Great planes: Piper J-3 Cub ARF

Model Airplane News, Jul 2001 by Onorato, Jun

A classic IMAA-legal sport plane for land or sea

The Piper J-3 Cub is probably the most widely recognized airplane ever built. It first appeared in 1935, and more than 5,000 of them were produced by the beginning of WW II! This enormously popular, two-man trainer is said to have introduced nearly 75 percent of WW II aviators to flying. The Cub proved to the general public that airplanes could be both safe and economical. Models of this classic airplane are offered by dozens of manufacturers and can probably be found at most RC flying fields on any given Sunday. Great Planes offers Cubs in several sizes, including this 81-inch-span, fabric-covered ARF that is made of high-quality materials. This latest entry into the world of Cubs is easy to build and fly.

Because I do a great deal of float flying, I decided to outfit the Cub with a pair of Great Planes Sport Floats. I knew it would be a natural on water.

THE KIT

The fine quality of this kit was obvious as soon as I opened the box. It is built using balsa and ply and comes fully covered with 21st Century fabric. The various parts are individually wrapped in cellophane bags. Everything is included except the radio, engine, propeller, spinner nut, fuel tubing and a pilot.

The kit also comes with a generous hardware package that includes hinges, an adjustable engine mount, preformed landing gear, self-adhe- A sive decals, gelcoated fiberglass , cowl, dummy engine, a vacuum-formed windshield and formed cockpit windows that pop into place from inside the fuselage for an attractive flush fit.

An excellent 24-page instruction manual loaded with photos and drawings guides you through the assembly procedure. There is no need for a plan. Although the instructions say this is 1/4-- scale, its 81-inch wingspan translates to approximately 1A.3 scale, but it's still IMAA-legal.

ASSEMBLY

First, I used a heat gun to remove the wrinkles from the covering; they came out with very little effort. Next, I removed the covering over the windows, painted the exposed wood surfaces with Cub yellow paint and fuelproofed the firewall and tank compartment. The firewall had already been fuelproofed, but I gave it a coating of thinned epoxy anyway.

After I removed the covering over the aileron servo openings, I glued the wing halves together with 30-minute epoxy. The wing joiner is made of three, 1 1/8 x 5 1/4-- inch plywood pieces that have a combined thickness of 11/32 inch. A piece of string passes through the wing from the aileron servo to the root; this is used to pull the aileron servo lead through the wing later on. I cut out the exit holes near the root and threaded the string through these holes before joining the wing halves. The wing is supposed to be built flat with no dihedral, but when I joined the halves, I taped them together tightly so there was no gap at the root ribs, and this resulted in a slight amount of dihedral.

Great Planes recommends an O.S. FS-70 4-stroke engine for the best scale appearance and sound, but 2-stroke engines ranging in size from .45 to .60 are acceptable alternatives. I chose the O.S. FS-70 4-stroke. Your choice of engines determines the location of the throttle servo and throttle pushrod exit on the firewall. An adjustable engine mount and drill template are included in the kit.

I attached the engine mount to the firewall with 8-32 socket-head capscrews and blind nuts. I used the Great Planes Dead Center Hole Locator to mark the location of the engine-mounting bolts then drilled and tapped the mount for 8-32 mounting bolts. To allow the exhaust to exit out the bottom of the cowl, I mounted the engine sideways. I also turned the carburetor around so the needle valve would be on top.

The Piper Cub kit includes a gelcoated fiberglass cowl that doesn't need to be painted. I trimmed the cowl to accommodate the engine and then attached the engine with 4 x 1/2--inch sheet-metal screws. The cowl in my kit was quite thin and turned up slightly at the edges, but it looked OK after I installed it. I painted the dummy engine black and silver and glued it to the left side of the cowl.

I attached the wing using two 1/4-inch dowels in the leading edge (LE) and two 14-20 bolts at the trailing edge (TE). The bolts thread into blind nuts that have been inserted into a factoryinstalled, 1/4-inch mounting plate. A precovered wing-bolt plate is also provided to reinforce the TE.

After I removed the covering from the slots in the fuselage, I epoxied the stab and fin into place. Everything lined up nicely without any cutting or shimming. I then added the tailwheel assembly.

The torsion landing gear comes in two prebent pieces and is attached to the fuselage with nylon straps and 2 x 1/2-inch sheet-metal screws. There are no cross-- braces. Covered landing-gear fairings come with hinged, 3/16-inch-thick wooden mounts already attached. I attached the fairing mounts to the fuselage with sheetmetal screws and connected the fairing to the landing gear with small rubber bands; this is an effective way to prevent the fairings from being knocked off when the landing gear flexes, but it is not very scalelike. Also, I thought the fairing mounts were a bit bulky, so after seeing how the landing gear was assembled, I removed them so I could attach the floats.