Physical description

Purpose built mechanical calculation machine for determining the drift of aircraft when operating in crosswind conditions. This device has an eyepiece and a prismatic periscope for viewing a distant feature being tracked. This device has calibrated rotary scales for height, speed (in KNOTS and MPH), and an adjustment to preset the 'crab angle' of the aircaft for cross wind compensation.

The device can be opened for cleaning or maintenance purposes.

Refer to the following extracts for information about bomb drift:

"Crosswinds brought into the bombing problem a new factor, "drift" In order to fly a given ground track in a crosswind, an aircraft had to "crab" into the wind; the angle formed between the aircraft's true heading and its ground track was called the "drift angle" In a crosswind, the bomb would impact directly behind the aircraft and along its longitudinal axis at the moment of release. But this meant that the bomb would strike the ground at some point downwind of the aircraft's ground track. Thus, in order to score a hit, the bomber had to fly a ground track that ran upwind of the target."

"Air resistance acting on a bomb after release caused it to lag behind the drop point and hit somewhere behind the bomber. The distance from a point beneath the aircraft at the instant of bomb impact to the point of bomb impact was called "trail." Trail increased as the bomber's airspeed increased or as its altitude increased. Furthermore, since different bombs encountered different resistance in the air, trail was also a factor of bomb shape."

Inscriptions & markings

A. M. Drift Recorder MKII REF. No 6B/190 No 2668/41 X/ 3606 3E4H28