AERODYNAMIC PARAMETERS

The aerodynamic coefficients would, in general, be a function of not only the flow angles but also of Mach number, angular rates, control surface deflections, and changes in thrust force due to the movement of the throttle lever arm. For example, Cm from Equation 4.2 can be expressed in terms of these independent response variables in a straightforward manner as

Cm = function of (a, q, 8e, M, FT)

= Cm0 + Cmaa + Cmq F Cmde de + CmMM + CmFjFT (4-6)

Here, the pitching moment aerodynamic coefficient is considered a function of several independent variables: AOA, pitch rate q, elevator control surface deflec­tion, Mach number, and thrust. The effect on the pitching moment, due to small changes in these response variables, is captured in terms that are called aerodynamic derivatives (e. g., Cm ). These terms can be considered proportionality constants. However, these constants vary with certain variables. The quantity is the non­dimensional angular rate (velocity) as can be seen from (rad/s) (m/2) (s/m) => rad. The small perturbation theory when applied to the simplification of the EOM leads to the fact that the aerodynamic forces and moments depend on some constants that are called stability derivatives. The parametric terms of Equation 4.6 are called aerodynamic derivatives, and often stability and control derivatives, as many of these derivatives govern the stability and control of the aircraft (dynamics), as will be shown later. For example, Cm signifies -—у when other variables are set to zero.