16:26 GMT26 January 2021
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    Traditional helicopters and vertical take-off and landing (VTOL) tilt-rotor designs such as the Bell Boeing V-22 Osprey provide considerable mobility to troops operating on the land or at sea, but have one major vulnerability – the tremendous amounts of noise they generate.

    Researchers from the US Army’s Combat Capabilities Development Command Army Research Laboratory have partnered up with engineers from Uber Elevate and academics from the University of Texas at Austin to experiment with constructing possible next-generation vertical take-off and landing (VTOL) aircraft capable of operating quietly using distributed electric propulsion.

    The prospective aircraft, expected to use rotors that are significantly smaller than those of traditional helicopters, are being considered for the huge advantage they may bring on the battlefield in missions such as cargo transport or surveillance, according to researchers.

    Engineers don’t expect experimental rotor designs to run completely silently, with electrically powered rotors showing a tendency in experiments to generate more “broadband noise”, ie, a form of loading noise caused mostly by a rotor’s ingestion of turbulence.

    The researchers are said to have assumed, and then confirmed in their field studies, that broadband noise becomes the dominant source of noise as rotors are scaled down. The study reportedly included examining a variety of electric VTOL rotor configurations, with a series of nine microphones recording noise generated both above and below a rotor hub.

    The engineers then analysed the noise produced using two customised computer programmes, which measured aerodynamic loads on the rotor blades in various settings, as well as the actual noise generated.

    George Jacobellis, a research engineer from the Army Research Laboratory, explained to Army.mil that “the noise you hear from these smaller rotors is generated through fundamentally different physical mechanisms” when compared with traditional, tried and true main rotor and tail rotor designs. “Traditional modeling techniques need to be improved to account for all of the noise generated so that vehicle designers can be aware of what will actually be heard,” he said.

    Jacobellis emphasised that “more work” would be needed to obtain “higher accuracy acoustic predictions” and compare model simulations with field experiments.

    Based on their studies, researchers have thus far found that stacked rotors (ie, co-axial, co-rotating rotors) with equally spaced rotor blades create the lowest amount of noise – equivalent to that of a traditional rotor. The engineers expect to continue experiments with axial spacing in hopes of producing a stacked rotor configuration that puts out noise levels below those of conventional rotors.

    The team published their findings in the Vertical Flight Society’s 76th Annual Forum Proceedings.

    The noise generated by approaching helicopters is one of the major potential weak points for highly mobile, technologically advanced militaries such as the US, particularly when facing a peer or near-peer opponent armed with advanced radar and anti-aircraft systems. Pentagon planners first experienced the limitations of air mobile warfare in Vietnam, where the US military lost more than 5,600 helicopters to North Vietnamese forces and Vietcong guerrillas over eight years.

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