Revolutionary Coaxial Propulsors for Ultra-Quiet eVTOL

Recently, the aerospace industry has witnessed a surge in interest in multirotor UAM (urban air mobility) systems for short-range personnel and cargo transportation for both civilian and military applications. This has resulted in significant capital investment and the development of numerous eVTOL (electric Vertical Take-off and Landing) aircraft. Soon, skies will be filled with busy buzzing multirotor aircraft, generating a new urban and suburban soundscape that is both different and louder than ever before. This intrusion of acoustic irritants into the last quiet recesses of life must be stopped if eVTOL are to be accepted by the public. Undesirably high sound levels are particularly problematic during hover and transition to cruise when the vehicle is close to people. During this time the propeller disk loading and rotational speeds are high creating high levels of annoying tonal noise. To address this, Harmony Aeronautics LLC will design and develop adaptable ultra-quiet coaxial propulsors for today’s diverse array of emerging eVTOL aircraft which will be 8 times (18dB) quieter than existing eVTOL propellers. Several in-house acoustic and rotor aerodynamic simulation tools have been developed. The acoustic code models the primary sources of rotor and propeller noise generation: thickness, loading, and broadband noise. The aerodynamic code predicts performance characteristics based on design parameters. These programs will be combined with high-fidelity 3D CFD simulation for a comprehensive analysis of propeller performance and rotor flowfield characteristics, and will be used to develop aeroacoustically optimized modular coaxial propulsors with minimal noise disturbances. A high blade solidity coaxial rotor configuration will be used to minimize the rotor RPM, as the blade tip speed is the primary driver of tonal noise. The impulsive unsteady loading caused by rotor-rotor and rotor-airframe interactions will be further attenuated using a novel double swept blade planform shape that dephases the acoustic effect of these interactions. Harmony Aeronautics has developed a hover-optimized prototype of the quiet coaxial rotor system and incorporated it into a sub-scale 22lb drone and a full-scale 550lb personal air vehicle. This was the result of participation in the Boeing sponsored GoFly challenge, a $2 million X-prize contest to develop a quiet, compact, high-endurance eVTOL personal air vehicle. The full-scale vehicle generated ~72dBA of noise in hovering flight at 50 feet, approximately 25 dBA quieter than a light helicopter in hover, and unprecedented for an aircraft of this size. The success of this quiet coaxial rotor demonstrates a promising solution to reducing eVTOL aircraft noise. Similar quieting techniques will be used to design propellers and integrate them into single coaxial propulsor units which will be scaled according to application requirements as demonstrated in the sub-scale and full-scale vehicles.