Category: Avionics & Panel

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Compass overhaul

I think I’ve finished the compass overhaul. The rebuild kit arrived from ACS yesterday. After carefully cleaning the glass with denatured alcohol, I installed the new gasket and the glass. With everything reassembled, I filled it with compass fluid. I had about half a pint from a rebuild kit I had bought several years ago, when I came across an old Airpath compass that ultimately turned out to be an oddball military model for which there are no replacement parts or gaskets. It was leaky around the glass, and I eventually figured out that the cork gasket was slightly oversized. From what I can tell, it’s supposed to apply pressure around the edges of the glass to seal it against the inner black rubber gasket. It was getting squeezed between the case and face plate. After trimming about a millimeter or so from the outside, I finally got it to seal up.

Normally you would submerge the compass in a container of fluid to work out all the bubbles. In fact, the Airpath manual specifies submerging it and pulling a vacuum for a few hours to ensure there is no trapped or dissolved air. I didn’t go to those lengths. I was able to work all the bubbles out and get the fill (or drain) plug in, and so far there’s no sign of leakage.

The new LED lamp works great. In a dark room there’s a nice red light that makes the compass quite readable. I’m happy with it.

I’ll let it sit on the bench for a few more days to ensure there’s no leak and no bubbles develop. If I do see any air, I’ll submerge it in a container and do it their way. I figure it won’t be getting mounted in a panel for at least a year or two, maybe longer, so I’ll have plenty of time to validate the quality of the rebuild. For a total investment of less than $71 I’ve got a completely rebuilt, well lighted Airpath compass. That total includes an extra nine little LED lamps that I’ll use elsewhere; if you don’t count those it’s under $65 total. Not bad considering I see dried-out junkers going for more than that on Fleabay.

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Closing out 2023

You think you’re about ready to perform some major step, then you step back and realize you’re not as close as you thought… I thought I was about ready to cut out the aileron, but I still had to get the bottom geodetic braces in as well as several of the plywood stiffening strips that keep the aileron bay and aileron from distorting when the fabric gets shrunk around it. Today I did that, I also got the leading edge plywood wrapped around the rest of the way and glued and clamped in place, and added the last of the wingtip reinforcements along with the gussets for all of those.

I started to shoot primer on the torque tube, but the can stopped spraying about halfway through. Super frustrating, as it’s a full can, It’s been in a cabinet in the garage for a while, so I suspect there was a chunk of unmixed solid that lodged in the valve or something. The spray tip is clear, the can is pressurized, but it refuses to spray. Bummer. I found a can of self-etch primer in another cabinet at home, so hopefully that works. I may end up with a torque tube that’s half gray and half white… oh well.

I’ve been cleaning up the Airpath compass while I wait for the new gaskets and diaphragm from Spruce. This one has a light and I didn’t want to use the incandescent bulb, and I’d already broken one of the wires anyway… so that path was blocked. I had ordered some LED bulbs from Amazon – they are commonly known as type 74 LED bulbs. The spec sheet showed they are about 0.7″ long and 0.23″ diameter. Those arrived today, and they do fit inside the bulb socket. I soldered the wires onto the new LED assembly “backwards”, or with the wires running along the body of the light so the tip goes in first and the connector end faces outward. This puts one of the three chip LEDs directly over the little red window that illuminates the compass. I had to remove about 1mm of the tail end of the lamp assembly so the little cover would close, but with that done it’s near perfect. I’ll get more pictures as soon as the paint I used to touch up the worn spots on the lamp cover is dry. The gaskets are supposed to be here on Tuesday, and I’ve got a can of mineral spirits I can use to fill it… not being willing to spend the extra money on UPS shipping for a pint or so of “compass fluid”, which from what I can tell is basically… mineral spirits.

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Progress on aileron #1, and a compass

It’s been a while since I was able to make it over to Stu’s place. Between being sick and various Christmas related activities, I think it’s been close to 2 weeks since I got anything done.

Today I went over and got some more aileron work done. I cut the trailing edge on the outboard end, and glued the aileron end plates and the remaining plywood pieces in place. Everything lines up well and I’m looking forward to cutting the aileron out of the wing for the next steps.

I also picked up an Airpath compass on eBay. This one is a C2350-L4-23M, I think. It’s in need of rebuild, which I knew would be the case when I bought it. The compass arrived completely dry, but the card rotates freely with no sign of binding. The compensation mechanism is in perfect shape, markings are clear, lubber line is fine. With some new gaskets, diaphragm, and fluid it will be perfectly serviceable. Even the paint cleaned up nicely with a little naptha and denatured alcohol.

I ordered the repair kit and a couple of replacements for missing screws from Spruce, This one is lighted and is designed to use a little GE incandescent bulb, but rather than use one of those I think I’ll retrofit it with an LED. Of course I could just leave it un-lighted, since the plane won’t be equipped for night flight, but I figure I’ve got parts on hand and time to spare, so I may as well fix it up with a red light.

While I was doing the aileron work, Stu was building a table for a 4′ x 4′ CNC router that should be here in a month or so. He’s understandably pretty pumped about that.

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Score!

I haven’t been doing much on the biplane recently, but I did manage to score an altimeter for it.  I’ve been watching the for sale ads on line for decent deals.  Someone offered an altimeter that worked, but failed an IFR certification due to out of tolerance leakage at 15,000′.  Well…  if I ever do finish the bipe and fly it, it certainly won’t be at anything over 10K, let alone 15K.  So for fifty bucks it was a no-brainer.  It’s a pretty nice altimeter, not one of the super cheap imported junkers that’s being sold now.  It’s really more altimeter than I need… 

My “new” used Shinko Electric altimeter

So…  I have an altimeter, and a quad oil temp / oil pressure / CHT / EGT gauge for monitoring the engine.  I will still need an airspeed indicator.  Most that I see for sale have speed ranges not appropriate for the Celebrity — I really don’t need or want a 200 knot ASI.  Then I’ll need a compass, of course; a slip/skid ball; a tachometer.  Whether I need a fuel gauge or not depends on what I decide to do for fuel tanks.  I’m thinking no electrical fuel gauges, all float or sight type.

I could just ditch all of it and go with a single instrument from Dynon or Garmin, but I just think such a thing would look out of place in a classic biplane.  I’m not sure what I’ll do for the front cockpit, though.  It would be nice to at least have airspeed and altitude there for passengers, but I’m not sure I’ll be able to score a deal like this again.  There’s always UMA…

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Sensor scored!

An update on the oil pressure sending unit.  I had found exactly one of the correct sending units for sale, a NOS Honeywell part that was listed on eBay for a couple hundred bucks.  I didn’t buy it, but I was watching the listing.  They dropped the price, I made a lower offer and they accepted.  I ended up paying roughly what a normal oil pressure sending unit would have cost from a source like Aircraft Spruce.  And, it’s got the exact output that the Westach quad gauge needs.  Now I won’t have to build a conversion unit but can just wire this one up directly.  These transducers are no longer in production, and if you can find one you’ll pay 8-10 times what I got this one for.  If it ever fails, I’ll probably replace it with a cheap one and a converter board — now that I have that figured out.

Update 5/4/17:  Transducer arrived, NOS as advertised with a nice sealed electrical connector.  This thing will likely outlive me.

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Some advance planning

A while ago, I picked up a used Westach quad gauge.  it looked like a great deal…  oil temperature, oil pressure, CHT & EGT in one round gauge.  What I found was that it’s an older gauge, though in excellent condition — almost like brand new.  The only problem is, it’s designed to use an oil pressure transducer that is no longer produced.  0 to 100 PSI, with an output of 0 to 50 mV.  You can get transducers like that, but they cost a few hundred bucks.

Fortunately, there are other transducers that are dirt cheap ($20 and below)   that can be adapted to work with the Westach gauge.  I found some that have a 0-5V output, and looking at the spec sheet they actually output 0.5 to 4.5V.  They also require a 5V supply, and I think the gauge supplies 12V nominal.  So…  we have two tasks.  Supply the transducer with 5V regulated DC, and convert the 5V ouptut to 50mV.  Easy!

Below is a schematic for a little interface board to do the job.  A common 78L05 regulator supplies power to the transducer.  A simple resistive voltage divider does the 100:1 voltage conversion.  I used a 100K Ohm fixed resistor, and a 2K Ohm trimmer.  That way you can calibrate the divider to account for resistor tolerances.  Apply 2.5V to the transducer side, and adjust the gauge side for 25 mV.  There will be some non-linearity near the high and low ends of the scale, but that’s OK in this case.  We don’t need an absolutely accurate oil pressure indication, more of a relatively good indication within a reasonable range.  once you get very low or very high, the exact numbers aren’t really important.

Really, it was a great deal.  A new similar gauge would cost over $800.  Individual small gauges to do the same jobs would cost several hundred and take up a lot more panel space.  I’m pretty happy with this one.