Building the Balsa USA 1/3 Scale PA-18 Super Cub

by Eric Pipenger

Like many others in this hobby, I got started by building my 1st model airplane. I was looking for a project that I could work on out in my shop. After months of assembly and help from fellow club members, I finished my first build and was rewarded with the satisfaction of seeing it fly for the 1st time. That was 2007 and many airplanes ago! I still have that Great Planes PT-40 trainer that I built. I learned to fly RC using that plane, and have built and flown many kits and ARFs since then. A fellow club member started building this Balsa USA 1/3 Scale PA-18 but lost interest in completing it. I was looking for a "Winter Build", and ended up taking over the project. I thought that I could complete it in one winter and have it flying by the summer of 2018. I didn't get a good start on it the first year. Other projects got in the way, then it was flying season again. The Super Cub got moved out of the way until October 2018.

11/8/2017 Here is what it looked like when I got it.....

I started to write a build thread on the "Forum" of this website, but I decided that maybe it wasn't the best place to put it. I will take another stab at it here.

2/18/2018 One of the 1st things I did was install the ailerons and flaps so that I could clamp them in neutral position to allow shaping the wing tips to meet the curve of the ailerons.

Then cutting the excess sheeting off the root of the wings so they could be mated up with the cabin top.

2/28/2018 The next step was to block the wings in position so that dihedral could be set, and the struts dimensions could be measured. The plans called for rib W-3 to be 3/4" higher than the root of the wing. I ran a string between the W-3 ribs (near wingtip) , and adjusted until the string cleared the root of each wing by 3/4". I also confirmed by measuring the distance from bench to bottom of each. When I was satisfied that the wing was where I wanted it, I measured between the struts attachment points to get the right length for each one.

3/18/2018 After the struts were installed, then I could measure up and fabricate the jury struts. I went ahead and installed the main gear since the struts attach to the gear frame. I ran into problems at this point. With the gear in place, it was nearly impossible to insert and tighten the 4-40 screws that hold the struts in place. After looking at several possibilities, I decided to add a 1/2" spacer to the attachments on each side. That was enough clearance to get the screws and driver in place. Throughout the project, I tried to anticipate how things were going to transport and assemble. I don't want to be looking at an hour to assemble or disassemble at the field! It may not be my everyday flyer, but I don't want it to be a "Hangar Queen" either.

I found the materials and hardware in the kit to be of good quality. However, I thought the manual to be lacking in several areas. I found myself scouring "Build Logs" and Forums all over the web to see if others had better pictures or descriptions of steps that I didn't fully understand.

Most of the 2017/18 winter was spent test fitting all of the components, ordering more servos, lighting kit, engine, and radio components. I decided on using the Promodeler DS305CLHV servos in the wing for the flaps and ailerons.

The original mounts for the wing servos were nice, but it was very difficult to install, adjust, or remove the servos.

I used a piece of 1"x2"x1/8" aluminum angle to fabricate a bracket for the servos that is then bolted to the ply mounting plate using 4-40 SHCS and T-nuts. I installed #4 bonded washers on the socket head cap screws to prevent them from vibrating loose. I can remove the entire assembly from the servo cavity in the wing very easily using a hex driver.

3/1/2018 In the above photo I am test fitting the stabilizer, elevator, and rudder. The rudder was covered using the Stewart Systems Superflite 104 polyester fabric and water borne adhesive. After watching several of the "How-To" videos on Stewart Systems website, I was convinced that this is the covering method that I was going to use for this project.

7/27/18 - After I completed building and fitting the main struts and the jury struts, I reassembled everything once again to check dihedral angles, test fit cowl, etc. For some reason, the cowl didn't seem to fit quite right. After checking measurements, etc., it appears that the cowl is longer from front to back than shown on the print. It is something that I will have to keep in mind down the road. One reason for the test fit was to measure clearance to see if the engine that I wanted to install would fit. Another reason was to try and decide where I was going to establish 0 degree incidence so that I can check wing incidence, engine down thrust, etc.. Also the position of the cowl will be affected because of the 2 degree down and right thrust of the engine. I continually ask myself if I am overthinking everything, but also realize that now is the time to iron it all out rather than after it is all covered and painted.

Before I ordered the engine, I downloaded the firewall template to determine what modifications would be needed for this engine to mount. I then calculated where the propshaft would exit the cowl using triangulation to determine how much I would have to raise the engine to compensate for the downthrust. Once I was satisfied that I could make it work, I borrowed a new Saito Gas 61cc twin cylinder from a friend to test fit and confirm before I bought the new engine. I had a Zenoah 62cc 2-Stroke that was meant to power the project, but I wanted the smooth running, sweet sound of the Saito 4 stroke. Also, seeing the cylinders and push rods through the cowl openings has got to look more realistic and add to the scale looks of the plane!

I saw a notice from Horizon Hobby that they anticipated a proposed 30% tariff on products coming in from China would increase their prices, so I ordered the SAIG61TS and a Spektrum 9104T receiver to beat the anticipated increase. I started receiving my parts from Horizon in September 2018, but it was January 2019 before I actually got around to installing.

Here is where I originally planned to install the receiver and light controller - behind the rear seat.

Shown here is the white "top hat" beacon that came with the package from Dave's RC Electronics.

Below, I am routing the wires for the landing lights and the navigation light in the left wing.

1/5/2019 - Began cutting the firewall and installing the engine. Since the carburetor extends behind the firewall, it required modification of the fuel tank compartment.

My helper, Jim, helped with the engine install. Overall the process went well.

1/21/2019 - The plans did not include landing lights, but the full scale PA-18 that I was using for reference had them, so I decided to modify the wing to include them as well. I used photos and measurements of the full scale to come up with the size and position of the lights.

I had one reflector that I robbed from a AA incandescent flashlight, and was able to affix one of my LEDs to the back of it. I cut and shaped the square frames from some thin aluminum sheets. I mounted the o-ring to a shaft and turned it in the lathe to cut a slot in the outside of the o-ring to fit it into the square frame. It took forever for me to find another incandescent AA flashlight reflector. All of the small flashlights that I dug out of toolboxes, junk drawers, glove compartments, etc. were all LED models, but I finally found one over at my in-laws house. I mentioned this to Dave at Dave's RC Electronics (where I bought my lighting kit), and he told me he carried them on his website! Duhhhh.

How did I miss that? I had searched eBay, Amazon, etc., but I didn't find the size I needed. Dave's would have been perfect!

Below is a photo of the full scale PA-18 landing lights that I used for reference. to make mine.

In the next phot, you can see the "rotisserie" frame that I used while sanding the fuse. I may or may not use it for covering and painting as well?

The next step you can see that the 1/32" ply has been cut and fitted for the top of the boot cowl, and I have started trimming and fitting the windshield .

The directions call for covering the larger parts first, then using the smaller, trimmed pieces to cover the smaller items. However, I decided to "practice" on some of the smaller items to begin with. I laid out the stab and elevator pieces to try and make the most efficient use of the fabric. I will be using Stewart Systems Superflite 104 Non-certified 1.8 oz. fabric 71" wide.

I will cover the bottom of each surface first

Here is one tacked and lightly shrunk. I found that you have to be very careful when using the iron to shrink the fabric. Too much will pull it too tight, and warp/or deform the shape.

I am using the Stewart Systems EkoBond waterborne fabric cement. Paint the balsa and ply areas where you want the fabric to stick, and then let it dry. (The fabric will not stick to wet cement.)

Once the cement has dried, you can stretch the fabric over the frame and use your fingers to smooth it in place. If you have a wrinkle, you can lift the fabric and reattach. Once you have it in place, you can iron over the painted frame areas and the glue will "melt" into the fabric tacking it into place.

I found that good scissors are necessary when working with this polyester fabric. A good pair of pinking shears is essential. For trimming and getting into tight spots, these sharp pointed scissors by Karen Kay Buckley are ideal.

2/23/2019 - The 71" wide fabric was just right for covering the wings. I was able to wrap the entire wing with one piece of fabric. I highly recommend having a helper for this step to help hold and pull the fabric into place. You will note that the hinges were already epoxied in place when I took over this project. Waiting until after the covering is on before gluing the hinges would make covering much easier, and then you would also be able to adjust the hinge gap as needed. When you add a couple thicknesses of fabric, 4+ coats of primer/filler, and several coats of base/clear, that all adds up!

Here is the covered left wing. Note that I only glued the leading edge and the perimeter so that the fabric can shrink nicely over the ribs. I started on the bottom first, the top. Getting the tip areas smooth is a bit of a challenge, but this fabric shrinks up nice, eliminating the wrinkles around the curved tip.

2/24/19 - The next 3 weeks I spent cutting and gluing the simulated stitches, pinking tapes for the ribs and perimeter tapes, and doilies for inspection ports and around strut mounts and servo hatches, etc..

I discovered this method for making the simulated stitches in a YouTube video by "James". Check it out at this link: https://youtu.be/JPLlDL74uCM . I found this video covers the subject well. He used a different cement to glue the string to the fabric, but the end result was similar. I used a polyester cord. (Kite string would work as well.)

Cutting doilies to reinforce the fabric around the strut mounts.

Test fitting servo hatch covers before covering them.

I modified the mounts where the jury struts attach to the bottom of the wing. By hinging this area, I can fold the jury struts against the wing, and the main struts can fold down on top of them for transport - without having to completely remove them.

2/28/2019 Applying pinked tape over the rib stitching.

Ribs and perimeter pinked tapes complete.

Door latch and handle mechanism

6/12/2019 - I fashioned the door handle out of a solid brass rod, then brazed some solder on it to give it a "chrome" appearance.

The mechanism inside operates similar to a garage door latch. I carved out an area in the balsa door for the mechanism. It will be covered with fabric and concealed.

When extended, the push rods extend out of the brass tubes in the side of the door, and into mating holes in the door frame.

Fuselage Covering

11/16/2019 I started on the bottom of the fuselage -

Then the left side.

and across the top over the center longeron. The side, vertical fin, and half of the top is all one piece of fabric.

Halfway done!

Trimming around the rudder hinges-

This is called "really getting into your work"! There is not too many RC planes that I have been able to get my head and shoulders inside to iron the fabric around the "D" window!

12/17/2019 - Pinking tape has been applied over frame members and all perimeters.

The most difficult part of this was getting the pinking tape to conform to the concave curve up to the vertical fin. Bias tape may have worked better, but a combination of heat and stretching, I was able to get it around the curve.

Struts getting a coat of primer:

I hand shaped covers for rudder cables outlets on fuselage out of aluminum sheet metal.

Cutting and forming the aluminum boot cowl was tedious work. I made templates out of manila folders first, and then transferred the shape to the aluminum.

Velcro straps sure come in handy to hold things in place while marking and drilling the holes for mounting screws.

12/21/2019 - Now that the wings are covered, it becomes evident that I am going to have to add some material to the cabin tops so that the aluminum fairings will fit over the aluminum gas tank covers. I also need to finish shaping the front of the cabin top to match the profile of the leading edge of the wings.

The fiberglass cowl came with lines molded in for the engine access doors. I have seen several modelers make the doors operable, using piano hinges on top and even scale straps to hold the doors closed on the bottom. This would make accessing the engine a lot easier and add another scale feature, but I was concerned about the losing integrity of the shape of the cowl. Instead, I opted to cut along the score lines on the bottom of the cowl to make the bottom of the cowl removeable.

12/24/2019

I chose to epoxy some light ply along the score lines and use blind nuts, machine screws and bonded washers to hold it all together.

1/15/2020

The instructions for the Stewart Systems EkoFill 620 calls for 2 cross coats brushed on to fill the weave of the fabric. I practiced on some of the smaller parts before tackling the wings and fuselage.

The EkoFill 620 dries very quickly, but I soon found myself running out of places to put the painted parts.

I started making templates for the aluminum access covers.

A trim iron works good to seal down the edges of the pinking tapes that have a tendency to curl a bit as the primer/filler dries.

I wrapped the ends of the struts with thread and then coated with epoxy resin to cover the steel straps on the ends of the struts.

Tip: a piece of fuel tubing over the blade of the Phillips screwdriver holds the #0 screws and helps to prevent the screwdriver from slipping off the screw and damaging the aluminum.

The fairings extend over the cabin top about 6 mm. to form a pocket for the root end of the wing.

I found that fitting the leading edge of the fairings and the windshield to be very challenging.

Here is the lower part of the cowl getting primer.

Here are the landing gear parts getting primer.

Upper cowl with primer.

Aluminum fairings and covers after coating them with aluminum primer.

2/9/2020 - Below, my friend Dan Bouman helping me out by spraying the primer coats for me in a paint booth that is normally reserved for full scale aircraft.

3-23-20 The lockdown begins! The governor orders a shelter-in-place order for the residents of Michigan due to the Covid-19. All the plans that we had made to get the plane painted would have to be changed .

3/25/20 - Since we weren't supposed to go anywhere, I decided to switch gears and work on something else for a while - Below, I have painted silver on the bezels / lenses before attaching them to the back of instrument panel.

3/26/20 - The lockdown continues! It now appears that it will be a while before the paint booth at the airport will be available, I decided to start on Plan "B" and build my own paint booth inside my shop at home! In retrospect, this is probably the better plan anyway. It gives me more flexibility since there will be a lot of time masking off areas between colors, etc. and I won't have to be transporting parts back and forth between each process.

I set up a 10' x 10' shelter inside my shop, and built a partial wall with a door that will also house a pleated air filter. A box fan set in the doorway will draw filtered air through the paint booth and exhaust it outside in the snow.

4/1/2020 - My wife and I had planned to fly to Florida on April 16th to drive her mom home after being a "Snowbird" for the winter. There was some rumors that some states were going to shut down or restrict travel, so we changed our flights and flew down on April 1st. The trip was short. One day to pack up and close up her place in Florida, then left early the next morning to get home before the governor decided to close the border.

4/7/2020 - The paint equipment that I ordered still hasn't arrived, so I went back to work on the instrument panel. I had a couple friends with 3D printers that offered to print these toggle switches and Navigation chart for me. Thank you Kyle and Jaime! I found the files on Thing-a-verse, and sent them the links. Here is a toggle switch before it was painted:

Here are some toggle switches and "circuit breaker" housings being painted:

Ready to install

Here the first switch is glued in place. ( using the tape as a guide)

Switches and circuit breakers installed:

4/9/2020 - The painting supplies finally arrived, and I finally was able to get ready to start putting some color on the plane! I wrapped the shelter with visqueen and installed LED tubes inside for lighting.

I finally decided the color scheme that I would be using on the plane. My friend Dan had recently completely redone a 1961 PA-18-150, and I had been using his full scale for a reference. After taking hundreds of detail pics of his, I grew to really liking his pattern and color scheme. Here is a side view of his that I used to eyeball the masking the of colors on my 1/3 Scale model. I saved a "flipped" image for reference when I masked off the left side.

4/11/2020 - After a few test strokes on cardboard boxes, I was ready to start on some smaller parts. Keep in mind that it has been years since I touched a paint gun, and this is my 1st experience with a HVLP sprayer. I am using a base/clear coat method. Here is the 1st coat of base applied:

I doesn't show up well in the photos, but the red and the gold paint are metallic.

Bridget at Robinson Graphics made the adhesive vinyl templates for the "N" numbers. 1514 is my AMA number, and EP is my initials.

Masking off the stripes was a very tedious and time consuming process. The full scale plane builders use a multi-layer mask for this process. We talked to the supplier that they use, and found that it was more involved than telling the computer to scale it to 1/3 scale. For some reason it would have to be re-drawn manually, and the masks would cost ~ $600. for my plane. About 1/2 way through this process, I was thinking maybe it would have been worth it!

I found that the vinyl "fine line" tape works the best for masking off the stripes. It does a nice job sealing off and preventing "bleed" resulting in nice crisp lines. However, it does require some care taken when removing. I found that pulling the tape at an angle towards the area I was removing helped to avoid "lifting" the base color. Sometimes I used the heat gun to help soften the adhesive a little when removing the tape.

Now the fuselage is pretty much completed with the base coat!

4/18/2020 - Now that the fuselage colors have been applied, the next step is masking off the leading edge and tip of the wings for the red on the leading edge and the pin stripes. I used 1/4" "fine line" vinyl tape.

But after it was on, I didn't like the looks of it, so I peeled it all off and used the 1/8" tape instead!

I installed the wings on the fuselage so that I could match up the stripes on the cabin top fairings.

4/23/2020 - Now that the clear coat is on, I can start cutting and fitting the acetate windows, all of the aluminum trim, fairings, etc.

I used my curve contour sanding block to hold the black fuel tubing while I slit it lengthwise to wrap the cabin brace supports.

I used some wax paper to mark the outline of the windshield on the upper boot cowl.

Then I transferred that shape to a piece of flat black adhesive vinyl to finish off the dash above the instrument panel.

4/24/2020 - Everything is starting to fall into place! Most everything had been test fit into place before it was painted, so now it is just a matter of reinstalling everything.

I used a screenshot of an online navigation chart, resized it to fit, and glued it to the tablet that I had made for this purpose on the instrument panel. The center of the left circle is the Sparta airport. The right one is the Greenville airport. Our RC flying field is located near the crosshair in the center.

Here it is on the completed instrument panel:

5/27/2020 - I was able to fit it into the back of my Buick Enclave for the first trip to the RC field! The rudder fits nicely along side the front seat, and the spinner clears the back window by a couple of inches! I left the struts assembled for this trip, but now that I have wing bags made, I can fold them flat for travel.

5/27/2020 - A pop up shower forced us into the shelter for cover. It did not last long, and we were able to get it back out on the runway after things dried out.

When I started this project, I had no idea it would take from the fall of 2017 to the middle of 2020 to complete it. Due to the polyester fabric and multiple layers of primer, base coat, and clear coat, it came in a little heavier than I had hoped, but at 35# dry, it is very solid in the air. The Saito 61cc 4-stroke pulls it along very nicely, and it has a pleasant 4 stroke sound in the air!

Thank you for reading my "build log". I hope you enjoyed it. I tried to share many detailed photos for those of you that may be working on a similar build. If you have any questions, please feel free to contact me : eric@wolverineskyhawks.com

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