With helicopters being a common sight and sound in any cityscape, sound reduction has been an ongoing area of rotorcraft research for decades. Starting with the shrouded Fenestron tail rotor, which first flew on an Airbus helicopter in 1968, to more recent advancements in blade design, such as the new H160’s innovative Blue Edge blades, Airbus Helicopters is continuously looking at ways to lower the volume of its rotorcraft.
Where does the sound come from?
The main source of sound from a helicopter is its rotor blades. Blades produce several types of sound. Some are due to air displacement, others are from forces acting on the air that flows around the blade, while others come from aerodynamic shocks on the blade surface. The engine, main rotor and tail rotor can generate sound, too, but this is mainly noticeable right around the helipad, and less from a distance.
The degree of sound from the individual sources depends considerably on both the flight condition and where the observer is in relation to the helicopter. When the aircraft is flying at cruise speed, a person is likely to hear the main rotor as the helicopter approaches. When it is passing overhead, the tail rotor and engines are predominant. In takeoff and approach, these individual sources of sound can change, due to the machine’s different power and thrust requirements.
During an approach or at moderate speeds in level flight, the vortex trail may intersect the paths of subsequent blades, producing impulsive noise that’s sometimes referred to as “blade slap.”
That being said, a person's perception of sound is just as important a factor in judging how quiet, or not, a helicopter is. People tend to be more bothered by impulsive, tonal, high frequency sounds, but the duration of one’s exposure to the sound also matters, irritants that Airbus is addressing with adjustments in landing procedures, for example.
What solutions exist for lowering a helicopter’s sound levels?
“There are many different tools in our toolkit that could each play a role in lowering helicopter sound emissions even further,” says Julien Caillet, sound expert at Airbus Helicopters. “These include improving the Fenestron, working on blade profiles, reducing rotor speed, integrating hybridization, exploring low-sound flight procedures, and inventing very specific solutions for reducing a helicopter’s perceived sound footprint in urban areas.”
Thanks to continuous evolution of the Fenestron shrouded tail rotor, the H130, H135, H145 and H160 are industry-wide benchmarks for low sound levels in their respective classes.
Another example is blade shapes. The H160, newest in the fleet, has innovatively shaped Blue Edge blades which are meant to reduce Blade Vortex Interaction (BVI) noise generation.
The H135, H145, H175 and H160 all include an automatic variable rotor speed control system that automates the rotor towards lower rotational speeds when the helicopter flies close to the ground.
At the same time, flight test departments at Airbus are working on low noise procedures, which aim at reducing sound propagated to surrounding populations by modifying flight trajectories.
Hybridisation, or hybrid-electric propulsion, which marries a conventional thermal engine with an electric battery system, could also see a reduction in noise emissions, particularly in flight conditions such as ground operations or hovering. “If we can reduce the rotational speed of the turbine by relying on an electric motor at certain phases of flight, the overall noise footprint of the helicopter would be lowered,” says Caillet.
Measuring perceived sound levels in urban areas
Most recently, in June 2020, Airbus Helicopters’ innovation and acoustics teams, supported by the French civil aviation authority DGAC, conducted a study to measure the sound levels of Airbus helicopters as perceived by urban residents in order to positively influence the design of future eVTOLs.
The project’s objective was to measure how buildings in particular affect this perception. The study took place at Airbus Helicopters' manufacturing site in southern France. The research team installed eight microphones at buildings strategically located around the site, which they then used to collect sound measurements from two flights carried out at 150 and 300 metres respectively.
“This project is another step towards predicting and mapping the environmental impact of our products with greater precision, which, in the longer term, could influence the technology being developed for future urban missions, such as urban air mobility,” says Caillet.
Initial analyses are currently underway in partnership with ONERA and Safran Helicopter Engines to measure how buildings can either filter or amplify sound.