[HN Gopher] Reverse-engineering an airspeed/Mach indicator from ... ___________________________________________________________________ Reverse-engineering an airspeed/Mach indicator from 1977 Author : picture Score : 59 points Date : 2023-01-12 18:28 UTC (4 hours ago) (HTM) web link (www.righto.com) (TXT) w3m dump (www.righto.com) | kens wrote: | Author here if you have any questions :-) | kennend3 wrote: | 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? | kens wrote: | 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. | tyingq wrote: | 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. | londons_explore wrote: | 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. | akelly wrote: | 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. | jonah wrote: | 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 wrote: | 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 wrote: | https://support.cessna.com/custsupt/contacts/pubs/ourpdf.pd | f... | | 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. | ddoolin wrote: | 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 wrote: | E-mail me (@gmail) if you'd like to chat about this. | tyingq wrote: | 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 wrote: | 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 wrote: | 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 wrote: | 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 wrote: | 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 wrote: | 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_Subst... | addaon wrote: | Although note that avionics fall under the "transportation" | exemption, and RoHS does not apply. | | That said, the supply chain has moved largely towards RoHS | compliance, so some avionics inherit that for supply chain | simplicity. Some don't; there's a cottage industry reballing | BGAs from lead-free to leaded solder for aerospace | applications. | mk_stjames wrote: | 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... ___________________________________________________________________ (page generated 2023-01-12 23:00 UTC)