ABSTRACT
Although simulation of nonlinear flight dynamics, in general, requires full sixdegrees-of-freedom (6DOF) equations of motion (EOMs), it is not always necessary to use very complex and coupled models in the design of flight control laws, dynamic analysis, and handling qualities analysis procedures. Simple models with essential derivatives that capture the basic characteristics of the vehicle normally yield adequate results. However, caution must be exercised in simplifying the model too much as this may not be able to represent the system dynamics properly. Some of the reasons why flight analysts prefer simplified models are (1) full complex models are difficult to interpret and analyze, (2) it is possible to separate the model equations into independent subsets without much loss of accuracy, (3) ease of linearization, (4) unnecessary details that have little effect on system dynamics are avoided, (5) excursions during flight test maneuvers can be restricted to apply the assumption of linearity, and (6) easy to implement software codes are available for handling qualities analysis and parameter estimation. The simplification of nonlinear EOMs into workable and easily usable linear models can bring about connections between various aircraft configurations via the transfer-function (TF) analysis and frequency responses. The aircraft may be of different sizes and wing-body configurations, but the dynamic characteristics may be the same. The TF analysis and zeros=poles disposition can be useful in assessing the dynamic characteristics of these aircraft, rotorcraft, missiles, UAVs (unmanned=uninhibited aerial vehicles), MAVs (micro-air vehicles), and airships. Simplified models are used in the design of autopilot control laws and in evaluation of handling qualities of the vehicle. The detailed effects of particular derivatives can be observed. Also, it becomes easy to obtain the aerodynamic derivatives of the vehicle from the flight data, using linear mathematical models in a parameter-estimation algorithm.
