§ 4. Referring to D. Many model builders make this mistake, i.e., the mistake of getting as low a centre of gravity as possible under the quite erroneous idea that they are thereby increasing the stability of the machine. Theoretically the centre of gravity should be the centre of head resistance, as also the centre of pressure.
In practice some prefer to put the centre of gravity in models slightly above the centre of head resistance, the reason being that, generally speaking, wind gusts have a "lifting" action on the machine. It must be carefully borne in mind, however, that if the centre of wind pressure on the aerofoil surface and the centre of gravity do not coincide, no matter at what point propulsive action be applied, it can be proved by quite elementary mechanics that such an arrangement, known as "acentric," produces a couple tending to upset the machine.
This action is the probable cause of many failures.
Fig. 5.—The Stringfellow Model Monoplane of 1848.
§ 5. Referring to E. If the propulsive action does not pass through the centre of gravity the system again becomes "acentric." Even supposing condition D fulfilled, and we arrive at the following most important result, viz., that for stability:—
The centres of gravity, of pressure, of head resistance, should be coincident, and the propulsive action of the propeller pass through this same point.
Fig. 6.—The Stringfellow Model Triplane of 1868.
§ 6. Referring to F and N—the problem of longitudinal stability. There is one absolutely essential feature not mentioned in F or N, and that is for automatic longitudinal stability the two surfaces, the aerofoil proper and the balancer (elevator or tail, or both), must be separated by some considerable distance, a distance not less than four times the width of the main aerofoil.[9] More is better.