Rotorcraft External Far-Field Noise Measurement Using a Hot Air Balloon

A new method is demonstrated for measuring the noise radiated by a helicopter using a hot air balloon as the measurement platform to support one or more microphones below the hot air balloon basket. Similar to studies with stationary ground microphones, the vehicle is flown past the microphone to gather forward flight external noise data. Because the helicopter and the hot air balloon are both moving in the same air mass when the acoustic measurements are acquired, the new method has many advantages that can be exploited to better investigate helicopter noise sources. One particular advantage is that acoustic data can be taken above the tip-path plane of the rotor without introducing reflections from the ground. Results are presented for a Bell 206B-3 helicopter for several flight conditions demonstrating the advantages and disadvantages of this new acoustic flight testing method.