Experimental Investigation and Numerical Study of a Helicopter Rotor Model

All rotorcraft operate in highly complex 3D and uns teady flowfield, dominated by strong vortices in th e wake. The wake geometry and its induced effects are still inadequately understood and difficult to pre dict [L EISHMAN , 2000]. The past decades have seen a continuing deve lopment of methods to predict rotor performance with increasing accuracy. The validatio n of numerical methods for prediction of performanc e, loads and flowfield of conventional helicopter roto rs remains an important subject in rotorcraft desig n. Unsteady free wake techniques (FELKER et al. ,1990; B ARON and BOFFADOSSI 1991, BRAMESFELD and MAUGHEMER 2008) appear to be fully attractive because of high computational efficiency. They do not require any prescribed wake model or wake tuning . However numerical methods do not yet represent a complete alternative to wind tunnel or flight tests . The aim of this study is to compare experimental results concerned with a helicopter model and the numerical simulation by an unsteady free wake method. Not o nly global parameters or mean velocity are considered, but also unsteady velocity flowfield and wake geometries are included in the study. A short description of wind tunnel investigations a nd a summary of the computational method used in the numerical simulation, are presented in the followin g, where some examples of both experimental and numerical results are also provided. The experimental investigations The experimental work provided the motivation for t his study and will be discussed first. All results concern with detailed tests performed on an isolate d helicopter rotor model in Agusta and DLR-NWB wind tunnels. The model configurations tested was desig ned as basic research tool in testing of rotors. I t consists of a fully articulated hub with four rectangular bl ades. Power was supplied to the rotor by a hydraul ic motor and rotor controls consisted of conventional swash plate and three 120° spaced actuators. One dedicat ed six component strain gauge balance provides measurement of aerodynamic loads. The rotor model was designed to be relatively simple to operate and mai ntenance free for testing in small wind tunnel and to be easily transportable to allow test in different fac ilities. The basic tests provide a survey of flowf ield and wake structure at different thrust and RPM, in hove r and forward flight. Velocity traverse measurement s of induced downwash flowfield have been done in different planes perpendicular to the rotor tip plane, in order to establish the wake structure. Nevertheless, the influence of different measurement techniques, bot h pneumatic and hot wire (single, X and 3D) probes ha ve been provided and their results have been corre lated together. Smoke filaments visualisations were also performed in steady and unsteady conditions, with t he aim to capture the tip vortex structure in the wake . The computational method In the present work a non linear unsteady vortex-la ttice method is used to afford the numerical simulations of the flowfield past the rotor. The scheme has been developed for the prediction of the load distribution and the geometry of the wakes on the b lades of general multi-rotor helicopter configurati ons by BARON and BOFFADOSSI (1991). Blades are assumed to have a negligible thickness and are simulated as rigid or cambered surfaces with arbitrary twist and taper distribution. They can undergo a general uns teady