Work on Quiet UAVs Shows Promise
Bettina H. Chavanne 
Military commanders readily admit that their enemies are getting smarter and more adaptive. Stories of insurgents surrendering to an unmanned aerial vehicle (UAV) are long gone. These days, even children recognize the nasal drone of a small UAV, which means that what used to be an easy mission can quickly be compromised.
Rick Gaeta, a senior research engineer at George Tech Research Institute (GTRI), calls noise an unwanted afterthought of UAVs. Every other element of the vehicle is integrated onto the platform before noise mitigation is considered. Once a UAV is fully built, "you're limited in what you can do to reduce noise," Gaeta says.
There are several options for noise mitigation. He and his team are working on what is officially called acoustic signature control, sponsored by an independent research program at GTRI and the Defense Dept. There are two primary targets for noise reduction--the propulsion system and the propeller.
A few years ago, DSO Laboratories of Singapore examined propeller performance and aeroacoustic signature. The reduction was a measly (at least for Gaeta's purposes) 3 dB., but the point was that there are technologies available to apply to every element of a UAV to shave off noise. The DSO study concluded that to reduce propeller noise, parameters would have to include rpm., propeller diameter, sweep and number of blades.
Most UAVs rely on efficient, two-stroke, fuel-burning engines for power, which Gaeta calls "a glorified lawnmower engine on steroids." Now think about how loud a lawnmower would be if it had 30 hp. A carefully tuned muffler can reduce sound, but only by 10-15 dB., Gaeta says. If you're looking to create a truly undetectable UAV, it would require "a radical reduction in noise from the baseline," equivalent to about 30-40 dB. "That's a challenge. That's significant," Gaeta says. "And it will require lots of innovative airframe propulsion integration and the right techniques to evaluate acoustic and aerodynamic performance."
Separating out how much noise is coming from the propeller and how much from the engine (whether powered by fuel cell, electricity or piston) is another challenge. It's possible to separate the propeller from the engine, but at that point you're sacrificing the propeller's cooling slipstream on the engine. Add a separate cooling element and you've piled on weight and increased noise signature again.
Gaeta and his team use two acoustic chambers at GRTI--the Anechoic Flight Simulation Facility and Static Jet Anechoic Facility--to measure acoustic and engine performance simultaneously. The facility has a 29-in. air duct that simulates flight while allowing for precise acoustic measurements. Gaeta used it to test various engine types for far-field noise. "If I do something to quiet [the engine], I can measure how I've affected performance and noise," he says. The next step was to go out into the field and do flyovers to measure the acoustic footprint on the ground and establish a correlation between the tests.
The constant evolution of technology has given rise to new testing techniques, allowing for better diagnostics and improving Gaeta's ability to characterize the acoustic signatures of vehicles currently operating. He's also worked with industry and the military to do a variety of tests, building on other programs to advance his research.
"This technology is an enabler," Gaeta says. "It allows you to do so many things. If I can fly 500 ft. off the ground with no one hearing me, at night, then I can install powerful optical devices that can get resolutions we don't have today."
He envisions applications that would be useful to police departments and Homeland Security. "We're pushing toward the development of a prototype that can lead to an acoustically undetectable UAV," Gaeta says.
Photo credit for AAI Corp.'s Shadow 200 UAV: U.S. Army
