US Army VTOL Stealth Helicopter
The U.S. Army Combat Capabilities Development Command (DEVCOM), Army Research Laboratory researchers, University of Texas at Austin and Uber partnered to research the acoustic properties of electric vertical takeoff / landing aircraft that use distributed electric propulsion to power flight. These eVTOL aircraft could be used in future aerial surveillance and cargo transport missions using smaller rotors than traditional helicopters. Read more for a video and additional information.

What the teams discovered is that stacked co-rotating rotors may be significantly quieter than traditional paired rotor approaches, this improving performance for a flying craft. Currently, no stacked co-rotating rotors have been employed in existing aircraft. The ultimate goal is to find a stacked rotor configuration that produces considerably lower noise than the conventional rotor.

SYMA RC Helicopter, S39 Aircraft with 3.5 Channel,Bigger Size, Sturdy Alloy Material, Gyro Stabilizer and High &Low Speed, Multi-Protection Drone for Kids and Beginners to Play Indoor-Red
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The noise you hear from these smaller rotors is generated through fundamentally different physical mechanisms. 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,” said Dr. George Jacobellis, Army research engineer at the laboratory’s Vehicle Technology Directorate.