Let's throw this in, if it's realistic enviroment that's the issue:
Is the Norden bombsight accurately modeled?
"To improve the calculation time, the Norden used a mechanical computer inside the bombsight to calculate the range angle of the bombs. By simply dialling in the aircraft's altitude and heading, along with estimates of the wind speed and direction (in relation to the aircraft), the computer would automatically, and quickly, calculate the aim point. This not only reduced the time needed for the bombsight setup, but also dramatically reduced the chance for errors. This attack on the accuracy problem was by no means unique, several other bombsights of the era used similar calculators. It was the way the Norden used these calculations that was different.
Conventional bombsights are set up pointing at a fixed angle, the range angle, which accounts for the various effects on the trajectory of the bomb. Looking through the sights, its crosshairs indicate the location on the ground where the bombs would impact if released at that instant. As the aircraft moves forward, the target approaches the crosshairs from the top, and the bombardier releases the bombs as it passes through them. One example of a highly automated system of this type was the RAF's Mark XIV bomb sight.
The Norden worked in an entirely different fashion, the "synchronous" or "tachometric" method. Internally, the calculator continually computed the impact point, as was the case for previous systems. However, the resulting range angle was not displayed directly to the bombardier or dialled into the sights. Instead, the bombardier used the sighting telescope to locate the target long in advance of the drop point. The calculator used the inputs for altitude and airspeed to determine the angular velocity of the target, and then used a rotating prism to attempt to keep the target centred. So while conventional bombsights waited for the visibly moving target to approach the fixed range angle, with the Norden, one waited for the visibly fixed target to approach the (hidden) moving range angle.
It was the difference between the two calculated values, the current range angle (to the impact point) and the current angle to the target measured from the telescope, that determined the direction and distance the aircraft had to travel in order to take it over the proper drop point. As the bomber approached the target, the difference between the range and target angles would be continually reduced, eventually to zero (within the accuracy of the mechanisms). At this moment the Norden automatically dropped the bombs.
The reason for this additional complexity was that the target generally did not stay centred in the sighting telescope when it was first set up. Instead, due to inaccuracies in the estimated wind speed and direction, the target would drift out of the crosshairs in the sight. To correct for this, the bombardier would use fine-tuning controls to slowly cancel out any motion through trial and error. These adjustments had the effect of updating the measured ground speed used to calculate the motion of the prisms, slowing the visible drift. Over a short period of time of continual adjustments, the drift would stop, and the bombsight would now hold an extremely accurate measurement of the exact groundspeed and heading. Better yet, these measurements were being carried out on the bomb run, not before it, it helped eliminate inaccuracies due to changes in the conditions as the aircraft moved. And by eliminating the manual calculations, the bombardier were left with much more time to adjust their measurements, and thus settle at a much more accurate result. Conventional systems would estimate wind speed using a drift telescope or dead reckoning, but this was time consuming to calculate, and did not offer anywhere near the accuracy of the Norden.
The angular speed of the prism changes with the range of the target; consider the reverse situation, the apparent high angular speed of an aircraft passing overhead compared to its apparent speed when it is seen at longer distance. In order to properly account for this non-linear effect, the Norden used a system of slip-disks similar to those used in differential analysers. However, this slow change at long distances made it difficult to fine tune the drift early in the bomb run. In practice, bombardiers would often set up their groundspeed measurements in advance of approaching the target area by selecting a convenient "target" on the ground that was closer to the bomber and thus had more obvious motion in the sight. These values would then be used as the initial setting when the target was later sighted."
http://en.wikipedia.org/wiki/Norden_bombsight
