FIG. 10. The figures given above each shape show the "drift" in lbs. of wooden bars of those shapes when placed in a wind blowing 40 miles an hour. The bars experimented with had a depth of 9 inches in the direction of the arrows and were 2 inches wide.
Automatic stability without doubt has attracted more attention from engineers and aviators than any other one problem connected with aviation. Since it is not possible for a model aeroplane to carry a pilot it is much more important that it should be naturally stable than any of its man-carrying prototypes. Automatic stability in a model of only two or three feet spread at the most, is quite a different proposition from that offered by a full-size machine.
FIG. 11. Flat and dihedral planes.
It would at first seem, that by placing the centre of gravity of the machine very low such stability could be secured. This is accomplished to a certain extent by setting the wings or planes at a dihedral angle. But if the angle is excessive, the aeroplane will fly with a pitching motion known as accentricity.
The centres of gravity, of pressure and of head resistance should be at the same point. The centre of thrust of the propeller should also pass through this point. In this will be found the secret of the successful model aeroplane. It is only arrived at by careful experiment and calculation.
Head resistance increases stability while weight and speed lessen it. When an aeroplane is gliding (traveling downwards) its stability is greater than when it is rising or flying horizontally. It is the least stable when rising.
CHAPTER III. PLANES AND RUDDERS. ELEVATORS AND TAILS.
In Chapter I it was explained that an aeroplane is fundamentally composed of a supporting surface, divided into one or two parts, usually the planes or wings, which cut the air in an oblique manner, driven by a propeller and motor. Before going further it is perhaps best to understand more exactly how the planes operate and support the machine in the air than it was possible to explain in the first chapter without confusion. A theoretical aeroplane consists of a flat surface or plane. When the propeller is set into motion the plane is driven through the air in an oblique manner and compels the gaseous molecules to glide under its surface. Since the plane is at an angle, the front edge being higher than the back, the air must necessarily leave at the rear in a downward direction. The air molecules in traveling under the surface exercise a resistance upon it which is really a pressure against the plane. When this pressure is resolved into its components, it is found to be made up of two forces, one horizontal, tending to retard the forward motion, and called the drift; the other, vertical and tending to lift the plane.