Balsa USA: Sopwith pup

Model Airplane News, Jul 2002 by Onorato, Jim

The landing-gear wires come already bent and need only to be wrapped and soldered. I made up the landing-gear fairings with the grooved spruce strips provided in the kit and attached them with CA. I then wrapped them in several places with heavy nylon cord. Be careful not to waste any of the grooved fairing material, as there is just enough to do the job. The plan shows a simple wire tailskid, but for a more scale look, I made a spring-loaded wooden one.

* Tail feathers. Construction of the tail feathers is conventional and straightforward. Build up the fin and rudder using 1/4-inch-balsa stripwood and 1/8-inch, laminated, die-cut parts to form the outline. Build up the stab and elevator with 3/8-inch balsa stripwood with diagonal bracing cut from 3/16-inch-square balsa. The diagonals have to be shimmed with 3/32-inch-scrap balsa so they end up centered between the thicker leading and trailing edges.

* Cabane struts, N-struts and wing setup. The most challenging aspect of building the Sopwith Pup is the fabrication of the cabane struts and N-struts and the setup of the wings. This has to be done very carefully to ensure that the wings are mounted properly. Fortunately, the instructions cover these steps very well, and if you follow them precisely, everything will come out OK. The cabane wires are bent, but the X-braces and N-struts must be bent to fit, and then wrapped and soldered. Spruce fairings have to be added after all the soldering has been completed. There is a lot to do here, but the result is well worth the effort.

* Radio and engine installation. I used a 7-channel Futaba radio (minimum 4-channel required) and five FMA servos in the Pup: three S35SMs for the ailerons and elevator and two S301s for the rudder and throttle. I Installed the aileron servos in the bottom wing just in front of the ailerons and connected them to the ailerons with the 4-40 hardware provided in the kit. I connected the ailerons in the top wing to the bottom wing ailerons with wire connectors with basswood fairings. I used semi-flexible pushrods (not provided) for the rudder and elevator.

I powered the Pup with a Saito FA-17083 3-cylinder, 4-stroke radial engine and used flexible exhaust header pipes to route the exhaust out the bottom of the cowl. The radial engine was a natural fit with the Pup, and its sound is very realistic. I also installed a JHM Aero Engineering DGS-3 On-Board Digital Glow Driver to make sure all three glow plugs stayed lit at idle. An 18x5 Zinger propeller and a 11/4-inch, B-style, Tru-Turn aluminum prop hub completed the power package.

FINISHING

I used a combination of covering materials to complete the Pup. The underside of the wings and fuselage and the fuselage sides were covered with antique white Super Coverite from Great Planes, and I used Balsa USA's olive drab Solartex on all of the upper surfaces and the fin. I covered the forward section of the fuselage with aluminum MonoKote from Great Planes, and I used red, white and blue MonoKote on the rudder and elevators. After applying the optional roundels available from Balsa USA, I sprayed the entire model with Top Flite flat, clear LustreKote. Finishing touches included the addition of rubber tubing cockpit coaming, a Williams Bros. machine gun, 65/8-inch Williams Bros. Vintage wheels and flying wires made from elastic cord. When complete, the Sopwith Pup weighed 15 pounds, 6 ounces. This didn't surprise me, considering that I had used the radial engine and the somewhat heavy wheels. Even so, the wing loading is still less than 18 ounces per square foot.