Rotor Wakes and Blade Tip Vortices

We have built new rotor test stands, new blades, with emphasis on heavily instrumented blades, new measurement techniques – it is mind boggling to me to see how well, and in how much detail, we can measure the flow field of a rotor. Modeling the flow field of a rotor is a terribly difficult problem. The airplane people have it easy compared to us. Capturing the details of the rotor wake and of what happens in the vicinity of the blade is very difficult.

Leone U. Dadone (1995)

10.1 Introduction

The significant physical features of helicopter rotor wakes, and some of the more advanced mathematical tools for modeling the wake, are discussed in this chapter. The understanding and prediction of the effects of the rotor wake is an important key to the successful prediction of blade loads and a host of other problems found in helicopter aero­dynamics. A helicopter rotor wake is dominated by strong vortices that are trailed from the tips of each blade. The nature of the rotor wake, in terms of its geometry, strength, and the aerodynamic effects produced on the blades, depends principally on the operating state and flight condition of the helicopter. In hover, the tip vortices follow nominally heli­cal trajectories below the rotor. This is perhaps the simplest operating state to understand, but even here the wake structure is relatively complicated. During forward flight, the rotor wake is skewed back behind the rotor by the oncoming flow, and a series of more complex interlocking, but nominally epicycloidal vortex trajectories are produced. Under these con­ditions, the increased mutual proximity of many of the vortex filaments results in stronger vortex-vortex interactions and complicated distortions to the evolving wake topology. At the lateral edges of the wake, the individual vortex filaments are found to roll up into a pair of merging vortex bundles, somewhat like those that would trail from the tips of a low aspect ratio fixed-wing.

The highly 3-D nature of a helicopter rotor wake, as well as the sensitivity of the wake to the geometric and operational parameters of the helicopter, means that the details of the wake flow are difficult to study experimentally, as well as to compute by means of mathematical models. Recent advances in experimental techniques have been substantial and now allow measurements to be made with a fidelity that was impossible only a few years ago. However, there are many physical phenomena involving the formation and evo­lution of blade tip vortices and rotor wakes that are still not well understood, and it is here that future research on helicopter aerodynamics must be focused. Landgrebe (1988) and McCroskey (1995) review the state-of-the-art capabilities in modeling helicopter rotor wakes, whereas Leishman & Bagai (1998) give an overview of key characteristic physical features of rotor wakes and some of the unique experimental challenges involved in their measurement.

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