is the "Offset Aiming Point" on the "Offset Aiming Point Bombing" chart.
We have shown you that in the "default bombardier's position" this sighting angle corresponds to the approximate center of the optically flat bombsight window in the B-26 plexiglass nose fairing.  Awkwardly, this window is partially obscured by the machine gun protruding through the nose fairing.  This makes it somewhere between difficult and impossible to see the target before it appears in the window.
To counter this problem, we have described how you can use the CTRL+Backspace control to move your vantage point forward of the nose fairing and, thereby, sight the target at some distance.  However, using this maneuver requires that the bombardier regain his default position before sighting his target.  The bombardier then has a choice of using the Offset Aiming Point or the Drop Delay method to determine when to release his bombs.  However, both methods require sighting either the target itself, or the offset aiming point through the 45 deg. sight point in the bomb-sight window.
So, we have developed a "sight picture" to tell you when you are in the correct (i.e. default:) position.
To do this, we positioned our B-26 above one end of the 8500' long Rwy. 35 at the Thermal test range (conveniently near the Group Commander's winter residence), in level flight at 8500' AGL

.  We then positioned the bombardier so that the other end of the 8500' runway appeared in the notch at the top of the bomb-sight window.

Note the white square in the visibility notch of the bomb-sight window, that is the other end of the 8500' long runway.  This is the 45 deg. sight angle point.  Note the framing carefully. Counting back from the edge of the plexiglass nose fairing, exactly seven rivets are visible along both the right and left stringers.  If your sight position is further forward, further back, higher or lower than this, either more or fewer rivets will be visible,  So, in level flight*, adjust your vantage point the match this sight picture and the notch at the top of the bomb-sight window will show you a point on the ground (the 45 deg. aim point) exactly as far away from the point directly beneath your aircraft as the altitude of your aircraft above the terrain.  Questions?

* Technically, this is the zero pitch angle of the aircraft and usually will not correspond to level flight.  Usually a pitch up of 2 or 3 degrees is required to maintain level flight.  This will displace the 45 deg. sighting point downwar by 2 or 3 deg., closer to the center of the bomb-sight window.
 
Last edit: 2 years 1 month ago by Westcoast. Reason: typos

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