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Traditional helicopters and tilt-rotor vertical take-off and landing (VTOL) designs such as the Bell Boeing V-22 Osprey provide considerable mobility to troops operating on land or at sea, but have one major vulnerability: the enormous amount of noise than to create.
Researchers from the United States Army’s Combat Capabilities Development Command Army Research Laboratory teamed up with engineers from Uber Elevate and academics from the University of Texas at Austin to experiment with building possible next-generation vertical take-off and landing (VTOL) aircraft. able to operate silently using distributed electric propulsion.
According to the researchers, potential aircraft, which are expected to use significantly smaller rotors than traditional helicopters, are being considered for the huge advantage they could bring to the battlefield in missions such as freight transport or surveillance.
Engineers do not expect experimental rotor designs to run completely quietly, with electrically powered rotors showing a tendency in experiments to generate more “broadband noise,” that is, a form of loading noise caused primarily by ingestion of turbulence on the part of the rotor.
The researchers are said to have hypothesized, and then confirmed in their field studies, that broadband noise becomes the dominant noise source when rotors are reduced. The study reportedly included examining a variety of electric VTOL rotor configurations, with a set of nine microphones recording the noise generated both above and below a rotor hub.
The engineers then analyzed the noise produced using two custom computer programs, which measured the aerodynamic loads on the rotor blades at various settings, as well as the actual noise generated.
George Jacobellis, a research engineer at the Army Research Laboratory, told Army.mil that “the noise you hear from these smaller rotors is generated through fundamentally different physical mechanisms” than traditional, proven main rotors and tail rotors. “Traditional modeling techniques need to be improved to account for all the noise generated so that vehicle designers can be aware of what will actually be heard,” he said.
Jacobellis pointed out that “more work” would be needed to obtain “more accurate acoustic predictions” and compare model simulations with field experiments.
Based on their studies, the researchers have so far found that stacked rotors (i.e. coaxial, co-rotating rotors) with equally spaced rotor blades create the lowest amount of noise, equivalent to that of a traditional rotor. The engineers plan to continue experiments with axial spacing in hopes of producing a stacked rotor configuration that emits lower noise levels than conventional rotors.
The team published their findings in the 76th Act of the Vertical Flight Society Annual Forum.
The noise generated by the approach of helicopters is one of the main potential weaknesses for highly mobile and technologically advanced militaries such as the United States, particularly in the face of an equal or nearly equal opponent armed with advanced radar and anti-aircraft systems. Pentagon planners first experienced the limits of mobile air warfare in Vietnam, where the US military lost more than 5,600 helicopters to North Vietnamese forces and Viet Cong guerrillas in eight years.
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