Aeroacoustic Flight Test Data Analysis and Guidelines for Noise-Abatement-Procedure Design and Piloting

This paper focuses on noise abatement flight procedure (NAFP) studies performed on an EC135 helicopter. Aeroacoustic flight tests performed in the framework of the DLR PAVE project have been analyzed in order to provide guidance for NAFP design. Flyability limits and noise sensitivity to main control parameters have been investigated in simulator and in flight in order to improve the NAFP design and optimization and to develop dedicated pilot displays. The torque appears to be a parameter governig the blade-vortex interaction (BVI) noise: piloting at very low torque or at high torque allows to avoid BVI noise. Thus, the engine torque display, available on almost all helicopters, can be used to avoid BVI conditions. It was also found that the very annoying Fenestron noise excess that appears when flying with low torque to avoid BVI, can be completely eliminated through side-slip. NAFP validation flight tests for the PAVE and Friendcopter projects are briefly presented. Examples of NAFP resulting in measured noise reductions close to 10 dB SEL are provided. A guideline to pilots explains how to perform quietly an EC135 complete landing approach with almost no BVI noise, no Fenestron excess noise, and possibly using only instruments commonly on board