TABLE OF CONTENTS Under stationary attached flow conditions, aerodynamic effects can be adequately described using time-invariant parameters and linear models. At higher angles of attack and aircraft undergoing stall, models are highly nonlinear, because flow separation and unsteady effects are dominant. Flow separation results primarily in a reduction of the lift curve slope before the maximum lift point, whereas a significant loss of lift occurs in the post-stall region. At such flight conditions the generated lift is influenced strongly by unsteady effects. Different types of flow separation occur depending upon the airfoil shape or wing configuration. The most common type is the trailing edge separation which is typical for most conventional aircraft having a turbulent boundary layer.
Unsteady aerodynamics has been a subject of extensive investigations using computational fluid dynamic methods, wind-tunnel tests, and semi-empirical models. The common approach has been based on indicial functions.22 26 Although such models provide a basis for analytical investigations of the complex flow phenomena, postulating them in an analytical form suitable for parameter estimation is difficult. Therefore, we follow the alternative approach of Refs. 27 and 28 to describe analytically the flow separation including stall hysteresis as a function of an internal state variable. Since the approach retains the statespace formulation, it is directly amenable to identification and validation from flight data, as has been validated on flight data in Ref. 29. For a review of unsteady aerodynamic modeling the reader is referred to Ref. 30.
In this section we draw heavily...
