Lower rotor noise thanks to a modified approach path
Helicopters do not have to be loud. This is the conclusion of scientists at the German Aerospace Center (Deutsches Zentrum für Luft- und Raumfahrt; DLR) working on the FRIENDCOPTER project. On 24 and 25 November 2009, this EU project was concluded with an official presentation in Braunschweig.
A total of 34 institutions were involved in the FRIENDCOPTER project, from helicopter manufacturers to university faculties and research agencies, with the aim of developing helicopters which are more passenger and environmentally friendly. Along with reduced fuel consumption and reduced noise and vibration in the cabin, one of the main objectives of FRIENDCOPTER was to lower noise during flight. The DLR Institute of Aerodynamics and Flow Technology (Institut für Aerodynamik und Strömungstechnik; IAS) at Braunschweig found a way to cut down the noise of helicopters, especially during the noisy landing phase, in a very simple and economical way. Even simply changing certain flight procedures can be of enormous value and reduce noise by up to 10 decibels.
During flight, the main rotor experiences numerous aerodynamic interactions. During descents, the noise is produced mostly by what is known as the Blade Vortex Interaction (BVI), the interaction of vortices in the air with the rotor blades. The tips of the rotor blades shed vortices and the following blades, due to the particular characteristics of helicopter flight, repeatedly cut through these vortices. This results in the familiar 'chopping' sound made by helicopters in flight. With the aid of thorough aero-acoustic flight experiments, simulations and optimisation with generic algorithms, the researchers were able to calculate that modified flight procedures when descending would significantly reduce rotor noise.