This summer, the UAV Icing Lab went on a joint campaign with UBIQ Aerospace at the icing wind tunnel facility of the Technical Research Centre of Finland (VTT). The main goal of the campaigns was to do experiments on ice protection systems for UAVs and to study the effects of icing on propellers. Among other things we used a high-speed camera system to capture ice shedding events.
Icing of unmanned aerial vehicles (UAVs) occurs during flight through clouds that contain supercooled liquid droplets. These cloud droplets are in a liquid phase but have a temperature below the freezing point. When these droplets collide with a surface, like the wing or the propeller of a UAV, they freeze and can accumulate into large ice accretions. This ice disturbs the airflow and can lead to severe performance penalties and lead to the crash of the UAV.
One of the most reliable tools to research icing on UAVs are icing wind tunnel tests. An icing wind tunnel is a special facility that can generate the same type of icing conditions that a UAV would encounter in real flight – but under controlled laboratory conditions. An icing wind tunnel typically has three main components: the wind tunnel, the cooling system, and the spray system.
A wind tunnel is the core of every icing wind tunnel. It is the part that generates the high wind speeds that match the flight velocity of UAVs. The spray system injects small droplets into the air stream of the wind tunnel. These droplets are equivalent in size and number to the conditions of icing clouds. The cooling system is required to lower the temperature of the air and droplets to sub-zero conditions. There are multiple ways of how this can be achieved. In our case, the entire wind tunnel was located inside of a large climate chamber that can be cooled down to -25°C. The combination of these three systems, wind tunnel, spray system, and climate chamber generates the same type of supercooled liquid droplets that a UAV encounters in flight.
During the test campaign this summer we used a high-speed camera to record ice shedding. Ice shedding refers to the process when the ice that has accumulated on a surface is removed. On a wing, this happens when an ice protection system is used that heats the wing and therefore breaks the bond between the ice and surface. On a propeller, ice shedding occurs when the ice accretion becomes so big, that shedding occurs due to centrifugal forces. We used a high-speed camera system to capture both of these events.
The high frame rate videos allow us to see exactly how the ice shedding process occurs. We can use them to identify different shedding processes that can occur. For example, on the ice protection systems we have cases where most of the ice melts and other cases where the ice breaks away due to the aerodynamic forces. Understanding these processes helps to develop efficient ice protection systems.
On the propellers, ice shedding is a dangerous mechanism, especially when ice sheds asymmetrically on the blades. When ice sheds form on one blade but not the other, it can lead to very strong vibrations that can damage the propeller and motor.
The data and knowledge gathered during these icing wind tunnel tests will be used in the UAV Icing Lab to advance our understanding of the effects of icing on unmanned vehicles. The data will also assist in developing better models and methods to mitigate the dangers of icing. This is an important contribution to help enable unmanned aircraft technologies for future applications like urban air mobility or advanced air mobility.
We like to thank the Erik Langørgen and the NTNU Department of Energy and Process Engineering for lending us the high-speed camera system.