Aircraft Pelorus, US Army Air Corps Type A-3
This is a rare find. We've only seen two in our many years of collecting aviation artifacts.
Here is an example of an Aircraft Pelorus, Type A-3, an early navigational aid used in aircraft of the US Army Air Corps. It has a serial number prefix of AC-41, evidence of a 1941 year of manufacture.
From Air Navigation Technical Manual TM 1-205 1940 US Army Air Corps for the Type A-2, which operates identically (the reference call-outs refer to the diagram in the photo gallery reproduced from this manual):
(1) The type A–2 pelorus is an instrument designed to measure bearings of celestial and terrestrial objects. It consists of a simple type of collimator rotatable in a housing mounted in gimbals for leveling. The assembly is mounted in the airplane on a mounting plate securely bolted to the airplane structure. This plate is mounted with the slot or keyway parallel to the center line of the airplane, thus putting the fixed lubber lines of the pelorus on this line. Means are provided for leveling the instrument, for rotating and elevating the collimator, and for measuring the angle of rotation.
(2) For terrestrial and low altitude celestial objects, the object is viewed through the glass plate (A) and the shade glasses, if necessary, and the plate rotated until the reflected image of the reticle is seen on the object. For high altitude objects, the observer looks straight down through the plate (A) seeing the reticle direct and a reflected image of the object. - - (3) The clamp (B) locks the scale (movable by means of four short pins) and clamp (C) locks the collimator assembly. Illumination of the reticle is provided (12–14 volts) through the socket (D) and bright ness by the rheostat (E). The screw (H) (two) serves to fasten the instrument to the mounting plate. (4) To replace the light bulb, unscrew the collar (I) and pull the lamp support assembly straight out. It is not necessary to unsolder any wiring.
b. Operation.—(1) To use the instrument, plug into the 12-volt circuit and adjust rheostat to full brightness. (2) Loosen clamp (B), set zero of scale on rear index (G), and tighten (B). (3) Level instrument, loosen clamp (C), and rotate upper assembly until object is seen through plate (A) in line with the bubble and the center of the filter plates (bubble toward the observer). Tilt plate (A) slowly until cross wires are seen, adjust brightness of reticle, and set vertical line on object. (4) To read azimuth, set scale to true heading on fixed index (G) and azimuth will be shown against index (F). (5) To measure relative bearings, set zero of scale on (G) and read on (F), or set 180° of scale on (G) and read on index under filters.
(6) To measure reverse bearings, set zero of scale on (G) and read on index under filters, or set 180° of scale on (G) and read on (F).
The instrument measure approximately 4 inches wide and 10 inches tall. The smoked glass sunscreens are damaged. The spirit bubble level is present and intact.
It is a wonderful artifact of the period that could use a good cleaning!