Leading edges of the empennage are capped with 1⁄8 × 1⁄4-inch balsa strips (at left) and rounded off. The one-piece elevator is attached to the stabilizer with flex nylon hinges at four places. The wing (above) has a single spar made of balsa capped with spruce strips, cardboard ribs and leading edge folds to obtain curved upper surface. Note 1⁄8 ply spar joiners front and rear at centerline.
Construction tips
Before we begin, take a look at these spe- cial tips for working with cardboard. Glue: Water base glue, such as white glue or Titebond, is recommended. Contact cement is not recommended since parts can- not be shifted when gluing surfaces. Folding: The scoring of the fold lines is done with a screening tool available at any hardware store. It consists of a handle with a 11⁄2-inch radius wheel at one end, which is run along a metal straightedge on the fold line.
Finishing: Cardboard gives a solid sur-
face with no open areas to cover and is non- porous. The easiest finishing method is to give two coats of clear dope, sanding lightly between coats with #400 sandpaper, fol- lowed by two coats of color dope. However, a wide variety of finishing materials may be used on the cardboard. Coverings such as Solarfilm, MonoKote and vinyl paper can be used. With any of these, it is recommended that the surface not be doped, which will re- sult in a better bond. Paper tape: All seams, joints and ex- posed edges of the model are covered with strips of gummed paper tape. Obtain a one- inch wide roll from a stationary store. Sim- ply cut a thin strip to length, dip it in water and smooth it over the seam.
Construction Be sure to note the direction of the corru-
gations when cutting out the cardboard parts. Score and fold cardboard parts as in- dicated on the plans. Empennage: The fin, rudder, stabilizer and one-piece elevator are each made from two pieces of 1⁄8-inch cardboard laminated together cross grain to give 1⁄4-inch thick sur- faces. Add a 1⁄8×1⁄4-inch balsa strip to the fin leading edge and round off. Add 1⁄8 × 1⁄4-inch balsa strips to the stabilizer leading and trailing edges and round off. Add a 1⁄8 × 1⁄4- inch balsa strip to the elevator leading edge and round off. Seal all raw edges with gummed paper tape. Hinge the elevator to the stabilizer with flex hinges at four places. Note that the fin has a tab on the bottom which is eventually glued into the slot in the top fuselage F3 piece. Wing: Make each wing spar from 1⁄4-inch
balsa, capped top and bottom with 1⁄4-inch square spruce and join the two spars togeth- er with 1⁄8-inch ply joiners, front and back. The M.20 wing has zero dihedral. Glue the right spar into the bottom of the right wing panel. Add all cardboard ribs W1 through W6. Glue a one-ounce weight to the right wing tip. Glue the inboard panel by ap- plying glue to the top of the W1 ribs, the top of the wing spar and the trailing edge of the wing between those ribs and fold the top wing surface down and pin securely in place until dry.
When dry, glue down the tapered out- board top wing panel. Build the left wing
panel in a similar fashion. Add the balsa tips to the wing and sand to shape. Make a line guide from 1⁄8-inch ply. Cut slots in the left wing balsa tip and glue the line guide in place. Cover the trailing edge and all seams with gummed paper tape. Fuselage: The flat fuselage sides are out- lined with a triangular symbol on the draw- ing. Line the upper and lower edges of each fuselage side with 1⁄8 × 1⁄4-inch balsa strips, as shown in the fuselage side view. The strips are recessed 1⁄8 inch in from the fuse- lage edges. Bevel the strips at the aft end of the fuselage so that the cardboard sides will come together.
Add cardboard supports to each fuselage side below the bellcrank and above the fuel tank. Be sure and cut holes in the left fuse- lage side for the bellcrank leadout lines and pushrod exit. Note that each fuselage side is scored and folded in one place, aft of the wing.
Make the firewall, FW, from 1⁄4-inch ply. Locate the mounting holes for the KM40 mo- tor mount on the face of FW. Drill the mounting holes and install blind mounting nuts on the back side. Drill a hole in FW for a fuel tubing exit. Drill two holes in FW and install blind nuts on the back side. These holes must align with the holes in C1 and are used to attach the removable bottom cowl section.
Lay the right side fuselage panel flat on the work board and glue the firewall FW in
Flat fuselage sides are lined with balsa strips (above left) recessed 1⁄8 inch from the edge and have cardboard supports for bellcrank and fuel tank ply mounts. Note the cutouts for the flying wires and the pushrod exit in the left side and notches in the bottom edge of both sides for the ply gear supports. The ply
FLYING MODELS
firewall has blind nuts for the KM40 engine mount and cowl attach. The bellcrank and fuel tank are attached to 1⁄8-inch ply supports (above right), which are glued to cardboard supports. The tank is held in place with rubber bands. The pushrod is made from 1⁄4-inch square spruce.
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