Why propeller icing prevents UAVs from operating in bad weather

**NEW PUBLICATION**  When flying in icing conditions, uncrewed aerial vehicles (UAVs) face severe dangers. The propeller of the UAV, which is creating the thrust for the UAV is especially sensitive to the accumulation of ice during flight in atmospheric icing conditions. To estimate the impact of icing on the propeller, we conducted experimental tests in an icing wind tunnel. The icing wind tunnel is a special facility whose main feature is a spray system, that can emulate the conditions in a cloud. The propeller was mounted on a test-bench that recorded the forces on the propeller. Also, a high-speed camera captured images of the ice on the propeller to get further details on the ice accumulation process. The propeller was tested across a wide range of icing conditions to separate the different influence factors on the icing on the propeller.

Performance of the propeller in icing conditions. Efficiency and vibration levels displayed.

The ice shapes start to grow on the leading edge of the propeller. This ice shape disturbs the airflow around the propeller. The disturbed airflow makes the propeller less efficient. If the propeller is no longer producing enough thrust, the UAV will no longer be able to continue to fly.

If the mass of the accumulated ice has grown to a critical points, parts of the ice will break off and thrown away from the propeller. The removal of ice improves the aerodynamic performance of the propeller in the briefly, until the ice has been formed again. But the blocks of ice breaking are also a risk to the UAV. The ice fragments can hit other parts of the UAV, like the empennage. More importantly, the ice shedding leads to an imbalanced mass between the propeller blades. This leads to very strong vibrations. In our testing campaign those vibrations exceeded the 10G measurement rage of the used sensor. This is the reason why focusing on the ice shedding process is very important to understand the risk of ice on the propeller of a UAV.

The analysis of the ice shedding has shown that the amount of ice that can form on the propeller is strongly dependent on the temperature. Lower temperatures lead to larger amounts of ice on the propeller, while at higher temperatures the amount of ice on the propeller is lower. The size of ice fragments that shed from the propeller were the largest at temperatures of -10 °C.

Propeller performance at different temperatures.

Another focus of the study was the aerodynamic performance of the propeller with ice accumulated on the propeller. Ice on the propeller reduces the thrust of the propeller and its efficiency. This could be measured by our test setup to see how different temperatures influence the propeller. Here it is clear that the loss in performance is the worst for temperatures of about -10°C. This hits a sweet spot between higher temperatures which have less amount of ice freezing on the propeller and lower temperatures where the ice shapes on the propeller are very streamlined and do not influence the performance of the propeller as much.

Predicted Performance of the propeller in icing conditions.

The whole range of measurements has been taken and merged into an algorithm to predict the performance of the propeller across a range of conditions. This can be used as a tool in flight simulators to predict the influence icing has on the flight of a UAV. This enabled the development of advanced path planning methods to optimize flight routes to reduce the impact of icing and to train autopilots to cope with icing on propellers.

Reference: Müller, N., Hann, R. (2022). UAV Icing: A Performance Model for a UAV Propeller in Icing Conditions. AIAA Atmospheric and Space Environments Conference. DOI: 10.2514/6.2022-3903

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