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In this paper, the control law design for flexible aircraft is discussed. First, the traditional procedure of decoupling rigid-body and aeroelastic dynamics with low-pass and notch filters is addressed, with focus on controller performance as well as the resulting stability margin issues. A procedure based on a unified formulation of the flexible aircraft dynamics for flight control law design is proposed. In this procedure, the aeroservoelastic dynamics is assessed in the loop, and the offline filtering process is avoided. The formulation is applied to the virtual aircraft generic narrow-body airliner, with improvements in closed-loop performance and stability margins.

This paper presents an expedient transonic correction technique to compute unsteady pressure distributions and aeroelastic stability in the transonic flow regime. The transonic correction procedure here is an improvement of the downwash weighting method proposed previously by several authors. The previous downwash weighting methods could provide pressure and/or force corrections to some extent by applying different weighting methods on the lifting-surface self-induced downwash resulting from aeroelastic structural displacements or prescribed motions. However,the resulting pressure/force solutionswere often foundto be inconsistent, becausethey all failed toinclude the proper transonic unsteady and out-of-phase effects. Our improved downwash correction method is a rational formulation to include proper transonic effects, as this formulation is based on a successive kernel expansion procedure established in accord with the formal pressure-downwash relation. Accordingly, the developed transonic correction procedureisaproperandrational onethatis expectedtoyield moreconsistent aeroelasticsolutions.This procedure is now a fully developed program, known as the transonic weighting aerodynamic influence coefficient procedure in the ZAERO software system, or ZTAW. Computed results by ZTAW for the unsteady pressures and aeroelastic stability boundaries for four selected wing planforms (AGARD 445.6, F-5, LANN, Lessing wings) are found to be in good agreement with measured data. In contrast to the computational-fluid-dynamics-based methods of computational aeroelasticity, the present procedure is proven to be far more computationally efficient and industrially viable while yielding comparable aeroelastic solutions.

A finite -difference Navier -Stokes code is used in order to study the linearity of aerodynamic loads with respect to the dynamic angle of attack in three dimensional transonic flow. Steady and unsteady pressure coefficients for prescribed rigid angle of attack motion are computed for a F -5 wing for which the method has been previously validated. The study is aimed at identifying the conditions under which approximate flutter analyses based on corrections to aerodynamic influence coefficients may be used. Results indicate that higher harmonics of the unsteady loads are present in the aerodynamic response and that the boundaries for linear behavior depend on the spanwise location along the wing.