The Second Boys' Book of Model Aeroplanes

by Francis A. Collins

Motor bases are of two general divisions. The "single stickers," or bases consisting of one member, are commonly called "spars," while the more complicated frameworks are designated as "built-up" frames. The spar type is, of course, the simplest to construct, and, as many believe, the most efficient of all forms. The simpler the design, as a rule, the less chance will there be of breakage. For the beginner the use of plain, honest sticks is, of course, to be recommended.

The built-up motor bases, on the other hand, make a more attractive model to the eye. There is besides an opportunity to reduce the weight of the frame while retaining the same strength. An examination of the models illustrated elsewhere will show to what an art such construction has been brought. By ingenious bracing it is possible to construct motor bases of strips one-sixteenth of an inch, or even less, in width, and yet render the whole sufficiently rigid to withstand the pull of powerful twin motors.

Your frame may be considerably lightened by using hollow sticks or shafts in place of solid members. A stick three-fourths of an inch wide formed of light lath one-eighth of an inch thick will weigh no more than a solid piece one quarter of an inch square. Such a member is stronger than the small, solid stick; it bends less readily under the pull of your motors, and renders the entire frame far more rigid. It will also be found much more satisfactory to mount the planes upon such a frame.

A little practice will enable you to make a very satisfactory stick of this pattern. Secure a light strip one-eighth of an inch thick and of a width one-eighth of an inch less than that of the stick you intend to build. A one-inch stick is probably larger than you will need. The following directions are intended for a stick three-fourths of an inch square.

First cut three square plugs one-half an inch square and one-fourth of an inch thick, and placing one at either end and one in the middle to form a core, build your frame about them. The edges should overlap and be glued continuously together, and the plugs fastened in position with brads driven from the middle of the four sides. When dry, cut away the glue, sandpaper and varnish.

Some model builders prefer a T-section-shaped spar, which is almost as light as the hollow stick, besides being easier to construct. We assume that you are working with eight-inch strips, which will prove heavy enough for ordinary purposes. Prepare one strip one-half an inch wide, the desired length of your base, and the second strip three-eighths of an inch wide, the thickness and length being the same.

Now fasten the smaller strip at the center of the long strip, glueing it first in position. When dry, drive a series of fine brads along the center of the back of the larger strip. Cut away the glue and sandpaper. The T-shaped stick will be found strong enough for all ordinary demands. The rubber strands of your motor may be carried either above or below it.

The H-shaped-section stick is rather more difficult to build, but it will be found somewhat stronger, weight for weight. If you are using one-eighth strips, cut two lengths one-half an inch wide, and a third length three-eighths of an inch wide. Fasten the smaller pieces to the middle of one of these strips, as in the case of the T stick, with glue and brads. When dry, attach the remaining strip opposite, glueing and nailing as before. Some builders prepare these strips by cutting out the grooves with a chisel, thus making a one-piece strip. This requires very careful workmanship, however, and is scarcely worth the trouble.

The motor frame may be still further lightened by using a trussed frame. The weight of this member may be cut in two in this way without sacrificing its strength. To build such a part secure two strips of wood one-eighth of an inch thick, one inch in width, and cut to the desired length. Now from the same material cut six blocks, one-half an inch in length, and set these at regular intervals along one side of the strip. They may be glued or nailed in position, or both. A small brad will hold them in place. In working with such delicate material it will be well to first drill the holes with a fine drill. Next fasten the second strip above them, nailing and glueing as before.