Solving Crossflow-Induced Fatigue in Foldable UAV Propellers – Introducing Mejzlik’s New 22" Design

Foldable propellers are widely used across multirotor UAV platforms because they support compact aircraft designs and make handling easier during both transport and operation. As UAVs spend more of their time in forward flight, the expectation for long-term stability and consistent mechanical behavior naturally grows.

Feedback from customers made it clear that commercially available foldable propellers did not deliver the level of longevity they expected. In several cases, users even reported catastrophic hub failures during flight. This highlighted the need to develop a foldable propeller capable of maintaining stable characteristics throughout extended operation. We began a focused engineering effort aimed at addressing this requirement in a reliable and data-driven way. The result is the second generation of our folding hub architecture designed around improved fatigue behavior, consistent mechanical response and long-term durability in forward-flight environments.

Establishing the Requirements for a New Foldable Propeller

Foldable hubs offer practical advantages, but their long-term performance depends on how well the internal components handle cyclic mechanical loads that appear primarily in forward flight. To ensure consistent long-term behavior, it was necessary to define clear requirements for a design intended for continuous use.

We therefore focused on identifying the parameters that most strongly influence hub longevity. Three areas proved essential:

• material pairing at the blade to hub interface

• geometry and load distribution within the hub assembly

• the clamping force that determines how firmly the blade is held in place

Understanding the Crossflow Challenge

Forward flight exposes foldable propellers to a specific aerodynamic regime known as crossflow. Unlike hover, where loads remain relatively uniform, crossflow introduces alternating forces that act on the blade root every rotation. Because the blade can rotate slightly around its connection to the foldable hub, these forces create repeated micro-movements that accumulate wear over time.

As a result, long-term stability depends on how effectively the hub can manage these alternating loads without allowing excessive micro-motion at the blade root, which would accelerate wear.

Developing and Validating a Crossflow Test Setup

To evaluate how different hub concepts behave in crossflow, it was necessary to create a test method that could reproduce the alternating loads seen in forward flight. We designed a setup that generates a controlled stream of accelerated airflow and exposes the test propeller to it at a perpendicular angle.

For the airflow source, we used our fixed-pitch 70×24 two-blade propeller, which produces a strong and repeatable flow when operated at defined conditions. The foldable propeller under test was positioned perpendicular to this airflow, allowing the hub to experience the alternating forces characteristic of crossflow. The method provides stable and repeatable conditions for comparing different hub configurations.

Using this setup, we tested multiple hub designs. Some configurations showed visible signs of wear after only a few hours of operation, while others lasted significantly longer. The final design withstood 100 hours in the test without meaningful degradation, providing a clear indication of improved long-term behavior. While other commercial propellers survived the test, their wear levels were noticeably higher, confirming that our new design maintains far superior stability under crossflow loading.

Beyond the crossflow method, the second-generation hub architecture also underwent additional mechanical evaluations including:

• tip-deflection testing

• overspin testing

• centrifugal load testing

These complementary tests provided a broader understanding of how each hub configuration behaves under both aerodynamic and mechanical stress.

Engineering Decisions Behind the Final Design

Testing made clear which hub characteristics had the strongest impact on long-term stability in crossflow. Two factors remained dominant: the material pairing between the blade bushing and the hub pin, and the clamping force that controls how firmly the blade is held in place. These parameters were refined to reduce wear at the blade to hub interface while keeping the foldable mechanism fully functional.

We also identified previously existing stress concentrations in both the upper and lower hub halves. These were systematically removed in the new design, improving mechanical strength and fatigue margins under repeated loading. The development process also included destructive load testing to verify structural strength and confirm the effectiveness of these changes.

The blades themselves were redesigned using prepreg material and manufactured through a hot-press process. This method increases stiffness, improves dimensional accuracy and results in a more precise leading edge, which is critical for consistent aerodynamic performance. Thanks to the updated hub architecture and modernized manufacturing approach, the new design also achieves a noticeable weight reduction compared to the previous generation.

Together these adjustments form the basis of the final hub configuration, which maintains stable behavior under alternating loads throughout extended operation.

Key Advantages of the Final Design

The resulting hub and blade configuration introduces several advantages relevant to long-term use in forward flight and beyond:

• Long-term stability in crossflow, with significantly reduced wear at the blade root during extended operation.

• Higher mechanical strength and improved fatigue performance due to refined geometry and the elimination of stress concentrations.

• More precise and consistent blade construction, achieved through prepreg hot-press manufacturing with improved stiffness and leading-edge accuracy.

• Fail-safe hub behavior, ensuring that the blade remains secured even if a mounting screw breaks.

• Broad motor compatibility and symmetric puller/pusher installation, reducing integration complexity and simplifying logistics for coaxial and multi-configuration platforms.

Final Remarks and Next Steps

The development of the new twenty two inch foldable propeller combined focused testing with targeted engineering adjustments to address the demands of forward flight. The final configuration demonstrates stable behavior under long-term crossflow loads and maintains its characteristics throughout extended operation.

Together, these characteristics make the new 22" folding propeller a forward-flight-ready solution that addresses one of the most persistent challenges in multirotor design. As platforms continue shifting toward higher speeds, longer missions and more demanding environments, this configuration gives developers a verified, future-proof basis for building reliable aircraft with confidence.

Building on this foundation, we are now transferring the same second-generation hub architecture to additional foldable propeller sizes in our portfolio, starting with 34" and 32" configurations. This approach ensures that the performance improvements achieved with the 22" design are consistently applied across a broader range of platforms.