**NEW PUBLICATION** Aircraft without ice protection systems will face severe issues when flying in icing conditions. To reduce the risk of losing the aircraft, the aircraft must either remain grounded when the risk of icing exists, or it must be equipped with an ice protection system. As of today, no mature ice protection system exists for small unmanned aircraft. To fill this gap, a new ice protection system was developed and thoroughly tested in an icing wind tunnel.
The occurrence of in-flight icing can significantly influence the performance of aircraft. Hence, every aircraft that is supposed to fly in or through icing conditions must have an ice protection system (a system that protects the aircraft from the adverse consequences of icing). This can be particularly important for unmanned aircraft. Drones, uncrewed aerial vehicles (UAVs), unmanned aerial systems (UAS), and urban air mobility (UAM) are often used as synonyms to describe unmanned aircraft – an area that is getting more and more popular.
Unmanned aircraft are typically more prone to icing than manned aircraft. Hence, ice protection systems are extremely important for unmanned aircraft to allow operation in all weather conditions. However, as of today, unmanned aircraft are not commercially available with an ice protection system. This means that unmanned aircraft will typically remain grounded when potential icing conditions are present (liquid droplets are in the air at temperatures below the freezing point).
An important parameter for the quality of an ice protection system for unmanned aircraft is the energy-efficiency. The more energy the ice protection system requires, the less the aircraft’s possible range. Since the available energy on board the aircraft is significantly lower for unmanned aircraft than for manned aircraft, this is a bigger issue for unmanned aircraft than for manned aircraft.
An example of a possible ice protection system for unmanned aircraft is D•ICE. D•ICE was originally developed at the Norwegian University of Science and Technology (NTNU) and is now commercially developed by UBIQ Aerospace. It is an electrothermal system that is designed to work best in de-icing mode. This means that after some time of ice accretion, the wing is heated by powering carbon fiber layers inside the wing. The heat melts some ice at the interface between the wing and the ice. When enough ice is melted, the ice will shed from the wing, as can be seen in the picture below.
To examine the performance of different settings of the ice protection system, tests were performed in the icing wind tunnel at the Technical Research Centre of Finland (VTT). An icing wind tunnel is a special wind tunnel that is equipped with a spray system and can be operated at constant temperatures below freezing. The wing was placed in the airstream that contains water droplets. After allowing ice to accumulate on the wing for four minutes, the heating zones are activated, and the de-icing procedure starts.
One of the key findings was that the heat flux provided to the heating zones is one of the operational settings of an electrothermal ice protection system that can be adjusted. Increasing the heat flux results in higher temperatures at the wing’s surface; hence, the ice starts melting faster. As a result, ice shedding happens faster when higher heat fluxes are used. While one might think that faster shedding also results in a more energy-efficient de-icing process, this is not always true. On the contrary, the results showed that – averaged over time – more energy was required for the de-icing when higher heat fluxes are used.
The heat flux provided to the heating zones is one of the operational settings of an electrothermal ice protection system that can be adjusted. Increasing the heat flux results in higher temperatures at the wing’s surface; hence, the ice starts melting faster. As a result, ice shedding happens faster when higher heat fluxes are used. While one might think that faster shedding also results in a more energy-efficient de-icing process, this is not always true. On the contrary, the results showed that – averaged over time – more energy was required for the de-icing when higher heat fluxes are used.
This means that for an encounter with a given duration, using lower heat fluxes for de-icing results in less energy used although the individual ice shedding times are faster.
While it should be said that the results might be different for different geometries and internal structures of ice protection systems, the study showed some potential ways to reduce the energy needs of ice protection systems. Using the results of the study to improve ice protection system operation modes will hopefully enable the flight of unmanned aircraft in icing conditions soon.
Reference: Wallisch, J., Hann, R. (2022). UAV Icing: Experimental Investigation of Ice Shedding Times with an Electrothermal De-Icing System. AIAA Atmospheric and Space Environments Conference. DOI: 10.2514/6.2022-3905