[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)