Commercial drone manufacturers have pretty much mastered small multi-rotor aircraft. Such aircraft are deployed regularly for surveying and cinematography. The quad and hexacopters deployed for lightweight commercial applications are ubiquitous and reliable. So now the industry is pushing forward into more challenging applications.
From long-range logistics to high-altitude endurance missions and military reconnaissance, the demands now being placed on drones are more challenging than ever. The industry is being forced to transition from small off-the-shelf, multi-rotor designs to Group 5 Unmanned Aerial Vehicle (UAV) systems.
For engineers and custom propeller manufacturers of UAVs, scaling up from multi-rotor to Group 5 propellers isn’t a simple matter of scaling up a CAD drawing. Engineers need to account for non-linear structural challenges, some of which can be severe. Moving into Group 5 territory means putting extra effort into addressing major aerodynamic and mechanical phenomena. Centrifugal force and blade flap take center stage.
Before an engineer can successfully address centrifugal force and blade flap, they must understand the differences between multi-rotor and Group 5 UAV propellers. For starters, blades from multi-rotor devices are relatively small. Their Group 5 counterparts can be impressively large.
With that in mind, we define the differences between the two as follows:
The applications for both types of propellers are as distinct as the propellers themselves. Small, multi-rotor craft need rigid, fixed-pitch UAV propellers capable of facilitating rapid RPM changes. By contrast, Group 5 UAVs operate more like traditional manned aircraft. Not only that, but they also perform similar tasks, including carrying heavier payloads and operating at altitudes where the atmosphere is thinner.
Scaling up from multi-rotor drones to Group 5 UAVs is an exercise in aerospace tyranny. Things don’t always work the way you want them to. On a small, multi-rotor craft, centrifugal force is easily managed by deploying carbon fiber composites or injection-molded plastics. The mass of the blade makes very little difference. But centrifugal force scales with radius and rotational speed.
A custom propeller for a Group 5 UAV requires a much larger diameter design to generate the necessary thrust. That means the stress on each blade – outward and away from the hub – increases to astonishing levels. If you know the math, you know it is unforgiving.
Forces extending outward to the tip of a blade offer significant challenges. But addressing those challenges doesn’t mean much if the blade is not securely attached at its root. A quick-release mechanism or a simple bolt typically secures the blade of a smaller commercial drone. Neither would be reasonable for a Group 5 blade.
A Group 5 UAV blade must be capable of withstanding tens of thousands of pounds of outward pull. If the root is too rigid, the concentrated pull stress will lead to catastrophic material fatigue. If it’s too heavy, it adds parasitic weight to the aircraft. Coming up with the strongest and most efficient design requires utilizing advanced geometric tapering and bonding techniques.
Centripetal forces pull blades outward while aerodynamic lift pushes them upward. One might expect the forces to equilibrate naturally. But that only happens under perfect conditions. Such conditions don’t exist in the Group 5 world. Group 5 UAVs often fly through turbulent, high-altitude air masses at impressive speeds. The result is a phenomenon known as blade flap.
Blade flap is a condition UAV propellers experience because of asymmetrical airflow. As a vehicle moves forward, the advancing blade is subject to a higher relative air speed compared to the retreating blade. Higher air speed means more lift; lower air speed means less lift.
This asymmetrical condition creates an inconsistent lifting force. The advancing blade wants to flap upward while the retreating blade tends to sag downward. This puts additional stress on both blades and creates challenges for maintaining stable flight.
Overcoming the challenges of high-stress Group 5 flight lies in engineering. Designers set aside simplistic design philosophies in favor of advanced aerospace engineering techniques. They rely on three primary strategies for designing robust and capable UAV propellers:
It would be great if engineers could scale from multi-rotor blades to their Group 5 counterparts simply by scaling up existing drawings. That’s not the way it works in reality. The transition from small, multi-rotor blades to Group 5 UAV propulsion systems requires significant technical and design enhancements.
Solving the unique problems that come with scaling up is a big part of what we do at Aerodine. We consider it both a challenge and an opportunity to help the industry continue pushing the boundaries of what Group 5 UAVs are capable of.
Why don’t scaled-up CAD drawings work for Group 5 UAV props?
Scaling up is non-linear thanks to the square-cube law. As prop dimensions double, centripetal forces quadruple. Simple scaling doesn’t work.
Do small drones suffer from blade flap?
Blade flap does occur with smaller, multi-rotor aircraft. But the blades are rigid enough to withstand the stresses. Making Group 5 blades equally rigid would add too much weight, so engineers must address blade flap through other means.
How do flapping hinges and elastomeric bearings protect Group 5 blades?
Both types of systems allow individual blades to pivot up and down. Group 5 propellers experience significantly less stress as a result of these designs.
How does altitude impact propeller structural design?
The higher the altitude, the lower the air density. UAV propellers must either spin faster or have a larger surface area to operate at high altitudes.
Is carbon fiber a common material for Group 5 propellers?
Yes. Carbon fiber is preferred thanks to its exceptional strength-to-weight ratio. However, engineers must utilize specialized protective leading edges for Group 5 UAV propellers to overcome carbon fiber’s inherent weaknesses.
