Important Points.—­It is absolutely essential that the weight be kept down as much as possible.  It is quite a mistake to suppose that weight necessarily means strength.  On the contrary, it may actually be a cause of weakness if employed in the wrong place and in the wrong way.  The heavier the machine, the more serious the damage done in the event of a bad landing.  One of the best and easiest ways of ensuring lightness is to let the model be of very simple construction.  Such a model is easier to build and more efficient when constructed than one of more complicated design.  Weigh every part of your model as you construct it, and do not be content until all symmetrically arranged parts which should weigh the same not only look alike but do actually balance one another. (Note.—­The writer always works out the various parts of his models in grammes, not ounces.) If a sufficiently strong propeller bearing weighing only half a gramme can be employed, so much the better, as you have more margin left for some other part of the model in which it would be inadvisable to cut down the weight to a very fine limit.

Details.—­To pass now to details, we have four distinct parts to deal with:—­

1.  The framework, or fuselage.

2.  The supporting surfaces, consisting of the main plane, or aerofoil, behind, and the elevator in front.

3.  The propellers.

4.  The motor, in this case two long skeins of rubber; long, because we wish to be able to give our motor many turns, from 700 to, say, 1,000 as a limit, so that the duration of flight may be considerable.

[Illustration:  Fig. 129.-Sections of backbone for model aeroplane.]

The Backbone.—­For the backbone or central rod take a piece of pitch pine or satin walnut 52 inches long, 5/8 inch deep, and 1/2 inch broad, and plane it down carefully until it has a T-shaped section, as shown in Fig. 129, and the thickness is not anywhere more than 1/8 inch.  It is quite possible to reduce the thickness to even 1/16 inch and still have a sufficient reserve of strength to withstand the pull of 28 strands of 1/16-inch rubber wound up 1,000 times; but such a course is not advisable unless you are a skilful planer and have had some experience in model-making.

If you find the construction of the T-shaped rod too difficult, two courses are open—­

(l) To get a carpenter to do the job for you, or

(2) To give the rod the triangular section shown in Fig. 129, each side of the equilateral triangle being half an inch long.

[Illustration:  Fig. 150—­Side elevation of model aeroplane.]

The top of the T or the base of the triangle, as the case may be, is used uppermost.  This rod must be pierced in three places for the vertical masts employed in the bracing of the rod, trussing the main plane, and adjusting the elevator.  These are spaced out in Fig. 130, which shows a side elevation of the model.  Their sectional dimensions are 1/16 by 1/4 inch; their respective lengths are given in Fig. 130.  Round the front edges and sharpen the rear.