Converting the 30″ Rubber Powered Dumas Piper J-4 to Electric RC – Part 3

Converting the 30 Rubber Powered Dumas Piper J-4 to Electric RC – Part 3

During our last session in the shop I finished building the basic structural components…wing panels, fuselage and tail surfaces…of the Dumas Piper J-4. This time we are going to fill in all those remaining details like mounting the motor, ESC , receiver and servos, hinging control surfaces, adding control connections and so on. When that’s finished the model will be ready for covering, which will be the subject of the next installment.

Last time I made up and installed those 1/4″ x 3/8″ balsa inner side rails that are slotted to permit both the servo tray and the motor mount tray to slide into place (and out again). I also cut out and assembled the 1/16″ plywood motor mount plate to the motor mount tray, reinforced it with two triangular 1/8″ balsa gussets, and then mounted the motor as you can see here using 3/8″ x 2-56 screws and ordinary hex nuts that I ZAP’d to the rear face to keep them in place. “Traditional” blind nuts would have been too bulky too fit easily. Off-camera I had to go through the drill of lining up the Cobra outrunner motor on the horizontal (thrust line) and vertical (centerline) reference marks I had already drawn onto the plate, and then double checking that the cowl would line up with all of it.

 

This is the motor mounted in place on the mounting plate/tray and slipped all the way back into place in the fuselage. Notice that I have made a cutout in the mount plate to permit the three motor power leads to fit around it neatly.

 

Now I have to create a way to make the motor tray assembly removable. I drilled a pair of holes in the fuselage (boot cowl) sides to permit a piece of 3/32″ O.D. aluminum tube to pass all the way through/across the fuselage while barely touching the upper face of the motor mount tray. I cut two short lengths of that tube just long enough to reach from the outer face of the boot cowl to the inside of each of those slotted rails, then slipped them into place over the “pass-through” wire you can see and aligned each into one fuselage side with the inner end flush with the slotted rail.

 

I left a bit of each piece of tubing protruding outside so I could fast-ZAP it with the wire still in place for alignment…without risking gluing the wire into the tube.

 

With the pass-through wire slipped out, this is what it should look like from the inside. See how the 1/16″ plywood motor mount tray is going to slip in and out without interference from those fixed parts of the aluminum tube?

 

The next step was to cut another piece of that 3/32″ O.D. tube long enough to reach across the opening where the motor tray will go and remain just clear of the fixed tubes at either end. Here I’ve replaced the pass-through wire to hold it in position.

 

Same game, except that now the motor mount tray is back in place. With that inner length of tube riding on the carry-through wire and resting lightly against the upper face of the tray, now I can use the tip of a screwdriver (or some other convenient tool) to accurately place a generous bead of thick ZAP all along the inner tube to bond it permanently to the tray.

 

When I remove the carry-through wire and pull forward on the tray/mount assembly it comes out and looks like this…

 

…and then I can put the whole deal back together whenever I want to. I bent over 1/8″ of one end of the wire to form a handle…the other end is cut flush with the outside of the fuselage. Later on I’ll work out just the right tension/drag on the wire to keep it from slipping out when I don’t want it to. With the wire seated in place like this the latch assembly will easily handle full throttle thrust and keep the motor assembly firmly connected to the rest of the airplane.

Speaking of the motor…not so long ago I got an email reminding me that I haven’t yet given you any specific info on the motor, ESC, servos etc. that I’m using. I’d planned to do this as part of the final assembly blog, but I can see that a lot of you might like that data now! The motor is a Cobra 2204/40 outrunner and the ESC is a Cobra 22 amp model with BEC. All the servos are Airtronics 94802’s and the receiver will be an Airtronics 92064 RH-61H 6 channel 2.4 Ghz. The battery pack (at least to begin with) will be a Venom 850 20C 3S LiPo.

 

Now I can fine-tune the shape and finish of the front cowl. Here I have made another of those custom sanding tools by shaping a piece of hard balsa to fit the curve I want the cooling air openings to follow and ZAP’d a piece of 320 grit paper to it. This part of the job demands a LIGHT TOUCH…take your time and you’ll be able to do it, too.

 

I have a couple pieces of masking tape on the opposite side holding the cowl in alignment while I drill for the little screws that are going to hold it in place as a part of the motor plate assembly. Removing that carry-through wire and then pulling forward on the assembled cowl will permit the entire assembly to slip forward and out for battery changes or maintenance. Normally the cowl will stay attached to the plate unless I need to take IT off to get at the motor.

 

Now I can do some careful finish sanding of the nose cowl to be sure it aligns exactly with the boot cowl (and the rest of the airplane).

 

Let’s work on the other end! Too late, I discovered that the sheet balsa center section of the fixed horizontal stabilizer was not wide enough to extend past the fuselage stringers. It must do that to provide an anchor for the inboard end of the covering tissue, so I added some extra 1/8″ balsa sheet and sanding it to match the existing surface. Now I’m marking a centerline and edge-guide lines that will permit me to assemble the stabilizer squarely into the pre-existing slot in the tail.

 

BUT…I won’t trust those guide lines on their own. I’m using the 90 degree corner of an ordinary drawing triangle to double check that alignment.

 

When I’m sure it’s right, I use a couple of pins to hold it temporarily in place and then apply a generous shot of thin ZAP to lock it all up tight.

 

I assembled the vertical fin the same way. Now that the little curved dorsal fin fairing on the leading edge is glued securely to that top center stringer, I can trust it to handle the stress of sanding to a finished shape and radiused edge with another custom sander.

 

Here’s the whole fixed tail surface assembly from the other side.

 

Back to the wing panels again. A while back I left the ailerons and the shaping of the aileron wells ALMOST finished. It turned out that I needed to cut the concave aileron well faces a bit deeper, so I made another custom sanding block and took out about 1/16″ more balsa from the center of the curvature.

 

Now that the ailerons fit, I’m going to set up the hinge installation. On the full scale J-4 the hinge centerlines are set back from the leading edges of the ailerons to get the center of rotation inline with an imaginary circle that defines the leading edge cross section. There is a rectangular cutout in the aileron for each hinge, and I have reproduced them here using that Paul Matt scale drawing as a reference. Here I’m using a 1/16″ drill bit in a pin vise to open holes for the 1/2A size Robart Hinge Points I’ll be using instead of flat tab hinges because they most closely match the full scale mechanical arrangement I’m trying to represent.

 

This is what the hinges look like dry-assembled in place. I’m going to cut off some of that extra length later.

 

I need aileron control horns that will fit this little (30″ span) model, and that means SMALL. Cut off servo output arms make excellent control horns for an installation like this. Watch…

 

I cut a slot in the aileron leading edge to match the cut-off end of the servo arm/aileron horn. You can see how the horn is going to slip right in place.

 

A generous bead of slow ZAP in the cut-out slot will form a neat bead around the nylon horn when it’s pressed into place and given a shot of ZIP KICKER. For an airplane this small and light that glued horn-in-slot attachment is plenty strong enough…BUT…I wouldn’t trust it on a larger model beyond, say, 1 1/2 to 2 pounds.

 

You may recall that I plan to use an aileron servo in each wing root, to be connected to the ailerons using small, flexible cable-in-tube assemblies. I installed a plate of 1/16″ balsa to create a control exit plate drilled to fit the yellow outer tube, and then fitted the tube through it and the 1/8″ holes I drilled in the appropriate ribs. Slow ZAP fixed the end of the tube to the inside face of the exit plate (I also ZAP’d the tube to each rib in turn so it won’t be able to shift around onside the wing under control loads and spoil the accuracy of my control inputs).

 

I cut an opening in the bottom 1/32″ balsa wing root sheet large enough to permit one of my Airtronics servos to lie flat on its side inside the wing. The servo is going to be stuck to a 1/64″ plywood plate that fits the opening, and the plate needs a ledge, or shelf to rest on. I cut several 3/16″ wide strips of 1/64″ plywood for that job.

 

They go in like this… I needed a very delicate touch for this job, so I gently pricked the piece of 1/64″ ply with a No. 11 blade just hard enough to let me lift it and hold it in place while I ZAP’d the joint.

 

Here are both wing panels with the servo openings framed to accept the servo mounting plates.

 

I’m using a “Goo” type adhesive to make a semi-permanent joint between the servo and its mounting plate.

 

All set up it looks like this. I have soldered a threaded brass mini-link end to the inner end of the aileron flex-cable, and threaded the nylon clevis into place. Here I’m using the little L-bend wire keeper that’s provided with the mini-link to lock the clevis to the servo arm. NOTE that this is a very tight installation, and I have trimmed the servo output arm as far back as I can to get the finished linkage assembly to fit into the wing. Watch…

 

I aligned that wire L-keeper and used a short piece of heat shrink tube over it to keep the clevis in place. With the other end of the flex cable already threaded into place in the yellow tube, you can see how the linkage-servo-mounting plate assembly is going to drop into place in the wing. I’ll wait to install these permanently until after the covering job is finished.

 

Back to the tail again. I’m using a 100-grit sanding block to put a radius on the leading edge of the rudder. I’ll do the same thing all the way around the laminated balsa outline. Tail surface outlines on airplanes like the Piper J-4 should always be given a 180 degree radius, never a taper. You are trying to replicate a tail surface made of welded steel tubing and covered with fabric. This balsa outline represents that steel tube.

 

On the tail surfaces I’m going to use ordinary DuBro small pinned hinges. On a thin structural edge like this I like to use a 1/32″ drill bit in a pin vise to define each hinge slot by drilling end point holes.

 

I CAREFULLY cut out the requisite hinge slots between the holes. If you are going to use mechanical hinges (instead of external plastic tape) to get a credible scale appearance, there is no substitute for the GENTLE touch you’ll need here. That comes with the territory of building very small airplanes.

 

Here I have the hinges installed in the rudder…note that what would have been protruding extra hinge tab material has been cut off along what will be the inside face of the fin trailing edge. The exposed surface (which would have “floated” inside the covered structure) would add extra weight but no strength. I have also added the rudder control horn, which I made from a servo arm and installed just as I did the aileron horns we already talked about.

 

This is the completed tail assembly with all the hinges dry-fitted into place. After the covering job is done I’ll neaten up the installation and close the gap between the surfaces to the correct scale distance.

 

Now it’s time for rudder and elevator control connections. I’m using a rigid wire-in-tube system here. Down inside the fuselage you can see the outer tube support made from a piece of 1/64″ plywood and drilled for the tubes which have been fixed in place with slow ZAP.

 

The tail end of that setup looks like this. After the covering job is done, I’ll slip the control wire into place from this end of the tube.

 

We also get to play with some nice fat (light) rubber wheels. I have bushed the wire axle with more of that aluminum tube to match the hole in each wheel. Here you can see how I’m using a couple of small washers ZAP’d in place to serve as an inner stop to keep the wheel aligned on the axle. I wanted to get all this work done before I do any covering/finishing on the rest of the strut.

 

She’s pretty much all built and trial-assembled, ready for the next step…tissue covering. I’ll disassemble everything before I start on that.

 

Here’s a look from another angle. The next time you see this airplane there will be a classic old-time dope-and-tissue covering job going on.

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Converting the 30in Rubber Powered Dumas Piper J-4 to Electric – Part 1
Converting the 30in Rubber Powered Dumas Piper J-4 to Electric – Part 2
Converting the 30in Rubber Powered Dumas Piper J-4 to Electric – Part 3
Converting the 30in Rubber Powered Dumas Piper J-4 to Electric – Part 4