diminished as much as possible by concentrating the ballast in the middle of the front edge of the plane, when it is found that the oscillations die out with great rapidity. If the ballast is distributed along the front edge, or if the plane itself be of too great weight in proportion to the total weight of the plane and ballast, the oscillations do not die away, but tend to increase in amplitude, so that instability results.
§ 6. Ballasted Aeroplane. Directional Stability.—The directional stability of an aerodone may be defined as its stability about a vertical axis. It is evident that if an aerodone were liable to rotational changes of position about a vertical axis, its stability in other respects would be impaired; if, for example, in an extreme case it were liable to slue completely round in a manner analogous to the "side slip" of a motor car, it might at any moment find itself travelling stern foremost, and its longitudinal stability would have vanished.
Furthermore, we have already seen that lateral stability is dependent upon freedom of lateral motion, and to some extent the constancy of angular position (about a vertical axis) has been assumed in the explanation given (§ 5).
In the ballasted aeroplane the directional stability is primarily due to the influence of skin friction. It is manifest that if there were no viscous connection between the plane and the air, if skin friction were absent in fact, there would be nothing whatever to restrain the plane from rotation about an axis at right angles to its surfaces, and any accidental irregularity in its form, such as a slight "wind" or twist, would result in it receiving a spin of continually increasing angular velocity. The influence of skin friction is to damp out any rotary motion that may be accidentally acquired. This influence is an appreciable factor when a plane is translationally at rest, but it is far more potent when the plane is in motion. The reason for this is that the skin friction varies approximately as the square of the velocity, and the difference between this quantity for the right and left hand "wings" of
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