TABLE OF CONTENTS Physical model structures are direct implementations of the equations of motion for the flight-vehicle system or subsystem components. These are wide ranging in complexity, depending mostly on the assumed level of coupling and system order. As always in system identification, the choice of model structure depends on the end application of the model, the frequency range of applicability, and the associated key vehicle dynamic characteristics. In the discussion of model structures that follows, the reader should refer back to Tables 1.1 and 1.2.
At the simpler end of the spectrum, the rigid-body flight dynamics of most conventional fixed-wing aircraft, tilt-rotor (e.g., XV-15 and V-22), and tandem rotor (e.g., CH-47) configurations are well represented by the decoupled classical three-DOF longitudinal and three-DOF lateral-directional dynamics equations. The unknown coefficients to be identified in the model structure are the conventional stability and control derivatives, which result from a Taylor-series representation of the aircraft aerodynamics. Model structures for fixed-wing aircraft configurations are widely available in the flight-dynamics literature. They can be expressed in either dimensional form (e.g., X u, X w, M u, M w, M q, M ?), as by McRuer, et al.56 and McLean,81 or in nondimensional form ( 
) as by Blakelock.146 (The dimensional form is in more common use now because it provides greater insight into the key dynamic characteristics.) These three-DOF model structures involve relatively few identification parameters (fewer than 20) and are generally identified to good accuracy if the recommended...
