The noise radiation from a helicopter is characterized by a strong directivity and a strong dependence on the flight conditions. During certain flight conditions a typical loud and popping noise can be generated due to interactions of the rotor blades with the tip vortex (BVI noise). By avoiding the flight conditions at which BVI occur it is attempted to cancel the BVI noise or to minimize it.
The prediction of helicopter noise is based on a helicopter specific noise database, which describes the noise radiation at certain flight conditions, so called noise hemispheres. In order to compute the helicopter noise of a flight procedure, the flight conditions along the flight path are computed based on a flight mechanical model and the corresponding noise hemispheres are selected from the database. In combination with an atmospheric model that takes into account wind, wind gradients and temperature gradients the noise footprint is then computed.
The database of noise hemispheres can be computed with the aid of numerical methods or they can be constructed from specific noise measurements. Experimental data is in general more accurate, because they include all noise sources and all relevant physics. However, the experimental data is expensive due to the many flight hours necessary. Furthermore the experimental data is only valid for one specific helicopter type.
The numerical computation of noise hemispheres is a current research topic, especially the influence of the tail rotor and shielding effects due to the fuselage. In order to assess the uncertainties and errors in numerically computed noise hemispheres and to verify new noise abatement flight procedures, accurate experimental data is essential. Therefore DLR is conducting flight experiments with the ACT/FHS research helicopter.
In order to provide guidance to the pilot on how to fly a noise abatement flight procedure, DLR has developed a so called “Tunnel in the Sky” pilot display, which guides pilots accurately along a prescribed flight path.