Hello and welcome back. If you follow the channel,
you know that we are reverse engineering an amazing military electro-mechanical computer,
the Bendix MG-1 Central Air Data Computer. We marveled at its mechanical complexity
when we opened it up in Part 1, and Master Ken explained how it works in Part 2. If you have
watched Part 2, you now know that synchros play an extensive role in this computer.
This episode will focus on these marvelous and famously expensive components. We will unbox
some new old ones, explain how they work, play with them in the lab, and show them in action in
other vintage aircraft instruments, which we are planning to eventually hook up to our computer.
[Marc] Master Ken, you got some toys?
[Ken] Yes, I got a couple synchros off eBay.
[Marc] All right!
[Ken] Our air data computer uses these weird Bendix synchros and I found the exact same model,
so we can hook them up, and see how they work, and hopefully connect them to the air data computer.
[Marc] Oh, they're Bendix too!
[Ken] Yeah, they're like the very same part number, which was hard to find.
So here, open one up!
[Eric] All right. So it's a cardboard box, and inside that box is...
[Ken] Special military packaging!
[Eric] ...A cardboard envelope that says Fragile on it. See, it says fragile.
[Ken] Wait, look at the other side, this is like...
[Eric] Oh, Method Two Packaged!
[Ken] Not just method one, we've got method two here!
[Eric] Packaged with desiccant, do not open until ready for use or inspection. Okay,
let's see if I'm ready for this.
[Ken] That fresh desiccant smell!
[Eric] Oh, that's not very fresh. (laughter)
[Ken] You look desiccated!
[Marc] That might be old, actually. How old is it?
[Ken] I don't know.
[Eric] Oh, we're not done yet though. So, underneath this package is another bag.
[Ken] It's 1987.
[Eric] Looking inside the bag, is some desiccant.
[Ken] Wow, fancy fancy little dots.
[Eric] And one of those little cards.
[Ken] Is blue good or bad?
[Eric] Blue is good.
[Ken] Okay, I got my money's worth on the packaging alone!
[Eric] Oh, it's taped shut.
[Marc] Okay, I'll give you the tools.
[Eric] Okay.
[Ken] My breath seems to have no humidity!
[Marc] It's because of your dry humor!
Oh, that's cute. Oh, that's the same one that's inside indeed!
[Eric] Yeah, exactly the same!
[Ken] Yes, it's like same part number, same everything. It's like authentic.
[Eric] And it comes with documentation!
[Ken] Oh this is the measurement specifications, to prove that it's valid. Look at this!
It's a strip chart showing how much error it has.
[Eric] Error in minutes.
[Ken] I don't have time for that!
[Eric] Plus or minus 30 minutes.
[Ken] So, it's like little wiggles right near the
center line. These are like serious parts with like...
[Marc] Yeah, MIL spec!
[Ken] They check, like, everything has been stamped off!
[Marc] So what do these MIL spec synchros do, and how do they work?
Let’s start by hooking one up, and showing you the final result.
So, Mr Ken, you are going to demonstrate how synchros work? Which is quite amazing.
[Ken] Yes. So, I have two synchros here. They're just wired - the outputs from one go to the other.
Three wires for three-phase signal, and then they each have 26 volts power.
[Marc] That's the cheap way to make an AC power supply. So that's 400 Hertz
connected to my HP power amplifier, that makes... How many volts does it need?
[Ken] So, it needs 26 volts RMS, which is about 75 volts peak to peak.
[Marc] Yeah.
[Ken] So it's fairly substantial. And now the cool part is, when I turn one synchro, the other needle
moves exactly to match. But it's bi-directional! I can also turn this one, and the first one moves.
[Marc] No electronics involved!
[Ken] Just the wires between them, and the power supply.
[Marc] I love those things. And then, you have the signals that go through the three windings.
[Ken] So, you can see the three phases here. Pink is highest, green is highest,
and blue... seems to be dead.
[Marc] Now we have blue.
[Ken] So, you can see the three phases: pink is highest, blue is highest,
green is highest. So, when they're in the same position, there's no current,
so there's no torque on it. But if you move either one out of position,
that creates torque that will move them to match. And if you hold one and push the other...
[Marc] You can you can feel it!
[Ken] It's pretty small, but you can feel it.
[Marc] All right!
[Ken] Synchros were very popular in the 40s and 50s. The Navy would use them for
transmitting signals all through the ship. They're used in aerospace, such as here.
It was a very convenient way of transmitting a rotation from one place to another.
[Marc] What? What kind of witchcraft is going on here? We just made a bi-directional position
transmission system with tactile feedback, and no electronics at all.
Looks like some alien technology from the future. But it’s actually very old,
and still in use today. Synchros date back from the early 1900’s, with one of
their first use being for the control of the Panama Canal valves, if you believe Wikipedia.
Inside, they are just little AC motors with three stator windings, at 120 degrees of each other,
and a single winding for the rotor. The rotor windings are connected to an AC power source,
with common frequencies being 50 or 60 Hz, 400 Hz or 1 kHz.
For avionics, 400 Hz is prevalent, and that’s what our computer uses.
The 3 stator windings pick up the signal induced from the rotor, and the relative amount and
polarity of the picked up signals depends on the position of the rotor. Conversely, the angular
position of the rotor can be inferred from the ratio of signals coming from the 3 windings.
Retrieving this position using modern electronic means is actually quite annoying:
you need sampling ADCs and a micro-controller and some code.
But retrieving said position with another synchro, without any electronics,
is a piece of cake. Just connect the rotor of an identical synchro to the same AC power supply,
and then connect all the windings of the receiver to the windings of the transmitter.
And the rotor of the receiver will assume and follow the position of the transmitter
rotor auto-magically, thanks to the miracle of motor magnetics.
It probably hasn’t been lost on you that the scheme is completely symmetric,
so you can also turn the receiver and watch the transmitter move, as we did in our experiment.
In practice, transmitter windings are optimized to produce a high signal, while receivers are
build to produce torque with some position damping, so they are not exactly the same.
Note that there is another closely related cousin to the synchro,
the resolver. We had encountered one recently in the Apollo CDU episode. It’s similar to a synchro,
except that it has two stator windings instead of three. The two windings are
at 90 degrees of each other. They are much more friendly to electronic reading systems,
since one gives you the sine and the other the cosine of the angle. Resolvers is what
the Apollo Guidance Computer uses, or more exactly the CDU, to read the inertial platform position.
But my favorite property of resolvers is that they make pretty circle figures on an analog scope.
Coming back to synchros, the one downside of this simple transmitter/receiver arrangement,
is that the received torque is very small. The receiving synchro can only move something light,
like a needle. To move anything heavier, you have to use a servo arrangement, where the receiving
synchro is assisted by a servo motor and some amplifying electronics. This is what happens here,
in this more complicated altimeter indicator. We’ll get to that in another episode, but for now,
let’s focus on the simple use of a directly connected synchro to move an instrument hand.
[Ken] So, I've wired up a synchro to the Mach computation part here, in the middle.
[Marc] So it's this pair of synchros, right?
[Ken] Right.
[Marc] Coarse and fine.
[Ken] And then those are wired to this connector on the front here. Which I've then wired to this
synchro here. So now, if I mechanically turn this, since we don't have the whole system working yet,
what you see, is the output synchro turning to match.
So, basically, I'm manually adjusting the Mach number, as we speed up through the speed of sound.
[Marc] Wow, i's going real fast!
It works, but I wanted real aircraft instruments. So I went out on eBay in search
for cheap vintage ones, that would hopefully have a compatible synchro actuator in them.
It’s pretty hard to predict what will be inside, but it looks like I lucked out on the first one.
[Marc] It's a temperature indicator, static air. So, I hope to put it at the output of
the temperature computer here. And inside, it is exactly what I wanted. So it's not
an instrument proper: it's a slave instrument. So all it has
is a synchro. And it's the right 400 Hertz 26 volts, the right standard.
So it's just a synchro driving a needle. And that is exactly what we need, since the measurement
is done by this part of the mechanical computer, over here.
And that is my achievement of the day.
On the left, my transmitter, on the right, my receiver, an air temperature indicator.
And here you go. And you can hear it singing an A at 400 Hertz. A very flat A...
Ooh, I'm getting negative. It's very cold!
And here's another marvel of engineering: a Mach number indicator. But this is a Mach number
indicator and repeater. So, in itself, it is a mechanical analog computer. It computes the ratio
of the Pitot pressure that comes through here, and pushes this thing down, and acts as a
super duper spring with little tuning pressure thingies. So that's the static pressure,
that's the Pitot pressure, they divide one with another, and it rotates the shaft.
I explained this instrument in the previous video, a very clever contraption indeed. This
one also has a repeater synchro connected to the hand, which is a transmitter for another
slave instrument, not really what I want. But, I think, I can use it in reverse as a receiver,
if I can find a way to disconnect it from the mechanical actuation.
And then, there is a synchro that picks the position up, and retransmits it to
something. But I want it as an indicator only. So I slightly bent this thing up to disconnect it.
And now this is free, and I think I will be able to use it as an indicator. So, the Mach number
would be calculated by the bigger computer, and just displayed. I'll see if that works.
And here's my second achievement of the day. That's my modified Mach number indicator. Oh, it
goes the other way around. Except from that, point three Mach, point four Mac, point five and six.
And that’s how I made the fastest flying bench ever, and also the coldest.