In-Flight and Ground Instrumentations for Dauphin's Fenestron Noise Investigation

The Fenestron had been developed, conceived and produced as an alternate solution to the conventional tail rotor in order to improve security and performance for rotorcraft, and was used for the first time at the end of the 1960s. The advantages of the Fenestron are numerous in terms of safety aspects, aerodynamic efficiency and noise reduction. Therefore, many rotorcrafts in the world use the Fenestron as tail rotor, for light, intermediate and medium weight helicopters. Nevertheless, even if this concept has demonstrated high performances for noise reduction, acoustic emissions could still be reduced in some peculiar flight configurations. The French Directorate for Civil Aeronautics Programs (DGAC) has funded a Fenestron Noise Study Program in order to understand and analyze the aero-acoustics phenomena involved in these configurations. This program is based on previous research dedicated to the development and the validation of ONERA's aerodynamic/acoustic computational tools for the analysis and prediction of noise generated by a Fenestron. The French Flight Test Center (CEV) at Istres and Eurocopter were involved in this research program which was conducted by the ONERA's Department of Computational Fluid Dynamics and Aeroacoustics. Flight tests with the research helicopter Dauphin 6075 of the French Flight Test Center at Istres have been preferred to wind tunnel tests which are unable to reproduce realistic in-flight conditions. The French Flight Test Center, Eurocopter and several departments of ONERA have been in charge of designing and building the aft equipment which was integrated and tested in flight on the helicopter. During a first campaign, onboard aerodynamic and acoustic measurements were performed to identify the flight conditions where the Fenestron is noisiest. A second test campaign began in March 2010 in order to measure helicopter acoustic footprint on the ground and to identify correlations between onboard and ground measurements in the noisiest flight conditions previously identified. The resulting experimental database will help to understand how the sound is generated by a helicopter Fenestron as well as to validate the aeroacoustic computation codes from ONERA which could be used to improve flight procedures and/or the shape of the Fenestron for noise reduction.