Historically, flight control laws have been designed based on the quasi-steady, mean axis flying qualities of an aircraft. However, as airplanes get larger and larger, flexibility and structural dynamics become more and more important. In order to address the influence of aeroservoelastic interactions in large aircraft, it is necessary to include structural dynamic and aeroelastic effects in the simulation tools used for control law design. This results in the generation of a dynamic ASE model with a large number of degrees of freedom for many flight conditions, which creates significant challenges for both the structural and flight controls engineers.

An additional difficulty in aeroservoelastic analysis stems from the fact that the modeling and analysis techniques that are most applicable to aeroelastic loads or flutter analysis are not necessarily those that are most useful for control law design. Traditionally, aeroelasticians have modeled the flexible aircraft in the frequency domain using modal degrees of freedom and generalized mass, generalized stiffness, and
frequency dependent generalized aerodynamic matrices. On the other hand, modern control theory is based primarily on the state-space approach, in which the aeroelastic airplane must be modeled as a first-order system of linear ordinary differential equations in the time domain. In addition, the aeroelastician typically works in a mean flight path coordinate system, and the flight controls engineer in a body axis
coordinate system.

The final challenge is to ensure that throughout the transformations from frequency domain to time domain, and from one axis system to another, the models remain consistent. This ensures that when a control law is designed based on the time domain state-space model, the same control law can be input into the aeroelastician’s frequency-domain analysis and comparable results can be expected.