6 ms·

Reverse-Engineering the Mechanical Bendix Central Air Data Computer

83 points by picture 3y ago

kens 3y ago
Author here for all your mechanical computer questions :-)
- CamperBob2 3y ago
  I'll point out that reverse engineering the CADC is not as easy as you might expect. Well, there goes that illusion. :) An amazing piece of work as usual, Ken. Thanks for sharing your hard-won insights on this gadget. The linear-endpoint wraparound hack was worth learning about all by itself. I'm surprised it took until 1954 before someone got around to patenting that, as it seems like a valuable general-purpose control technique. I can imagine a CORDIC-like algorithm that takes advantage of something similar to avoid clamping. You have to find a way to power this thing up as a static demonstration piece.
  - kens 3y ago
    Thanks! We're powering up the unit piece by piece. CuriousMarc got a vintage pitot tube vacuum simulator so we'll use that to simulate the different pressures.
- mgsouth 3y ago
  Based on a quick search, it looks like air impedance (the mysterious air density x speed of sound calculation) affects turbine stability. So needed for engine control?
  - kens 3y ago
    Thanks, that makes sense. Do you have a link that discusses this?
    - mgsouth 3y ago
      Wow, what a rabbit hole... I started by searching for "turbine air impedance". Seems to be a common topic, but maybe just for design modeling? And maybe an outdated parameter to use? One of the front-page hits is a recent (2021) paper from Whittle Laboratory [1] "Modeling Turbine Acoustic Impedance" [2]. Abstract says in part "Impedance boundary conditions are an influential yet uncertain parameter in predicting the thermoacoustic stability of gas turbine combustors...A parametric study of turbine stage designs using the analytical model shows acoustic impedance is a weak function of degree of reaction and polytropic efficiency. The design parameter with the strongest influence is flow coefficient, followed by axial velocity ratio and Mach number. We provide the combustion engineer with improved tools to predict impedance boundary conditions, and suggest thermoacoustic stability is most likely to be compromised by change in turbine flow coefficient." Many search iterations later [5], I found a recent paper discussing "fuel flow rate fluctuations which depend upon the air side impedance at the fuel injection location" [3]. Maybe it's for controlling inlet shock waves? However, I couldn't find anything obiously linking "Air inlet control systems" to "air impedance". Here's one paper discussing such a system, with an extremely detailed transfer function [4]. Maybe it's using another term for impedence, or something close? Edit: Considering this was cutting-edge control hardware for military equipment, I guess some lack of public details is to be expected. On the other hand, on Ebay there's a Fuel Control Unit for a J-79 engine as used in the B-58 Hustler [6]. [1] Yes, that Whittle. https://en.wikipedia.org/wiki/Whittle_Laboratory https://en.wikipedia.org/wiki/Whittle_Laboratory [2] https://www.mdpi.com/2504-186X/6/2/18 https://www.mdpi.com/2504-186X/6/2/18 [3] "Analytical modelling of flame transfer functions for technically premixed flames" https://journals.sagepub.com/doi/full/10.1177/17568277221094403 https://journals.sagepub.com/doi/full/10.1177/17568277221094... [4] "Complex Control System for an Aircraft Supersonic Inlet" http://www.wseas.us/e-library/conferences/2009/rodos/SYSTEMS/SYSTEMS75.pdf http://www.wseas.us/e-library/conferences/2009/rodos/SYSTEMS... [5] Just so you know, "b-58 engine computer" only brings up automotive stuff. [6] https://www.ebay.com/itm/325676156863?_trkparms=amclksrc%3DITM%26aid%3D777008%26algo%3DPERSONAL.TOPIC%26ao%3D1%26asc%3D20230823115209%26meid%3D5af4cefff0304c96bd723aeb9765b577%26pid%3D101800%26rk%3D1%26rkt%3D1%26sd%3D325676156863%26itm%3D325676156863%26pmt%3D0%26noa%3D1%26pg%3D4375194%26algv%3DRecentlyViewedItemsV2SignedOut&_trksid=p4375194.c101800.m5481&_trkparms=parentrq%3A0c6fc5d618b0ab9711f1f22affff6958%7Cpageci%3Ae1b69473-6567-11ee-bb79-8eca912930b8%7Ciid%3A1%7Cvlpname%3Avlp_homepage https://www.ebay.com/itm/325676156863?_trkparms=amclksrc%3DI...
- dekhn 3y ago
  Are the synchros in this similar to what the Rosenblatt Perceptron used to adjust and show weights: "This machine was designed for image recognition: it had an array of 400 photocells, randomly connected to the "neurons". Weights were encoded in potentiometers, and weight updates during learning were performed by electric motors.[2]: 193 "
  - kens 3y ago
    I couldn't find details on the Perceptron's motors, but I expect they were DC motors, rotating the potentiometers higher or lower as needed. The synchro is useful if you have a rotation in one location and one to match the rotation at another location. But with the Perceptron, you don't have a specific rotational angle you want, just "more" or "less".
- cesarb 3y ago
  How are these "zeroed"? That is, how did they make sure that the "zero" position for all gears is correct when assembling the device?
  - kens 3y ago
    I'm not sure how the devices were zeroed. This will probably be an issue when we put it back together. I think some of the gears could be loosened on the shaft so they could be rotated to the correct point. The manual defines the zero points for all the synchros, i.e. 0° means X pressure or Y temperature, so I guess you'd adjust everything to those points.
OldGuyInTheClub 3y ago
I'm struggling to wrap my head around manufacturing so many parts to the needed tolerances and having them work under the temperature swings and forces seen in flight. Debugging these analog computers would involve a large number of engineering disciplines! Impressive technology and excellent article.
- Animats 3y ago
  It's not that hard. All the components here are common and well-behaved - gears, cams, bearings, shafts, mounting plates, and synchros. Only the mounting plates, shafts, and cams are fully custom. Everything else is an off the shelf part. It's like designing electronics - you use standard components, but boards are custom for the job. Design is mostly topology plus constraints to make it fit and keep vibration and backlash under control. There are mechanical equivalents to proto boards, sort of like Lego gearing, but all metal. W.M. Berg used to be the main US maker. So you make up all the functionality on a breadboard, test with inputs and outputs, then rearrange for production. The mechanical design is like medium duty clockmaking. Teletype machines are in roughly the same scale - the parts can be handled without tweezers and are not too fragile. (And all custom, with far more levers than gears.) It's possible to go smaller but everything becomes more fragile and wears faster. It's roughly the same technology as naval fire direction computers [1], but smaller and more automated. [1] https://en.wikipedia.org/wiki/Mark_I_Fire_Control_Computer https://en.wikipedia.org/wiki/Mark_I_Fire_Control_Computer
  - OldGuyInTheClub 3y ago
    When I was in basic research I thought that concept development and prototyping represented the hard part of the work, requiring actual brain power. Show it works and the rest is "engineering." Then, I moved to industry where products have to meet size, weight, and power restrictions, be manufacturable, testable, deployable, and usable by people who have other things to do. Aircraft usually come down so add serviceability. For most space applications, it's even more stressing. Even within engineering disciplines there's a tendency to "throw the design over the wall to manufacturing." It usually leads to bad news. The transition from TRL(Technology Readiness Level) 4 to TRL 7 in NASA's parlance is called the "Valley of Death" for good reason. https://mkainsights.com/insights/fundamentals-operations/trl/ https://mkainsights.com/insights/fundamentals-operations/trl...
creer 3y ago
The synchros run the cockpit displays. Sounds like an intuitive match. But the computed values are also used in other systems: engine control, targeting, that sort of thing. Is the signaling to these also through the synchros? Is there some kind of standard signal format for these?
- creer 3y ago
  Seems to be mostly synchros. With more views in the videos around this one: https://www.youtube.com/watch?v=bbOMHQMohc0 https://www.youtube.com/watch?v=bbOMHQMohc0
- kens 3y ago
  The signaling is indirect. The air data computer is connected to another box, the air data converter. The air data converter is designed for a specific type of aircraft. It converts the synchro outputs from the air data computer into the formats required by the particular aircraft. These are probably synchro outputs as well, but might need to be scaled. (I don't think there is a standard.) The converter would also distribute the signals as needed, with a connector for each device that receives the data.
  - creer 3y ago
    Ah, yes some distribution box sounds fair. Thank you! This is yet another awesome series!
andrehacker 3y ago
Ken: big fan of your and Marc's work, would be so great if you guys could give the Ken&Marc treatment to the "Nortronics NAS-14V2 Astroinertial Navigation System", The R2-D2 unit from an SR-71. Well, here's hoping.
- kens 3y ago
  We're working on the similar star tracker from a B-52. Of course, if anyone wants to send us a SR-71 unit...
garba_dlm 3y ago
WOW all I can think is how all those gears have all those ratios ratios which are all somewhere in the stern brocot trees and which can be written as finite continued fractions I wish I could think about what this means in terms of compilers
jihadjihad 3y ago
That was an awesome read. Would love to see a similar breakdown of the (even older) Norden bombsight [0]. 0: https://en.m.wikipedia.org/wiki/Norden_bombsight https://en.m.wikipedia.org/wiki/Norden_bombsight
ggm 3y ago
I have a copy of "computing mechanisms and linkages" by Svoboda, MIT radiation labs press. I always wondered what functional applications cams had, and now I know (it was a library sale book, one of a huge series published at the end of ww2)
simne 3y ago
Medieval, say wind :)

83 points by picture 3y ago

kens 3y ago

Author here for all your mechanical computer questions :-)

CamperBob2 3y ago
I'll point out that reverse engineering the CADC is not as easy as you might expect. Well, there goes that illusion. :) An amazing piece of work as usual, Ken. Thanks for sharing your hard-won insights on this gadget. The linear-endpoint wraparound hack was worth learning about all by itself. I'm surprised it took until 1954 before someone got around to patenting that, as it seems like a valuable general-purpose control technique. I can imagine a CORDIC-like algorithm that takes advantage of something similar to avoid clamping. You have to find a way to power this thing up as a static demonstration piece.
- kens 3y ago
  Thanks! We're powering up the unit piece by piece. CuriousMarc got a vintage pitot tube vacuum simulator so we'll use that to simulate the different pressures.
mgsouth 3y ago
Based on a quick search, it looks like air impedance (the mysterious air density x speed of sound calculation) affects turbine stability. So needed for engine control?
- kens 3y ago
  Thanks, that makes sense. Do you have a link that discusses this?
  - mgsouth 3y ago
    Wow, what a rabbit hole... I started by searching for "turbine air impedance". Seems to be a common topic, but maybe just for design modeling? And maybe an outdated parameter to use? One of the front-page hits is a recent (2021) paper from Whittle Laboratory [1] "Modeling Turbine Acoustic Impedance" [2]. Abstract says in part "Impedance boundary conditions are an influential yet uncertain parameter in predicting the thermoacoustic stability of gas turbine combustors...A parametric study of turbine stage designs using the analytical model shows acoustic impedance is a weak function of degree of reaction and polytropic efficiency. The design parameter with the strongest influence is flow coefficient, followed by axial velocity ratio and Mach number. We provide the combustion engineer with improved tools to predict impedance boundary conditions, and suggest thermoacoustic stability is most likely to be compromised by change in turbine flow coefficient." Many search iterations later [5], I found a recent paper discussing "fuel flow rate fluctuations which depend upon the air side impedance at the fuel injection location" [3]. Maybe it's for controlling inlet shock waves? However, I couldn't find anything obiously linking "Air inlet control systems" to "air impedance". Here's one paper discussing such a system, with an extremely detailed transfer function [4]. Maybe it's using another term for impedence, or something close? Edit: Considering this was cutting-edge control hardware for military equipment, I guess some lack of public details is to be expected. On the other hand, on Ebay there's a Fuel Control Unit for a J-79 engine as used in the B-58 Hustler [6]. [1] Yes, that Whittle. https://en.wikipedia.org/wiki/Whittle_Laboratory https://en.wikipedia.org/wiki/Whittle_Laboratory [2] https://www.mdpi.com/2504-186X/6/2/18 https://www.mdpi.com/2504-186X/6/2/18 [3] "Analytical modelling of flame transfer functions for technically premixed flames" https://journals.sagepub.com/doi/full/10.1177/17568277221094403 https://journals.sagepub.com/doi/full/10.1177/17568277221094... [4] "Complex Control System for an Aircraft Supersonic Inlet" http://www.wseas.us/e-library/conferences/2009/rodos/SYSTEMS/SYSTEMS75.pdf http://www.wseas.us/e-library/conferences/2009/rodos/SYSTEMS... [5] Just so you know, "b-58 engine computer" only brings up automotive stuff. [6] https://www.ebay.com/itm/325676156863?_trkparms=amclksrc%3DITM%26aid%3D777008%26algo%3DPERSONAL.TOPIC%26ao%3D1%26asc%3D20230823115209%26meid%3D5af4cefff0304c96bd723aeb9765b577%26pid%3D101800%26rk%3D1%26rkt%3D1%26sd%3D325676156863%26itm%3D325676156863%26pmt%3D0%26noa%3D1%26pg%3D4375194%26algv%3DRecentlyViewedItemsV2SignedOut&_trksid=p4375194.c101800.m5481&_trkparms=parentrq%3A0c6fc5d618b0ab9711f1f22affff6958%7Cpageci%3Ae1b69473-6567-11ee-bb79-8eca912930b8%7Ciid%3A1%7Cvlpname%3Avlp_homepage https://www.ebay.com/itm/325676156863?_trkparms=amclksrc%3DI...
dekhn 3y ago
Are the synchros in this similar to what the Rosenblatt Perceptron used to adjust and show weights: "This machine was designed for image recognition: it had an array of 400 photocells, randomly connected to the "neurons". Weights were encoded in potentiometers, and weight updates during learning were performed by electric motors.[2]: 193 "
- kens 3y ago
  I couldn't find details on the Perceptron's motors, but I expect they were DC motors, rotating the potentiometers higher or lower as needed. The synchro is useful if you have a rotation in one location and one to match the rotation at another location. But with the Perceptron, you don't have a specific rotational angle you want, just "more" or "less".
cesarb 3y ago
How are these "zeroed"? That is, how did they make sure that the "zero" position for all gears is correct when assembling the device?
- kens 3y ago
  I'm not sure how the devices were zeroed. This will probably be an issue when we put it back together. I think some of the gears could be loosened on the shaft so they could be rotated to the correct point. The manual defines the zero points for all the synchros, i.e. 0° means X pressure or Y temperature, so I guess you'd adjust everything to those points.

OldGuyInTheClub 3y ago

I'm struggling to wrap my head around manufacturing so many parts to the needed tolerances and having them work under the temperature swings and forces seen in flight. Debugging these analog computers would involve a large number of engineering disciplines! Impressive technology and excellent article.

Animats 3y ago
It's not that hard. All the components here are common and well-behaved - gears, cams, bearings, shafts, mounting plates, and synchros. Only the mounting plates, shafts, and cams are fully custom. Everything else is an off the shelf part. It's like designing electronics - you use standard components, but boards are custom for the job. Design is mostly topology plus constraints to make it fit and keep vibration and backlash under control. There are mechanical equivalents to proto boards, sort of like Lego gearing, but all metal. W.M. Berg used to be the main US maker. So you make up all the functionality on a breadboard, test with inputs and outputs, then rearrange for production. The mechanical design is like medium duty clockmaking. Teletype machines are in roughly the same scale - the parts can be handled without tweezers and are not too fragile. (And all custom, with far more levers than gears.) It's possible to go smaller but everything becomes more fragile and wears faster. It's roughly the same technology as naval fire direction computers [1], but smaller and more automated. [1] https://en.wikipedia.org/wiki/Mark_I_Fire_Control_Computer https://en.wikipedia.org/wiki/Mark_I_Fire_Control_Computer
- OldGuyInTheClub 3y ago
  When I was in basic research I thought that concept development and prototyping represented the hard part of the work, requiring actual brain power. Show it works and the rest is "engineering." Then, I moved to industry where products have to meet size, weight, and power restrictions, be manufacturable, testable, deployable, and usable by people who have other things to do. Aircraft usually come down so add serviceability. For most space applications, it's even more stressing. Even within engineering disciplines there's a tendency to "throw the design over the wall to manufacturing." It usually leads to bad news. The transition from TRL(Technology Readiness Level) 4 to TRL 7 in NASA's parlance is called the "Valley of Death" for good reason. https://mkainsights.com/insights/fundamentals-operations/trl/ https://mkainsights.com/insights/fundamentals-operations/trl...

creer 3y ago

The synchros run the cockpit displays. Sounds like an intuitive match. But the computed values are also used in other systems: engine control, targeting, that sort of thing. Is the signaling to these also through the synchros? Is there some kind of standard signal format for these?

creer 3y ago
Seems to be mostly synchros. With more views in the videos around this one: https://www.youtube.com/watch?v=bbOMHQMohc0 https://www.youtube.com/watch?v=bbOMHQMohc0
kens 3y ago
The signaling is indirect. The air data computer is connected to another box, the air data converter. The air data converter is designed for a specific type of aircraft. It converts the synchro outputs from the air data computer into the formats required by the particular aircraft. These are probably synchro outputs as well, but might need to be scaled. (I don't think there is a standard.) The converter would also distribute the signals as needed, with a connector for each device that receives the data.
- creer 3y ago
  Ah, yes some distribution box sounds fair. Thank you! This is yet another awesome series!

andrehacker 3y ago

Ken: big fan of your and Marc's work, would be so great if you guys could give the Ken&Marc treatment to the "Nortronics NAS-14V2 Astroinertial Navigation System", The R2-D2 unit from an SR-71. Well, here's hoping.

kens 3y ago
We're working on the similar star tracker from a B-52. Of course, if anyone wants to send us a SR-71 unit...

garba_dlm 3y ago

WOW all I can think is how all those gears have all those ratios ratios which are all somewhere in the stern brocot trees and which can be written as finite continued fractions I wish I could think about what this means in terms of compilers

jihadjihad 3y ago

That was an awesome read. Would love to see a similar breakdown of the (even older) Norden bombsight [0]. 0: https://en.m.wikipedia.org/wiki/Norden_bombsight https://en.m.wikipedia.org/wiki/Norden_bombsight

ggm 3y ago

I have a copy of "computing mechanisms and linkages" by Svoboda, MIT radiation labs press. I always wondered what functional applications cams had, and now I know (it was a library sale book, one of a huge series published at the end of ww2)

simne 3y ago

Medieval, say wind :)