7 ms·

Reverse-engineering an airspeed/Mach indicator from 1977

105 points by picture 4y ago

kens 4y ago
Author here if you have any questions :-)
- jonah 4y ago
  How would this unit be calibrated? It seems that wear an aging on the motors and potentiometers would change their characteristics slightly over time.
  - kens 4y ago
    I assume there is a detailed testing procedure specified by the FAA, and they would replace any bad components. But because of the servo loop feedback, most of the components aren't critical to accuracy. (Note that the internal power supplies are entirely unregulated.) If the motor slows down, for instance, it will still end up at the right location. The potentiometer is really the critical part, but it shouldn't change very much. And if it does change, as long as the resistance changes uniformly, it will still be okay.
    - inoffensivename 4y ago
      https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=45979 https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf... This is an example of the pitot/static inspection procedure for a Cessna Citation Sovereign. The procedure itself begins on page 9 of the PDF.
- kennend3 4y ago
  Nice article and cool you are here for questions. > The unit is powered by 26 volts, 400 Hz, a standard voltage for aviation. Any idea as to the history as to why this is the case? I assume higher voltage is useful for lower amps? But why 400 hz? Seems odd to use such a high voltage if components such as this must have internal transformers?
  - londons_explore 4y ago
    Higher frequency allows transformers and induction motors to use less steel in their cores for the same power. In aviation, that matters because weight matters. The downside is iron losses become bigger (heat lost in transformers). In a plane that typically doesn't matter because you aren't worried about losing a couple of watts of electrical power. In today's world, it is irrelevant because all voltage conversion is done solid state (which is easier from DC), and all motors (of new designs) are brushless and therefore prefer to run from DC.
  - kens 4y ago
    Others have described why 400 Hz is useful. But I'll mention that this frequency is very inconvenient for using the box on the ground. Fortunately CuriousMarc had some vintage HP boxes that we could use. We ended up hooking up an HP 3310A function generator to an HP 6824A DC power supply amplifier to produce the power for the indicator.
    - raverbashing 4y ago
      Ah good. I was going to ask that question (does anyone has a 400Hz AC power supply lying around? :) - well I guess the manufacturers have but not many more people)
  - akelly 4y ago
    Higher frequency means smaller and therefore lighter transformers, which is very important on aircraft. Nowadays a DC voltage supply would be better, but DC to DC voltage converters didn't exist when the 24V 400Hz standard was created. For mains voltage we use 50-60 Hz because lower frequencies work better with very large AC generators in power plants and, and lower frequencies are more efficient to transmit long distances.
  - tyingq 4y ago
    I know on the ground, 400Hz was a better match for turbofan electric generators. Which were lighter and more efficient than normal ICE generators. And so used a lot when the equipment was mobile.
  - 4y ago
mk_stjames 4y ago
There is something about this era of higher-end electronic assemblies that just ticks boxes for me aesthetically- metal can transistors, raw fiber boards with no solder mask, axial lead film capacitors, perfect leaded (or sometimes silver) solder joints...
- asmithmd1 4y ago
  Some electronics are still made that way. Bartel Amps has created a new ultra-premium category that he calls "masterbuilt". Take a look at the construction details: http://bartelamps.com/wp-content/uploads/2017/12/IMG_5727_crop_800x800.jpg http://bartelamps.com/wp-content/uploads/2017/12/IMG_5727_cr... http://bartelamps.com/signatureseriesdetails/ http://bartelamps.com/signatureseriesdetails/
  - mk_stjames 4y ago
    That point-to-point style is more reminiscent of 1940s-50s era radio construction than the 1970's post-transistor but pre-integrated-circuit PCB I was getting at. But very cool nonetheless.
ddoolin 4y ago
This is awesome. I've seen a much simpler one of these (and other things) behind the firewall and was pretty amazed at how much is really back there. To see that the systems are as reliable as they are, and that they tell you when they aren't (e.g. this ASI's fault checking), is really cool from my perspective and I believe has carried forward to more modern avionics. I really freaking love avionics. I'd love to get into software development at an avionics company but I have no clue where to start with that.
- addaon 4y ago
  E-mail me (@gmail) if you'd like to chat about this.
- dadadad100 4y ago
  Avionics, sadly is not like this anymore. When I started in avionics (late 70s, when this instrument was state of the art) I was fixing some radios that still had tubes, a mechanical crystal carriers that got moved to change frequency. In 10 years it’s was all chips. In a few more years it was glass cockpits. I saw the future and moved into programming. But for a while at least I programmed flight simulators to use my avionics knowledge. But now MSFS is as good as the software in any of those sims. Of course, being in a cockpit built with actual aircraft hardware with a full motion base and a visual system focused at infinity is an experience I’m glad I had. And I repaired a lot of instruments like the OP has for the sim.
  - Nextgrid 4y ago
    > now MSFS is as good as the software in any of those sims Are you sure? I'd expect the primary purpose of a pro simulator is to be a training aid in which case the simulation needs to be extremely accurate especially at edge-cases such as near/at stall speeds, various combinations of faults, etc - the dangerous stuff you'd actually want to train for and can't easily/safely replicate in a real aircraft. I'm sure consumer-grade sims do a great job at replicating routine flight, but I'd bet good money a pro sim is much more accurate at replicating let's see a stall condition.
tyingq 4y ago
Great writeup! I didn't see it mentioned, but I assume the fast motor with reduction gears is helpful for a damping effect. To slow any twitching and show a sort of short term average speed.
rootusrootus 4y ago
They sure did make old electronics pretty. At least sometimes, as in the case of this device that probably cost as much as my car. I'm curious about the soldered wires, though. Especially in a device intended to be used in an airplane. No concerns about brittleness? I was under the (probably mistaken) impression most vehicular wiring was clamped or crimped these days for that reason. Maybe those aren't really soldered, but just look kinda like it?
- dboreham 4y ago
  A crimp is done to a connector so you have to consider the reliability of the connector+crimp. Those soldered wires are direct onto the terminal so no connector.
  - rootusrootus 4y ago
    I was thinking it might be specific to the use case, for sure. And maybe these are solid core wires, I can't really make it out from the picture.
- madengr 4y ago
  Crimping on cars is probably just to save money, and some of the wire they use is garbage. Solder is pretty soft, especially lead based. Silver solders are more brittle and we don't use them for high reliability applications. Gold also has to be removed as it will embrittle the joint. For wires soldered to a PCB you can stake the wires with an electronics grade RTV, then bundle with twine, staked with epoxy. I have used low-outgassing RTV for space applications that costs $600 a tube.
- blamazon 4y ago
  One possible explanation: leaded solder is more elastic than non-leaded. (less brittle) Modern mass produced products generally have had lead engineered out of them, including in the solder. One popular product specification in this domain is RoHS, the EU Restriction of Hazardous Substances Direction, which largely bans lead and 9 other hazardous substances: https://en.wikipedia.org/wiki/Restriction_of_Hazardous_Substances_Directive https://en.wikipedia.org/wiki/Restriction_of_Hazardous_Subst...

105 points by picture 4y ago

kens 4y ago

Author here if you have any questions :-)

jonah 4y ago
How would this unit be calibrated? It seems that wear an aging on the motors and potentiometers would change their characteristics slightly over time.
- kens 4y ago
  I assume there is a detailed testing procedure specified by the FAA, and they would replace any bad components. But because of the servo loop feedback, most of the components aren't critical to accuracy. (Note that the internal power supplies are entirely unregulated.) If the motor slows down, for instance, it will still end up at the right location. The potentiometer is really the critical part, but it shouldn't change very much. And if it does change, as long as the resistance changes uniformly, it will still be okay.
  - inoffensivename 4y ago
    https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf?as_id=45979 https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pdf... This is an example of the pitot/static inspection procedure for a Cessna Citation Sovereign. The procedure itself begins on page 9 of the PDF.
kennend3 4y ago
Nice article and cool you are here for questions. > The unit is powered by 26 volts, 400 Hz, a standard voltage for aviation. Any idea as to the history as to why this is the case? I assume higher voltage is useful for lower amps? But why 400 hz? Seems odd to use such a high voltage if components such as this must have internal transformers?
- londons_explore 4y ago
  Higher frequency allows transformers and induction motors to use less steel in their cores for the same power. In aviation, that matters because weight matters. The downside is iron losses become bigger (heat lost in transformers). In a plane that typically doesn't matter because you aren't worried about losing a couple of watts of electrical power. In today's world, it is irrelevant because all voltage conversion is done solid state (which is easier from DC), and all motors (of new designs) are brushless and therefore prefer to run from DC.
- kens 4y ago
  Others have described why 400 Hz is useful. But I'll mention that this frequency is very inconvenient for using the box on the ground. Fortunately CuriousMarc had some vintage HP boxes that we could use. We ended up hooking up an HP 3310A function generator to an HP 6824A DC power supply amplifier to produce the power for the indicator.
  - raverbashing 4y ago
    Ah good. I was going to ask that question (does anyone has a 400Hz AC power supply lying around? :) - well I guess the manufacturers have but not many more people)
- akelly 4y ago
  Higher frequency means smaller and therefore lighter transformers, which is very important on aircraft. Nowadays a DC voltage supply would be better, but DC to DC voltage converters didn't exist when the 24V 400Hz standard was created. For mains voltage we use 50-60 Hz because lower frequencies work better with very large AC generators in power plants and, and lower frequencies are more efficient to transmit long distances.
- tyingq 4y ago
  I know on the ground, 400Hz was a better match for turbofan electric generators. Which were lighter and more efficient than normal ICE generators. And so used a lot when the equipment was mobile.
- 4y ago

mk_stjames 4y ago

There is something about this era of higher-end electronic assemblies that just ticks boxes for me aesthetically- metal can transistors, raw fiber boards with no solder mask, axial lead film capacitors, perfect leaded (or sometimes silver) solder joints...

asmithmd1 4y ago
Some electronics are still made that way. Bartel Amps has created a new ultra-premium category that he calls "masterbuilt". Take a look at the construction details: http://bartelamps.com/wp-content/uploads/2017/12/IMG_5727_crop_800x800.jpg http://bartelamps.com/wp-content/uploads/2017/12/IMG_5727_cr... http://bartelamps.com/signatureseriesdetails/ http://bartelamps.com/signatureseriesdetails/
- mk_stjames 4y ago
  That point-to-point style is more reminiscent of 1940s-50s era radio construction than the 1970's post-transistor but pre-integrated-circuit PCB I was getting at. But very cool nonetheless.

ddoolin 4y ago

This is awesome. I've seen a much simpler one of these (and other things) behind the firewall and was pretty amazed at how much is really back there. To see that the systems are as reliable as they are, and that they tell you when they aren't (e.g. this ASI's fault checking), is really cool from my perspective and I believe has carried forward to more modern avionics. I really freaking love avionics. I'd love to get into software development at an avionics company but I have no clue where to start with that.

addaon 4y ago
E-mail me (@gmail) if you'd like to chat about this.
dadadad100 4y ago
Avionics, sadly is not like this anymore. When I started in avionics (late 70s, when this instrument was state of the art) I was fixing some radios that still had tubes, a mechanical crystal carriers that got moved to change frequency. In 10 years it’s was all chips. In a few more years it was glass cockpits. I saw the future and moved into programming. But for a while at least I programmed flight simulators to use my avionics knowledge. But now MSFS is as good as the software in any of those sims. Of course, being in a cockpit built with actual aircraft hardware with a full motion base and a visual system focused at infinity is an experience I’m glad I had. And I repaired a lot of instruments like the OP has for the sim.
- Nextgrid 4y ago
  > now MSFS is as good as the software in any of those sims Are you sure? I'd expect the primary purpose of a pro simulator is to be a training aid in which case the simulation needs to be extremely accurate especially at edge-cases such as near/at stall speeds, various combinations of faults, etc - the dangerous stuff you'd actually want to train for and can't easily/safely replicate in a real aircraft. I'm sure consumer-grade sims do a great job at replicating routine flight, but I'd bet good money a pro sim is much more accurate at replicating let's see a stall condition.

tyingq 4y ago

Great writeup! I didn't see it mentioned, but I assume the fast motor with reduction gears is helpful for a damping effect. To slow any twitching and show a sort of short term average speed.

rootusrootus 4y ago

They sure did make old electronics pretty. At least sometimes, as in the case of this device that probably cost as much as my car. I'm curious about the soldered wires, though. Especially in a device intended to be used in an airplane. No concerns about brittleness? I was under the (probably mistaken) impression most vehicular wiring was clamped or crimped these days for that reason. Maybe those aren't really soldered, but just look kinda like it?

dboreham 4y ago
A crimp is done to a connector so you have to consider the reliability of the connector+crimp. Those soldered wires are direct onto the terminal so no connector.
- rootusrootus 4y ago
  I was thinking it might be specific to the use case, for sure. And maybe these are solid core wires, I can't really make it out from the picture.
madengr 4y ago
Crimping on cars is probably just to save money, and some of the wire they use is garbage. Solder is pretty soft, especially lead based. Silver solders are more brittle and we don't use them for high reliability applications. Gold also has to be removed as it will embrittle the joint. For wires soldered to a PCB you can stake the wires with an electronics grade RTV, then bundle with twine, staked with epoxy. I have used low-outgassing RTV for space applications that costs $600 a tube.
blamazon 4y ago
One possible explanation: leaded solder is more elastic than non-leaded. (less brittle) Modern mass produced products generally have had lead engineered out of them, including in the solder. One popular product specification in this domain is RoHS, the EU Restriction of Hazardous Substances Direction, which largely bans lead and 9 other hazardous substances: https://en.wikipedia.org/wiki/Restriction_of_Hazardous_Substances_Directive https://en.wikipedia.org/wiki/Restriction_of_Hazardous_Subst...