Balsa USA: Sopwith pup

Model Airplane News, Jul 2002 by Onorato, Jim

In 1916, the Sopwith Aviation Co. Ltd. completed the prototype of a singleseat fighting scout, designed by Herbert Smith. This new plane bore a striking resemblance to the two-seater Strutter (also designed by Smith), but it was appreciably smaller and was powered by an 80hp rotary engine. It was a military version of a single seater that had been built in 1915 as a personal transport and aerobatic aircraft for Harry Hawker, the Sopwith Co.'s test pilot. The pilots who flew this aircraft apparently looked upon it as an offspring of the Strutter, and they nicknamed it the "Pup." Although authorities tried to persuade everyone to use "Sopwith Type 9901" as the official name, the most famous fighter and best flying airplane of WW I will always be remembered as "the Sopwith Pup."

THE KIT

Balsa USA's 1/4-scale Sopwith Pup kit uses conventional built-up construction with aircraft-grade plywood, lite-ply, balsa, hardwoods and plastic. The kit contains a very substantial hardware package and a 31-page, photo-illustrated construction manual with detailed, step-by-step building instructions. The manual is very well done and includes numerous photos to guide you through the building process. Other items provided include four sheets of rolled plans, prebent landing gear, cabane strut wires and an ABS cowl. An optional aluminum cowl is available, as are decals.

CONSTRUCTION

I used Balsa USA thin and gap-filling CAs for most of the balsa construction and Great Planes Epoxy on the plywood.

* Wings. As with any biplane, most of the building revolves around the wings, so this seemed like a logical place to begin construction. The upper and lower wings are built directly over the plan. The building process for both wings is quite similar; the main difference is in the center sections. The lower wing has a short center section and 8-bay end panels, while the upper wing has a longer center section and 7-bay end panels. When completed, they both have 77-inch wingspans.

The lower wing houses two aileron servos that control the ailerons in both wings. The wings have four balsa spars. The two forward spars are 1/4x3/8-inch balsa, and the rear spars are 1/4-inch-square balsa. Shear webs are used between both front and rear spars. The die-cut ribs are 3/32-inch balsa, as are the false ribs. These, by the way, really give the wings a great scale look when they're covered. Ailerons should be built directly over the plan at the same time as the wing panels. The wingtips are built up and should be added before you remove the panels from the building board. Wing-strut mounting pads and aileron connector pads are made of 1/8-inch aircraft plywood. The center section of the lower wing is sheeted with 3/32-inch balsa, but do not add the bottom sheeting until after the wing has been mounted on the fuselage.

* Fuselage. The fuselage is built up with 1/4x3/8-inch balsa stringers and 1/8-inch lite-ply formers and doubters. After building the two sides directly over the plan, I removed them and glued them to the 'A-inch lite-ply forward top crutch that was pinned over the top view. The addition of the formers and 1/4x/8/8-inch-balsa crosspieces completed the basic fuselage "box."

Before I proceeded any further, I added 1/4x3/8-inch-balsa diagonal braces to the bottom of the fuselage to add a little torsional rigidity. A note of caution here: before you install the firewall, measure the length of the engine, the engine mount and the cowl that you plan to use to see whether the firewall must be recessed. The optional scale aluminum cowl that I used was shorter than the plastic one that's provided, so I had to recess the firewall about 3/4 inch to prevent the engine's thrust washer from sticking out too far beyond the cowl. As a matter of fact, I initially set up the Pup with the included ABS cowl and had to recess the firewall after I had decided to use the aluminum cowl. If you use the longer cowl, you can always use spacers to move the engine forward if necessary.

With the fuselage upside-down on the building board, I put the lower wing in place and made sure it was square to the fuselage and that both tips were the same distance off the board. When I was satisfied with the fit, I installed the wing hold-down bolts and dowels. Then, I installed the two basswood landing-gear mounting blocks-one in the wing and the other in the bottom of the fuselage just behind the firewall. The two landing-gear blocks are not the same, so make sure that you install the one with the narrower groove in the lower wing. Next, I removed the wing from the fuselage and sheeted the bottom of the wing center section and the bottom of the fuselage behind the firewall.

The forward section of the fuselage is round on the top and sides, and die-cut formers, fairings and 1/4-inch-balsa stringers are used to develop the proper shape. The two basswood cabane-mounting blocks should also be installed at this time. The forward deck and the side panels between the firewall and the first former are covered with 1/3 2-inch plywood. I attached these with USA Gold thick CA from Balsa USA, following the procedure described in the instruction manual.