Hello and welcome back. If you follow the channel, you know that we not so recently
opened an incredible mechanical computer, the Bendix MG-1 Central Air Data Computer.
Inside was an electro-mechanical marvel of an analog computer, of insane complexity. Master
Ken has been working on reverse engineering it ever since, with an eye on making it work again.
[Ken] So, I've been doing a lot of research on this, and I can explain pretty much the
ideas of how it works. So, it takes two air inputs, static pressure and total pressure,
it has two pressure transducers here, that convert the pressures to rotations. And
that does a lot of calculations with gears, and then produces outputs for
things the aircraft needs to know, like your air speed, your altitude, your Mach number,
the air density. This is information that goes to the pilot, goes to the engine controller,
goes to the wing controller, goes to the weapon systems. And so, it's doing some mathematical
computations to compute these things, based on the inputs of temperature and pressure.
[Marc] Oh my, we are barely three minutes in and we are already in dire need of elevator music.
It’s remarkable how much information you can gain from just measuring three parameters of the air
surrounding an aircraft: the static pressure, the pitot pressure and the air temperature.
Out of that, you can calculate your air density, your mach number, altitude,
rate of climb, apparent airspeed, true air speed and a smattering of other things.
Traditionally, the static air pressure is measured from a port on the side of the aircraft,
in a region as representative as possible to unperturbed air. You are trying to sample air
pressure at rest, which is going to give you your altitude, and also serve as the main reference
pressure to later calculate other quantities when combined with the other measurements.
But first, static pressure will tell you at what altitude you are flying, and for that,
you don’t need a complicated computer. You can make a good mechanical altimeter with surprisingly
little means, and without any electronics. Just hook up a needle to a sealed barometric capsule
that expands when the pressure gets lower. Put the right amount of gears, springs, levers and
clever mechanical corrections, and voila, you made a relatively precise altimeter. We’ll get
deep into the innards of one, when we try to repair a broken altimeter in a future episode.
The Pitot, or total pressure, is obtained from a tube pointing in
the airstream towards the front of the aircraft. It is called the pitot tube,
after its inventor Henri Pitot. Or Pitott, I guess, if you try to pronounce it in English.
But since it’s one of the very few words I can pronounce correctly, I’ll keep calling it Pitot.
Pitot pressure is larger than the static pressure, because it includes the effect of
the air stream ramming into the tube. It is the pressure felt at the nose of the plane,
or seen by the leading edge of the wing. By taking the difference between the pitot
pressure and the static pressure, you can tell how fast you fly. A speed indicator instrument
can compute this difference mechanically, once again using a barometric capsule,
but this time subjected to ram air on the inside, and static air on the outside.
There is a catch though. The speed indicated by this instrument, rightly called indicated
air speed or IAS, is not the true air speed or TAS. Although the IAS will be close to
the TAS at zero altitude, it underestimates your actual true speed the higher you fly,
because the air gets less and less dense. If you want to know your true air speed,
you have to do a more complicated calculation, involving the air temperature and the mach number.
Speaking of Mach number, you can also get that by
dividing the dynamic pressure by the static pressure, something you can do mechanically.
This time, it takes two barometric capsules, one for each pressure,
and a clever lever arrangement to calculate the proper ratio. We will also dig into a Mach meter
in a future episode, and you can clearly see the two barometric pressure capsules inside.
But despite their awesome mechanical cleverness, these simple pressure-based instruments were
not good enough for the high performance jet aircrafts developed in the 1950s and 1960s.
As you approach or surpass the speed of sound, relationships between air density,
speed and pressure become complicated, and you need a true computer to derive precise values.
Here are the standardized equations that our Air Data Computer solves. It takes 5 inputs:
the static pressure, the total pressure, air temperature,
along with an angle of attack correction and an airplane specific correction.
From that, it computes a whole range of parameters to a very high degree of accuracy,
including the Mach number, true air speed, and air density.
That information is used throughout the aircraft, by the pilot instruments of course,
but also by the jet engine control system and the weapons system.
[Marc] All right, and it does square roots and logs and all that stuff, right?
[Ken] So, yeah, it's pretty amazing how it does this all,
with these gears and differentials and cams. So basically, you have logarithmic
cams that convert the rotation into the log - and I think it's this one.
[Marc] Okay, so that's log?
[Ken] And then, it has differentials that can do addition and subtraction. So, by adding two logs,
and then putting it through an exponential cam, that's how you get multiplication.
[Marc] I think we see that one better over... where is this one? Over here, this one.
[Ken] Yeah, so these are differentials, kind of like the differential in your car. It has two
inputs, rotational, and then an output that gives you the sum or the difference of those rotations.
[Marc] Okay.
[Ken] And then, I believe this is the exponential cam that converts your result
back into a linear value. So, that's how you get multiplication or division.
[Marc] Right, and then...
[Ken] And then, these are kind of amazing. These are metal cams that have 20 little screws, so you
can warp this middle surface to the exact shape for a particular function. And then, there's a
follower that rides on it, to get an input based on the rotation of the cam. And then,
that goes into a differential, and so that's used for various functions. And then,
by turning the screws, you can tweak it to get the function exactly right.
[Marc] Here's the little cam.
[Ken] Don't turn the screws!
[Marc] I can push it, and I don't know if you can see it...
[Ken] You can see the gear turning here, as you move that. So that's input into a differential.
[Marc] Yeah, I can see the differential right here.
[Ken] So, another interesting part of this, is how the outputs come from
this. Because they don't want rotational outputs, they want electrical outputs,
that can go to the pilot's instruments, other parts of the system. And so, that's done with
these synchros. And the synchro turns the rotation into a three-phase electrical signal.
And then, these wires go to these connectors that go to other parts of the aircraft. And then,
there's matching synchros on the other side, that can recreate those rotations.
[Marc] Is that a synchro here?
[Ken] Yeah.
[Marc] These are all synchros?
[Ken] Yeah, they're basically basically almost all synchros. There's a few rate encoders and...
[Marc] Is that where the output of the calculation comes out, here?
[Ken] Yeah, through these. These three connectors are the electrical connections
to the rest of the aircraft. This one is actually an input for temperature.
They measure it with a special platinum probe. And then, you can kind of see,
this whole sector, here, is slightly disconnected from the rest of it. It's a separate module.
[Marc] Oh, that's the temperature module!
[Ken] Yeah, this whole module is basically just to convert the resistive temperature into a rotation.
By servo loop, it adjusts the potentiometer to perform a Wheatstone bridge, to make it
balanced. And then, that rotation is used for the calculations in the rest of the module.
[Marc] As you can tell, this was recorded last Christmas - yes,
that’s how backed up I am editing the videos…
[Marc] Yeah, I was thinking,
are those here used for reading the results of the calculation? The synchros themselves?
[Ken] For the most part, yes. So, to get more accuracy, in a lot of cases they use a fine
synchro and a coarse synchro, where every 90 turns of the fine synchro corresponds to a
turn of the coarse synchro. You can see they're in pairs here.
And then, these weird springs on top, these are to prevent backlash. So if there's like any
play in the gears, the spring forces it into one direction, so it won't wobble back and forth.
[Marc] Mm-hmm, okay.
[Ken] And then the...
[Marc] There are some in here, hidden, too. Are those results also?
[Ken] This one, I think is a... Yeah, this is a rate generator. So,
it basically generates a voltage proportional to the shaft speeds, so it calculates the derivative.
[Marc] Ah, okay. And then, there must be motors, because these things are servoed, right?
[Ken] Right. So, the pressure sensor has basically a bellows,
but the force in the bellows isn't enough to turn these shafts. So, instead, it's a servoed system
where the bellows moves a magnetic pickup. And then, there's an amplifier board here,
that amplifies the error to drive a motor, that brings the pressure sensor back into balance,
and then also provides the driving force for all the rest of the gears.
[Marc] Yeah, it's like the gyro in the Apollo thing. You don't
want the sensor to do any mechanical work.
[Ken] That's one of the big problems with analog geared systems like this is,
you lose some torque every time you go through the gears. So, to get any motion at the end, you have
to have pretty good force at the beginning. And that's what the motors here provide.
And then, these boards here are basically the error amplifiers. They use magnetic
amplifiers to generate the motor driving signals. I think it's an H-bridge.
[Marc] The famous magnetic amplifier.
[Ken] Yeah, this thing is powered by 115 volts AC at 400 Hertz.
And then, the synchros themselves have a separate 26 volt AC 400 Hertz supply.
[Marc] What are the red thingies over here?
[Ken] These are potentiometers,
that are driven by the gears, to provide a resistive output for a couple signals.
[Marc] Okay. So, some signals AC, other signals are DC?
[Ken] Some are three phase AC synchro, and some are just simple resistance...
[Marc] Okay
[Ken] ...by turning the potentiometer. Which is sort of
very brute force way of getting a resistive output.
[Marc] All right, okay!
Thank you, Master Ken, for our first dive into the Bendix Air Data Computer. There is of
course a more in depth blog article on Ken’s website, which I will link in the Doodly-Doo.
I realize that I have not yet explained synchros, which are magical components and central to this
computer, and also very prevalent in vintage aircraft instruments. They allow to measure
and transmit a rotational movement from one place to another with amazing precision and
simplicity. We’ll take a closer look at synchros in the next part of this series. See you then!