US Army studies eVTOL acoustics in the quest for silent helicopters
The US Army is working to better understand the acoustic properties of eVTOL aircraft and co-axial rotors as part of efforts to design quieter, stealthier helicopters.
Helicopters have many advantages, but one of their more obvious drawbacks is the loud wop wop noise they make in flight. This is caused by what is known as blade-vortex interaction, where the rotor blade keeps running into the wake vortex formed by the leading blade.
That's the simple explanation, but the aerodynamics involved in having a number of long, slender, and flexible blades spinning at high speed in close vicinity to one another is incredibly complex. The U.S. Army Combat Capabilities Development Command (DEVCOM) and the Army Research Laboratory, in collaboration with Uber and the University of Texas at Austin, are studying the acoustics eVTOL vehicles that use multiple small, distributed rotors, in the hope that it may one day be possible to produce an all but silent helicopter for the battlefield.
The basic idea is that smaller rotors have different sound signatures than larger conventional rotors because the sounds are created by different physical mechanisms. A conventional rotor's noise comes from thickness noise, where the air is displaced by the rotors, and loading noise from the lift and drag forces of the air as it flows over the rotor blade. These together produce what is known as a tonal noise.
Meanwhile, the smaller rotors on an eVTOL tend to create noise through turbulence to produce broadband noise. This scales differently from tonal noise and at some point might dominate it, according to the researchers.
The study involved field tests using a number of rotor configurations on a test stand with electric motors, as well the use of computer models. For the tests, the stands were wired with nine microphones and the noise recorded for comparisons with the models, which were run using two programs – the Rotorcraft Comprehensive Analysis System (RCAS) and the PSU-WOPWOP ... and no, WOPWOP isn't some tortured acronym.
"RCAS computes the aerodynamic loads, or forces, on the blades as well as the bending and twisting of the blades," says Dr. George Jacobellis, an Army research engineer at the laboratory’s Vehicle Technology Directorate. "This information is necessary to use as inputs to PSU-WOPWOP, which computes the noise generated by the rotor. Connecting the two programs required a significant amount of work, which was done by our group as well."
Along studying the acoustics of eVTOL rotors, the team also looked at co-axial co-rotating rotors, also known as stacked rotors. As the name suggests, instead of the rotor blades being on the same plane, they are set at different heights on the axis. What the team found was that equally spaced rotor blades made the lowest noise level, or about that of a conventional rotor.
"I do think that a stacked rotor can be beneficial for eVTOL applications," says Jacobellis. "The added degree of freedom for the design will allow for gains in efficiency and control over the acoustic signature, which has been shown in the results. More investigation is needed, however, to quantify the noise reduction with axial spacing."
The research was published in the Vertical Flight Society 76th Annual Forum Proceedings.
Source: US Army