https://www.righto.com/2023/01/reverse-engineering-airspeedmach.html
How does a vintage airspeed indicator work? CuriousMarc picked one up for a project, but it didn't have any documentation, so I reverse-engineered it. This indicator was used in the cockpit panel for business jets such as the Gulfstream G-III, Cessna Citation, and Bombardier Challenger CL600. It was probably manufactured in 1977 based on the dates on its transistors.
You might expect that the indicators on an aircraft control panel are simple dials. But behind this dial is a large, 2.8-pound box with a complex system of motors, gears, and feedback potentiometers, controlled by two boards of electronics. But for all this complexity, the indicator doesn't have any smarts: the pointers just indicate voltages fed into it from an air data computer. This is a quick blog post to summarize what I found.
(Score: 0) by Anonymous Coward on Sunday January 15, @03:51AM
Yes, once upon a time, radium paint was used to make self-illuminating dials. Per this nice overview of aircraft instruments and maintenance, https://www.cessnaflyer.org/maintenance-tech/item/1152-aircraft-instrument-systems-a-brief-guide.html [cessnaflyer.org]
The local aero museum had some problems with a WWII Bell P-39 that was rescued from the bottom of a lake...iirc there were problems bringing the instruments (radium dials) back into USA where they were made.
(Score: 2) by JoeMerchant on Sunday January 15, @03:59AM
For lower speed aircraft the airspeed gauge is pretty simple, we even ran one on little 2lb drones - open tube forward of the wing to a pressure sensor, calibrates fairly easily and makes a much better feedback to engine speed control loops than anything like GPS speed over ground.
Україна досі не є частиною Росії Слава Україні🌻 https://news.stanford.edu/2023/02/17/will-russia-ukraine-war-end
(Score: 4, Interesting) by driverless on Sunday January 15, @09:46AM
Referenced in the comments is this fabulous YouTube channel [youtube.com] full of teardowns of this sort of stuff, alongside other things that fly like missiles. God knows where he gets all this stuff.
Given his open invitation to send him military gear for teardowns, I hope to see lots of surplus Russian stuff being dissected in the future.
(Score: 2) by Username on Sunday January 15, @12:13PM (11 children)
I assembled some boards for parker-lord, about two years ago, that had melf resistors that I guess to be from the late 90s. There were no dates on it, but the reel/tape fell apart fairly easy and the parts did not wet to the pad due to oxidation on the endcaps.
So 1977 might just be those components. Could very well have been made in 1980s. If this was parker, maybe 2020s.
(Score: 2) by kazzie on Sunday January 15, @12:34PM (10 children)
The article mentions 26V 400Hz as being a common power standard for aircraft. Is that still the case today? Is it limited to a paricular sector (e.g. light aircraft, lilitary, or whatever)?
The reason I ask is that a co-worker was recently adamant that transformers are of no relevance to aeronautical engineering, yet this old device uses one.
(Score: 2) by Username on Sunday January 15, @03:06PM
I never designed or tested boards, I just made them. No idea what voltage they used. But most had large inductors and transformers on them. Some coils as large as my fist. I can only guess that it's for power, ac to dc. The boards I made had the mosfet type, not the can type transistors. So a little bit more modern in that aspect. Other difference is the resistors were melf for surface mount. No color markings either. Just this brownish red coating on the melfs. So you had no idea what was what after they came out of the tape. This was late 90s early 2000s era tech, probably made for older planes. Did make some newer stuff for raytheon though. Still had transformers on them. They were the brick looking ones. Could see small coils inside the part when doing xray. Their stuff also had buck converters powering ICs like normal motherboards.
(Score: 2) by JoeMerchant on Sunday January 15, @03:13PM (5 children)
I worked in an Eaton Aerospace Systems Controls Division as an intern in 1986 (my motto for the place was: "We make toggle switches!") The running joke around the place was: which is more painful, wall current or aircraft current? Aircraft current, of course, because it has 380 more Hertz.
The generators that run off of APU turbines run 24000 RPM (400Hz) in part because it's more efficient than gearing them down to spin 3600 RPM, also because they're smaller than a lot of the big 60Hz gensets that would literally fly apart at 24K, and because: well, aircraft, we can make 'em to take the speed.
As for voltage, I think there were a whole lot of "standards" generally around the 12-24-48-96V levels, 26 would just be 24V near the top of a +/-10% tolerance window.;
Україна досі не є частиною Росії Слава Україні🌻 https://news.stanford.edu/2023/02/17/will-russia-ukraine-war-end
(Score: 2) by maxwell demon on Sunday January 15, @09:06PM (1 child)
So you had 20 Hz wall current?
The Tao of math: The numbers you can count are not the real numbers.
(Score: 2) by JoeMerchant on Sunday January 15, @10:18PM
Ha, yeah, the factory employees weren't that good at math, and also didn't always remember whether aviation AC was 400 or 440Hz... they did know where the bar was on Friday night, though.
Україна досі не є частиною Росії Слава Україні🌻 https://news.stanford.edu/2023/02/17/will-russia-ukraine-war-end
(Score: 0) by Anonymous Coward on Monday January 16, @06:08AM
Did the punnier ones ask "which hurts more?"
(Score: 2) by VLM on Monday January 16, @03:47PM (1 child)
115 Vrms 400 Hz is (was?) very popular.
Aside from transformer core size issues, the smoothing cap to turn 400Hz into DC is going to be a lot smaller than the smoothing cap to turn 60 Hz into DC.
(Score: 2) by JoeMerchant on Monday January 16, @06:22PM
It's all kinds of efficient, until you're trying to build hydro-electric generators for places like Lake Meade.
Україна досі не є частиною Росії Слава Україні🌻 https://news.stanford.edu/2023/02/17/will-russia-ukraine-war-end
(Score: 3, Informative) by janrinok on Sunday January 15, @03:24PM
It was certainly a standard among UK military aircraft when I was still flying and, as they all seem to use the same ground power supplies at both military and civilian airfields I imagine that plenty of civil aircraft used it too.
Whether it is still the most common voltage I do not know, but an aircraft that needs special ground equipment just to land at an airfield and take off again isn't going to sell very well.
(Score: 3, Informative) by JoeMerchant on Sunday January 15, @10:22PM (1 child)
Here's a reference for a modern-ish regional jet: https://www.smartcockpit.com/docs/Bombardier_CRJ_200-Electrical.pdf [smartcockpit.com]
Seems pretty standard to me.
Україна досі не є частиною Росії Слава Україні🌻 https://news.stanford.edu/2023/02/17/will-russia-ukraine-war-end
(Score: 2) by kazzie on Monday January 16, @11:06AM
That's a pretty useful document, as it not only lists AC and DC systems on that aircraft, but also several transformer rectifier units that generate DC from the AC buses. I'll mention this to my colleague later, thanks.
(Thanks also to earlier commenters sharing their experience.)