High Advance Ratio Effects on the Aerodynamics and Blade Dynamics of a Slowed Helicopter Rotor
Final Report Abstract
A recent doctrine in rotorcraft development is the pursuit of higher flight speeds without sacrificing hover capabilities or lowering overall vehicle performance. One approach to achieve this is through novel “compound” helicopter configurations that employ a slowed main rotor and additional components that provide lift and forward thrust to unload the main rotor. The aerodynamics and structural dynamics of compound rotorcraft are complex, especially on the retreating blade side and for operation at high advance ratios, defined as forward flight speed divided by blade tip speed. At high advance ratios, the inboard sections of the retreating rotor blade experience reverse flow from the sharp to blunt edge of the rotor blade, leading to flow separation, vortex shedding, and interactions between vortices and the rotor blades that induce dynamic blade loading and reduce rotor performance. This project focused on the effects of advance ratio on the reverse flow field, identifying dominant flow structures in the reverse flow region, and characterizing their potential effects on the main rotor. To reduce the impact of reverse flow on the overall vehicle performance, a novel asymmetric lift-compound configuration was realized and its performance benefits studied. This research was conducted at the University of Maryland in College Park, MD, USA. The approach was to carry out a series of wind tunnel tests with a sub-scale instrumented rotor test stand in the Glenn L. Martin wind tunnel and conduct performance, structural load, and flow field measurements in the reverse flow region. The first test successfully characterized the dominant reverse flow structure, the reverse flow dynamic stall vortex, and determined its dependence on radial blade station and advance ratio. The vortex strength and size were found to increase towards the rotor hub and with increasing advance ratio. Reduced time, i.e. the number of semi-chords a blade section travels in reverse flow, was proposed as a means for efficient data reduction of the reverse flow dynamic stall vortex strength. Vortex circulation over relative radial station yields a linear dependency over reduced time for all examined radial stations and advance ratios, which can be used effectively for aerodynamics model creation to improve prediction accuracy of comprehensive rotor code tools used in preliminary rotorcraft design. A secondary vortex in the reverse flow region was studied in a different test campaign. Contrary to a previous hypothesis, we found that the formation of this vortex was linked to dynamic stall induced by a blade-vortex interaction and subsequent vortex detachment from the blade upon entering the reverse flow region. The reverse flow topology varies significantly with advance ratio and radial station, including strong interactions of the dynamic stall and blade tip vortices. Despite these varying flow morphologies and a substantial strength of the dynamic stall vortex, the dominant aerodynamic loads in the reverse flow region are still caused by the reverse flow dynamic stall vortex, and its characterization in the present work serves as the basis for unsteady aerodynamics modelling that will inform future high-speed rotorcraft designs. In a final wind tunnel test, a novel asymmetric lift-compound configuration was investigated, which aims to compensate reverse flow-related performance penalties through substituting rotor lift on the retreating blade side by adding a fixed-wing. Based on performance, blade deformation, and flow field measurements, the impact of operational and rotor parameters on performance and aerodynamics of the system was examined and mutual interaction effects between rotor and fixedwing were analyzed. The combination of asymmetric rotor trim and aft shaft tilt led to a substantial increase in rotor lift (+79%), while peak lift-to-drag ratio of the compound rotorcraft was improved by up to 60% at an advance ratio of 0.5. These results highlight the effects of adding asymmetric rotor trim and a fixed-wing to a high-speed rotorcraft and demonstrate that significant improvements in overall vehicle performance are feasible. The present work contributed to the development of high speed compound helicopters by testing a promising novel configuration and successfully investigating the morphology and properties of dominant reverse flow features relevant for an improved design of future high-speed helicopters.
Publications
- “Three-Component Reverse Flow Measurements on a Mach-Scale Rotor at High Advance Ratios,” in AHS International 74th Annual Forum & Technology Display, Phoenix, AZ, USA, May 14-17, 2018, pp. 1–11
L. Smith, A. Bauknecht, X. Wang, A. Lind, and A. Jones
- “Wind tunnel test of a rotorcraft with lift compounding,” in 45th European Rotorcraft Forum, Warsaw, Poland, Sept. 17-20, 2019, pp. 1–20
A. Bauknecht, X. Wang, and I. Chopra
- “Wind Tunnel Test on a Slowed Mach- Scaled Hingeless Rotor at High Advance Ratios,” in Vertical Flight Society 75th Annual Forum & Technology Display, Philadelphia, PA, USA, May 13-17, 2019, pp. 1–10
X. Wang, A. Bauknecht, S. Maurya, and I. Chopra
