Inside Unmanned Systems

JUN-JUL 2016

Inside Unmanned Systems provides actionable business intelligence to decision-makers and influencers operating within the global UAS community. Features include analysis of key technologies, policy/regulatory developments and new product design.

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37 unmanned systems inside June/July 2016 ENGINEERING. PRACTICE. POLICY. FOR A VARIETY OF REASONS, the quiet drone is fl ying to the forefront. by Rachel Kaufman "IF I CAN CHOOSE between two UAVs and they do the same thing—but one is quieter—I'm going to take the quiet one." Rick Gaeta, director, Unmanned Systems Development Center at Oklahoma State University T he problem of noise has long vexed drone operators. Many small multiro- tor aircraft are relatively quiet on their own and flying at height, but in large numbers or closer to the ground they could pollute the air- space with buzzing rotors. Or so some of those unhappy about drones would have you think. Concerned citizens aside, there are a number of reasons why quieter vehicles would be ben- eficial to operators. In a defense context, a loud unmanned aerial system (UAS) announces its presence. Similarly, any intelligence-gather- ing mission would be jeopardized if the target could hear an approaching surveillance drone. Already, some vehicles are noisier than others. "You have a wide range [of noise levels]," says Rick Gaeta, director of Oklahoma State University's Unmanned Systems Development Center and a research professor in the school of mechanical and aerospace engineering. Fixed- wing vehicles are usually quieter than those that use rotors for power, but everything makes some sort of noise. "Just the fact that it moves through the air, it will make noise," he said. Gaeta has spent a lot of time over the years learning how to make vehicles quieter. His group works on a number of projects for de- fense and commercial applications, though not all are related to making quieter drones. He also believes noise is going to be a key issue for buyers. "If I can choose between two UAVs and they do the same thing—but one is quieter— I'm going to take the quiet one," he told Inside Unmanned Systems. But getting to that quiet, it turns out, is a complicated thing. Tougher Than It Looks First, pure amplitude—"loudness"—is not al- ways easy to measure. For example, if you're trying to test the loudness of a certain type of propeller, "If you just put it on a stand and spin it up, that's not the same noise it'll make when it's actually f lying." Further, human beings perceive different noises in different ways. "There's a difference between annoyance and detection," Gaeta said. "So noise [is] somewhat subjective." A multirotor vehicle, with its higher frequency noises coming from propellers spinning at high RPMs, is likely to be louder and more annoying than a fixed-wing drone. But because the atmosphere ab- sorbs higher frequencies, it's not audible from as far away. "If I had it 100 feet [away] you'll be more sensitive to it being there. You're going to be more annoyed by it," Gaeta said. Even external factors like cloud cover and terrain affect how loud a drone sounds from the ground, which can confound measurement techniques and wreak havoc with computer models that predict noise. Still, Gaeta and his group have had some success. Lately he's been testing new material combinations to find lightweight but strong construction materials that can also acousti- Some people fi nd noises above 75 db very loud and annoying. Source: https://www.chem.purdue. edu/chemsafety/Training/ PPETrain/dblevels.htm fast fact

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