The Violent Vs.The Silent Aspects Of Vortex Ring State And The Particular Case Of The Double-Swept H160 Blade

Abstract

In this study numerical simulations are used to examine the rotor wake effects on the lift generation of a generic main rotor in low-speed descending flight. The VRS envelope is mapped for three different disc loadings, and the reduction using the hover induced velocity is demonstrated. From an operational standpoint, it is important to explain how pilots may involuntarily develop high sink rates during final approach. Therefore, besides the known dynamic part of the VRS phenomenon, such as thrust and moments fluctuations, we focus on the quasi-static drop of lift with increasing rate of descent, and the associated loss of rotor heave damping. It is shown that the loss, or even inversion, of the rotor heave damping is dependent on disc loading, and lightly loaded helicopters are more prone to develop unexpected rates of descent, even though the dimensionless VRS boundaries are fairly the same for all disc loadings. The effectiveness of raising the collective to arrest the descent rate, depending on the severity of VRS, is also shown. This part is related to an open debate as to whether the pilot should raise or lower the collective inside VRS. Finally, the H160 rotor with its double-swept blades is compared to the generic rotor with straight blades in the same flight regime. It is found that thanks to a strong link between rate of descent and blade elastic twist the H160 rotor shows a much better behavior in heave damping and less thrust fluctuations than the generic rotor.