P-39 Aircobra

Model Airplane News, Jul 2003 by Garwood, Dave

Cavazos Sailplane Design

A radical design for its day, the Bell P-39 Airacobra was designed around a weapons system. The design started with an Oldsmobile 37mm cannon, which was fired through the propeller hub. To fit the cannon inside the nose, the engine was placed behind the cockpit; from there, it drove the propeller by means of a long shaft that ran under the pilot's seat. The Airacobra played a distinguished role in ground attack and close-air support for ground units as an early tank buster. It served with distinction in Russia, North Africa and the Pacific theater until more powerful fighters began to replace it in 1944.

Now you can own a replica of this seldom-modeled aircraft. Cavazos Sailplane Design recently acquired the Slope Scale line of high-performance, power-scale sailplanes (PSS) and upgraded it with improved fuselage molds, a new airfoil and advanced production methods.

KIT CONTENTS

First out of the box, the epoxy/glass fuselage is exquisitely molded and very scale looking. The molded detail now includes canopy rail markings, wing fillets and an engine air-intake scoop. To compress and strengthen the composite construction, the lapped-seam fuselage was manufactured with an inflatable bag inside the mold.

The kit includes hot-wire-cut, pink foam wing-cores sized to a new airfoil that preserves high-speed attributes and improves performance in light lift. Also included are precut balsa vertical and horizontal stabilizers, balsa wingtip blocks and sheet balsa for the wing; hard balsa leading-edge and balsa sub-trailing-edge stock; and hardware (including aileron torque rods, threaded aileron control wires, a snake-type elevator pushrod, elevator control horn, clevises and threaded wire parts).

Three 11x17-inch drawings and an eight-page instruction booklet are also included. Slope Scale planes are designed for experienced builders and fliers, and the instructions assume that the builder is familiar with foam-core wing sheeting and fiberglass finishing. If you've built foam-and-fiberglass planes before, you'll be pleased with the discussion of alternative building and finishing materials and flight setup tips. If you're new to this construction method, you may want to get some advice.

CONSTRUCTION

Stage 1. Contemplation and planning: two hours. You'll decide either to permanently mount the wing on the fuselage or to build a removable wing. The permanent wing mount makes a stronger aircraft that's better able to resist damage during cartwheel landings, but it requires you to cut a hatch in the fuselage for radio access. The removable wing option produces a model that's easier to transport, is nearly as strong and requires you to make the wing-mounting parts but doesn't need a hatch. The removable-wing version will be slightly heavier because the servos are mounted farther back, and more nose weight will be needed to balance the plane. Each wing mount has its advantages, and light and heavy Slope Scale planes fly well in suitable lift conditions. I decided to go with the removable wing option.

The kit supplies a conventional wire-in-tube pushrod and a nylon control horn for elevator control, but it spoils the scale look of the tail; I prefer internal elevator-control horns on scale planes. You may also decide that you'd like the leading-edge material to be tougher than the hard balsa supplied in the kit, so that your plane will survive landings on unfriendly terrain.

In building my Aircobra with a removable wing, I installed an internal elevator control and used basswood leading edges. My list of additional materials included pieces of plywood; a threading tap; nylon bolts for the wing mount; wire and brass tube for the elevator horn; and 24-inch-long, 1/2x1/2-inch basswood sticks for the leading edge.

Stage 2. Building: 22.5 hours. Begin by sheeting the wings, add the leading- and trailing-edge sticks, trim them with a small plane, and shape them with long sanding blocks. While the glue is setting up on the wing parts, wet-sand the fuselage, let it dry, spray it with primer and fill the pinholes.

Shape and sand the tail parts. The kit supplies P-63F King Cobra vertical fin parts. I made a pattern for my P-39Q Airacobra tail by enlarging a drawing from the "P-39 Airacobra in Action" and by cutting new vertical stab parts out of 3/16-inch balsa sheet. If you've decided on an internal control linkage, build and install an elevator-control horn.

Trim and sand the wing halves, cut out the ailerons and then hinge them. Install the aileron torque rods and join the wing halves with epoxy, paying attention to the dihedral and wing-sweep angles. After a suitable cure time, wrap the wing center joint with fiberglass tape, and epoxy the joint. While the epoxy is curing, wet-sand the fuselage again.

If you build a fixed-wing version, you'll need to cut an access hatch, following the instructions, so you'll be able to install both servos in the nose. For the removable-wing version, I installed the aileron servo in the wing and secured it to wooden mounting rails with screws. I installed the elevator servo inside the molded canopy with a blob of silicone glue. I connected the elevator-control horn to the servo with a 1/4-inch-diameter birch dowel that had threaded rods epoxied into holes at each end. For an external elevator-control horn, follow the instructions to install the snake-type pushrod, threaded parts and molded control horn supplied in the kit.