Simulation of Rotor in Hover: Current State, Challenges and Standardized Evaluation

Abstract

A helicopter is subjected to a wide variety of flight conditions with disparate objectives; these include efficient cruise and hover, and high-performance maneuvering. The hover condition remains a very important design consideration because it represents the true value of the helicopter, and it is a limiting design point in terms of power requirements. Accurate numerical prediction of rotor-blade aerodynamic parameters such as thrust, torque and efficiency requires an accurate modeling of the vortex wake. Despite tremendous advancements in the ability to preserve the tip vortex from first principles, the capability to consistently and reliably predict performance parameters for a new rotor-blade has still been elusive. These challenges provide the motivation for the AIAA Applied Aerodynamics Technical Committee Rotorcraft Simulation Working Group's efforts to evaluate hover simulation capabilities across government organizations, industry and academia. The AIAA Applied Aerodynamics Rotor Simulation Working Group aims to bring together government, industry and academic participants to evaluate and further rotor-in-hover performance predictions. The invited session at SciTech 2014 is the first step to assess different approaches for the prediction of baseline S-76 rotor planform. Future plans include a full workshop at SciTech 2015 to evaluate prediction of performance for S-76 planforms with various tip-shapes. MotivationA helicopter is subjected to a wide variety of flight conditions with disparate objectives; these include efficient cruise and hover, and high-performance maneuvering. Next generation aircraft are increasingly required to improve performance in all areas. Achievement of significant advances in rotorcraft capability requires advanced concepts such as complex rotor tip shapes and active rotor control. The hover condition remains a very important design consideration because it represents the true value of the helicopter, and it is a limiting design point in terms of power requirements.The flow field around a rotor, whether in forward flight or hover, is difficult to model due to the presence of strong vorticity. The flow phenomena for a rotor differ from that for a wing in forward flight, because of the differing influence of their respective wakes. For a wing in forward flight, the generated tip vortex and the vortex sheet are quickly convected away from the wing, and the influence of the shed wake on the flow field in the vicinity of the wing is small. For an adequate numerical simulation of a wing in forward flight, it is sufficient to capture the generated tip vortex in the vicinity of the wing.In contrast, in the flow field around a rotor, the strong vortex wake system lingers in the vicinity of the rotor. In hover, the strong tip vortex coils beneath the rotor, and significantly alters the effective angle of attack seen by the rotor. The schematic in figure 1 shows a sketch extracted from a smoke study of a model rotor in hover [1].Accurate numerical prediction of aerodynamic parameters such ...

Validation of the Steady-State Hover Formulation for Accurate Performance Predictions

“…The current state of the art of hovering rotor calculations is represented by the studies within the AIAA Hover Prediction Workshop formed in 2014 [8]. The selected test case was the S-76 rotor which was experimentally tested by Balch [9], and was chosen due to availability of data for different tip shapes -swept-tapered, rectangular and anhedral.…”

Single-shot pressure-sensitive paint lifetime measurements on fast rotating blades using an optimized double-shutter technique

“…Surface pressure measurements on rotating blades are vital in order to understand inherent flow phenomena (Gorton et al, 1995), for the characterization of aerodynamic loads and performance data (Lorber et al, 1989) and due to their inevitable use for the validation of computational fluid dynamics (CFD) codes used for prediction purposes (Hariharan et al, 2014). Benchmark experiments in rotorcraft research used for CFD validation either provide integral balance data only (Balch and Lombardi, 1985) or employ wall tappings providing data at discrete point-wise locations (Lorber et al, 1989;Tung and Branum, 1990).…”

Accurate Predictions of Rotor Hover Performance at Low and High Disc Loadings

“…Group [14,15] was established in 2014. The 1/4.71 scale S-76 rotor blade [5,6] was selected for assessment because of its public availability and data set with various tip shapes.…”