Conceptual Flexible Aircraft Model for Modeling, Analysis and Control Studies Andr´ e Lu´ ıs da Silva, * Pedro Paglione † and Takashi Yoneyama ‡ Technological Institute of Aeronautics (ITA), S˜ao Jos´ e dos Campos, SP, 12228-900, Brazil In this paper, we present models of conceptual flexible aircraft suitable for aerodynamic, flight mechanics and flight control studies. The aircraft is representative of a medium size civil jet transport and it is developed in three configurations of increasing flexibility. The model integrates 6 degrees of freedom rigid and structural dynamics, which are obtained via finite elements and modal decomposition (up to 9 flexible modes are considered). An incremental unsteady aerodynamic model is determined via the Doublet Lattice method. The aircraft possesses 8 aerodynamic control effectors, suitable for control of the rigid and flexible modes. A rigid body aerodynamic model is also considered. A brief numeric discussion about flutter is also presented. I. Introduction Every aircraft is, in fact, a flexible body and its motion should be evaluated by the mechanic of continuous bodies. However, an aircraft is commonly treated, in the flight mechanics, as a rigid body. In other way, in the structural dynamics, the rigid body modes are commonly neglected. Such separation is acceptable when there is sufficient separation between rigid and flexible modes natural frequencies. However, trends in civil transport aircraft industry are leading to aircrafts with longer fuselages, larger aspect ratios, smaller thicknesses, composite material structures (characteristics that are already present in some unmanned air- craft, such as the NASA Helios aircraft). These configurations lead to aircrafts that are more flexible than usual, so the flexible and rigid body modes natural frequencies tend to approximate, and integrated models for flight mechanics and structural dynamics need to be considered. One of the earlier works in the flight mechanics of flexible aircraft is due to Milne (1964). 1 In this work, the concept of mean axis reference frame (MRF) is introduced, in order to promote inertial decoupling between rigid and flexible modes; the steady strip theory is applied to determine the incremental aerodynamic loads due to flexibility; the structural dynamics is determined by tension-deformation relationships in control points and displacements compatibility. The approach is applied to the longitudinal dynamics. In Dusto et al. (1974), 2 the MRF is chosen to write the equations of motion, while the aerodynamic frame is chosen to write the aerodynamic forces. In Cavin and Dusto (1977), 3 the Hamilton principle is invoked to determine the motion equations, the MRF is considered and the structural dynamics is modeled via finite elements. In Karpel (1981), 4 unsteady aerodynamics via rational function approximation (RFA) is adopted, and a procedure to include traditional control and stability derivatives in the RFA is presented. In Waszak and Schmidt (1988), 5 under some assumptions, the coincidence between the MRF and the usual rigid body attached frame (RBRF) is considered; the equations of motion are derived via the Lagrange formalism; the structural dynamics is represented via the modal decomposition; the incremental aerodynam- ics is determined via the quasi steady strip theory for the longitudinal dynamics. The major contribution is a flexible aircraft flight mechanics model that extends the traditional rigid body model, with flexible and rigid body dynamics connected only via incremental aerodynamic forces and moments and generalized forces in the flexible modes. The same methodology is found in Ref. 6. In Buttrill et al. (1987), 7 the MRF and the RBRF are considered distinct, the similarity with the rigid body models is lost. It is shown that the * Doctorate Student, Electronic Engineering Division, Systems and Control, andretaura@yahoo.com.br † Professor Associado, Aeronautical Engineering Division, Flight Mechanics, paglione@ita.br ‡ Professor Titular, Electronic Engineering Division, Systems and Control, takashi@ita.br 1 of 27 American Institute of Aeronautics and Astronautics