Gregory's ServerThe Bendix CADC is an analog computer used by fighter planes in the 1950s. It computed airspeed, Mach number, and other important parameters. I reverse-engineered how it performed these calculations with tiny gears, differentials, and cams. 1/12
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The central "Mach section" computes log static pressure, log pressure ratio, impact pressure, and Mach number. This diagram shows how the calculations are performed with differentials β and cams. 3/12 The outputs from the CADC are transmitted electrically to other parts of the aircraft. The CADC receives pressures from the aircraft's pitot tube. One problem is that the airflow over the aircraft distorts the static pressure reading. The fix is to apply a Pressure Error Correction factor. This factor is determined by the Compensator, an external unit. 6/12 The Compensator uses a 3-D cam to generate the correction factor, sent to the CADC as a synchro signal. The CADC uses a servo loop to generate a rotation corresponding to this factor, then multiplies it with a differential. 7/12 An amplifier board uses transistors and magnetic amplifiers to drive the motor that turns the shaft. They didn't use a printed circuit board, but components soldered to metal pegs. Here's the schematic I reverse-engineered. 8/12 The CADC is a boring cylinder from the outside. You wouldn't expect that amazing mechanical complexity would be found inside. 9/12 For more information, see my blog post: https://www.righto.com/2024/02/reverse-engineering-analog-bendix-air.html 10/12 You can see the Bendix CADC in action in this thread: Credits: thanks to Joe for providing the CADC. I worked on it with https://twitter.com/curious_marc and https://twitter.com/TubeTimeUS; Marc made a video of the CADC:  @foma_econlodge The CADC computes airspeed, Mach number, temperature, air density, and other "air data".  @kenshirriff : Wow! You are a legend! π²π²π² Thank you, kind Sir, for this knowledge! @kenshirriff I understand the idea / how a function is encoded in these cams, but how you'd manufacture them isn't clear to me and isn't coming up easily in search (I'm getting a lot of cam shaft results, which seem relatively simple compared to the cams in an analog computer). Do you know about any resources on how such cams were made? @ChateauErin I think they would machine the cams the same way they would make other metal parts with a complex shape. E.g. using a milling machine and a template. @kenshirriff thanks. I don't know why I didn't think of milling machine; part of me knew that they were plenty old. "and a template" was something I absolutely didn't consider except abstractly (I'd imagined something kind of pantograph-y) so this should hopefully be enough to send me down that rabbit hole  @kenshirriff do you know if this is the same Bendix as the Bemdix G-15 (?) that Usagi Electric is restoring?  @kenshirriff Thank you for sharing this. I wonder if the reason for doing things this way was for radiation hardening. |
The basic idea is that specially-shaped cams convert values to logarithms. Differential gears add and subtract these values. Other cams can take exponentials or other functions. 2/12