# NONDIMENSIONAL STABILITY DERIVATIVES

The nondimensional stability derivatives are the partial derivatives of the force and moment coefficients in lines 1, 3, and 4 of Table 4.1 with respect to the nondimen­sional motion variables in lines 5, 6, and 7. The notation for these is displayed in Ta­bles 4.2 and 4.3. Each entry in the tables represents the derivative of the column heading with respect to the row variable.

Since ax differs from a only by a constant (the angle between the zero-lift line and the x axis), then Дax = Да, Э/Эал = Э/Эа, and no distinction need be made be­tween these two derivatives.

NONDIMENSIONAL EQUATIONS

It is possible with the definitions given in Tables 4.1—4.3 to make the equations of motion entirely nondimensional, and such equations have been widely used in the past, especially for analytical work (see Etkin, 1972 and 1982). The prevailing cur­rent practise in design and research, however, is to use the dimensional equations and program them for calculation on a digital computer. We are therefore not including the nondimensional equations in this book. There is no real loss in so doing however, since any analytical results that are obtained with the dimensional equations can sub­sequently be expressed, for maximum generality, in nondimensional form. Examples of this are contained in Chaps. 6 and 7.

Table 4.3

Lateral Nondimensional Derivatives

 cv c, Cn /3 C>e c«. p c» C’r c„ r Cv Cl,- C„, /3 Cy\$ C,