Building Scale Models.—Models of well-known machines should be built to correct proportions, if as perfect a resemblance as possible is aimed at. The best way to do this is, of course, to adopt a definite scale. The particular scale will depend principally on the size the builder requires his model; but the size of the prototype must, of course, be considered, because the large machines differ so much in point of size.
Taking the span or width across the planes as the base from which to start, it is assumed that the width of the model is desired to be from 25 in. to 35 in., which is perhaps the best all-round minimum and maximum to adopt. Then having decided on the prototype, multiply the span of the real machine by a fraction, which brings the model span somewhere between the two figures. For instance, suppose it is desired to model an Antoinette monoplane, the span of which is about 46 ft., and multiplying by ¾ the model span becomes 34½; therefore the scale is ¾ in. to the foot.
If the model is to be a Blériot, then as the original has a span of 28 ft., the model may be built to a scale of 1 in. to the foot. The Wright machine has a span of 41 ft., so a model to ¾ in. to the foot would have a span of 30¾ in. In this case, perhaps, 1-in. scale would not be considered too large. Odd scales such as ⅞ in. to the foot can, of course, be adopted; but whatever the scale is to be, the model should be set out full-size on a sheet of cartridge paper, and the scale drawn accurately at the foot. The ribs should be built up as in [Fig. 36].
In designing a rubber-driven model, absolute scale must of necessity be departed from, except in the principal measurements and in the distance of centres apart of spars and other important members, which if not reproduced in their proper form and position would mar the otherwise correct appearance of the machine. Many of the spars will, of course, need to be increased in cross-sectional dimension in order to make them of sufficient strength. Some efficient wing plans are given by [Fig. 37].
Stated briefly, there are essentially three kinds of model aeroplanes. First, the scale model, which is a reproduction to scale of a real machine; second, a modified copy of a large machine, which is so designed as to resemble in general form some well-known prototype, while retaining by means of a suitable motor, generally twisted rubber, some ability to fly; third, a machine which does not in any way follow the lines of full-size machines, and is built for flight only.
The first of these is essentially an exhibition model; it is more often built either to illustrate points in the design and construction of large machines, or to demonstrate the functions of the various parts to technical classes, etc.
Scale models, as a rule, are unsatisfactory flyers, and if they fly at all the flight is so short that little can be learned from their performance.
Some serviceable types of bearings are given by Figs. [38] and [38A], on p. 31.
CHAPTER IV
Practical Construction:
Carving Air-screws
One of the most important units of an aeroplane, whether full-size or model, is the screw, since excellence of design with regard to the other portions of the machine are rendered void if the means of converting the power of the engine into work are inefficient.