Decreasing the ‘whir’ of whirlybirds
The external noise generated by helicopters creates negative public opinion despite the usefulness of these aircraft. In addition, the noise generated within the cabin impedes conversations and comfort. The ‘Integration of technologies in support of a passenger and environmentally friendly helicopter’ (Friendcopter) project was designed to reduce both internal and external helicopter noise for enhanced environmental compliance and public acceptance. The project focused on reduction of noise on the ground, engine and cabin noise reduction and highly innovative active rotor blade noise control. In order to reduce the acoustic footprint on the ground, the researchers developed a new noise footprint model with automated optimisation routines for noise abatement. The optimisation procedure resulted in an up to 50 % decrease in the acoustic footprint, corresponding to a 10 decibel reduction in noise at specific observer locations. There are currently no silencing technologies for turboshaft engines on serial helicopters. Thus, the Friendcopter project developed a truly ground-breaking manufacturing process for integrating acoustic liners in composite engine materials that reduced not only engine noise but manufacturing time and cost as well. Researchers identified the gearbox as the critical source of cabin noise and thus modified the gearboxes using cutting-edge active technologies along with passive ones. In addition, to further mitigate residual cabin noise, the researchers employed active noise reduction in advanced cabin panelling. Finally, the most innovative concept was active rotor blade control technology by distributed actuation. The researchers investigated three different technologies based on so-called intelligent or active materials and validated them experimentally, demonstrating technical feasibility. In summary, the Friendcopter project made significant advances in decreasing helicopter noise both within the cabin and on the ground. The use of active materials should tremendously enhance noise reduction and vibration damping, leading to environmental compliance and generation of public support. Given the critical importance of low-flying helicopters for emergency situations such as rescue missions, medical evacuation and law enforcement, the Friendcopter project should have positive impact on continued and improved functioning of these whirlybirds.