the aeroplane, for a given rate of spin, is greater the greater the velocity of flight.
If, as suggested in § 5, the mica plate have a slight twist or wind, the pressure reaction on either the right or left hand "wing" will be in excess, and, as we have already seen, this results in the aeroplane setting itself diagonally to the line of flight, so that the torque about the axis of flight due to the defect of symmetry is balanced by that due to the end effect. Under these conditions the permanence or otherwise of direction depends upon the alignment of the resistance resultant, and
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Fig. 6.
the gravity component which forms the propulsive force. Thus if (Fig. 6) these two forces are not in the same straight line they constitute a couple, and give rise to a torque about an axis normal to the aeroplane, and the flight path will not be straight. The same considerations apply if the initial want of symmetry is due to other causes, such as an unsymmetrical distribution of ballast, an unsymmetrical variation of the smoothness, etc., etc., the most frequent cause of trouble, however, when mica is employed, is that stated.
So long as the plane is carefully constructed from a plate of mica that is a sufficient approximation to a true plane, the turning moment that arises about a vertical axis is never of{[c|10}}
