100.1 PITCH-UP PHENOMENON CHARACTERIZATION BY MEANS OF HW AND PIV MEASUREMENT TECHNIQUES D. Barbagallo, F. De Gregorio, N. Tino CIRA – Italian Aerospace Research Centre Abstract An experimental campaign was performed in the 24ft open jet subsonic Wind Tunnel at DERA-Farnborough Research Centre in the framework of the Brite-Euram project named “HELIFLOW”. The project objective was to improve experimental and theoretical tools for helicopter aeromechanics and aeroacustic interactions. The present work describes the experimental investigation performed by means of Particle Image Velocimetry (PIV) and Hot Wire Anemometer (HWA) techniques to qualify (investigate) the rotor wake behind a 1:2 scaled helicopter model in forward flight at different advance ratio values (= 0.0150.05) at cT/=0.08 thrust level and at S=2° shaft angle. The present research work will address effects due to low speed interference named “pitch-up”, during transition from hover to forward speed. The phenomenon results from the changes of the pitching moment associated with the main rotor wake impinging on the rear of the fuselage, tailplane and tail rotor of the helicopter. Quantitative measurements of flow field have been performed, resulting in a map of the flow in the zone of the tailplane (TP) with a clear individuation of tip vortex, velocities behaviour, frequency characteristics, flow status on the tailplane and rear fuselage. A comparison of the results obtained with the two techniques will be showed. This measurements have also detected the position of the rotor wake impinging on the tailplanes and it will be compared with results of the previous tests performed to measure the force acting on the tail planes in order to detect the advance ratio at which the tail planes were effected by rotor downwash. Nomenclature : Rotor solidity CIRA : Italian Aerospace Research Centre cT: Non dimensional thrust coefficient DERA : Defence Evaluation and Research Agency FFT: Fast Fourier Transform HWA : Hot Wire Anemometer Kt: Tangential Sensitivity Coefficient PIV : Particle Image Velocimetry TP: Tail plane U,Vx: Axial Flow Velocity Component Ueff: Effective Velocity Vy: Transverse Velocity W,Vz: Vertical Flow Velocity Component S: Shaft angle : Advance Ratio 1. Introduction The development of analytical methods to predict a rotor performance are increasing in accuracy. Nevertheless, none of these is self-contained; they all require the characteristics of the wake in order to have correct downwash distribution. The rotor data can be obtained by surface pressure transducer [1, 2]. In the past the rotor wake geometry has been investigated by means of flow visualisation techniques [3-5] and by LDV [6-8]. A detailed investigation of the wake flow structure has been made by means of HW technique [9-11]. Anyway, in spite of the growing capacity of analytical models, the development of new helicopters is impossible without time-consuming and expensive test. The more complex aerodynamics and structural dynamics of rotorcraft specifically require adequate tests addressing such effects as interference between rotors and fuselage components and the coupling between aerodynamic and aeroelastic characteristics.