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

Updated: Jul 23

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.