Forward Flight Wake

Abstract

During translational motion of the helicopter, when the rotor is nearly horizontal, the rotor blades see a component of the forward velocity as well as the velocity due to their own rotation (Figure 5.1). In forward flight the rotor does not have axisymmetry as in hover and vertical flight; rather, the aerodynamic environment varies periodically as the blade rotates with respect to the direction of flight. The advancing blade has a velocity relative to the air higher than the rotational velocity, whereas the retreating blade has a lower velocity relative to the air. This lateral asymmetry has a major influence on the rotor and its analysis in forward flight. Thus the rotor blade loading and motion are periodic with a fundamental frequency equal to the rotor speed Ω. The analysis is more complicated than for hover because of the dependence of the loads and motion on the azimuth angle. As a consequence of the axisymmetry, the analysis of the hovering rotor primarily involves a consideration of the aerodynamics. In forward flight, however, the lateral asymmetry in the basic aerodynamic environment produces a periodicmotion of the blade, which in turn influences the aerodynamic forces. The analysis in forward flight must therefore consider the blade dynamics as well as the aerodynamics. This chapter covers a number of aerodynamic topics that are familiar from the analysis of the rotor in vertical flight. In particular, we are concerned with the momentum theory treatment of the induced velocity and power in forward flight. Then the rotor blade motion and its behavior in forward flight are considered in Chapter 6.