Jetoptera has conducted a series of tests to measure the noise signature of its patented 200 lbf-class Fluidic Propulsive System (FPS™) thrust augmentors as a subcontractor to Paragrine Systems, under a United States Department of Defense-funded research collaboration. The testing compared the Jetoptera FPS™ to a conventional reciprocating engine-driven propeller propulsion system and found an up to 15 dBA lower Overall Sound Pressure Level (OASPL) output based on equivalent power and before application of any additional noise mitigation technologies. Acoustic treatment will increase its advantage to more than 25 dBA OASPL over a rotor on a similar power basis.
The tests were conducted as part of Paragrine’s development of advanced parafoil platforms with highly efficient, low-noise propulsion as an important objective. Jetoptera has been selected for the program because of FPS’s maturity and unique potential for integration into innovative propulsion system configurations. This round of testing evaluated an FPS geometry whose aerodynamic performance Jetoptera has previously demonstrated on a 7-meter wingspan unmanned aircraft used as flight test bed, in multiple VTOL and hovering flight tests, and in the University of Washington wind tunnel.
Optinav, Inc., conducted the acoustic evaluation by using its BeamformX Acoustic Array System, which is a sophisticated instrument that precisely and accurately measures emitted sound profiles and pinpoints the precise sources of audible vibration for actionable solutions. The data analysis was performed by Dr. Robert Dougherty, whose scientific contributions were recently recognized by the AIAA with the Aeroacoustics 2020 Award.
In addition to testing the FPS, the same setup and procedure was applied to a 11-kW internal combustion engine-driven 1.25-meter diameter propeller turning at maximum RPM with a resulting blade tip speed of Mach 0.46. Broadband and narrowband noise measurements were performed with directivity resolution. The test results show up to 15 dBA lower noise performance of the FPS™ versus the propeller, before any FPS™ noise abatement, based on equivalent power level. With the data collected and considering the directivity and atmospheric absorption normal day, we can now predict that at 300 meters the FPS™ would produce less than 50 dBA even before abatement hence, significantly lower than the 65 dBA goal of UAM communities. The FPS will be the most silent propulsor in the skies.
“Unlike a propeller, the FPS™ can take non-round shapes for perfect integration with the wing and therefore is more amenable to further noise abatement via passive and active acoustic abatement techniques. The team applied a technique of placing the thrusters inside various enclosures which further reduced noise emissions by another up to 5 dB, at a modest cost to the thrust performance,” said Dr. Andrei Evulet, CEO of Jetoptera. “This is a very significant lower noise emission level compared to any propeller system, on a same thrust rating comparison, as the inevitable tonal noise from the propeller or rotor dominates the acoustics of such a legacy system, be it electric, piston or turbine driven,” he continued. “We have just demonstrated how our propulsion method will be quieter by a significant margin, even before we incorporate abatement methods. We also compared the aeroacoustic data collected to the noise signature of some advanced propellers that have been developed and reported in the past five decades by NASA and others and the FPS™ was found to have the potential to be up to 20 dB lower and having no tones, an advantage that cannot be matched by any rotor or propeller. Our next steps include the use of the FPS™ in conjunction with an Upper Surface Blown (USB) Wing, which will further reduce the noise levels, as NASA studies have confirmed over several decades of research.”
Jetoptera thanks Paragrine Systems, LLC, who made this acoustic evaluation test possible. We look forward to advancing our collaboration and our unique combination of technologies.