5 ms·

Reverse-engineering a three-axis attitude indicator from the F-4 fighter plane

212 points by zdw 2y ago

kens 2y ago
Author here if there are any questions...
- farseer 2y ago
  How accurate you think this instrument was compared to the ic based sensors found in your typical smartphone nowadays?
  - dfox 2y ago
    This instrument is only an display that shows data coming from another device that does the actual measurements. One of the reasons why the threewire synchro interface is used is that it is surprisingly accurate as long as you don't care about the fact that it is “slow” by modern standards. The same interface was used to direct artillery and similar things that require significant accuracy to be effective.
  - echoangle 2y ago
    Well this is just an indicator, the accuracy from the actual IMU would also need to be considered. The indicator itself probably isn’t the main source of inaccuracy once the IMU has drifted a bit.
  - kens 2y ago
    According to a paper on navigation sensors, commercial grade sensors have gyroscope drift of 0.1º/s (which is consistent with iPhone data), while navigation grade sensors have a drift of <0.01º/hour. I couldn't find specific numbers for the F-4's inertial navigation system, but I assume it is navigation grade. So the aircraft gyroscopes would be orders of magnitude better than a smartphone. For the azimuth, the F-4 used a flux valve compass, which must be much better than the relatively poor compass on a smartphone. Of course, the smartphone sensors are orders of magnitude cheaper and smaller. [1] https://doi.org/10.1186/s43020-019-0001-5 https://doi.org/10.1186/s43020-019-0001-5
    - bigiain 2y ago
      Drone flight controllers can use 3 axis magnetic field sensors combined with their 3 axis gyros and accelerometers, and use various sensor fusion algorithms to produce higher accuracy compass headings which shouldn't have _any_ long term drift, since the magnetometer readings can compensate over time for any short term gyro drift. I'm not sure how accurate the magnetometers are, and how accurate the calculated compass heading can be, but I recall a project where I used a pair 6DOF accelerometer/magnetometer breakout boards to measure the angle change between the body and the grind setting adjustment ring to datalog my coffee grinder, and I was getting 10bit resolution degrees which all _seemed_ to accurately track the small sub 1 degree adjustments when dialing in the grind. Although I'll admit to not taking the time to understand all the math involved to get from the raw 3 axis acceleration and microTesla magnetic measurements to compass heading, I just copy/pasted sample Arduino code which gave me 1 decimal place degrees. But my grinder has 100 "notches" around the 360 degrees of the grind adjustment ring, and my data logger at least looked like it was accurately/repeatably showing changes down in the quarter to half a notch range. So while I don't think $50 worth of stuff from Adafruit will get you <0.01deg/hour gyro drift rate, with magnetic calibrations and sensor fusion you could probably hit at least 0.1deg long term precision/accuracy, so over 10 or more hours you'd have "navigation grade" accuracy. Which is probably "good enough" for anything traveling at less that a couple of hundred MPH. 0.1deg error is about 1km every 600km - so if you were sailing from Sydney to San Francisco (with no GPS or astro nav equipment) which is ~12,000km, you'd be within 20km or so when you got there. Which seems "close enough" for anything except delivering nukes.
- wezdog1 2y ago
  In civil aviation specifically jets, your AI pitch on the horizon does actually reference body angle not level flight. I wonder if it's a function of the precision and scale of mechanical AIs and also the wider flight regime of military aircraft.
AIorNot 2y ago
pretty awesome to see the engineering details involved! -thanks. as a software person I always wonder how they handle bugs and QA when building complex pieces of hardware like this
- dev_tty01 2y ago
  The strangest concept for modern software engineers is that it had to ship bug free and it could never be updated with firmware patches. Shipping under those constraints brings a certain level of focus not experienced in modern design.
  - consp 2y ago
    So basically like designing and building a bridge?
  - drtgh 2y ago
    I think the key is that in those days you didn't launch a product until you were absolutely sure it was going to work well, it was prototyped and debugged before it was launched. At least that is the impression one get with classical tech, solid reliability.
  - mpenet 2y ago
    My dad used to work on certifying, servicing and making custom instruments for planes, subs, prototypes of all kinds of that era (60s to mid-90s). His “lab” was basically all about testing and simulating environments for the instruments. He had tons of sayings about not having room for error in his line of work. This is as close as you can get from “building bridges” and to this day I don’t think I have seen this level of attention to detail/perfection in any other profession. His job involved electronical engineering , mechanical engineering and programming amongst other things, not to mention a deep knowledge of the physics of these environments. Back then also the tools or source of information that were available to them were quite crude compared to what we have now. His spare time was all about flying, pimping his ham radio gear with all kind of “home made” electronics, build antennas and messing with computers. I guess he’d qualify as a “Hacker” nowadays.
  - 2y ago
genter 2y ago
Thanks for including ridiculously high res images. And it amazes me how many analog tricks they used. Modern day would be a couple lines of code.
- syndicatedjelly 2y ago
  The 1950s were a time in computing where it wasn't a given that digital computing was clearly "better". We still hadn't developed methods of mass-producing reliable, fast, and cheap microelectronics and controllers. So for high-reliability applications, analog computing was THE solution. In 1954, Rex Rice wrote this piece about preferring a simple plugboard as the means of programming a computer, versus any sort of abstraction with a programming language (https://dl.acm.org/doi/10.1145/1455270.1455272 https://dl.acm.org/doi/10.1145/1455270.1455272). So it was still very much up for debate, whether high-level programming languages were even the right solution for the problems being faced. But I agree with you, our forefathers were simply geniuses to have figured out how to manipulate the physical world to produce mathematical computations. Early in his career, my dad had to disassemble and reverse-engineer some Soviet-made aerospace devices, and he still fondly recalls how superbly engineered and precise the Soviet devices were. I wish there was more information out there about Soviet computing, but the winners do write history after all.
  - spitfire 2y ago
    It's interesting that you note the unreliability. I always assumed tubes were unreliable, but thought anything solid state (even those card based systems) would be "reliable enough" to start taking for granted. But then you look at it and think Yeah, obviously they're not going to have MTBF times in the millions. It's going to be hundreds of hours - once a week, or maybe every few weeks between real hard crashes. How would that change your behaviour.
  - somat 2y ago
    My understanding the surprising factor that settled the debate for analog vs digital computers. was that digital computers require far less precise electronics. the voltage in an analog computer has to be exactly 5.2648 volts, every resister, capacitor, transistor has to be a high precision part, the more complex the computer the higher precision required. while in a digital computer "close to 5 volts" is good enough. this makes the components cheaper, more reliable and smaller. and as such the digital computer won. This is why I have my doubts about existing designs for quantum computers, the quantum algorithms we are trying to solve in hardware require an analog design and we rejected analog computers before, mainly because they were unable to scale. This inability to scale is the same problem we see with quantum computers.
Amir6 2y ago
Fun fact is these airplanes are still being used as the backbone of Iranian Airforce and the very same unit was being used before they upgraded the avionics a couple of years ago on some variants.
Scene_Cast2 2y ago
I've wanted to add such an indicator to my car's dash (I already added a boat compass, which I find quite useful and aesthetic). Unfortunately, electronic indicators of any kind are much more rare than vacuum powered ones or all-glass cockpits.
- Onavo 2y ago
  What you need my friend is a ring-laser gyro.
- dumbo-octopus 2y ago
  Boat compass on the dash is awesome, I might have to borrow that. Any issue with interference from the vehicle itself?
  - Scene_Cast2 2y ago
    Yeah. My compass (a Ritchie) has two axis calibration at the bottom; I ended up maxing out one of the axes (so it's still a bit off). Also, it tends to shift by a decent amount when the car is pointing up or down steep hills.
- bigiain 2y ago
  I am currently pondering the idea of building a modern electronics "replica" one of these, with 3d printed sphere halves containing stepper motors, magnetic rotary encoders, and a 6dof compass/gyro imu. If you put an Arduino or ESP32 inside to drive those, you could have simple slip rings that only needed to supply power through the roll and pitch axes. (Only pondering though, I have had the same idle thoughts about making my own Russian Soyuz mechanical navigation instrument too from this other writeup of Ken's https://www.righto.com/2023/01/inside-globus-ink-mechanical-navigation.html https://www.righto.com/2023/01/inside-globus-ink-mechanical-... but somehow the idea of making replica soviet vintage tech isn't as appealing as it was a few years back...)
  - lttlrck 2y ago
    That would be a wonderful project. It'd be cool (albeit inefficient) if there were a way to use induction and remove the slip rings altogether.
bargle0 2y ago
I bet the engineers responsible for this would be so stoked that someone figured out how they solved all these problems.
- deleted 2y ago
  [deleted]
dmitrygr 2y ago
kens@ is a treasure we do not deserve.
- kens 2y ago
  Thanks! Wait, you're the Linux/4004 guy, aren't you? That project was truly amazing.
  - dmitrygr 2y ago
    Thank you :)
liendolucas 2y ago
Asking just out of curiosity/ignorance. The author mentions that the F35 has a completely digital touchscreen to basically do anything on the aircraft (I assume). I can also image a powerful gun damaging it, then how does pilot manage if that screen stops working at all? Compare the same situation in the F4. The hit would only break/damage the instruments on that line of fire, correct? So in one case you would be totally screwed while in the other one you would partially lose some instruments, right? I must obviously not be taking into account something (or many things) for the F35, but in my mind having a 100% digital aircraft seems pretty scary.
- StableAlkyne 2y ago
  Generally, if the cockpit is getting hit with damage to the instruments, there is a very good chance the pilot has also been injured or killed, and doesn't care about the instruments anymore. In old gun fights (which just don't happen anymore), shots were likely to come from behind (so, they intersect the pilot) or the top (so, through the canopy if they're hitting the instruments). This has to do with the orientation both planes are probably in if one is shooting at the other. Go back farther and you get shots from the front, not from fighters (head-ons are very difficult to pull off outside of videogames) but from bomber tail gunners - very old planes from WWII even had bulletproof glass in front of the pilot for this reason. If the F35 has gotten into a gunfight, the pilot has fucked up, it's not a dogfighter and wasn't designed to be one. Even nowadays, if the missile or flak pops next to the cockpit and has managed to damage the instruments, there is a very strong chance the shrapnel has also hurt the pilot to the point that they're not flying home that day. This is the most likely way for the F-35 to be damaged in the modern era. There are obviously scenarios where the instrument panel gets damaged but the pilot is okay, but it's such a low probability scenario that they likely deemed it to be less harmful than the benefit they foresee in a glass cockpit.
  - liendolucas 2y ago
    Thanks for replying! As other mentioned I was missing/not considering the most important case that the pilot is assumed to be dead and that the plane is not supposed to receive such fire.
29athrowaway 2y ago
Crazy to think that all that technology was built by people using slide rules.

212 points by zdw 2y ago

kens 2y ago

Author here if there are any questions...

farseer 2y ago
How accurate you think this instrument was compared to the ic based sensors found in your typical smartphone nowadays?
- dfox 2y ago
  This instrument is only an display that shows data coming from another device that does the actual measurements. One of the reasons why the threewire synchro interface is used is that it is surprisingly accurate as long as you don't care about the fact that it is “slow” by modern standards. The same interface was used to direct artillery and similar things that require significant accuracy to be effective.
- echoangle 2y ago
  Well this is just an indicator, the accuracy from the actual IMU would also need to be considered. The indicator itself probably isn’t the main source of inaccuracy once the IMU has drifted a bit.
- kens 2y ago
  According to a paper on navigation sensors, commercial grade sensors have gyroscope drift of 0.1º/s (which is consistent with iPhone data), while navigation grade sensors have a drift of <0.01º/hour. I couldn't find specific numbers for the F-4's inertial navigation system, but I assume it is navigation grade. So the aircraft gyroscopes would be orders of magnitude better than a smartphone. For the azimuth, the F-4 used a flux valve compass, which must be much better than the relatively poor compass on a smartphone. Of course, the smartphone sensors are orders of magnitude cheaper and smaller. [1] https://doi.org/10.1186/s43020-019-0001-5 https://doi.org/10.1186/s43020-019-0001-5
  - bigiain 2y ago
    Drone flight controllers can use 3 axis magnetic field sensors combined with their 3 axis gyros and accelerometers, and use various sensor fusion algorithms to produce higher accuracy compass headings which shouldn't have _any_ long term drift, since the magnetometer readings can compensate over time for any short term gyro drift. I'm not sure how accurate the magnetometers are, and how accurate the calculated compass heading can be, but I recall a project where I used a pair 6DOF accelerometer/magnetometer breakout boards to measure the angle change between the body and the grind setting adjustment ring to datalog my coffee grinder, and I was getting 10bit resolution degrees which all _seemed_ to accurately track the small sub 1 degree adjustments when dialing in the grind. Although I'll admit to not taking the time to understand all the math involved to get from the raw 3 axis acceleration and microTesla magnetic measurements to compass heading, I just copy/pasted sample Arduino code which gave me 1 decimal place degrees. But my grinder has 100 "notches" around the 360 degrees of the grind adjustment ring, and my data logger at least looked like it was accurately/repeatably showing changes down in the quarter to half a notch range. So while I don't think $50 worth of stuff from Adafruit will get you <0.01deg/hour gyro drift rate, with magnetic calibrations and sensor fusion you could probably hit at least 0.1deg long term precision/accuracy, so over 10 or more hours you'd have "navigation grade" accuracy. Which is probably "good enough" for anything traveling at less that a couple of hundred MPH. 0.1deg error is about 1km every 600km - so if you were sailing from Sydney to San Francisco (with no GPS or astro nav equipment) which is ~12,000km, you'd be within 20km or so when you got there. Which seems "close enough" for anything except delivering nukes.
wezdog1 2y ago
In civil aviation specifically jets, your AI pitch on the horizon does actually reference body angle not level flight. I wonder if it's a function of the precision and scale of mechanical AIs and also the wider flight regime of military aircraft.

AIorNot 2y ago

pretty awesome to see the engineering details involved! -thanks. as a software person I always wonder how they handle bugs and QA when building complex pieces of hardware like this

dev_tty01 2y ago
The strangest concept for modern software engineers is that it had to ship bug free and it could never be updated with firmware patches. Shipping under those constraints brings a certain level of focus not experienced in modern design.
- consp 2y ago
  So basically like designing and building a bridge?
- drtgh 2y ago
  I think the key is that in those days you didn't launch a product until you were absolutely sure it was going to work well, it was prototyped and debugged before it was launched. At least that is the impression one get with classical tech, solid reliability.
- mpenet 2y ago
  My dad used to work on certifying, servicing and making custom instruments for planes, subs, prototypes of all kinds of that era (60s to mid-90s). His “lab” was basically all about testing and simulating environments for the instruments. He had tons of sayings about not having room for error in his line of work. This is as close as you can get from “building bridges” and to this day I don’t think I have seen this level of attention to detail/perfection in any other profession. His job involved electronical engineering , mechanical engineering and programming amongst other things, not to mention a deep knowledge of the physics of these environments. Back then also the tools or source of information that were available to them were quite crude compared to what we have now. His spare time was all about flying, pimping his ham radio gear with all kind of “home made” electronics, build antennas and messing with computers. I guess he’d qualify as a “Hacker” nowadays.
- 2y ago

genter 2y ago

Thanks for including ridiculously high res images. And it amazes me how many analog tricks they used. Modern day would be a couple lines of code.

syndicatedjelly 2y ago
The 1950s were a time in computing where it wasn't a given that digital computing was clearly "better". We still hadn't developed methods of mass-producing reliable, fast, and cheap microelectronics and controllers. So for high-reliability applications, analog computing was THE solution. In 1954, Rex Rice wrote this piece about preferring a simple plugboard as the means of programming a computer, versus any sort of abstraction with a programming language (https://dl.acm.org/doi/10.1145/1455270.1455272 https://dl.acm.org/doi/10.1145/1455270.1455272). So it was still very much up for debate, whether high-level programming languages were even the right solution for the problems being faced. But I agree with you, our forefathers were simply geniuses to have figured out how to manipulate the physical world to produce mathematical computations. Early in his career, my dad had to disassemble and reverse-engineer some Soviet-made aerospace devices, and he still fondly recalls how superbly engineered and precise the Soviet devices were. I wish there was more information out there about Soviet computing, but the winners do write history after all.
- spitfire 2y ago
  It's interesting that you note the unreliability. I always assumed tubes were unreliable, but thought anything solid state (even those card based systems) would be "reliable enough" to start taking for granted. But then you look at it and think Yeah, obviously they're not going to have MTBF times in the millions. It's going to be hundreds of hours - once a week, or maybe every few weeks between real hard crashes. How would that change your behaviour.
- somat 2y ago
  My understanding the surprising factor that settled the debate for analog vs digital computers. was that digital computers require far less precise electronics. the voltage in an analog computer has to be exactly 5.2648 volts, every resister, capacitor, transistor has to be a high precision part, the more complex the computer the higher precision required. while in a digital computer "close to 5 volts" is good enough. this makes the components cheaper, more reliable and smaller. and as such the digital computer won. This is why I have my doubts about existing designs for quantum computers, the quantum algorithms we are trying to solve in hardware require an analog design and we rejected analog computers before, mainly because they were unable to scale. This inability to scale is the same problem we see with quantum computers.

Amir6 2y ago

Fun fact is these airplanes are still being used as the backbone of Iranian Airforce and the very same unit was being used before they upgraded the avionics a couple of years ago on some variants.

Scene_Cast2 2y ago

I've wanted to add such an indicator to my car's dash (I already added a boat compass, which I find quite useful and aesthetic). Unfortunately, electronic indicators of any kind are much more rare than vacuum powered ones or all-glass cockpits.

Onavo 2y ago
What you need my friend is a ring-laser gyro.
dumbo-octopus 2y ago
Boat compass on the dash is awesome, I might have to borrow that. Any issue with interference from the vehicle itself?
- Scene_Cast2 2y ago
  Yeah. My compass (a Ritchie) has two axis calibration at the bottom; I ended up maxing out one of the axes (so it's still a bit off). Also, it tends to shift by a decent amount when the car is pointing up or down steep hills.
bigiain 2y ago
I am currently pondering the idea of building a modern electronics "replica" one of these, with 3d printed sphere halves containing stepper motors, magnetic rotary encoders, and a 6dof compass/gyro imu. If you put an Arduino or ESP32 inside to drive those, you could have simple slip rings that only needed to supply power through the roll and pitch axes. (Only pondering though, I have had the same idle thoughts about making my own Russian Soyuz mechanical navigation instrument too from this other writeup of Ken's https://www.righto.com/2023/01/inside-globus-ink-mechanical-navigation.html https://www.righto.com/2023/01/inside-globus-ink-mechanical-... but somehow the idea of making replica soviet vintage tech isn't as appealing as it was a few years back...)
- lttlrck 2y ago
  That would be a wonderful project. It'd be cool (albeit inefficient) if there were a way to use induction and remove the slip rings altogether.

bargle0 2y ago

I bet the engineers responsible for this would be so stoked that someone figured out how they solved all these problems.

deleted 2y ago
[deleted]

dmitrygr 2y ago

kens@ is a treasure we do not deserve.

kens 2y ago
Thanks! Wait, you're the Linux/4004 guy, aren't you? That project was truly amazing.
- dmitrygr 2y ago
  Thank you :)

liendolucas 2y ago

Asking just out of curiosity/ignorance. The author mentions that the F35 has a completely digital touchscreen to basically do anything on the aircraft (I assume). I can also image a powerful gun damaging it, then how does pilot manage if that screen stops working at all? Compare the same situation in the F4. The hit would only break/damage the instruments on that line of fire, correct? So in one case you would be totally screwed while in the other one you would partially lose some instruments, right? I must obviously not be taking into account something (or many things) for the F35, but in my mind having a 100% digital aircraft seems pretty scary.

StableAlkyne 2y ago
Generally, if the cockpit is getting hit with damage to the instruments, there is a very good chance the pilot has also been injured or killed, and doesn't care about the instruments anymore. In old gun fights (which just don't happen anymore), shots were likely to come from behind (so, they intersect the pilot) or the top (so, through the canopy if they're hitting the instruments). This has to do with the orientation both planes are probably in if one is shooting at the other. Go back farther and you get shots from the front, not from fighters (head-ons are very difficult to pull off outside of videogames) but from bomber tail gunners - very old planes from WWII even had bulletproof glass in front of the pilot for this reason. If the F35 has gotten into a gunfight, the pilot has fucked up, it's not a dogfighter and wasn't designed to be one. Even nowadays, if the missile or flak pops next to the cockpit and has managed to damage the instruments, there is a very strong chance the shrapnel has also hurt the pilot to the point that they're not flying home that day. This is the most likely way for the F-35 to be damaged in the modern era. There are obviously scenarios where the instrument panel gets damaged but the pilot is okay, but it's such a low probability scenario that they likely deemed it to be less harmful than the benefit they foresee in a glass cockpit.
- liendolucas 2y ago
  Thanks for replying! As other mentioned I was missing/not considering the most important case that the pilot is assumed to be dead and that the plane is not supposed to receive such fire.

29athrowaway 2y ago

Crazy to think that all that technology was built by people using slide rules.