A device called the aerial screw, invented by Leonardo da Vinci 500 years ago, may hold the key to quiet drones.
The Federal Aviation Administration receives thousands of complaints each year about the characteristic loud noise caused by drone propellers cutting into the air. That noise is also a problem in other countries: Canada and the UK, for example, have reported sharp increases in similar complaints over the past several years.
Experts believe that the noise pollution caused by these unmanned aerial vehicles will worsen as they are increasingly used for package delivery, photography, emergency response and more.
„Da Vinci's vision of the vane screw—a precursor of sorts to the modern helicopter—inspired our investigation. The idea was to bring together historical inspiration and modern computation to redesign a silent modern drone,” says Rajat Mittal, professor of mechanical engineering and professor of mechanical engineering at Johns Hopkins' Whiting School of Engineering. Suryansh Prakar holds a PhD in Mechanical Engineering.
Team members were aware that other research groups had explored loop-shaped propellers that were less noisy than traditional propellers with flat, thin blades and angled edges. The characteristic whirring sound produced by traditional propellers is the result of „tip vortices”—small swirling eddies of air intersecting the flat, angled blades. Loop propellers spread those eddies around, muting the sound.
Mittal's team thought da Vinci's design, with its screw-like shape and single blade, might also be silent.
To find out, researchers need to build a model, which requires more precise measurements and select design features. They found a project where aerospace engineering students at the University of Maryland analyzed the design of an aerospace propeller, including its radius, curvature, pitch, shape and number of spirals.
„Using that project as a starting point, we created a 3D model of the da Vinci aerial screw's looped shape, and then used our simulation software called ViCar3D to simulate the flow of air around the rotor as the drone hovered in place. The software predicted the air velocity and pressure patterns around the propeller,” he says. Prakar.
The pressure generated on the surface of a rotating screw turns into sound, so the team used Farasat's formulation — a theory devised by a NASA scientist in the late 1970s to predict sound levels based on simulated airflow patterns. Rotor They then simulated a loop propeller under the same conditions.
The da Vinci propeller was, in fact, slightly noisier than the loop propeller at any given rotation speed, but the aerial screw produced more lift—an upward force that opposes the downward pull of gravity. The researchers also knew that the amount of lift needed to be constant for common drone tasks such as delivering packages, so they got their answer by calculating the noise produced by da Vinci and loop propellers when producing the same amount of lift.
„The da Vinci propeller produced much less noise to produce the same amount of lift,” Brakar says.
The team plans to run more simulations to model the noise level of the propellers as the drones grow larger and operate at higher speeds.
„We expect similar results in noise reduction; however, the aerodynamic efficiency of the da Vinci's propeller will be lower when compared to traditional propulsion because not all parts of the spiral propeller are optimized to generate the same lift force. Despite this potential loss in efficiency, noise reduction is more important than aerodynamic efficiency. These propeller shapes are useful for applications,” Prakar says.
The researchers presented their work at the 76th Annual Meeting of the Division of Fluid Dynamics in Washington, DC in November.
This work was funded by the US Army Research Office.
Source: Johns Hopkins University