Segment-wise Measurement of Helicopter Approach Noise with a Reduced Microphone Setup

In order to measure the noise footprint of a helicopter approach procedure, microphones are usually distributed over a large area. In this paper, however, a method is utilized where the measurement of the noise footprint is conducted in parts by distributing the microphones over a limited area and executing the same approach procedure, while the position of landing point is shifted. The flight tests are conducted with the EC135-ACT/FHS helicopter. Guidance to the pilots is provided by use of a "Tunnel-in-the-Sky" head down pilot display. Since the same flight procedure is executed multiple times it is possible to conduct a statistical analysis on the accuracy and reproducibility of the approach procedure in terms of position, velocity and generated noise. This statistical analysis shows that the geometric accuracy is in the order of 6 meters in the horizontal plane and 12 meters in the vertical plane. The variations in the measured sound exposure level from one approach to another are in the order of 3-7.5 dB(A). The approach procedure is also executed with excessive cyclic pilot control inputs in order to identify an upper limit of the extra noise generated by unsteady flight. However, these approaches are indistinguishable, in terms of the sound exposure level or sound pressure level, from the approaches with normal pilot control inputs.