4 ms·

The Bendix Central Air Data Computer (CADC)

62 points by wglb 4y ago

kens 4y ago
Any questions about this system? (I know everyone hates Twitter threads...)
- T3OU-736 4y ago
  I wonder how reliable those were. Additionally, since they are mechanical in nature, were they a limit to aircraft performance (seems unlikely, but figured it is worth asking).
  - kens 4y ago
    I don't know about the reliability in practice, but the official spec called for a "reliable life of 1000 hours with reasonable servicing" and a life of 5000 hours with replacement of parts. I don't think they put a limit to aircraft performance since you're not changing altitude or speed very fast compared to the speed of a gear. The spec called for testing acceleration up to 10g.
- kens 4y ago
  Also, to the submitter: thanks for fixing my typo in the title. Of course I make a typo that is highly visible at the top of a popular tweet.
- digdugdirk 4y ago
  Absolutely beautiful collection of machinery, though just a single brief thought about the tolerance stackup in that thing will inevitably give me nightmares for weeks. Any idea what that stackup looks like, and how the resultant error in calculations/readings was communicated to the pilot?
  - kens 4y ago
    The official spec had lots of maximum allowed tolerances and how to test for them. E.g. the true airspeed had to be +/- 4 knots from 150 to 300 knots and +/- 1% and less than 7 knots above that. Mach number had to be +/- 1% but no greater than 0.02 or less than 0.006 of the real Mach number.
- sreevisakh 4y ago
  Any plans to publish this on your blog? It's a much more elegant source of information. This also belongs there with the other great articles.
  - kens 4y ago
    Yes, I'll get it on my blog at some point. But honestly Twitter is a lot less work and reaches a lot more people.
    - wglb 4y ago
      Second vote for a blog!
- vajrabum 4y ago
  I used to work on avionics of a mostly newer vintage than this but I love this sort of technology. Thanks for sharing and if you get around to digitizing the manuals I'm sure they'd be fun to look at too.
- somat 4y ago
  There is white powdery detritus around some of the components. Some sort of galvanic reaction? https://pbs.twimg.com/media/FhUHb-6WYAAMayN?format=jpg&name=4096x4096 https://pbs.twimg.com/media/FhUHb-6WYAAMayN?format=jpg&name=...
  - kens 4y ago
    We're not sure what it is. There's also a resistor with leads that look like they have been attacked by fungus, but it's probably corrosion. It's unclear why just a couple of components would have this issue.
- Anon4Now 4y ago
  Any idea how it compares to the hydraulic computer designed for the Pratt & Whitney J58 engine used on the SR-71 Blackbird? I saw this video (linked below) wherein a test engineer talked about the engine design. He touched a bit on the hydraulic computer (at the 6:50 mark), and I thought it was ingenious and fascinating. https://youtu.be/MJrXUh0eZjw https://youtu.be/MJrXUh0eZjw
livinginfear 4y ago
I had always wondered what the avionics in the early F-111 looked like. Thanks for documenting this!
- 1letterunixname 4y ago
  The avionics in the F-111 are the same as those in the B-1A and F-35: pure gold-plated MIC failed projects at the expense of mere billions of your tax dollars at work. ;) (Sorry: ~2 terabucks in the case of the F-35.) Meanwhile, the A-10, F-16, and F-15 were wildly successful.
1letterunixname 4y ago
From their earliest days, most automatic transmissions are analog computers. This is how most control systems operated pre transistor. Using vacuum tubes in an avionics computer seems inherently risky, plus the power consumption of electronics back then would've made mechanical designs cheaper and more survivable.
userbinator 4y ago
Bendix is one of those huge companies that has manufactured a lot of different products over time, although for me and probably many others, it's more closely associated with starter motors and brakes than computers. https://en.wikipedia.org/wiki/Bendix_Corporation https://en.wikipedia.org/wiki/Bendix_Corporation In automotive communities, "the Bendix" is still used as a slang term to refer to the drive gear of a starter motor.
- jeremy_wiebe 4y ago
  Interesting. I only know Bendix from air brake systems. Used to install them on semi trailers.
- fumeux_fume 4y ago
  In the audiophile community, Bendix is known for their excellent vacuum tubes.
thedrbrian 4y ago
Not the same computer but OmegaTau have an interesting podcast about analog computing http://omegataupodcast.net/159-analog-computers/ http://omegataupodcast.net/159-analog-computers/
ddulaney 4y ago
If you’re interested in some of the individual components, there’s a great 1950’s US Navy training film on the individual components of analog fire control computers here: https://m.youtube.com/watch?v=s1i-dnAH9Y4 https://m.youtube.com/watch?v=s1i-dnAH9Y4 The ideas are the same, but the technology is about a decade earlier than the CADC, so everything is noticeably more primitive.

62 points by wglb 4y ago

kens 4y ago

Any questions about this system? (I know everyone hates Twitter threads...)

T3OU-736 4y ago
I wonder how reliable those were. Additionally, since they are mechanical in nature, were they a limit to aircraft performance (seems unlikely, but figured it is worth asking).
- kens 4y ago
  I don't know about the reliability in practice, but the official spec called for a "reliable life of 1000 hours with reasonable servicing" and a life of 5000 hours with replacement of parts. I don't think they put a limit to aircraft performance since you're not changing altitude or speed very fast compared to the speed of a gear. The spec called for testing acceleration up to 10g.
kens 4y ago
Also, to the submitter: thanks for fixing my typo in the title. Of course I make a typo that is highly visible at the top of a popular tweet.
digdugdirk 4y ago
Absolutely beautiful collection of machinery, though just a single brief thought about the tolerance stackup in that thing will inevitably give me nightmares for weeks. Any idea what that stackup looks like, and how the resultant error in calculations/readings was communicated to the pilot?
- kens 4y ago
  The official spec had lots of maximum allowed tolerances and how to test for them. E.g. the true airspeed had to be +/- 4 knots from 150 to 300 knots and +/- 1% and less than 7 knots above that. Mach number had to be +/- 1% but no greater than 0.02 or less than 0.006 of the real Mach number.
sreevisakh 4y ago
Any plans to publish this on your blog? It's a much more elegant source of information. This also belongs there with the other great articles.
- kens 4y ago
  Yes, I'll get it on my blog at some point. But honestly Twitter is a lot less work and reaches a lot more people.
  - wglb 4y ago
    Second vote for a blog!
vajrabum 4y ago
I used to work on avionics of a mostly newer vintage than this but I love this sort of technology. Thanks for sharing and if you get around to digitizing the manuals I'm sure they'd be fun to look at too.
somat 4y ago
There is white powdery detritus around some of the components. Some sort of galvanic reaction? https://pbs.twimg.com/media/FhUHb-6WYAAMayN?format=jpg&name=4096x4096 https://pbs.twimg.com/media/FhUHb-6WYAAMayN?format=jpg&name=...
- kens 4y ago
  We're not sure what it is. There's also a resistor with leads that look like they have been attacked by fungus, but it's probably corrosion. It's unclear why just a couple of components would have this issue.
Anon4Now 4y ago
Any idea how it compares to the hydraulic computer designed for the Pratt & Whitney J58 engine used on the SR-71 Blackbird? I saw this video (linked below) wherein a test engineer talked about the engine design. He touched a bit on the hydraulic computer (at the 6:50 mark), and I thought it was ingenious and fascinating. https://youtu.be/MJrXUh0eZjw https://youtu.be/MJrXUh0eZjw

livinginfear 4y ago

I had always wondered what the avionics in the early F-111 looked like. Thanks for documenting this!

1letterunixname 4y ago
The avionics in the F-111 are the same as those in the B-1A and F-35: pure gold-plated MIC failed projects at the expense of mere billions of your tax dollars at work. ;) (Sorry: ~2 terabucks in the case of the F-35.) Meanwhile, the A-10, F-16, and F-15 were wildly successful.

1letterunixname 4y ago

From their earliest days, most automatic transmissions are analog computers. This is how most control systems operated pre transistor. Using vacuum tubes in an avionics computer seems inherently risky, plus the power consumption of electronics back then would've made mechanical designs cheaper and more survivable.

userbinator 4y ago

Bendix is one of those huge companies that has manufactured a lot of different products over time, although for me and probably many others, it's more closely associated with starter motors and brakes than computers. https://en.wikipedia.org/wiki/Bendix_Corporation https://en.wikipedia.org/wiki/Bendix_Corporation In automotive communities, "the Bendix" is still used as a slang term to refer to the drive gear of a starter motor.

jeremy_wiebe 4y ago
Interesting. I only know Bendix from air brake systems. Used to install them on semi trailers.
fumeux_fume 4y ago
In the audiophile community, Bendix is known for their excellent vacuum tubes.

thedrbrian 4y ago

Not the same computer but OmegaTau have an interesting podcast about analog computing http://omegataupodcast.net/159-analog-computers/ http://omegataupodcast.net/159-analog-computers/

ddulaney 4y ago

If you’re interested in some of the individual components, there’s a great 1950’s US Navy training film on the individual components of analog fire control computers here: https://m.youtube.com/watch?v=s1i-dnAH9Y4 https://m.youtube.com/watch?v=s1i-dnAH9Y4 The ideas are the same, but the technology is about a decade earlier than the CADC, so everything is noticeably more primitive.