Estoppey D-series
A key feature was the use of a pendulum to keep the bombsight correctly oriented towards the ground even as the aircraft maneuvered, and dashpots to keep it from swinging around in turbulence.
The Sperry proved difficult to produce in quantity, and in 1938 the Army ordered another production run of a much more basic update of the D-4, designated as the D-8.
On leaving military employment in 1926, he immediately began demanding fees from both the Army and the Navy, eventually receiving several thousand dollars.
The speed and current altitude were then entered into the bombsight, which would move a pointer fore and aft to indicate the correct spot to drop the bombs.
If the plane is flying in a wind that is 20 kilometres per hour (12 mph) on its nose, its ground speed is only 130 km/h, and in this case, the bombs will travel only 720 metres (2,360 ft), missing by over 100 m.[1] The solution is to use a stopwatch as the aircraft approaches the target.
The bomb aimer would pick a suitable object on the ground along the line of approach, perhaps the target itself, and then time it as it passed through two markers on the sight set at a fixed angle.
Taking this measurement was not a trivial process, especially as almost all of these systems had to be operated by the pilot while looking downward through the sights and trying to keep the plane level at the same time.
Estoppey came up with a system that would keep the sight correctly perpendicular to the ground using a pendulum, and then stabilized its motion using dashpots so it did not swing about after moving.
[2] Estoppey then went to work for the Air Service Engineering Division at McCook Field, where he continued refining the design and introduced the first production-quality version as the D-1 in 1922.
The Navy was also impressed, but only to the point of using a small number while they waited for newer developments of their own being carried out by Carl Norden to stabilize the Mark III.
The bombardier would first watch the target move in relation to a vertical metal wire, and direct the pilot left or right until any sideways drift was seen to stop.
Because it was so similar to the earlier models, three prototypes were ordered and rapidly delivered, but the Army concluded they too represented too small an improvement to consider production, especially as they were now awaiting much more advanced designs.
Those advanced designs failed to emerge, and over the next five years, the Army ordered several batches of the D-4, amounting to 230 examples, with the Navy adding another 40 for low-altitude work.
The bombardier would then adjust the initial estimate by turning dials to change the direction or speed until objects could be seen unmoving in the sight.
While this offered unparalleled accuracy in theory, in practice the D-5 proved less accurate than the D-4, and the Air Corps recommended that "no further experimental work be done on the D-5 sight.
Although these were worthy updates, the expected imminent arrival of the more advanced designs from Sperry meant there was no reason to buy the D-7 in the meantime.
Meanwhile, the Navy would promise to deliver a certain quantity of sights for a given month, so the Army would adjust their aircraft orders to match, only to receive a different number and either have missed the opportunity to produce more than a month, or have to deliver them with what one Air Corps member called "this damn D-8 sight that I would just as soon have a couple of nails and a wire.
"[8] After enormous arguments and inter-service rivalries, the issue of the Norden was ultimately solved unexpectedly when the Navy concluded it couldn't hit ships anyway.
[6] With no more devices being built on his designs after that date, he turned his attention to the Navy, claiming that the Norden Mark XI "fall within the disclosures and scope of my patents."
The Navy disagreed, noting that they purchased the Mark XI specifically because it allowed attacks on moving targets, something the Estoppey never solved.
The sights consisted of several sets of thin metal wires on the right side of the case, a long longitudinal one running the length of the device that was used to compare the motion of the target to the aircraft's line of travel, and three horizontal ones used to time the drop, two at the bottom of the device just above the longitudinal wire, and a second at the top of the case.
[12] With the drift zeroed out, the operator then consults a table printed on the side of the case that listed the approximate angle of the drop from that altitude and their current airspeed.
[16] The system also included a dial at the back of the case that could be used to manually adjust the timing by moving the sights forward or rearward.
