The U.S. Army has announced that its researchers are working with the Uber research lab on a quiet and efficient VTOL, or vertical take-off and landing operation, for its fleet of next-generation aircraft.
At present, the clandestine movement of troops and supplies is the priority of modernizing the army for future vertical lift aircraft.
The US Army Combat Capabilities Development Command (DEVCOM) Army Research Laboratory, researcher, worked with Uber and the University of Texas at Austin to study the acoustic properties of electric vertical take-off and landing aircraft that provide distributed electric propulsion for propulsion flight use.
According to a team paper recently published in the Vertical Flight Society’s 76th Annual Forum, these eVTOL vehicles can help the army with critical tasks such as aerial surveillance and cargo transportation. However, they have smaller rotors than traditional helicopters. As a result, eVTOL rotors may emit a different sound signature that researchers need to consider.
Initial experiments with this concept have shown that stacked rotating rotors are significantly quieter than conventional paired rotor approaches and improve the performance of an aircraft. So far, stacked, co-rotating rotors have not been used in existing aircraft.
“The sound you hear from these smaller rotors is made by radically different physical mechanisms,” said Dr. George Jacobellis, Army Research Engineer in the Laboratory’s Vehicle Technology Directorate. “Traditional modeling techniques need to be improved to take into account all of the noise generated so that vehicle designers know what is actually being heard.”
Standard helicopter noise simulations mainly focus on predicting thickness noise and charging noise as they are the dominant noise sources for large helicopters.
Thickness noise is caused by the displacement of air through the rotor blades, while loading noise occurs when lift and drag forces act on the air flowing around the rotating blades. Together they form what experts call sound noise.
In contrast, army researchers hypothesized that eVTOL rotors produce more broadband noise, which is related to noise caused by turbulence, than tone noise.
“We didn’t know whether broadband noise was important or not, but we did know that audio and broadband noise are scaled differently,” Jacobellis said. “We thought if rotors got smaller there would eventually come a point where broadband noise would be the dominant source.”
The team confirmed their hypothesis in their research study, which not only measured the acoustic properties of various eVTOL rotor configurations, but also assessed the modeling capabilities of helicopter noise simulations for eVTOL rotors.
During the field test, the researchers set up a test stand with two electrically driven rotors and recorded the noise generated above and below the rotor plane with nine microphones arranged in a circular arrangement that surrounded the rotor hub.
For the simulations, the team used Rotorcraft’s comprehensive analysis system in conjunction with a separate program called PSU-WOPWOP, a routine noise prediction code named after an onomatopoeia for the sound of the helicopter blades.
“RCAS calculates the aerodynamic loads or forces on the blades and the bending and twisting of the blades,” Jacobellis said. “This information is required in order to be used as input to PSU-WOPWOP, which calculates the noise produced by the rotor. The combination of the two programs required a considerable amount of work, which was also done by our group. “
The researchers modeled the broadband noise in PSU-WOPWOP using the Pegg method, one of the two settings in the simulation program, and the other, the Brooks method.
While Pegg’s method makes broadband noise predictions based on experimental data from the entire rotor, Brooks’ method takes into account the unique distribution of lift along the blade.
“[These technologies] show promise in their ability to predict eVTOL rotor noise, but we need to do more work to get acceptable accuracy, ”said Jacobellis. “The next steps are to implement the Brooks method for acoustic predictions with higher accuracy and to compare the transient loads between simulation and experiment in order to check the accuracy of the simulations in predicting the transient loads. The Brooks method should be better able to capture unique configurations and load distributions. “
The researchers also found that coaxial co-rotating rotors or stacked rotors could potentially provide better performance and lower noise than a traditional rotor. Unlike traditional rotors, which use single-plane blades, stacked rotors place the blades in multiple levels.
According to the results of their study, stacked rotors with evenly spaced rotor blades produced the lowest noise level, roughly on par with a conventional rotor.
By studying various values of axial spacing, Army researchers believe they can uncover a stacked rotor configuration that produces less noise than the conventional rotor.
“I think a stacked rotor can be beneficial for eVTOL applications,” said Jacobellis. “The additional degree of freedom for the design enables efficiency and control gains in the acoustic signature that was shown in the results. However, more research is needed to quantify the axial clearance noise reduction. “