The general equations of motion for an airplane with a number of spherical fuel tanks are derived. These equations are applied to two cases with two fuel tanks located in the plane of symmetry. The calculated motions show that the airplane motion may be greatly changed by considering the motion of the fuel and, in particular, the small-amplitude residual oscillations may result. The same type of analysis may be applied to arbitrarily shaped tanks; therefore, the most general conclusions as to the effects of the fuel motion on airplane dynamics also apply for arbitrarily shaped tanks.