• 28 Apr 2025 09:06 PM
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Harmony Advances Quiet Propeller Technology

By Alex Scerri
Vertiflite, May/June 2025

Vertiflite recently caught up with Moble Benedict, an associate professor of aerospace engineering at Texas A&M University and founder of its Advanced Vertical Flight Laboratory (AVFL) with 20 years of experience in VTOL aircraft concepts. He is also the CEO of Harmony Aeronautics, a Texas-based engineering services startup, that was a Phase I and II winner of the GoFly Prize with its Aria coaxial helicopter (see “Flying Solo: GoFly Advances Single-Passenger Air Mobility Solutions,” Vertiflite, May/June 2019).

Aria is a high technology readiness level (TRL) compact rotorcraft, designed to minimize noise and maximize efficiency, safety, and reliability. As noise minimization remains a key objective in eVTOL design, we took the opportunity to get some insight into this research. This interview was lightly edited.

Vertiflite: Moble, can you give us the “elevator pitch” for your noise minimization design?

Benedict: With the GoFly challenge, the objective was a quiet and compact personal flying vehicle. We put a lot of effort into quieting the rotor. I say “rotor” because we started with a coaxial helicopter configuration, with collective and cyclic pitch control. We have an aeroacoustics partner from Penn State, Prof. Eric Greenwood, who we teamed up with for this project. Starting with small-scale models from a 22-lb [10-kg] drone, we finished with a 550-lb [250-kg] full-scale aircraft. When measuring the noise on the various scale iterations, we achieved a 15–16 dBA noise reduction, which is a very significant decrease compared to standard rotors producing the same thrust. For the full-scale aircraft in hover flight, we measured 73 dBA at 50 ft [15.2 m].

Vertiflite: Can you give us some comparison?

Benedict: Well, with a mainstream light helicopter, even when standing at around 100 ft [30 m], the noise level will be close to 100 dBA. Don’t forget that the decibel scale is logarithmic. With every 10 dB the perceived noise approximately doubles, so our aircraft is 3–4 times quieter [than a comparable helicopter].

Vertiflite: Where did this research lead?

Benedict: At the same time that we founded Harmony Aeronautics with the plan to commercialize this innovation, we saw that there was not such a huge market for a coaxial helicopter. That drove our focus to develop a quiet propeller system that could be used in eVTOL [electric vertical takeoff and landing aircraft] applications. We applied for a Phase I Small Business Grant with AFWERX [an innovation unit of the US Air Force]. In the first phase, we started by developing the design tools and software and built a subscale, 18-inch [45.7-cm] diameter propeller and did an array of tests in an anechoic chamber at Penn State, using the collected data to validate our design tools.

In Phase II, the Air Force wanted to see if we could further tweak the design tools and build a full-scale version of the propeller system, which we did. This was a coaxial propeller system with an 8-ft [2.43-m] diameter. That is about the same size as Beta’s Alia aircraft's lift propellers, which we used as a representative aircraft that could use our system, being around a 6,000-lb [2.7-tonne] aircraft with four propellers. Each of our coaxial propellers would produce around 1,500 lb [6.7 kN] of thrust, plus margin.

The big effort was building the stand and test rig. It is 25-ft [7.62-m] high, fully instrumented with load sensors to measure forces on the upper and lower propellers. We can actively change the vertical separation between the propellers. This allows us to measure all the performance parameters at the different distances to find the optimal value. Besides separation distance, we also tried different numbers of blades and pitch angles to give us a full spectrum of results. With one configuration, we measured a noise level of around 85 dBA at 50 ft, while producing 1,000 lb [4.5 kN] of thrust. The noise level was only slightly higher than the original concept aircraft, although the propeller operates at a much higher disc loading and thrust.

Vertiflite: Does noise minimization impinge on efficiency?

Benedict: That’s a good question. For an isolated propeller we obtained a figure of merit around 0.78 (from test data), which is pretty good. So, we are happy with the noise versus efficiency balance.

Vertiflite: How do you achieve this noise minimization?

Benedict: The first task was to reduce the noise of each propeller separately. The main target is limiting the tip speed while producing the same thrust. To do this, you must increase the solidity (the ratio of total blade area to disk area) of the propeller. That would result in a propeller that requires high torque and low rpm, which increases the load on the motor. Doing a detailed parametric study with our aeroacoustics tools, we came up with the optimal airfoil, which also changes from the root to the tip of the blade.

When you think about noise from a helicopter, one source will be the loading noise, which is there because of the aerodynamic force distribution on the blade, and that will always be there as long as you are producing lift. You also have unsteady loading noise in forward flight. For coaxial rotors, the unsteady loading noise is also present in hover due to the interference between the two rotors.

Another source of noise is the thickness noise, which we can minimize by reducing the thickness of the blade, to displace less air as the blade cuts through it, but then of course you have to consider the impact on other required properties of the blade, especially on its structural rigidity.

There is also the broadband noise, like a white noise, that occurs at all frequencies, which is due to turbulent air coming off the system. This is a strong function of the angle of attack and can be reduced by controlling the airflow on the blades. One of our innovations is blade shaping to reduce the unsteady loading noise in the coaxial system.

Vertiflite: Did you consider a shroud?

Benedict: It is not that obvious. There has been some research done in the past to study the effect of shroud on the noise. One of the main parameters is the length of the shroud or duct, which has to be high to minimize the noise emission. There are some scenarios where the shroud could amplify the noise. Shrouds can have a useful function of protecting the propeller, but these are usually short and don’t do much for noise reduction. We normally want to bring down the frequency to one that produces less irritation to the human ear, but that increases the wavelength, meaning you would need a much longer shroud which in turn increases weight and drag.

Vertiflite: Is there a market for your product?

Benedict: All the market leaders like Joby, Beta and Archer have their proprietary propellers. However, there are still a number of new companies that may not have the [research and development] capacity to do all this themselves. They would certainly be a good target for our product. As you can imagine, now there is even a bigger demand for local USbuilt aircraft and subsystems, and we can be a part of that growth.

Vertiflite: What other projects do you have in the pipeline?

Benedict: One of things we are really keen on is the quadrotor biplane, which takes off vertically, turns 90 degrees and flies like a biplane. We presented our concept to the US Department of Defense (DoD) tech scouts, and they were impressed with the simplicity of the technology and its efficiency when flying in biplane mode.

[In] February, we were one of the 11 GoAERO Stage 1 winners and also secured funding from NASA as innovation awardees [see “GoAERO Awards $500,000 to Stage 1 Teams,” Vertiflite, March/April 2025]. Our concept is a 600-lb [272-kg] quadrotor biplane aircraft that can carry one person. Of course, we are seeing how to integrate quiet and efficient proprotor technology into that aircraft as well.

About the Author

Alex Scerri started his aviation career in 1994 flying for Air Malta. He joined Emirates in 2006, flying several aircraft types, including the Airbus A380 as captain, and worked in the fleet technical office. He is currently flying the A350 for French Bee in Paris. He has been writing on advanced air mobility (AAM) and aviation safety since 2020, including for Vertical Magazine.