Aperiodic Wake Developments

There are many applications in helicopter aerodynamics for which quasi-steady or periodic wake methods are unsuitable because the physics of the rotor wake cannot be assumed globally periodic. Such unsteady environments include rapid changes in pilot control inputs, some types of rotor wake-airframe interaction problems, flight operations near the ground, maneuvering flight conditions, autorotational flight, or when descending flight near the vortex ring state (VRS). These problems often result in various forms of disturbances in the wake that result in instabilities. The instabilities are aperiodic with respect to the rotor frequency, and so they can change fundamentally the physical structure of the rotor wake topology. Examples of wake perturbations that result in instabilities have been shown previously in Fig. 10.15. Furthermore, the rotor wake is often found to be aperiodic in hovering flight. This behavior has its source in natural instability modes of helicoidal vortex filaments.

Accurate simulations of these types of problems requires fully time-accurate wake meth­ods, which raises substantially the complexity of the modeling. Of particular practical interest is the rotor behavior under maneuvering flight conditions, for which a fully ade­quate wake model continues to elude helicopter analysts. A maneuver often sets a limit to the normal flight envelope of a helicopter so the prediction of the rotor airloads under these conditions forms an important part of the design process (see Section 5.9). Under these conditions the pilot uses cyclic pitch control inputs to place the helicopter in a variety of nonsteady flow states. The wake develops in a time-dependent (aperiodic) manner, and the inflow through the rotor acquires a history or hereditary effect. This problem is similar to the unsteady airfoil problems discussed in Chapter 8, although for an entire rotor the problem is considerably more 3-D and clearly much more complex.