As unmanned aerial vehicle (UAV) capabilities have evolved, so have the missions for which these vehicles are deployed. Performance expectations have increased accordingly. For instance, modern performance is no longer exclusively a matter of flight duration or payload capacity. Certain types of missions require sensitive acoustic signatures.
A loud drone is one that is easily noticed. So whether a UAV is deployed for crime fighting in a dense urban environment or on the battlefield for intelligence, surveillance, and reconnaissance (ISR), keeping noise to a minimum is a must. Drones with silent flight capabilities are now the standard for certain types of operations.
We achieve silent flight by engineering low-acoustic UAV propellers. By manipulating carbon fiber layups and designing advanced blade geometries, we can now suppress UAV noise at its source: the props.
Noise is the obvious enemy of silent flight. From an engineering standpoint, we look at noise through the lens of acoustic signature. The acoustic signature is composed of two elements:
Traditional UAV propellers made of injection-molded plastics, metals, or wood are limited by material stiffness. The blades undergo micro-deformations under the high RPMs drone blades are known for. Those deformations create flutter that introduces unreliable air pressure changes, leading to higher decibel levels and a harsh, high-frequency whine that can be heard over impressively long distances.
Replacing injection-molded plastics, metals, and wood with composite materials goes a long way toward making UAVs quieter. But achieving silent flight doesn’t actually begin with materials. It begins with geometry.
Blade geometry determines how air exits the trailing edge of a propeller. Low-acoustic designs call for engineering that manages the tip vortex – the swirling air that comes off the tip of a prop. This is the point at which the most noise is generated. From a design standpoint, there are three things to consider:
Blade geometry that takes advantage of how air interacts with propellers can help reduce noise significantly. When combined with better materials, silent flight becomes quite achievable.
Even though geometry can reduce noise by manipulating how air interacts with blades, engineers must also account for how a blade performs under load. This specific area is where our expertise in carbon fiber layup becomes an asset in stealth.
We can engineer silence into a layup. Consider anisotropic tailoring. Where metals and other materials are isotropic (they behave the same way in all directions), carbon fiber is anisotropic. That means we can manipulate its behavior based on the direction of the fibers in a given layup. For example:
Another property we can manipulate is vibration damping. Because carbon fiber offers a high stiffness-to-weight ratio, it also offers acoustic properties not found in other materials. By making a blade stiffer, we also make its natural frequency higher. We can push a blade’s resonant frequency beyond its operational RPM range, thereby ensuring that a small drone doesn’t sound like a much larger aircraft.
Composite materials matter because we cannot do the same things with their traditional counterparts. Propellers made of metal or wood cannot easily be made silent. Likewise, propellers made of traditional injection-molded plastics cannot withstand the punishment of stealth flight.
The core of a UAV propeller also affects its acoustic signature. A hollow prop makes more noise because its internal structure acts as an echo chamber. It is like the body of a guitar. A hollow blade actually amplifies both tonal and broadband noise.
The flip side is that solid blades weigh more. They are also harder to manipulate. So we can address both issues using sandwich core construction. This involves creating blades comprising a carbon fiber skin with a specialized foam or honeycomb core in between. Rather than amplifying sound, the core dampens it. Meanwhile, the blade maintains its lightweight properties without compromising strength.
Silent UAV flight is sometimes about stealth and other times about not being a public nuisance. Yet, regardless of the need for silent flight, accomplishing it requires precision manufacturing. A blade that is out of spec by even a fraction can easily become an acoustic nightmare. So manufacturers need to get it right. Here is how we do it at Aerodine:
Payload capacity and flight duration are still critical performance factors for UAVs. But as missions evolve, silent flight is also becoming a top priority. We help accomplish it through the custom engineering and manufacturing of composite UAV propellers.
What is ‘blade slap’ and how is it prevented?
Blade slap is a noise-generating phenomenon that occurs when a propeller blade interacts with a neighboring blade’s vortex shed. It can be prevented by using high-modulus carbon fiber to increase stiffness and reduce structural deformation.
Why is the trailing edge so important to noise reduction?
How air leaves the trailing edge of a propeller determines whether it creates turbulence. To achieve silent flight, we want as little turbulence as possible.
What is aeroelastic tailoring?
In the context of designing UAV propellers, aeroelastic tailoring involves manipulating a carbon fiber layup to cause the finished blade to intentionally twist under load. This allows us to reduce noise by tightly controlling the angle of attack.
How do serrated trailing edges help with silent flight?
Serrated edges break up coherent turbulent structures so that they dissipate more quickly. Think of an owl’s wings. Those wings naturally reduce noise, making the owl one of the stealthiest predators on Earth.
