Building the Balsa USA 1/3 Scale PA-18 Super Cub
Updated: Jul 23, 2021
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.