CLOSED-FORM UNSTEADY AERODYNAMIC THEORY FOR LIFTING ROTORS IN HOVER AND FORWARD FLIGHT.

A theory of unsteady aerodynamics (i. e. , of induced flow) is offered for a lifting rotor in hover and forward flight. The induced flow is expressed azimuthally by a Fourier series and radially by Legendre functions. The magnitude of each term is determined from first-order differential equations in either the time or frequency domain. The coefficients of the differential equations depend only on the wake skew angle. The forcing functions are user-supplied, radial integrals of the blade loadings. In a nonlifting climb with quasi-steady aerodynamics, the theory gives results almost identical to those of Loewy theory but with improved values of the wake apparent mass. The theory implicity includes both dynamic-inflow theory and the near-wake approximation to the Theodorsen function. Finally, comparisons with other theories and experimental data show the theory to be accurate in the range of 0-12/rev (reduced frequency less than one).