Page:Aerial Flight - Volume 2 - Aerodonetics - Frederick Lanchester - 1908.djvu/30

§ 5 to speak, be eating its own up-current, and consequently it will experience an excess of pressure, and a turning moment will arise tending to elevate the end L of the plane. When the plane acting under the influence of this couple has assumed a laterally inclined position (Fig. 5), the forces W (the weight of the plane) and F (its pressure reaction) will have a tangential resultant, and the plane will proceed to slide downhill, that is in the direction indicated by the arrow; this motion will result in an alteration of course which will continue until the direction of flight is again parallel to the end edges of the plane, when the turning couple about the axis of flight will vanish.

If the initial disturbing cause be some want of symmetry of the plane, such as a slight twist or "wind" (a defect to which

Fig. 5.

mica is extremely liable), then the final position of equilibrium is such that the edges of the plane are slightly diagonal to its line of flight, the couple due to the want of symmetry being then balanced by an equal and opposite couple due to the added reaction on the one or the other end of the plane. It is obvious that the same causes that result in the plane setting itself "square" to the line of flight in the case of a symmetrical plane result in it automatically setting itself at the appropriate angle in the case of a plane of defective symmetry.

It is evident that the motions of the plane after a disturbance will not be dead-beat as has been suggested, but that owing to the moment of inertia about the axis of flight, there will be a period of oscillation while the plane is settling down to its position of equilibrium. In practice the moment of inertia is {[c|8}}