"INTRODUCTION The information in this Item will introduce the reader to the subject of the theories of oscillatory aerodynamics and their application in aircraft design. Sections 3 and 4 concern the basic principles that underline linearized theory. Sections 5 to 8 are devoted to the various methods of solution for aerofoil sections and wings in simple harmonic motion in subsonic and supersonic flows. Sections 9 and 10 consider the practicalities of semiempirical methods and applications of unsteady aerodynamics to flutter@ atmospheric disturbances and active control systems. From as early as 1920 (Reference 38) the subject of unsteady aerodynamics has been studied with particular effort to determine@ mathematically@ the pressure distributions on wings oscillating with elastic deformations. The integrated forces are necessary in the investigation of the phenomenon of dynamic aeroelastic instability known as flutter@ which occurs above some critical equivalent airspeed. The problems associated with aeroelastic instabilities received growing attention as flight speeds increased@ and in recent years flutter considerations have often been a major influence in the design of the primary wing structure. It is therefore important that the unsteady aerodynamic forces be evaluated with some confidence. For subsonic or supersonic flow linearized theory is known to perform adequately for thin wings. However@ even on the basis of linearized theory@ the computation of aerodynamic forces on three-dimensional wings@ for all the mode shapes@ is time consuming. Therefore@ in the initial phase of aircraft design@ flutter calculations are sometimes made by a ""strip theory"" analysis in terms of aerodynamic derivatives from two-dimensional theories. ESDU Item Nos 81034 (Reference 40) and 82005 (Reference 41) present some relevant two-dimensional data for subsonic and supersonic speeds respectively. For transonic flow non-linearity in the aerodynamics leads to large errors in the computed flutter speed when@ say@ 0.8<M<1.1. Another phenomenon that restricts the application of the linearized theories is the occurrence of flow separation. Both types of flow r??gime are beyond the scope of this Item@ but they are discussed briefly in Sections 3.3.1 and 3.3.2. In contrast to the aeroelastic instability of an aircraft structure@ mentioned above@ the critical loads that determine structural strength are often those encountered in a gust or atmospheric turbulence. This is particularly true of civil aircraft which have only modest manoeuvring requirements. The calculation of structural loads due to atmospheric disturbances is another important area in which unsteady aerodynamics finds an application. The mathematical models used in such studies usually include a representation of structural deflections@ but in less detail than for flutter calculations. The fact that the response of an aircraft to a gust is non-sinusoidal places a different emphasis on the modelling of the unsteady effects. In recent years@ the advent of active control systems designed to alleviate gust loads or to suppress undesirable flutter characteristics has broadened the spectrum of interest in unsteady aerodynamics. As such systems can demand very rapid control movements compared with those normally input by a pilot@ the overall performance of an active system will depend to some extent on the rate of growth of the aerodynamic forces. A vast amount of technical literature has built up over the years@ and a comprehensive survey up to the middle fifties is provided in Reference 21. A useful textbook on the subject is Reference 16. This introduction to unsteady aerodynamics does not attempt to review all subsequent developments@ but aims to provide a general background@ to illustrate current linearized theoretical methods and to discuss their applications."