FIG. 7. Showing the disturbance created by a small spar on the back of a plane.
It is obvious then that bodies which are to move through the air with the least resistance possible should be given such a shape that the stream lines of air will flow around it smoothly and not leave a dead region behind. In other words, the stream line flow of the air shall keep the same contour as the surface.
The ichthyoid or fish-like form is of such a shape. This is illustrated in Fig. 8. Its greatest diameter should be about two-fifths of its entire length from the head. All struts, stanchions, etc., should be given this shape.
FIG. 8. Diagram illustrating the ichthyoid shape and how smoothly it slips through the air without creating an eddy.
This shape is very interesting because of its probable origin, for a glance is sufficient to tell that it not only resembles a fish but also the body of a bird.
Weight is an all-important item in model aeroplaning. How to obtain the maximum strength with the minimum of weight is undoubtedly the most difficult problem which the aviator has to solve. Weight is a much more important factor in model aeroplanes than in the case of full-size machines because models do not fly fast enough to possess a high weight-carrying capacity.
FIG. 9. Of the three shapes shown above, the round one will slip through the air with the least disturbance and resistance. A bar of wood like (A), 2 inches square, showed a "drift" of 5.16 lbs. when placed in a breeze blowing 49 miles per hour. Turning it as shown by (B) changed the "drift" to 5.47 lbs. A round bar, 2 inches in diameter, like (C) showed 2.97 lbs. "drift" under the same conditions.
It is only by the constant use of a pair of scales and an accurate knowledge of materials with the ability to combine them in the most efficient manner that the weight and strength may be kept in harmony. Such knowledge and experience come only with practice. They may, however, be acquired by any one. In this regard, a notebook forms an almost indispensable aid to the experimenter. After a machine has been built an accurate record of every flight and of every alteration or change in material should be made.