Smoke Pump Controller

Obviously I have too much time on my hands.

While reading some posts on Van’s Air Force about wiring up smoke pump control switches, I thought there must be a better way than most of the ideas presented. I know if I were putting smoke in my airplane (and I fully intend to do so), I’d want an ARM switch and a pushbutton on the stick or throttle. What I would NOT want is to have to keep my finger (or thumb) on that switch all the time.

So, I designed a little circuit board with an ultra simple microcontroller to act as a multi-mode switch. It does everything I want it to do…

  • An indicator that uses a bi-color LED to show the ARMED / ON status
  • On/off toggle action with a simple “click” of the pushbutton
  • Constant ON mode while the pushbutton is held
  • Sufficient to control a 10 A pump with 100% duty cycle
  • Easy and intuitive to use
  • Simple and inexpensive to produce
  • Small and light weight, easy to mount wherever there’s space

The switching MOSFET was really the only part I was concerned about. I contacted Marvin at Smoke System Helper, who helped with testing to validate that it will drive a 7A pump constantly with no detectable rise in temperature. All the parts are rated for 10A or more, and the MOSFET has a ridiculously low Rds(on), less than 4 mOhm at the 5V gate drive voltage used. It’s rated for 160A at 30V, so it’s snoozing. The max current limit is actually imposed by the terminal block.

Overall I’m very pleased with the result. The case is not perfect — no mounting tabs — but really, the thing weighs so little that some double-sided foam tape will hold it in place, maybe with a zip tie for high vibration environments. My plan for the Celebrity is to tuck it behind the panel.

Tempus fugit

Another year draws quickly to a close, with what feels like not a lot of progress on this project. I have a tall stack of ribs, but really nothing else. I had hoped to have a good start on the tail surfaces by now, but haven’t even started on them. We’ll need the basement for a family Christmas party, so no laying a 4×8 sheet of MDF over the island down there to start laminating pine. No worries, though. I have located some nice clear pine boards at Menard’s that will rip down nicely into 10′ long lamination strips. I have an order of Finnish birch ply queued up on Aircraft Spruce’s web site, ready to place the order once I’m sure I have all the stuff on there that’s going to be expensive to ship. As soon as we’re finished with the post-party cleanup, I’ll have the basement available to work in through the rest of the winter since the garage will just be a bit too cold for epoxy.

I also got a great idea from one of the guys on the Biplane Forum. I’ve been pondering how to store the wing ribs. I think I’ll build short pieces of main and rear spars out of pine and non-certified plywood, just to assemble all the ribs and clamp them together. That way I can surface sand them all together, and maybe if I’m lucky use that assembly to run them through the table saw for trimming as well. Maybe.

Musing.

Haven’t done much in the past few days, mostly because I’ve had the workbench cleared off to fix a recalcitrant robot vacuum.  I did unroll some of the plans and take a good long look at the tail surface sheet.  Sixteen feet or so of paper, mind you…  not the easiest thing to manage on a 6′ countertop!  I wish that had been split into two sheets. 

It looks like I can build the entire fin and rudder out of pine and plywood, so that’s my next project, I think.  I’ll go look for a suitable 10′ length of pine at Menard’s that can be ripped down into decent quality lamination strips.  Once the saw is set up for that I’ll make as many as I can, since I’ll have a bunch of other laminations to do as well for the wingtips and fuelage.  Then I’ll look at what to do for the elevator and stabilizer spars. 

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

Slow progress

I’ve knocked out a few more aileron ribs, two at a time.  I’m about halfway through them and trying to speed things up a little, so I don’t die of old age with a half finished airplane.

Yesterday I decided to use up a piece of obviously bad capstrip Aircraft Spruce saw fit to ship me.  This piece has a large chunk missing out of one edge, part of a knothole or pitch pocket or something.  Part of it is in no way suitable for aircraft use or much else for that matter.  But – there’s enough good wood there to use it for false ribs, so I made one of those.  That went OK, but it’s apparent that I will need to soak the top capstrip in HOT water for the false ribs.

I’m looking forward to starting work on the tail surfaces.  I’m planning to get out to the garage and clear off the workbench this week, lay out the plans and see exactly what I will need to get started.  The wood called out is white pine, so I’ll start checking the local places for suitable pieces of white pine or Douglas fir…  a little heavier, but I know Menard’s sells some good boards from which I can cut suitable pieces for the laminations.

What next?

It’s a little early for this debate, but given the long lead times for ordering wood…  not too early.  I could possibly be done building the wing ribs in a month or two, if I get on it with a purpose.  If I’m going to need a bunch of spruce, I probably need to get the order in now.  So the question becomes — what next?

  • I could build the wings.  It looks like the wood for the spars will cost me around $8-900, plus substantial shipping.  I’d also need to extend my workbench by at least 4 feet, so that’s another project in itself.  But I’d have wings, man, which would look bitchin’ cool hanging in the garage or hangar.
  • I could start on the fuselage.  It’s a big piece that would be a huge leap forward.  I haven’t calculated the wood cost yet, but it’s a significant amount of birch ply and spruce.  Again, I’d have to build another workbench to extend what I have by 8 feet or so (and rearrange the garage).
  • I could start on the tail feathers.  I could do all the work on my existing workbench.  After looking at the parts list, most of the wood specified is pine; I’d need some aircraft ply as well.  The elevator & stab. spars are specified as spruce, but I could source suitable quality Douglas fir locally and substitute that.  I’ve seen boards at Menard’s with growth rings and grain slope that meet specs, with enough defect-free wood to be usable.  That means all I would need to ship from Aircraft Spruce or Wick’s would be a couple sheets of plywood.  Plus, building the tail would give me some valuable experience with large/long glue-ups that I’ll need to do for the wing spars, and if I screw it up it’s relatively cheap pine instead of very expensive spruce.

So…  tail feathers it is.  Now I just need to consult the plans and see what size sheets of plywood I will need to order.

Strength vs. weight vs. work (rib geodetics)

While waiting (still) for my spruce to be shipped, I did some figuring…

Building from plans means not getting the wood factory cut and grooved.  There are just some pieces that you can’t buy off the shelf.  The Celebrity plans call for rib geodetic braces made of 3/32″ x 3/8″ Sitka spruce.  ACS and Wicks sell spruce capstrip down to 1/8″ thickness, but not 3/32″.  So the choices are to plane or sand down the entire length of the 1/8″ x 3/8″ strips, or use them as is and just sand the ends to fit the grooves in the top and bottom capstrips.  So how much extra weight will we end up with if we just use 1/8″ thick pieces?  It’s 1/32″ thicker, so we’ll do the math…

1/32″ x 3/8″ wide x 1464″ of capstrip = 17.25 in3 of extra spruce.  Sitka spruce weighs about 28 pounds per cubic foot (or 1728 cubic inches), so 17.25 / 1728 = .009928 ft3 x 28 lb gives us a little over a quarter of a pound, less whatever gets sanded off on the ends and scrap, etc.  So…  around four ounces or less; I can live with that.  Still, I’m thinking I may set up a spindle sander with a 3/32 gap to do the ends.  If it works well enough I could run the entire lengths of the strips through it, then so much the better.  Of course that would probably mean having to buy a new power tool.  Oh, no!  🙂

 

Flying wires vs. lift struts

I emailed Dave at Fisher Flying Products asking about the lift struts I have seen used on several Celebrities that I’ve seen in pictures.  I still have not seen an actual Celebrity “in person”, nor any other completed Fisher design for that matter – just a partially built single seat ultralight at Oshkosh.  It seems you have a choice between lift struts or flying wires.  Personally, I like the look of flying wires a lot better.  It just has that classic wire-braced biplane look.  I know that a cylinder shape, such as a wire, has much higher drag than streamlined tubing.  A cylindrical object will have 10 times the drag of a streamlined shape of the same frontal area.  The wires, however, will be much thinner than struts, 5/32″ — meaning that they would produce drag roughly equivalent to 1-9/16″ wide streamlined struts.  BUT…  the plans call for 1-1/8″ and 1-1/4″ round tubing for the lift struts.  The flying wires would have far less drag than those.  I could probably replace the round aluminum tubing with smaller streamlined steel and pick up some drag reduction there, but the fact remains — I just don’t like the look of the lift struts.

It’s not like this airplane will be a speed demon no matter what I do, so I’m not really worried about what may be a small drag penalty for the wires.  If I wanted to fly faster and more efficiently without regard for anything else, well, I have an RV-12 for that (acknowledging that “faster” is entirely relative here).  It looks like I can have the wires made to my specs by Aircraft Spruce, swaged and tested with professional equipment so I don’t have to worry about getting it perfect on my first try.

It’s official!

I received a quart of T-88 epoxy yesterday, so the first batch of glue is covered.  Today the mail carrier brought the plans, tightly rolled and somewhat bent from their trip from the nether regions of Canada.  I will be building Celebrity serial number CE164.  So now I have a number of jobs that need to be completed…

  • Get the plans unrolled to flatten out so they’re usable
  • Get an inventory of the drawings, figure out how and where to store them so that they’re protected.  I may add tabs or some sort of externally visible identification so I’ll be able to quickly locate a drawing.  One of the irritations encountered while working on the RV-7 was sorting through a pile of drawings to find the one I was looking for.   That was a time waster.  These drawings are in at least four or five different sizes, a few of them 16′ long.  Fortunately, it does not SEEN as though I’ll need to frequently switch between several drawings as I did while building the RV.
  • Replace the workbench top with fresh MDF.  It’s a good solid bench, but the old hardboard top is scrap.  As a bonus, a 4×8 sheet of MDF will be one foot wider than the bench, giving me a 1×8′ cut-off.  That will make two 1×4′ wing rib construction jigs.
  • Figure out where to begin construction.
  • Order up some wood!

I’ll start with the wing ribs.  They use only two sizes of stock, and construction is simple and repetitive.  I can build the first two or three out of locally sourced pine, just to get the jigs built and get my process sorted out.  I can scrap them or use them as wall decorations in my office.  The jigs do not need to occupy the entire work bench, so the bench can be used for other things while working on the ribs.

Sourcing wood

While waiting for the plans from Fisher, I asked for and received a PDF copy of the bill of materials for the airplane.  While I’m sure there may be some errors and omissions, it’s a good place to start in my efforts to obtain the materials I need to start building.

Much of the wood is aircraft grade Sitka spruce (naturally), and that I’ll be buying from expert and trusted sources like Wicks and/or Aircraft Spruce.  There are a few (very few) other sources for aircraft spruce in the country, but those two seem to be the biggest and most readily available.  There certainly aren’t any near here, so no running down to the local supplier with the truck to pick out my own bits and pieces.

There is, however, also quite a bit of pine used in areas where spruce is not required and there is some money to be saved.  A good little bit of money, in fact.  For instance, the outer frames of the fin, stabilizer, elevators and rudder are all laminated from 1/8″ x 3/4″ pine, as are the wingtip bows.  You could use spruce as well, of course, but spruce doesn’t come cheaply.  Just the pieces to make those laminations would cost over $155, plus truck freight, from ACS.  Wicks seems to be a little cheaper for those pieces, but still well over $125 plus freight.

Finding clear, straight- and tight-grained pine won’t be easy, nor the boards cheap – compared to the so-called “stud grade” garbage typically sold at big box stores.  That stuff is mostly more suited for a pulp mill than anywhere else.  I’ll have to re-saw any pine boards I do find down to size, probably recovering only a small fraction of the wood as usable stock.  Still, even if I could only get a few strips (these are 1/8″ x 3/4″, remember) from a typical select grade 1×6 from a local lumber yard, it could still cut the cost down by  a hundred bucks or so.  So, I think I’ll visit a couple of the local lumber yards to see what I can find.  I have the option of slicing pieces from millwork like baseboard and flooring too, so I’m hopeful that I’ll be able to find something.  I printed a chart to help me quickly figure out acceptable grain slope on two axes.  That and my Incra ruler should help sort out any good boards I may find.

The worst case would be not finding anything usable at all.  That just means I would instead use aircraft grade spruce for all the parts called out as pine in the plans.  It would be somewhat more expensive, but certainly no compromise of quality, weight or difficulty.  I figure about an extra $700-750 added to the build cost if I have to go all spruce, maybe a bit more or less.  It’s not enough to derail the project, but it is enough to see what I can find and maybe make some sawdust.