Transient Dynamics of Helicopter Rotor Wakes Using a Time-Accurate Free-Vortex Method

A second-order accurate predictor-corrector type algorithm has been developed to obtain a time-accurate solution of the vortical wake generated by a helicopter rotor. The rotor blade flapping solution was fully integrated with the wake geometry solution using the same time-marching algorithm. The analysis was used to predict the locations of wake vortex filaments under transient flight conditions, where the rotor wake may not be periodic at the rotational frequency. Applications of this analysis include prediction of the rotor induced velocity field and blade airloads during transient flight and maneuvers. The stability of the rotor wake structure is important from the perspective of free-vortex wake models. The wake stability was examined using a linearized stability analysis, and the rotor wake was shown to be physically unstable. Therefore, the stability of the numerical algorithm is an important consideration in developing robust wake methodologies. Both the stability and accuracy of the numerical wake solutions algorithms was rigorously examined. The straight-line vortex segmentation used in the present analysis was shown to be second-order accurate. The overall numerical solution was also demonstrated to converge with a second-order accuracy. A technique for increasing the order of accuracy for high resolution solutions is also described. Along with a formal (mathematical) verification of solution accuracy, the numerical solution for the rotor wake problem was compared with experimental results for both steady-state and transient operating conditions. The steady-state wake model was shown to give good predictions of rotor wake geometry, induced inflow distribution as well as performance trends. Under transient conditions, such as those following a pitch input during a maneuver, the time-accurate wake model was shown to correctly model the dynamic response of rotor wake. In axial descent passing through the vortex ring state, the present analysis was shown to properly model the associated power losses as shown by experimental results. The present analysis was also shown to give improved predictions of wake distortions during simulated maneuvering flight with various imposed angular rates of the rotor

Aperiodicity Correction for Rotor Tip Vortex Measurements

The initial roll-up of a tip vortex trailing from a model-scale, hovering rotor was measured using particle image velocimetry. The unique feature of the measurements was that a microscope was attached to the camera to allow much higher spatial resolution than hitherto possible. This also posed some unique challenges. In particular, the existing methodologies to correct for aperiodicity in the tip vortex locations could not be easily extended to the present measurements. The difficulty stemmed from the inability to accurately determine the vortex center, which is a prerequisite for the correction procedure. A new method is proposed for determining the vortex center, as well as the vortex core properties, using a least-squares fit approach. This approach has the obvious advantage that the properties are derived from not just a few points near the vortex core, but from a much larger area of flow measurements. Results clearly demonstrate the advantage in the form of reduced variation in the estimated core properties, and also the self-consistent results obtained using three different aperiodicity correction methods

Rotor Hover Performance and Flowfield Measurements with Untwisted and Highly-Twisted Blades

The flowfield and performance characteristics of highly-twisted blades were analyzed at various thrust conditions to improve the fundamental understanding relating the wake effects on rotor performance. Similar measurements made using untwisted blades served as the baseline case. Twisted blades are known to give better hover performance than untwisted blades at high thrust coefficients typical of those found in full-scale rotors. However, the present experiments were conducted at sufficiently low thrust (beginning from zero thrust), where the untwisted blades showed identical, if not better, performance when compared with the highly-twisted blades. The flowfield measurements showed some key wake differences between the two rotors, as well. These observations when combined with simple blade element momentum theory (also called annular disk momentum theory) helped further the understanding of rotor performance characteristics