Help me gut my altimeter!

[SimSupervisor]

I didn't know about those linear servos, looks quite interesting. The gears outside do lots of revolutions, might be perfect. It would certainly be more interesting than most other zero-sensing methods I have tought about.

[xplanematt]

The problem with a servo is that you have an effective resolution of *maybe* 1,000 steps or so...and that's for a high-end servo in ideal conditions. The problem with the altimeter is that the bellows makes extremely small movements to make the needles move quite a bit. With a stepped worm drive, your resolution is limited only by the gearing of the stepper. Higher resolution just means more steps required for movement of the needles. The only other piece of the puzzle, as you say, is detecting the zero/home position. I don't think this would be a huge problem to overcome, and would certainly be worth the hassle for the smooth and precise movement that would be possible with the stepper. A light sensor behind a tiny hole in the altimeter face is one possibility, or an optical switch that is interrupted by the bellows linkage at the precise point at which 0ft/sea-level is indicated.

Matt

[SimSupervisor]

The below linkage had been severed, I'd think by corrosion. I still need to confirm the dial gears are functional. I am working on my tachometer right now, but this is next up my list.

You are right, it is going to require quite a small, tiny precision movement. I am pretty sure I will have to get some gearing going. Thing is, I have no local store where to get hobby gears.

[SimSupervisor]

after some cleaning, and trying to see if the first gear was stuck or what, it appears the gear is no longer on its shaft. I don't know if it happenned before I got it or is it me trying to see if I could make it turn, but its no longer in place.

I had tried prior to that to see if I could get into the gearbox, it is held in place by two brass screws. Thing is, one of them is stuck in place and is getting damaged by trying to unscrew it.


here's what the front plate just under the faceplate looks like. I decided to go ahead and just drill the bolt down, and am getting to the insides.

here's the inside of the gearbox:


plus two gears shown bellow. The small one goes on the right side, you see discoloration where it goes, and I believe the bigger one is supposed to be part of the small one, and give the below's access to the motion. Everything else works fine (except the middle needle, that would go with the below's gear)


Can a set of those two gears (in one piece), plus a gear that would fit the specs of the below's gear (and interface with the "faster" needle) be made/purchased somewhere?

Or should I simply gut it out and find something else? It woudl be a shame, as most of the gears are still functionning.

[hyamesto]

Simsupervisor: That´s it your name?
Let me disassemble my altimeter to better understand what is missing in your setup, but basically, you have a 1:10:100
gear train (1 is connect with 100 ft needle, 10 with 1000 feet needle, and finally, 100 with 10.000 feet needle).
I don´t think you can use that gear train. If your sim is based in a C-150, you can´t fly over 13.000 feet (at least in the real plane), and using this gear train with a stepper, to calibrate to zero, you need at least 100 or 200 turns of the 100 feet needle.
Another thing: the altimeter that you have got has a max operation range of 20.000 feets. (no need to use the full scale of the 10.000 feet needle)
There are better solutions.
I´m building my instruments from raw materials, cutting gears in acrylic, others pieces with plastic or MDF with a CNC laser, and using steppers and servos. (cheap service from a laser CNC shop near my home)
For the altimeter, i use 1 stepper for the 100 feet needle, a gear reduction of 1:10 for the 1000 feet needle, a servo for 10.000 needle, and another servo for the kollsman setting, drive by an encoder. To zeroing, the altimeter needs to turn 10 or 20 times the 100 feet needle when i turn on the control card, before start Flight Sim.
Regards.
Horacio.

[SimSupervisor]

That sounds interesting, but I don't know that I can accurately print gears... I hope to find somewhere to buy what it takes to make a functionnal gearbox.
But I have to agree, this gear train is not in good shape, I might have to use something else.

I did complain to the used aviation parts supplier where I got this altimeter about the poor state of some of the instruments he sold me (this wasn't the worst, you should see my airspeed and my vertical speed indicators!) and he said he would send me other instruments soon. I hope it includes a replacement altimeter. I wasn't expecting to get that much corrosion inside the instruments. Aren't they supposed to be sealed??!

Regarding the kollsman indicator, I am not sure to follow you... isn't the Kollman setting an input from the instrument to the flight sim? Maybe I completely miss the point, but normally you would setup the reference barometric pressure and leave it as is, which in turn would infere a proper value to the instrument, to display as accurately as possible the altimeter....

I was thinking of reusing the current knob and disc for display, and interface a potentiometer to the knob's gear for input trough the Arduino, letting the software know what is the current Kollsman setting.

[hyamesto]

Tomorrow i will try to show you my proposed solution. (If i have time...), and why i am using an encoder and not a potentiometer to set the kollsman, as well a test to understand that...
About the corrosion, the real instruments are not sealed. They are interconnected with plastic tube to a static port open for sensing the pressure surrounding the plane, and to the pitot tube.
The system must use a moisture trap, and regulary, you must purge the condensed water with a valve.
If the plane which took the instruments, was flying near the sea, the salt is very corrosive, or maybe, the plane crashed on water, and the instruments are discarded.
By the photos, all the instruments you have, seems to be scrapped, and sold as ornament. (like mine instruments)

In advance, check this post for your others instruments to come....

http://www.mycockpit.org/forums/showthread.php?t=26848

As you can see, based on reply of your posts, very few people are using real instruments, due to the complexity of adapting to a software Flight sim.
And when you get the NAV instruments (VOR, ILS) to set the OBS, everything done up here, will be the easiest part....

Regards.
Horacio.

[SimSupervisor]

Less people working with actual instruments is yet another reason to post about it! Share the knowledge!!!

I have a Narco VOA-4 VOR/ILS Nav converter/indicator that I might try to interface later. Its all transistor circuitery from the 70s, and wasn't anywhere near being used on a cessna 150 that I base my cockpit on; so I am uncertain if I'll use up one of the empty instrument bay for that, or if I'll use its chassis for another instrument (I badly need an attitude indicator)

in the meanwhile, I am out of luck for gears on my altimeter, so far...

[hyamesto]

It´s more easy to build a 737... a couple of monitors behind a panel..........
I'm short on time, but wait a few days while I get a good video camera to show you what I have build and how is working with Flight Simulator.
And i will give you some ideas to replace the instruments.
My collection of real instruments is ornament now, only buyed to see the internals and to study to interface (like your tachometer) or find a way to replicate with scratchbuilding ones. (The altimeter is the next...)

Don´t hurry, building my cockpit took to me 10 years, and never is finish.

For example, i made an Attitude Indicator, or Artificial Horizont, simple like the original cessna, and weeks laters, a friend give me a real broken one from an Aero Commander 500, with Flight director needles and two warning flags (gyro and steer). I want to remake mine, adding the Flight Director and the flags too..
Wait for photos and videos.

Regards.
Horacio.

[SimSupervisor]

I found a clock mechanism that I can dismount to have access to triple centered shafts if I can't manage to work the original gearbox.

here's what the clock shaft gears look like on the top/below separation plane inside my altimeter. I would modify the gearbox casing to hide my gears and still use the Kollman dial, and will have to make the clock shafts shorter (not an issue), but could manage my three needles from there.







I still need to manage my own gearbox to get the 1:10:100 ratios.


How do you manage your own gears?

I can't wait to see your instruments in action.