A novel technique used in control engineering for formulating a high-quality, finite-state, unsteady aerodynamic model by applying Bode plot methods is presented. Indicial response functions for both fixed- and rotary-wing applications are obtained using these finite-state, unsteady aerodynamic models. It is shown that the rotary-wing indicial response function has a fundamentally different characteristics when compared to fixedwing indicial response. The rotary-wing indicial response function is oscillatory in nature, while the fixed-wing indicial response function is nonoscillatory. Furthermore, it should be emphasized that this is the first literary presentation of rotary-wing indicial response function. © 1986 American Institute of Aeronautics and Astronautics, Inc., All rights reserved.