Segmentwise Measurement of Helicopter Approach Noise with a Reduced Microphone Setup.

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 the 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 on the order of 6 m in the horizontal plane and 12 m in the vertical plane. The variations in the measured sound exposure level from one approach to another are on the order of 3-7.5 dB(A). The approach procedure is also executed with excessive cyclic pilot control inputs 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 approacheswith normal pilot control inputs. [ABSTRACT FROM AUTHOR]