Page:Aerial Flight - Volume 2 - Aerodonetics - Frederick Lanchester - 1908.djvu/178



§ 83. Introductory.—Lateral and directional stability relate to the equilibrium of the aerodone in respect of its three degrees of freedom involved by motions other than those in the phugoid plane. Thus lateral stability is concerned primarily with rotation about the axis of flight; that is, a motion resembling the

Fig. 75.

rolling of a ship; if such a motion does not give rise to a restoring couple an aerodone may turn over, otherwise "capsize," and so lose its equilibrium. Directional stability involves rotation about a vertical axis and motion of translation at right angles to the phugoid plane, these two kinds of motion being associated in any change of course.