Balsa USA: Piper cub: A civilian J-3 becomes a military L-4

Model Airplane News, Apr 2001 by Yarrish, Gerry

The venerable Piper J-3 Cub is one of the all-time classic aircraft designs. During the early part of WW II, Piper converted its basic civilian J-3 into a military observation and liaison aircraft by replacing the top of the cabin with transparent Plexiglass that extends well behind the pilot's seat. So configured, the bright yellow Cub became the olive drab L-4 Grasshopper.

The Balsa USA 1/4-scale Piper Cub kit is also a classic and has been around for a very long time. Its excellent flight characteristics, relatively accurate scale outline and plug-in wing panels make it a popular choice for first-time giant-scale modelers. Balsa USA recently re-engineered the kit to bring the quality of its die-cut parts up to today's high standards. I built the original kit many years ago, so I thought the new Cub would be a good candidate for kit-bashing the military I-4 version. Built either way, the J-3 is a good starting place for anyone who wants to try an IMAA-legal model or to enter scale competition.

THE KIT

Inside the well-organized box, you'll find a lot of balsa, spruce and plywood. If you have previously assembled only ARFs, you may be surprised by the amount of lumber, but the kit is fairly easy to assemble. All the hardware, the formed music-wire landing gear, the clear window material and the vacuum-formed engine cowl are of good quality. The instruction manual is photo-- illustrated and has check-off-as-you-go boxes next to each step, so you can quickly resume where you left off. Items you'll need to complete the kit include radio gear and engine, engine mount, 16-ounce fuel tank, pushrod material, hinges, 1/4-scale Cub-style wheels, 1 1/4-inch tailwheel assembly, covering material and paint.

The manual suggests that you begin construction with the fuselage sides. I, however, like to limber up by building the tail surfaces first. These are made of die-cut balsa and lite-ply outline pieces with balsa sticks used as ribs. The stab and rudder outlines are laminations of two balsa layers (top and bottom) and a lite-ply center. After each part has been assembled and sanded smooth, you glue them together to form the control surface. Though a bit time-consuming, this technique produces strong, warp-resistant parts. I used Du-Bro flat, pinned hinges throughout.

The fuselage is of typical stick construction and has four 1/4x1/8-inch main longerons. Begin by building the upper cabin and window frames and then add the 1/4-inch-balsa cabin side pieces. Then add the main longerons followed by the uprights and diagonal supports. Again, as you would with any model, be sure to build a left side and a right side.

After you have installed the lower landing-gear support doublers on each side, connect the two sides by installing the large plywood landing-gear support plate and the lite-ply cabin formers. Pull the tail ends together, glue them, then assemble and install the upper fuselage formers and the lower cross-pieces. Make sure that the structure is straight and true before you add the cabin top.

This is where I began the L-4 conversion. If you build the stock J-3 Cub, you must sheet the cabin top and add the parts for the small skylight opening. Since the L-4 has a transparent "greenhouse" cabin top, I omitted the sheeting and installed thin spruce strips in its place to act as window support frames. Before you attach the cabin top to the fuselage, add the plywood firewall and the tank-- support pieces as well as the forward fuselage sides and the bottom pieces.

Install your engine mount before you attach the firewall to the fuselage; this makes it much easier to install the blind nuts. Temporarily attach the cabin top and support it with scrap balsa sticks. After making sure that the top is correctly positioned, install the forward cabane strut wires, and attach them with grooved blocks at the bottom and with a screw and two solder tabs at the top of each strut. When you have done this, add the remaining window frames, the instrument-panel former and the forward plywood decking between the firewall and the cabin.

Next, install the top fuselage stringers and the rear window frame sheets. Here again, I modified the fuselage so I would be able to add the large L-4 side windows. To do this, I replaced former F-6 with a spruce framework to support the upper stringers. I also removed former F-7 and installed a thin plywood "field-radio" support shelf. I replaced former F-8 with a new, longer one that I angled back about 30 degrees; it becomes the rearmost window-frame support. When the new parts and former were in place, I installed the top cross-pieces and the other scale window-support frames. The rest of the modifications were cosmetic changes to improve the model's scale outline.

To complete the fuselage construction, install the tail surfaces and the fuselage side stringers, attach and solder the landing-gear wires, add the tail-support wires, and frame and hinge the entry door.

WING

Wing construction is simplified by its flatbottom ribs. Pin the bottom LE and TE sheeting and the forward bottom spar into place. The aft bottom spar requires 3z-inch scrap balsa shims to lift it off the board. I used several ribs as "spacers" to correctly position the aft spar before I pinned it into place. The root rib is a balsa and lite-ply lamination that must be assembled before you glue it into place. It has to be angled outboard slightly to accommodate the wing's dihedral angle. Install the wing-attachment blind nuts and the alignment dowels before you glue the root rib into place.