DeHavilland Twin Otter

Model Airplane News, May 2002 by Eisele, Anton

A sport-scale model of DH's workhorse

The Canadian de Havilland Turboprop Twin Otter was designed for use as a regional commuter and for other special applications. The plane has a 65-- foot wingspan, can carry up to 12 passengers or a load of 4,400 pounds and can operate from short runways. This 12-scale design of the Twin Otter makes a great 4-channel model that can be powered by a pair of geared Speed 480 electric motors or small .10-size glow engines. Because the model has a high-lift wing, I decided not to use flaps, but the ailerons are large enough to be used as flaperons if you're looking for STOL capability. If you go this route, you'll need to modify the wing for dual-aileron servos and use a radio with flaperon programming. If you're really into scale, there are lots of websites that can give you information on color schemes and scale detailing.

CONSTRUCTION

The wing with its constant chord is easy to build and has 3 degrees of dihedral on each panel. Start by cutting out the ribs from the material listed on the plan. Cut the top and bottom sheeting from a sheet of 1/16x3x36-inch balsa, and pin the bottom sheet to the plan. Glue the bottom 3/8 x3/16-inch balsa spar onto the leading-edge sheet so that only half of the spar width is on the sheeting. Glue the ribs into place and then glue on the top spar. Use the dihedral gauge on the plan to set the dihedral angle of the root ribs.

Now add the top leading- and trailing-edge sheeting, the 1/4-inch balsa at the end of the aileron ribs, the capstrips and the shear webs. Be careful not to sheet the area where the nacelles will go. Flip the wing and glue on the remaining bottom sheeting and the capstrips. Sand the leading-edge sheeting so it's even with the ribs and then glue on the 1/4-inch leading edge. Before the wing halves are joined, shape the leading edge to the rib profile.

Cut out the nacelle sides from 1/8-inch plywood and glue them into place. Next, install the bellcranks and aileron pushrods, and route the motor wiring; you'll need to cut a small hole in the webbing at the nacelle for the motor wires.

Join the wing halves with two plywood braces, sheet the center section of the wing and build the ailerons as shown on the plan. I mounted the aileron servo on a plate that slides into 4-inch hardwood rails, and I secured the plate with a single screw.

NACELLES

Glue formers N1, N2, N3 and MA onto the nacelle sides, and sheet the top and bottom with 12-inch-wide strips of 1/16-inch balsa. Build the cowl by gluing formers N1A, N2A and N3A onto the lfa-inch plywood cowl floor, and plank them with 1/16-inch balsa strips. Make the front section of the cowl out of 1/4-inch balsa and then sand it to shape. Screw the cowl to the front of the nacelle, and peg it to the rear of the nacelle.

Glue the motor-mounting plate and the 1/4-inch and 3/8-- inch-square hardwood rails into the nacelle. The motors are held in place with a 1/2-- inch-wide metal strip that's screwed into the 3/8-inch hardwood rail.

FUSELAGE AND TAIL ASSEMBLY

The fuselage construction is unique; the sides are built of /4-inch balsa sticks as front and rear sections (I find it easier to make the sections square and true that way). I then add the formers and longerons to give the fuselage its shape. Begin by building the two fuselage sides over the plan and gluing in the 1/4-inch balsa wing saddle. When they're dry, remove the fuselage sides from the plan, clamp them together and sand them to make them identical.

Join the aft halves with 1/4-inch-square balsa sticks, former F9 and the 1/8-inch balsa fin platform, using the plan to get everything straight. Next, join the front halves by gluing formers FS and F6 onto one of the fuselage sides and then gluing the other side onto the formers. Again, make sure everything is straight and true and then add the horizontal stringers.

Next, glue the front and back fuselage sections together and add F4 (note that it fits on the front of the fuselage box, not in between the fuselage sides). Build the box that goes between F3 and F4 from B1 and two Sls and fit them into F4.

Epoxy the 5/32-inch-i.d. brass sleeve onto F3, glue F3 onto the box and sheet the area between F3 and F4 with 1/16-inch balsa. Note that the sheeting goes only as high as B1.

Glue the two 1/8-inch plywood main landing-gear plates into place between formers FS and F6. The main landing gear is made of 1/8-inch-diameter wire, and the front gear is made of 5/32-inch-diameter wire. Now add the rest of the formers and the 1/8x1/4-inch balsa stringers. Note that the number of stringers decreases toward the rear of the fuselage.

Build the nose by gluing the balsa platform and the two 1/8-inch balsa stringers onto former F1. Now glue on former F2, sheet the nose with 1/16-inch balsa and then add the balsa nose block. Glue the assembly to F3. Place the wing on the fuselage and make the holes for the 3/16-inch-diameter wing hold-down dowels by drilling through F5 and into the leading edge. Remove the wing, and glue the dowels into it. Build the front deck and the battery access hatch out of 1/8--inch balsa. Glue B2 onto the front of F4, fill the top part with soft balsa and sand it to shape. Carefully sand the entire fuselage, and set it aside.