Before the era of digital electronic computers, mechanical analog computers were found everywhere. From the relative simplicity of bomb sights to the complexity of fire control computers on 1940s battleships, all the way to 1950s fighter planes, these mechanical wonders enabled feats which were considered otherwise impossible at the time.
One such system that [Ken Shirriff] looked at a while ago is the Bendix Central Air Data Computer. As the name suggests, it is a computer system that processes air data. To be precise, it’s the mechanism found in airplanes that uses external sensor inputs to calculate parameters like altitude, vertical speed, Mach number and air speed.
This diagram shows how the values are computed. The differential numbers are arbitrary numbers. (Credit: Ken Shirriff)
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This particular electrical-mechanical avionics package by Bendix is a prime example of these systems. The basic concept is rather straightforward: sensors like pitot tubes provide the raw input to the computer, which runs a number of equations on this data through the use of intricate differential gears:
The components that compute log free air temperature. D12 is not part of this equation. (Credit: Ken Shirriff)
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In a sense this is not dissimilar to the way a digital computer is programmed using software instructions. Instead of register values being set and memory addresses written, carefully crafted gears and cams move to transfer exactly the right number of rotations on the output, or whatever output may be required to implement the desired equation. Many details can also be gleaned from the (now expired) 1954 patent.
Since the avionics will never be changed for the life of the airplane (barring upgrades), having such functionality ‘hard-coded’ can be seen as a feature, as it’s likely to shake off EMPs and general EMI. In an older Hacker News thread this air data computer was discussed, with some references to similar systems and a link to a 1953 US Navy training video for an analog firing solution computer.
Despite the general robustness of these analog electrical-mechanical computers, they were phased out by the late 1960s for all-digital versions. Offering faster calculations in a much smaller package was an attractive feature that mechanical computers just couldn’t compete with, especially not in the field of avionics.
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- Source: https://hackaday.com/2023/10/19/reverse-engineering-the-mechanical-bendix-central-air-data-computer/
