Deals with derivatives of forces, moments and of other quantities characterizing such forces and moments. The term derivative designates the partial derivative of a function with respect to an independent variable. These derivatives appear in the terms of the Taylor series representing the variations of functions with the independent variables. Is restricted to first-order terms. Terms of higher order would require additional definitions of higher order. The aircraft is assumed to be rigid. However, most of the definitions can be applied to the case of a flexible aircraft.