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NOISE REMAINS A TOP CHALLENGE for Making Air Taxis a Reality
ensure blades are designed to promote low noise instead of using existing helicopter blades. For multi-rotors, the aerodynamic interactions are essential because they alter the aerodynamic loads and contribute to broadband noise as well as create additional blade vortex interactions. Multi-rotors offer different possibilities to lower the noise, e.g., rotation direction and phasing strategies for quad-rotors have shown that some decent noise reduction can be achieved. However, the variability of the different configurations complicates the noise issue. Also, the possibility of several different UAM vehicles operating at the same vertiport is an important issue and contributes to broadband noise. Finally, the effect of the environment (e.g., urban canyons) has to be included in the studies. It is estimated that we need to get to a 15 dB reduction compared to a helicopter of the same weight class to reduce noise to the level of automobile traffic. In order to achieve significant noise reduction, noise must be accounted for in preliminary/conceptual design as well as operation parameters. It is also unclear if new FAA noise certification regulations will be needed, and if they are, significant delays should be expected.
Uber recently announced it will be launching UAM service for key cities in 2023. Is this timeline for aerial taxis realistic? Let’s go back in history. In 1943, a LIFE Magazine writer proclaimed that “After the war is over… the helicopter may well become the average man’s flying machine to be used — not right away but inevitably — much as the average man uses his automobile.” The cover photo featured aviation pioneer Igor Sikorski with an early helicopter followed by illustrations of a post-war commuter leaving his home via helicopter for a day in the office. Seventy-seven years later why has this prediction not yet been realized? There are several reasons — chief among them: the helicopter still faces safety concerns and it is not easy to control compared with the automobile. Rotor noise also remains a persistent problem. Rotor noise (when tip Mach numbers are below the transonic value) can be divided into thickness noise (due to the fluid displacement by the rotor blade), loading noise (due to the rotating of the blade forcing lift and drag), blade-vortex interaction (due to the blade interacting with the tip vortex of the preceding blade), and broadband noise (due to ingestion of atmospheric turbulence, interactions with rotor wake turbulence, and scattering of turbulence over the trailing edge). UAM vehicles have lower Mach numbers and Reynolds numbers compared to a regular helicopter resulting in higher amounts of loading noise and, most importantly, broadband noise. That’s why it is crucial to
By Tasos Lyrintzis, Ph.D.
Dr. Lyrintzis is a distinguished professor and chair of the Aerospace Engineering Department at Embry-Riddle. He is an AIAA Associate Fellow, an ASME Fellow, and a Boeing Welliver Fellow. 15 dB reduction
It will certainly take time and patience to resolve the many important noise issues before UAM vehicles achieve industry commercialization. Significant investment is needed to support research and development in this area.
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