Hello and welcome back.
If you follow the channel, you know that we are working on an amazing electro-mechanical
computer, the Bendix MG-1 Central Air Data Computer, from an early supersonic fighter
jet.
We marveled at its mechanical complexity in Part 1, Master Ken told us how it works in
Part 2, and we explained how its many synchros work in Part 3.
We also started to play with my new collection of aircraft instruments that we will need
in order to display our computer’s results.
But we will also need to test our Air Data Computer.
And for that, the preferred method is to use is a Pitot-Static test set.
It is a contraption that can precisely set the pressures in the two computer air input
ports, the static pressure and the Pitot pressure.
But these are precision safety items, still widely used today, and are therefore very
expensive, even very used and very vintage.
So, I was going to build my own on the cheap instead, with a vacuum pump and two old eBay
altimeters to monitor the pressure in each port.
The first eBay altimeter, despite being the worst looking of the two, worked flawlessly.
But the second one arrived with one missing hand.
It was ultimately found rattling at the bottom of the meter.
The hand was definitely attached in the eBay picture, so I suspect that a combo of poor
packaging, and the legendary, extra-careful FedEx handling, damaged it in transit.
Not to worry, the vendor was very apologetic, we got our money back from FedEx, and I even
got to keep the broken instrument.
So let’s turn this mishap into an opportunity: maybe we can try our hand at repairing it,
and learn a thing or two.
[Marc] Okay, go for it Mike.
So, we're checking to see if there is any radium in the thing.
Geiger counter, and it gets nothing.
[Mike] Yep, nothing above background.
[Marc] So, what about the needle that's down there?
All right, no radium.
So we're safe, we can work on that one.
So, of course, I tried to mount the needle back on first, as the shaft it mounted on
did not appear to be damaged, and it’s just simple a pressure fit, like in a watch hand.
But that did not work at all.
The altimeter would just not budge.
So, either I had remounted it wrong, or something else was amiss.
So, this is my altimeter repair number two.
I put it all back together, and put the needle, the main hands, where they belong.
But somehow I did it wrong, and it's not working as it should.
Yeah, it's not connected.
Yeah, this was annoying, it's missing a part here.
So, this has been somewhat tampered with.
I found all kind of stuff that didn't belong, this for example.
So, I'm taking a piece of plastic from the flimsiest - ah there you go - flimsiest bag
I can find.
I got one needle.
And the second one
And then the third one.
Now of course, I probably completely messed up the calibration, but I don't really care,
as long as it's close.
Okay.
So I get the face off, get that disc off, and...
No!
So what gives?
That goes out, and this is working fine.
I am wondering if there's a simple way to take it out, but there is not.
You have to go and undo this.
There we go, got that out.
And then, the whole thing should lift up.
There we go!
And that should be fine, or is something wrong about it?
I wonder if this is turning.
Oh!
So, I don't know that this instrument ever worked...
Oh yes!
So I know what the problem is: it's in the mechanism itself.
These two are disconnected.
So, I think I have a problem in the mechanism.
Which will be good, because then, we have to take the mechanism out...
...and I sure can.
There we go.
Empty!
It's an entirely mechanical thing.
So, that's my altimeter, and I thought it had electronics inside, but it doesn't.
So there's the front glass, the case, the front face, and the mechanism, which is just
a pressure sensor.
Pressure disc, stack of two.
And the air fills the cavity, and that's it.
There is a vacuum in there, it compares it to the vacuum - it's all mechanical.
So, although the practical realization of this instrument is insanely difficult, its
principle is very straightforward.
It relies on a couple of sealed metal capsules with just vacuum in them, known as aneroid
capsules.
The whole enclosure of the instrument is also sealed, that’s why there is a gasket behind
the front glass portion.
The static port is connected via a tube to the back of the enclosure, and it fills the
instrument with the static pressure.
The top and the bottom of the aneroid capsules are springy.
So, as the static air pressure surrounding them diminishes, the capsules expand.
They push on a lever which rotates a shaft, and that mechanical rotation is amplified
by a gear train, and displayed by the hands of the instrument.
In addition, there is a barometric adjustment knob that will zero the instrument at the
current ground level pressure, a very important piece of information that the control tower
will relay to you.
So, here it is.
Oh yeah.
Oh, I see the problem!
Okay.
So, the whole case is sealed, and the air from the pressure intake comes and fills
the case.
And this is at vacuum, so it will expand, move this lever, which will then...
So it has all kind of calibrated springs and magical mechanical things, but in the end,
it should move this wheely thingy.
And that one is bent down, and is not moving the wheel.
So, I have to re-engage those two.
Oh, no!
It's worse than that!
Oh, I see what's wrong with it.
It has sprung out of its pivot!
All right, so this is out of the pivot, so I have to...
Okay, so that might be too difficult, I might want to do that in the binocular microscope.
Delicate operation, under the microscope we go, or I'm not going to see it correctly.
So, I'm back from the microscope, and the news is not great.
The bearing here is actually either broken, or it's missing a ball.
I think, they used a steel ball in between a sapphire cup and a screw, and the ball is
nowhere to be seen.
So, probably the whole thing went bad, in the same incident that dislodged the indicator
hand in the first place.
So, I need to get a steel ball, or...
I don't know what to do.
This has been poorly shipped, and probably died in shipment.
Okay, I've put it all back together, minus the hands and a few screws, and ordered some
steel balls.
And when they come from China, I will see if we can attempt to replace it, and repair
- save the thing.
But that will be difficult.
All right, my steel balls have arrived, and they are absolutely minuscule: half millimeter!
I hope that's the right size.
And that's certainly going to be a challenge to install.
That's definitely an 'under the microscope' operation.
I don't know that you'll be able to see anything, nor that I will be able to see anything.
I'm going to try to use gravity to my advantage.
So, a long fight with a tiny ball and a delicate mechanism ensued.
Under the microscope, it became painfully apparent that the original pivot did not include
a ball, which would have been much too fragile a solution anyhow.
The pivot was a tip extension, machined at the end of the screw, and the tip just broke
off in the shipping incident.
Normally the tip engages in a receiving hole in the jewel, and the pressure is adjusted by
the screw.
By some stroke of luck, the broken off tip created a depression in the adjustment screw,
so sticking a ball in there may work in a pinch, for our restricted use.
But the proper repair would necessitate procuring the special pivot screw.
Don’t fly with this instrument.
After a couple of trials, I managed to put my ball between the broken screw and the jewel,
restoring a temporary low friction pivot.
You can see it quite well here.
But whether it will be strong enough or low friction enough is anyone’s guess.
Okay, somehow, I managed to put my ball in there, and the pivot, and I think now the
wheel engages.
Whether this will be good enough to make it work again, I dunno.
We are gonna try!
There we go.
Tada!
This goes...
29.48 today - something like that - that is the right pressure today.
All right!
Phew, phew, phew...
I don't have the correct tool, so I do it by hand.
1000 feet hand...
And, the only problem, is that you don't know how to test it until it's closed.
So, let me close it up.
All right!
And now the question is, will it tell our altitude?
Pump on!
No, not working.
Oh!
And that, my friends, is probably the sound of a pivot that popped the ball, and broke
again.
So, I took it all apart again, put another ball in, and applied a little more pressure
this time.
If it does not work, I’ll have to find or machine the correct pivot screw.
Or give up, in the name of the the eBay and FedEx Gods.
Okay, one last try.
I took it all apart, and put it back together again, making sure everything is loose.
The axle had stepped out.
Oh!
Yeah yeah yeah!
Yes!
We are having altitude!
Oh, I repaired it!
I don't know how far we can push it.
All right!
Wow, I had almost given up on that one!
There we go!
Now, I suppose the precision is completely shot, but vaguely, it'll tell me 20,000 feet
from 30,000 feet.
Much better!
Okay.
All right.
Perseverance did it on that one, excellent!
Well, we didn't return quite to zero, but good enough.
Yeah, I think I didn't wind the spring strong enough, but hey, that will do it for me.