The main source of sound from a helicopter comes from its rotor blades. Blades produce several types of sound. Some are due to air displacement (thickness noise); others (loading noise) are from forces acting on the air that flows around the blade—these are caused by lift and drag, for example. Still other sounds come from aerodynamic shocks on the blade surface, or interactions with turbulent inflows of air.
The engine and gear box can generate sound, too, but this is more of the up-close variety—noticeable around the helipad, but less if you’re an observer some distance away. Heavier helicopters tend to be less quiet because of the larger amount of thrust they require—they have more weight to lift, after all.
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 and is approaching, a person is likely to hear the main rotor as the helicopter comes towards him. When it is closer, and 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.
Different flyover positions produce different types of sound from individual parts of the helicopter.
And helicopters can be more or less quiet in certain flight conditions, such as during an approach. This is due to blade vortex interaction (BVI), a type of loading noise. Each main rotor blade also produces a strong tip vortex whose trajectory travels downstream from the rotor in an approximately epicyclical manner. In approach and sometimes at moderate speeds in level flight, the vortex trail may intersect the paths of subsequent blades, producing impulsive noise that’s sometimes referred as “blade slap.”
When a helicopter is making an approach, the main rotor blades can move into the path of a vortex produced by the blades ahead, producing a sound called “blade slap.”
The future of sound
VTOL aircraft already fly over some of our cities. Helicopters transport medical patients to urban hospitals. Sound is part of the world we inhabit. The next step, says Tomasz Krysinski, Research & Technology Director at Airbus, is the “design for sound” of new aircraft, particularly to address the urban air mobility market.
One focus is exploring new configurations for air vehicles, to leverage on possibilities that could trigger significant benefits in sound reduction. The CityAirbus and Vahana VTOL vehicles with their multiple propellers, now in the demonstrator phase, will need to meet a level of sound acceptance that will be significantly more stringent to operate above cities.
“It’s not only about designing silent products, it’s also about designing silent procedures for our products,” says Krysinski. To this end, the company is also working on another kind of sound reduction: optimising takeoff and landing procedures, and lowering real-life operational sound. BVI occurs in certain glide slope angles and not in others, so by designing a landing procedure with a steeper glide slope, sound can potentially be lowered. This is just one of the solutions being considered. Climbing procedures, too, may come under scrutiny.
“Certification for sound is done under flight conditions that are imposed by authorities,” says Krysinski. “They are not necessarily the procedures flown by our customers daily. We are working on impacting the real sound people hear coming from our products.”
*VTOL: vertical takeoff and landing