Author: dale

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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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Not entirely dormant

I haven’t posted any updates to speak of this month.  I did get out to the garage and spend a couple of hours sanding down rib geodetic stock..  What a pain in the ass.  I really wish I’d gone a different route than I did on this stuff.  I just don’t know what that route would have been.  Anyway, today I started taping stacks of geodetic strips together and cutting braces.  I’ll do as many as I can before starting to build ribs again, so hopefully when I can return to building I can get some momentum going.

I have to be honest…  I look at some of the single-seat parasols and think, “Hey, I could be building half as many wings…”  But I just need to stick t this.  It will be worth it in the end.  I’m working on an idea to semi-automate the sanding of the geodetic stock, I just need to fabricate a couple of guides and use a stepper motor to feed the stock through my sanding rig.  That could increase throughput and give more consistent results, while I get other things done.

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Pause button

I haven’t had much opportunity to do any construction for a few weeks now.  The garage is a disaster with a remodel project going on, and I’ve been busy selling my side business.  Hopefully in another week or so I’ll be able to get back to it.  The first order of business will be to try out the steamer to soften up capstrip for the ribs.

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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.

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Fixin’ and cookin’ ribs

Last night I glued the two separated ribs back together.  I cleaned off/out all of the old epoxy, sanded everything clean, mixed up some new T-88 and brought them in overnight.  Just to be extra sure of a good glue cure I’ve got them warming up under a heating pad.  I took one of the others to the EAA chapter meeting, no one seemed to see anything wrong with it…  although the reaction varied from “Wow, that’s cool” to something closer to “You’re nuts, why would anyone do that?”

It’s cold, I’m not looking forward to the next session of shaving down geodetics for the next batch of ribs.  I’ll have to go out tomorrow and do it though.

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Second guessing

Not me.  I made my choice and I’m happy with it.  But I swear I have not had this much well-intentioned second guessing since I announced my engagement to the woman I’ve been married to for nearly four decades.

“You should build a Hatz.”  No, I love the look of the Hatz but it’s not LSA.  “Oh, a Hatz Bantam.”  Nope, sorry.  I’m tired of aluminum, it’s got a welded fuselage (which I did not want to build) and at the time I was looking was designed ONLY for a Jabiru engine, nothing else.  “Oh, you should build a Murphy Renegade.”  Ummm, nope.  Two-strokes are simply not an option  for me.  And on and on, with every non-LSA, or steel fuselage, all aluminum or single seat variation on the planet mentioned.

“Oh, just go find someone’s abandoned project.”  No thanks, I think I can do fine at creating my own problems, no need to try to find what someone else did wrong or “different” along the way.  I’m not traveling the country inspecting half-built (or wrecked) airframes to see if they’re suitable.  And would YOU buy that plane that was built but has been sitting for a few years, and has never flown?  Maybe there is a reason it’s never been given an airworthiness certificate.

Holy crap.  Can’t a guy pick an airplane and build it without everybody on the planet having a better idea?  No matter.  I’m building on.  I learned my lesson the first time around.  It’s really easy to get wrapped up in so much well meaning advice that you spend a few thousand hours on something like this, and never finish it.

 

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A little setback

This morning I went out to check on the two ribs I had glued last night.  The epoxy was set, but just a little “tacky”.  It’s pretty cold out in the garage, as the outside temperature has dropped into the 20s and low 30s.  I figured it was cured enough to be OK, and pulled the ribs from the jigs so I could compare the first rib out of the new jig to the others.  It’s a perfect match, so I set them on the bench and went inside.

A couple of hours I went out again to do some cleanup.  I found the two new ribs had pulled apart at the leading edge!  The glue was still pliable enough that the “spring” from the wood very slowly (judging by the long strings of epoxy still joining the pieces) pulled them apart from the main spar forward.  Fortunately I caught it before the glue had completely finished curing.  With some wiggling I was able to remove the geodetics from the capstrips and clean off as much of the excess epoxy as possible without carving into the wood.  I put the ribs back into the jigs, and will re-glue them tomorrow.  I’m pretty sure I can salvage and repair these two, but if I’m not 100% confident in them they’ll be scrapped.

I really want to figure out an arrangement to steam the capstrips.  Soaking in cold water doesn’t really seem to do much good, which is why I had not pre-bent these.  Trying to soak them in hot water works for about 10 minutes until it’s no longer hot water.  Two out of the six or eight I’ve bent in my former have split during that process.

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Second jig built

Yesterday I did a little shop cleanup.  Not a lot, just enough to get done what I wanted to do.  I cut a length of 3/4″ x 3/4″ pine and cut a bunch of 1-1/2″ blocks for the second rib jig.  I got the work bench partially cleared off.  The box from the R/C Cub went underneath, and the plans sheet and wings were moved to one end of the bench.  I now have roughly half the bench covered by model airplane wings, while the other half is being used to build the real thing.  ‘Murica, as my kids would say.

So, I built the second wing rib jig.  I used the first rib from the first jig and blocked everything up.  I used some plastic sheet Lisa had picked up underneath it to prevent the glue sticking things together.  When it was all finished, I mixed up 20cc of epoxy and glued up two ribs.  That’s going to work as long as it stays cold, but the glue was gelling by the time I was finished.  If it warms up at all there won’t be time to do two at a time, unless I can find a way to pre-glue the capstrip grooves a lot faster.  I wonder if thinning out a portion of the T-88 and brushing it into the grooves would work.  For that matter, I wonder if pre-gluing the grooves is even necessary.

This morning I pulled both ribs from the jigs.  You can’t tell them apart, and you can’t tell them from the other four I had done.  Any differences between the six ribs I have done are slight enough they can be removed by sanding.  So, now I can double my production rate.

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More ribs

Rib #2 matches rib #1 to within plus or minus a millimeter or two everywhere.  I spent some time with an X-Acto knife and sanding block cleaning the excess epoxy off of the first rib, so it’s looking pretty good now.  Tonight I put #3 in the jig and glued it.

I was planning to make the second jig tonight also, but that’s going to just have to wait a little while.  The garage is getting too cluttered to work in.  I’ve got numerous tasks partially completed, and it’s a real mess.

  • I need to finish grooving the capstrips, first off, so I can use the table saw for other things. (done!)
  • I need to get the rest of the geodetic stock shaved down to final size.  I got my strip cutting jig from Rockler today, so we’ll see if the table saw can be used to help with that or not.
  • I need to cut more wood blocks for the jig.  With the spindle sander, I can make sure they are perfectly square. (done!)
  • There is some cleanup to be done, including two workbenches that are piled high with so much crap that I’m working with a rib building jig hanging over the sides on one end of the bench.  It’s an eight foot long bench and I’m using less than a foot of it.  Ridiculous.
  • I will need a place to store these ribs so they don’t get damaged.  And I definitely have to find a better solution for the geodetic braces.  The paper cups I have been using were a terrible idea.  They take up enough room that they’re a pain in the ass to use, and they tip over, fall off the bench and spill parts everywhere at the slightest bump or touch.
  • I have the table saw and spindle sander sitting halfway out in the middle of the floor, need to get a permanent location figured out.

That will at least get me to where I need to be to keep building ribs.  At some point I’m going to need another eight feet of workbench, but that can wait until I have the ribs and tail feathers built.  Pretty sure I can do all that on the 3 x 8 that I have now.

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#2 rib glued

I spent some time cutting geodetic braces.  I stacked five of the thin capstrips, laid out the parts along them, and cut them with the bandsaw.  I now have enough parts to make half a dozen more ribs.  It’s slow going with the geodetic capstrip, because each one has to make three passes through the spindle sander to shave them down to 3/32″  Holy crap that’s tedious.  I may try using the table saw when the thin strip rip jig arrives.  Even if I can get them close enough to just make a single pass on the sander it would be a huge improvement.

I tried a different technique for applying the glue.  First I dry-fit all of the parts to make sure I didn’t end up with a brace with glue all over it that needed to be trimmed.  I marked the top and bottom capstrips with a pencil to indicate where the groove needs to be glued.  Then I pulled them out of the jig and used a trimmed-down popsicle stick to apply epoxy to the grooves.  Back into the jig, with the front and rearmost vertical pieces in place since those are impossible to install with the capstrips in the jig.  Then I proceeded as usual, installing all the other parts with epoxy.  It worked like a charm, and resulted in no glue running down where I didn’t want it.  We’ll see how this rib looks when I pull it out of the jig tomorrow…  and we’ll see how closely it matches the first one.  In theory they should be identical, but I’ve  never done this before, so…