Photo-history of the aerocompass

This is the first decent aerocompass.

On hearing of pilots' compass-troubles Captain Creagh-Osborne, of the Compass Department, put a submarine compass in a bedding of horsehair to counter the effects of the aeroplane's vibration and took it up in Colonal Cody's plane. This was only a year after Cody's first powered flight and made Creagh-Osborne the first Naval Officer to fly. Yippee.

This little beauty is from 1915, when Creagh-Osborne was working in close communication with a few manufacturing firms. Their joint design has fancy new ways of dealing with vibration (rubber, springs, blah), a vertical and simplified reading card to make things easier for those pressured pilots, and an inverted pivot to make the card spin in a more groovy way.

Of course, the RAE (precursor to RAF) over at Farnborough wanted to have a go too, and came up with this rather inelegant effort, also in 1915. It has a much slower period (i.e. the needle oscillates much more slowly) than those of the Admiralty Compass Observatory; this was the RAE's solution to the wild behaviour of the needle when the aeroplane turns. Known as 'northerly turning error' this problem was due to the fact that when the aeroplane banks, the needle begins responding to the vertical component of the earth's magnetic field. We're not interested in that component. It really sucks. It means that when you turn towards the north the compass under-indicates the turn, so that you turn too far and may even enter a spin, and when you turn towards the south it over-indicates it so that you don't turn enough. The idea of the slow period compass is that it registers turns so slowly that these problems don't have time to take effect, and the pilot doesn't end up constantly over-correcting his course.

The ACO, however, began employing two Proper Scientists in about 1918. With a military-industrial-academic complex under full swing, what should appear but an aperiodic compass! This was really very cool. The needle didn't oscillate in a silly way when the plane had finished a turn, and yet responded quickly. By now the compasses also has a better way of dealing with the problems caused by flying at high altitudes. There the low pressure and temperature would cause the liquid to contract, and so result in the formation of bubbles, which screw up the movement of the needle system. At first they used expansion chambers; then Creagh-Osborne asked his manufacturing buddies to put in an airtrap, which was a little better. Really what they wanted to do was make the liquid out of pure alcohol, but then this would cause the paint inside to dissolve, so that a) the markings couldn't be read so well and b) little bits would get into the liquid. It really was tricky business.

From 1922, separate compasses for the plane's observer/navigator and its pilot were designed by the ACO. Above is a rather lovely observer's compass, complete with azimuth for taking bearings and serving as a bombsight, and the picture before was a pilot's compass. This 'observer' entity existed for a short, 50 year period and then dissappeared again, poor chap.

Then they had the Second World War, and all sorts of other stuff happened.