Converting the 30″ Rubber Powered Dumas Piper J-4 to Electric RC – Part 1

Converting the 30 Rubber Powered Dumas Piper J-4 to Electric RC – Part 1

Last time I finished up with the construction of the TF 100″ span Stinson SR-9 and explained that I’ll be waiting for happier weather  before you get to see flight photos. Right now it’s time for something a lot smaller, but no less interesting. If you are familiar with my work you already know how I feel about those little ’40’s and ’50’s-vintage stick-and-tissue models. They pretty well defined “Model Building 101” for a whole generation, and it’s a good bet that most of today’s for-real, old time expert model builders got their start doing things like cutting little wing ribs out of a sheet of 1/16″balsa. The experience  of building and flying (or trying) in those days was mostly about what we all hoped aeromodeling might one day come to be about…the reality of being able to do more than just imagine  controlled, reliable, repeatable flights.

In those days it took a real master modeler to build one of those balsa and tissue rubber powered free flight kits right and then actually get it to fly…and it still does. These days, though, we have another option. Conversion of any of those models…either  from a dusty old shelf or from a new laser-cut balsa kit…to use contemporary electric power and RC is totally practical, and not that difficult if you are willing to learn some of those traditional building skills. Knowing how to pay attention to detail and to appreciate the value of patience is what a lot of that is about.

The new high-tech stuff makes all the time you might spend working on an airplane the old way a pretty safe investment. It sure beats turning your beautiful new airplane loose so a wild snake of twisted rubber could jerk it up and over and down into the hard ground before you had time to figure out what was going on. You can fly those old model designs really well on rubber power, but it takes an artistic touch to do it right. Check out an article on the Flying Aces Club if you want to see what I’m talking about. What I want to share with you here is the alternative of using electric power and tiny servos to turn the results of your efforts into a successful flying machine.

There are dozens of classic balsa stick and tissue scale models in production as kits, and probably hundreds around in the form of reproduced plans. What does it take to turn one of them into a miniature radio controlled gem of scale aeromodeling? The quick answer is, you have to make a lot of the decisions yourself. This article will show you how I did it when I converted the Dumas 30” Piper J-4 to electric powered radio control. The minimum demands of re-engineering a classic stick and tissue rubber model so it will fly reliably are that you provide a place to mount the motor, battery and radio system and create some sort of moveable control surfaces. You can add as little as possible to the original design, keeping weight to a minimum to permit very slow flight, or you can accept more weight and higher speed in the air and add features that increase the scale fidelity and/or esthetic appeal of the airplane. You can always count on touches like lightweight sheet balsa skins, aileron control, scale rib spacing, steerable nose/tail wheels and perhaps simple scale engine detail to add plenty of charisma. There are no clear boundaries between simple and scale…you can always find something else to eliminate and reduce weight a little more, just as you can also get inventive and find plenty of ways to add interest without increasing weight too much. As a modeler experienced enough to be doing a kit conversion, you have to choose just how far you want to go, and what you want the finished model to do.

 On this project I chose to go beyond basic changes like the addition of working control surfaces, building a motor mount, and finding ways to increase structural strength to deal with greater thrust and flight loads than a rubber powered airplane would have to deal with. I knew right away that on this model I wanted to add a lot of detail and structural modification to suggest the appearance of the full scale Piper J-4 without getting involved in exacting scale replication. This is Scale for Fun, not for Scale Masters…and…before we start cutting wood, I want to remind everyone that this mini-series is not just about the dumas Piper J-4. There’s no way I could ever choose an airplane that all of you might want to build, nor will I ever be able to write about all the designs you’d like to see me work on. What I want to get across is that these techniques of deciding what to change, how to do it, and why can be applied to a very long list of little airplanes that are waiting out there for you to choose.

 

Wanna’ build a model airplane? Dumas has invested serious effort in creating a rich selection of real stick-and-tissue kits…mostly for rubber-powered free flight, but a few bigger ones for electric RC as well…that combine old time build-it-from-balsa design with contemporary laser cutting of quality wood and clear, comprehensive plans and building instructions. They also made the decision to put an accurate image of the model inside on the box cover, which is something the old kits didn’t usually do.

One of the options they offer is laminating the flying surface outlines (tips) from balsa strips. This is a real improvement over the “standard” cut-out balsa sheet outlines furnished in the kit…if you want to use it. The outline drawings you see here are the pattern outlines you’ll need if you decide to make those laminated parts.

 

One of the things they don’t do is provide EXTRA 1/16″ x 1/8″ balsa strip to make the laminated parts. (The 1/8″ x 1/16″ strip you’ll find in the box is for other things…they should explain that). I ALWAYS cut my own strips. Buying pre-cut balsa strip is way too expensive. Here I’m using an ordinary Master Airscrew balsa stripping tool to cut 1/8″ strips from a sheet of light weight, medium-soft 1/16″ balsa sheet. These will become the tail surface and wing tip outline structures.

 

Not all laminating strips are created equal. I’m test-bending two to make sure they deflect equally. This means that they won’t be fighting each other when I form them into a part. (Cutting all your strips from a single, uniform sheet of balsa is a good way to get them all the same.)

 

Use a new (sharp) razor blade or knife against a firm surface to cut strips to the required length. Dumas specifies these measurements for you in the instructions.

 

Soak the pre-cut laminating strips in a 50-50 mixture of water and ammonia until they become noticeably softer and “bendier”. The little scraps of wood at the bottom will be used as “pinning blocks”.

 

I have cut the various laminating forms from scrap 1/8″ plywood and pinned them to the building board over plastic wrap. The cream colored stuff in the cup is my favorite laminating adhesive, Titebond glue. I prefer brushing it into place. ( I happen to have a contact allergy to the formaldehyde in the glue, which is why I’m wearing those latex gloves.)

 

This part is pretty much self-explanatory. I’m using the pinning block to PRESS and FORM the wet balsa strips tightly around the form, working an inch or so at a time and then adding a new block.

 

All the laminates laid up in place and drying look like this. Overnight is a safe drying time.

 

A long, wide sanding block is the best way to get both surfaces of each laminated outline really smooth…which means NO glue bumps or raised edges.

 

I’m starting to construct the horizontal tail. On this model the stabilizer and elevator have to become separate surfaces, so I have drawn new leading and trailing edge positions on the plan using red ink so you can see where I have made changes. The 1/8″ balsa sheet horizontal stabilizer center section has to be trimmed short by 1/8″ short to compensate for the 1/8″ sq. trailing edge I’ll add.

 

Here’s the entire horizontal tail surface assembly with just a few 1/8″ x 1/16″ balsa “ribs” still to be added. I’ll separate the elevator by cutting the two laminates at the tips AFTER I have the entire assembly block sanded as flat and smooth as I can get it.

 

This is the vertical fin/rudder assembly getting its sanding job. In this shot I have the assembly flat on a full sheet of 100-grit paper. This “double-sided-sanding is a good technique to learn.

 

I’m going to modify the wing in several important ways. Adding ailerons is one. Here I have cut a piece of 3/8″ sq. balsa to match the length of the new aileron well cutout,, and I’m using another of those “custom sanding blocks” (a piece of metal tubing) to cut the trailing edge to the concave outline I derived from my scale reference drawings. Details like this are going to vary from model to model…it’ll be UP TO YOU to dig up the necessary data to do YOUR conversion.

 

Here’s the finished aileron well trailing edge all shaped to match the contour shown on my scale reference drawing. (This one is from Paul Matt). I’ll make the aileron to fit it later.

 

The original kit design calls for a one-piece wing that’s permanently glued in place. For a number of reasons this won’t work on my conversion (I’ll explain them in detail as we go along). The “big” change is redesigning the wing to become two panels that attach to the upper fuselage sides just as on the full scale j-4. I discovered that I might need an extra pair of R-2 ribs, so I traced them from a laser cut kit part onto a sheet of 1/16″ balsa sheet and cut them out “the old way”.

 

The original kit design does not include any sheet balsa on the wing leading edge. I chose to add 1/32″ balsa sheet…for reinforcement and scale appearance…from the leading edge back to the first 1/16″ sq. spar on the upper surface. I duplicated this on the bottom surface to match the appearance of the full scale airplane. The kit design does not provide for a corresponding bottom/front spar, so I added one. I marked the location on all the ribs with a pencil and here I am making the new spar notch in one rib using a razor blade for the cross-grain cut.

 

A No. 11 knife blade is the best way to finish that final cut along the grain.

 

The next step was to relieve/trim back the outer edge of each rib from the L.E. back to the 1/16″ sq. spar where the new 1/32″ sheet will fit. I have already made the cut on the bottom.

 

Here’s an original rib at the top and one I have recessed for the leading edge sheeting at the bottom. You can see where the new 1/16″ spar will go to provide a mounting base for the 1/32″ sheet skin and the relief cuts top and bottom for the sheet itself.

 

I put a piece of 1/32″ balsa sheet on the plan…under the protective plastic film…to space the rib noses off the surface during the initial assembly, then pinned both bottom spars in place and began to assemble all the wing ribs in position per the original instructions.

 

Here’s what that looks like with several ribs in place. The extra rectangles of 1/32″ sheet are spacers for where the center rib bay will later get its own 1/32″ sheet skin. You can also see the extra inboard end ribs I’ll be adding as doublers to handle mounting stresses on the removable wing root.

 

The outboard ribs all need to be trimmed short at the rear to fit the new aileron well leading edge we just made. You can see where I have marked this one for cutting.

 

This is a better look at the preliminary wing structure with the aileron well cutout defined. I’ll leave the corresponding section of trailing edge just as it is for now and cut it free later.

 

Now all the ribs are in place, but the upper spars are not, yet. The left wingtip laminate has been trimmed to length and is ready to assemble to the rest of the structure.

 

Now the tip outline is in place (angled up even with the top surface of the wing per the kit plans) and I have added both upper wing spars.

 

I used the 1/8″ x 1/4″ balsa leading edge as provided. Here I’m rough-trimming it prior to fine sanding the upper surface to blend with the slope defined by the upper front rib edges so the 1/32″ balsa sheet will lie neatly against it.

 

This is what the trimmed, sanded L.E. should look like when it’s ready for the 1/32″ sheet to be added.

 

Before I close up the wing leading edge…the part ahead of the two forward 1/16″ sq. spars…I need to build in some spar webs that will become part of my wing panel mounting structure. Here you are looking at the inboard end of the left wing, upside down and from the rear. I have cut a couple of pieces of 1/16″ balsa sheet to fit flush with the outer edge of each spar and snug against the face of each rib. The grain is vertical…at right angles to the spars…and that’s important for strength. I am using clothespin clamps to hold the web tight against the front face of both spars while I apply fast ZAP to keep them there. The open space that results forms a pocket 1/16″ deep by 1/4″ high. I have opened a 1/16″ x 1/4″ slot in both adjacent ribs to permit a 1/16″ x 1/4″ plywood mounting stub spar extending from the fuselage to slip into the opening. My next step will be to add two more identical spar webs across the back of the spars to create a secure mounting pocket for the stub spar to slip into. You’ll get a chance to se this in action later.

 

As with the tip laminates, I’m using Titebond glue brushed into place to secure those wing skins. I’ll spray each wing skin sheet lightly with water just before installing it. While the wetting allows the sheet to conform easily to the curvature of the wing surface, this combination of water based glue and damp wood will result in a super-tight, durable bond as well.

 

This is the 1/32″ balsa sheet wing skin in place on the left wing panel. the rear edge of the sheet lies flush with the back of the top 1/16″ sq. spar and the front edge ends just forward of the leading edge. I’ll sand it top a round cross section later.

 

To make that result in an accurate, straight wing panel I cut several 1/4″ x 1/8″ hard balsa “clamping strips” and used a bunch of clothespin clamps to lock the sheet FLAT against the spar and L.E. while the glue-and-wet -wood dried overnight. Yes, I did the other wing panel at the same time.

 

Here’s what all that looked like from the bottom after all the clamps and braces went away.

 

Off-camera I added the bottom L.E. sheets the same way. With those dry, I started to skin the entire inboard rib bay with 1/32″ balsa sheet. In order to achieve a smooth outer surface without having to cut into the structurally important spars, I have INSET each piece of 1/32″ sheet between the various spars.

 

To avoid using pins to secure that delicate 1/32″ balsa sheet around the wing root, I used wide masking tape to keep it all in place while the glue dried.

 

I tapered the rear edge of the top sheet to blend smoothly with the trailing edge. This part of the building job is one where I took issue with the choices Dumas made…even for a rubber powered model. As designed, all the wing ribs taper down to a point where they meet the trailing edge. A close look at this part of the structure will show you where that has to result in a trailing edge top surface that will be nearly impossible to cover with tissue (or film) without having it adhere randomly along the top of the trailing edge and cause gross puckers and wrinkles. I’m filling the “gap” between the tails of each pair of ribs with more of the same 1/16″ x 1/4″ balsa the T.E.is made of.

 

Now I’m using the good ol’ sanding block to blend the wing root rib face with all that 1/32″ sheet I added.

 

The next step is to blend the entire wing panel into a smooth, aerodynamically (and esthetically) clean surface. This is where SPEED means nothing and ACCURACY counts for everything. The only magic here is care and patience…blending all those edges correctly is up to you.

 

I’m doing the same thing where the 1/32″ leading edge sheet and top spar joint meets the open tops of the ribs. This is one of those places that’ll really look like —- if you don’t get it right. When it IS right, you should be able to run your fingertips lightly across the edge where the sheet meets the open ribs and feel NO step or bump of any sort.

 

This is what the aileron cutout area should look like from the bottom when you’re done with it.

 

Same game for the finished wing panels. The panel I’m holding has been finish sanded; the other has not. Like those puzzle pictures…can you spot all the differences?

 

As promised, I cut out those portions of the trailing edge that are to become part of the ailerons and set them aside. The next step is to make an entirely new aileron leading edge that will match up with the concave cut-out aileron wells I made earlier. Later on I’ll go into specific detail about how those two parts work together to get close-to-scale aileron appearance and control response.

 

Remember the scale drawing of the aileron cross section from Image 14? Here I have begun the job of shaping the 3/8″ sq. balsa aileron leading edge block to match it. (Remember…all this “extra” aileron structure is built from extra balsa that I added to the Dumas kit.)

 

More sanding…in this case I’m using the top of my building board as the block, with a full sheet of 100 grit resting flat on it. You can see where I’m sneaking up on the curvature described by that scale drawing.

 

This is what that looks like with all the shaping on the aileron leading edge almost done.

 

With the aileron leading edges just right, I’ve pinned the left one down over the plan. You can see how the rest of the wing panel will fit around it, but I’m going to trust the plan and leave it out of the way for the next step.

 

I have pinned one of those cut-off aileron trailing edges in place and I’m assembling one of the trimmed-off ends of the wing ribs to become an aileron rib.

 

This is the entire left aileron, tip gussets included, all glued up. There’s a lot of finish shaping and sanding yet to do to make it match the rest of the wing panel.

 

We’ll finish that off-camera. Now it’s time to start work on the fuselage, and the first part of that job is to figure out how to modify the original Dumas one-piece design to accept those removable, separate wing panels we’ve been working on. What you see here is the fuselage side view from the kit plan with a couple of photocopies laid over it. Originally the “white”, non-detailed wing cross- section-shaped portion of the structure would have been glued directly to the parallel 3/32″ sq. balsa strips at the top of the rest of the fuselage. I’ve placed a sheet of balsa over the drawings to show you where I’m going to incorporate those 3/32″ sq. strips into an entirely new “wing root base” component that will e part of the fuselage. (Actually, there’ll be two of them…left and right.)

 

I tack glued two pieces of medium-hard 3/32″ balsa sheet together and contact cemented a pattern cut from one of those photocopies to one side. Here I’m using my old Dremel scrollsaw to cut two identical wing root base plates at once.

 

Here’s one of those root base plates ready to go. The paper pattern is still stuck to the other one.

 

I have built the two fuselage side frames over the plan exactly per the Dumas design…except for the inclusion of the base plates as part of the fuselage structure.

 

Just as the instructions say, I’m sanding both fuselage side frames dead-smooth on each side prior to assembling them. This is another place where I like to true-up structure by moving the entire part over a fixed abrasive surface.

 

Again exactly per instructions I have assembled both fuselage sides with the built-up fuselage center frame, which is installed to help you keep everything squared off. At this point the wing center section/root base plates and so on are being built as part of the fuselage, not as part of the wing. Always use a square to check your work on a job like this.

 

Now I’m adding the first of the 3/32″ sq. balsa fuselage cross members and 3/32″ sheet formers exactly per the box-stock instruction sequence.

 

Here’s a shot down inside the fuselage at the wing trailing edge station just to show you what it’s supposed to look like.

 

I’m assembling the nose formers, again exactly per the book.

 

Formers 2 and 2A get laminated like this…the offset will help define the shape of the cowl.

 

The instructions don’t mention it, but I’m sanding a bevel into the corner where 2/2A meets the lower longeron in order to get a properly fitted joint.

 

Here’s the fuselage assembly so far.

 

Now I have added the A-5, A-6 and R-1 parts that were originally to have been part of the wing, to finish defining the wing center section /cabin top portion of the fuselage. I have also cut out and installed the 1/16″ plywood wing center section stub spar that becomes a permanent part of the fuselage. There’s no pattern for this anywhere on the plans…I just measured the dimensions involved and drew it out myself.

 

And here’s how that all goes together. There’s more work to do here, but what you see is the load-bearing heart of the new wing center section/cabin top structure. I changed the free flight dihedral angle shown on the plans to one degree per panel…correct scale for the Piper J-4. We’ll talk more about this when it’s time to set the airplane up for flight.

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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