ABSTRACT
Apart from the equations of motion (EOM) discussed in Chapter 3, the quantification and analysis of aerodynamic models is most important for flight mechanics analysis. The forces and moments that act on an aircraft can be distributed and described in terms of the so-called aerodynamic derivatives. The stability and control analysis is an integral part of the aircraft design cycles, design evaluations, and design of control systems. In this analysis, aerodynamic derivatives, often known as stability and control derivatives, form the basic inputs. For the conventional stable aircraft configurations, these derivatives directly ensure the adequate levels of stability and controllability. For unstable aircraft configurations, these derivatives form the essential input parameters for the design of control laws, which, in turn, ensure the desired stability and controllability characteristics of the aircraft. The stability derivatives also play a very important role in the selection process of the aircraft configuration, as will be seen in Section 4.6. This aerodynamic model then becomes an integral part of the EOM, which in totality represents the aerodynamics and dynamic behavior of the vehicle. The accuracy of the results of any flight mechanics analysis depends on the degree of completeness and approximation of aerodynamic models used for such analysis. Due to the assumption of linearity often made, the validity of these (the EOM and aerodynamic) models is limited to a small range of operating conditions, which are generally in terms of AOA and Mach number; often the excursion in AOSS is assumed to be very small. In general, aerodynamic derivatives are applicable to small perturbations of the motion about an equilibrium, and this leads to quasi-static aerodynamic derivatives. Large-amplitude motions/maneuvers require sophisticated and more complex aerodynamic modeling. In this chapter, we specifically deal with aerodynamic modeling that basically consists of aerodynamic coefficients, which, in turn, expand to (or encompass) aerodynamic derivatives. Certain stability and control aspects (of aircraft) are captured in certain aerodynamic derivatives. Primary knowledge of aerodynamic coefficients, especially of aerodynamic derivatives, can be obtained from some analytical means like DATCOM, CFD, and/or wind tunnel experiments. When flight tests are conducted on an airplane and if it is instrumented properly with sensors, then one can measure dynamic responses (p, q, r, AOA, AOSS, Euler angles, linear accelerations) of the aircraft when certain maneuvers (Chapter 7) are performed. These response data can be processed using parameter estimation methods to obtain these aerodynamic coefficients/derivatives (ACDs), as discussed in Chapter 9. The interplay of aerodynamic coefficients, derivatives, and EOM at the top level is shown in Figure 4.1.
