Converting the 30in Rubber Powered Dumas Piper J-4 to Electric RC…Extra Details and Flight Report

Converting the 30in Rubber Powered Dumas Piper J-4 to Electric RC…Extra Details and Flight Report

I’m going to begin this flight report by catching up on some of the last-minute details I don’t want to miss telling you about, as well as by sharing a story about this airplane that might at first seem to be more about not flying a new model than getting the results of your latest building project into the air the way you always intended it to. Let’s begin with that.

Remember Installment No. 4 of my Master’s Workshop column/online blog that pretty well finished up the description of how I modified the kit design and built the model? There was a lot going on. I was beginning to explain how to get that sort-of-intricate nose assembly (designed to provide easy access inside the airplane without spoiling its scale appearance) apart and back together again at the field to install a freshly charged LiPo and get checked out for another flight.  Well, all that worked exactly as designed, but…when I slipped and slid and pressed it all back into place for a test run, the motor wouldn’t start. I thought, “You’ve  gotta’ be kidding! I need to get this thing into the air, get some photos, and write a report to finish the story I’ve started to tell.”

Yes, the motor had always run just fine every time I checked it out during construction, along with the Airtronics receiver and servos that operate on power from the linear BEC that’s part of the Cobra ESC. This time the radio stuff still worked OK, but the motor (and maybe the ESC) did not. I removed the cowl retaining pin, pulled the front of the airplane open/apart again, and…everything worked. So…it’s all connected right, nothing’s loose, I checked that I’m using a fully charged LiPo pack (which ought to eliminate the possibility of the low voltage cutoff feature of the Cobra ESC keeping the motor from running when there’s not enough power available for a safe flight)…just as it is supposed to be. Back together again (very carefully) and…the motor won’t start.

Those of you who have been around E-powered RC aeromodeling for a while can see what’s coming. So could I, but I was also fortunate enough to see that I was becoming impatient. I have to make this thing work! That’s when the voice of very hard-earned experience said “STOP! Pull out the battery, set it all aside, and go work on something else”.  Letting impatience control your work at a time like this pretty well guarantees that you’re going to break something.  I hung the J-4 in its new place on my shop wall and transferred my attention to something else, which happened to be getting my newly built (big) Stinson SR-9 test flown and trimmed out.  Several days later, after I had stopped feeling as if the little Piper was out to get me, I put the little airplane back on the work bench and within a few minutes I had discovered the intermittent failure that was screwing everything up. Inside the original heat shrink tubing on one of the power leads to my motor was a cold solder joint that always made contact, allowing the motor to run, unless I put any kind of side load on that power lead. That sort of thing can…and does…happen even with good quality equipment. This is the sort of mess-up that results in a lot of equipment getting sent back for service…and if you’re unsure of how to deal with it, that’s what you should do. But, if you’re confident that you can inspect, discover and re-solder/resleeve a bad connection, go for it. That’s what I did. This is the kind of judgment call that becoming an experienced aeromodeler will demand of you as you gain experience. Half an hour later the J-4 was checked out and flight-ready, and early the next day I took advantage of the morning calm, got together with my good friend (and master model airplane photographer) Gary Ritchie and made a satisfactory test flight and photo session with the model.

 

Here’s a close look at the completed model as built from the Dumas 30” span Piper J-4 rubber-powered free flight kit and modified a lot…all finished, checked out and ready for a trip to the flying field. That’s a copy of the Paul Matt Piper J-4 scale reference drawing beneath the model. It’s easy to see where I’ve designed structural modifications to add the scale size ailerons, elevator and rudder. Built per the plan, the rubber powered model would weigh in on the order of two or three ounces. As you see it here mine came in ready to fly at 10.5 ounces. That works out to a wing loading of 6.9 oz. / sq. ft. (or 6.75 expressed as a cube load). I could have built this model an ounce or two lighter, but at the expense of the attention to scale detail we’ve talked about already. That’s OK with me, because along with the slightly higher wing loading I get the option of being able to fly the model safely in light winds of 5 mph or so.

 

Wait a minute…not so fast! Before we go flying I want to show you a few more shots I made of some of that extra stuff I added. We talked about hinging the control surfaces but I didn’t get to share any of the close-up details. There are several good “hinge slot” devices on the market that work as advertised, but they are way too big for use on a model like this one. I used pinned, tab-type hinges on the tail surfaces (yes, I could have used those flexible plastic CA hinges here, but I like these). I used a fine wire drill to define the ends of each hinge location, then used a sharp No. 11 blade to slice out a slot of the required thickness/depth to create each working hinge slot.

 

I used Robart Hinge Points (the smallest size) on the ailerons because they represent the appearance of the full scale airplane so well…but they’re too thick to work on the tail surfaces. It’s the same game, though, when I use a drop or two of machine oil to flood the working/mating portion of every hinge to keep whatever adhesive I’m going to use for installation from grabbing where I don’t want it to. You can also see where I’ve cut off one end of the hinge that would otherwise have protruded to the rear into the “open” portion of the aileron where it would have contributed no strength, looked crude through the clear doped tissue covering, and left useless extra weight on the model.

 

Back to the elevator and those tab hinges…I’ve pre-oiled this one and added a bead of slow cyanoacrylate to ONE end of the hinge.

 

I keep switching subjects here so I can use the best and clearest of my photos. Now we’re back to the rudder, with the lower tab hinge in place. If you look closely you can see that I have taken advantage of that “oil job” I did and chipped/broken away the slight squeeze-out of adhesive that would otherwise have clogged the hinge.

 

Remember the slide-in/removable receiver and servo tray I designed to fit into and out of the cabin area through the large opening I left in the F-1 (firewall) former? Here I’m using a mini-Allen wrench to install and tighten one of the two socket head sheet metal screws that lock the servo tray in place. NOTE that I have purposely left the clear plastic out of the cabin side windows not only for cooling airflow but also so I can access stuff like this inside the model  that would otherwise be hard to get at owing to the two-piece wing/integral cabin roof I designed into the conversion to improve scale appearance.

 

I’m leaving out the clear plastic glazing on those two side windows, but the windshield is going in just as you’d expect. Before that happens, I have to finish any interior detail that would otherwise be difficult to install. This is the first of two diagonal fuselage frame members that would be critically stressed steel-tube structure in the full-scale J-4. Here it’s purely for scale effect and I’m using (very light) 1/8” K&S aluminum tubing. Can you see the hole I’ve already drilled on the left side of the top cowl for the second piece of tube?

 

Time to finish up all the remaining linkages and connections. Here I have pre-fitted the little E-Z Link into the hole I already drilled into the custom aileron horn and (with the aileron servo, which happens to be out of the picture, plugged in, turned on and centered at the transmitter) I can visually align the aileron itself at the “neutral” position and tighten down the set screw in the link assembly to lock that flexible cable in position. Later, when I have finished all the control connections and double-checked all the neutrals and throw limits, I’ll trim off the excess cable length.

 

It took me a while to decide exactly how I would attach the outboard ends of the struts to the wing panels. As you can see, I’ve already made openings in the wing surface to accept and position the strut ends. The question becomes, “What’s the best way to stick them in place?” This is one of those trick deals where there is no single right answer. If I glue them permanently in place, handling flight loads and maintaining alignment is easy, but in the event of a poor landing (or worse) everything will break loose and probably take part of the underlying structure with it. I could rig up a screw-in-place fixture with some sort of breakaway feature, but that would almost certainly add extra weight I don’t want to deal with. In the end I decided on “tack-gluing” the connections with just enough Roket Hot to wet the joints…the strength of the adhesive and the doped tissue it is bonding the strut ends to is enough to handle normal flight loads, but will almost certainly tear loose under the stress of an impact without breaking up the structure beneath the covering. (If you’re concerned that this would be a poor choice on a larger/faster/heavier model, I agree with you).

 

You may recall that I put on the last light/thinned coats of clear dope after the control surfaces were hinged/mounted. That saved me from having to handle loose ailerons, an elevator and so on while they were coated with wet dope, but it also just about guarantees that some of that clear dope is going to get into the various hinge gaps and dry there. I’m using a hand-held piece of 320-grit production paper to clean out the aileron-to-aileron well gap. You have to develop a feel for this, but that’s not difficult if you’re paying attention. The goal is to get rid of anything that drags or blocks control surface movement.

 

Here’s another last-minute detail. I’m using ordinary DuBro lightweight wheels with standard aluminum wheel collars…but…I want to disguise them with scale hubcaps made from very light aluminum. For this model I used the cut-off bottoms of soft drink cans trimmed to size to take advantage of the built-in convex dished surface. Here I have trimmed one of the hubcaps to size, cleaned up the edges and test-fitted it only to discover that the wheel collar is too thick/deep to allow it to rest on the wheel rim where it belongs. I decided to fix that by using another cut-and-try technique that’s appropriate for a model this small and lightly loaded. I used an ordinary rotary sanding drum to “face-off” about 3/64” of material from the collar…enough that the hubcap will ride on the wheel rim and clear the collar so the wheel can rotate freely. You’ll see later what that looks like all put together.

 

ALL of the “rag wing/fabric covered single engine Pipers (and a lot of other light airplanes as well) used some sort of external bracing on the tail surfaces. This little J-4 would probably fly OK without them, but real world handling stresses are another thing. Not only do these nearly-scale tail brace wires look good, they can save a lot of “OOOPS…” moments like when you’re anxious to get your plane out of the car before the wind comes up. I’m going to use some very light craft-store plastic-coated steel “beading wire” to represent the full scale round section solid steel wires.

 

I’ve been doing this stuff for a VERY long time, but even so there are times when my fingers are too big and get in the way. A little surgical clamp works just fine at a time like this.

 

For a very delicate application like this there are times when I don’t count on being able to control the flow of cyanoacrylate even through a fine tip…creating a drop/bead on the end of a clean tool (like this screwdriver) allows me to ensure that there’s just enough AND NO MORE “loose” to go anywhere other than exactly where I want it.

 

All the tubes are in place through the vertical fin, both sides of the horizontal stabilizer and here on the lower tailpost. Now I can lock down one end of the cable at one specific starting point (here I’ve used the opposite side of the horizontal tail) and then thread the free end through each brace point /reinforcing tube in turn. I can fine-tune each section of the tail assembly for exact alignment and then lock them all in place with more cyanoacrylate at each cable/tube junction in turn.

 

All done, that looks like this.

 

Here’s another look. You can also see that I’ve hooked up the micro link to the rudder horn and added that tiny black tubing retainer that comes with it.

 

All that fussing to get the cowl, motor mount and so on RIGHT pays off. Here I have removed the wire retaining pin and pulled the entire cowl/motor/prop assembly forward on those slide rails I built into the fuselage. What’s going on now is that my Venom Pro Charger is hooked up to the AC-DC power supply (out of the frame) and the Cell Balance Block Plus that come with it and is running a normal LiPo charge/balance cycle on one of the Venom 20C 3S 850 mAh LiPo packs that I have chosen to fly this model.

 

How many of the details I’ve just described can you spot in this photo? This shot really shows off the semi-translucent appearance of classic tissue covering sealed with several coats of clear nitrate dope. Unlike the grossly oversized plastic prop that comes with the kit (just right for rubber-powered flight, but awkward looking), this 6-3 prop is nearly scale size and matches the output of my Cobra 2204/40 outrunner motor perfectly. (This happens to be an old Tornado Plasticote wood propeller…a genuine antique from the 1950’s. I have a weakness for using stuff like that). A GWS, APC or similar prop of the same size will work just fine on your model.

 

At the field and just about out of excuses to arm the system and get her into the air. This shot does an excellent job of showing off the combination of vintage stick-and-tissue construction with the sun just shining through the covering and the suggestion of “is it real?” that comes with all that scale detail I spent the time to include.

 

Wanna’ go for a ride? The cabin door on a Piper J-4 is always on the right-hand side, but you get the idea. And yes, I need to take her back to the shop and make a nice instrument panel to go in there.

 

After a few circuits of the field to get all the trims tweaked just right, I felt comfortable bringing the J-4 close in and low so photographer Gary Ritchie could grab some detailed in-flight shots.

 

This airplane is small enough that with any appreciable wind or turbulence I would not leave it “hands-off” in a low fly-past like this…but…as long as you maintain a smooth touch and fly it gently, the impression of scale flight is excellent.

 

Sunlight through clear-doped, dyed tissue! This is a big part of what building and flying vintage model the old way is all about. Plastic covering (in my opinion, anyway) just doesn’t measure up at a time like this.

Be sure to check out some other posts in the Rubber Power-to-Electric RC category.