With sizes ranging from the massive Global Hawk aircraft to handheld machines, UAVs are becoming increasingly important to the U.S. military for tasks such as surveillance and reconnaissance.
UAVs are often tasked with flying close to their targets to gather data effectively and may evade enemy detection with sophisticated techniques such as radar stealth, IR stealth, and special camouflage. Aeroacoustics researchers at the Georgia Tech Research Institute (GTRI) are investigating the sources of noise in UAVs to quiet the aircraft and give them an additional form of stealth.
“With missions changing, and many vehicles flying at lower altitudes, the acoustic signature of a total tactical UAV has become more and more critical,” said GTRI Senior Research Engineer Rick Gaeta.
Gaeta, an aeroacoustics specialist, is working with a U.S. Department of Defense-sponsored GTRI research team to find ways to reduce a UAV’s sound footprint. The researchers have characterized UAV noise using both ground-based methods and vehicle flight tests.
The researchers’ central task involved characterizing the acoustic signature of a UAV’s propulsion system, typically consisting of a piston engine and a propeller but sometimes also electrically or fuel-cell powered. Researchers sought to determine how much noise comes from the engine’s exhaust as opposed to the spinning propeller.
While seemingly straightforward, isolating engine noise from propeller noise is complex because removing the propeller from the engine also removes the cooling source—the propeller wash. Switching to another cooling source typically adds unwanted noise, in turn complicating sound measurements. To operate the engine without a propeller, GTRI investigators were forced to seek out quiet ways to provide both substitute cooling and a load for the engine to spin.
Complexity also arose when attempting to measure acoustic and engine performance simultaneously, which was key to making the correct design trade-offs. Researchers then utilized two special acoustic chambers at GTRI’s Cobb County Research Facility—the Anechoic Flight Simulation Facility and the Static Jet Anechoic Facility.
The flight simulation facility features a unique chamber with a 29-in air duct capable of simulating forward-flight velocities while also allowing precise acoustic measurements. Gaeta’s team then built a special dynamometer capable of driving a small UAV engine and a propeller, allowing them to test both engine and acoustic performance, thereby providing data for UAV design trade-offs.
“The dynamometer measures torque, rpm, and thus horsepower,” Gaeta said. “In addition, it measures axial thrust all in an anechoic or low-sound-reflecting environment, allowing for simultaneous acoustic and propulsion system performance measurements.”
Investigators are also studying how to quiet an unmanned aircraft so that its own sound does not interfere with the task of monitoring ground noise using airborne sensors. For example, a concept that rendered a UAV acoustically undetectable might also affect the UAV’s IR and radar signatures. Changes in those signatures could interfere with the aircraft’s ability to evade hostile IR detection equipment.
In addition to ground-based research methods, GTRI investigators have acoustically measured UAVs in the field, where real atmospheric and meteorological effects modify the acoustic signature reaching the ground. They acquired considerable data using GTRI-owned-and-operated UAVs and also traveled to U.S. military installations and made measurements of UAVs being flown there.
“We’ve been able to learn a lot from piggybacking on other flying programs,” Gaeta said. “These efforts have helped us to develop optimal methods for capturing UAV acoustic data and to find the best ways to process it for analysis.”
Based on its findings, the research team identified specific acoustic measures that could lead to truly covert, low-altitude UAVs.
“Our next step is to put our findings into a prototype for testing,” Gaeta said. “We believe that we have the means to make tactical UAVs much quieter.”