How is an UAV affected by in-flight icing and can we simulate it accurately?

**NEW MASTER THESIS** One of the master students of the UAV Icing Lab has finished his thesis work. Johannes Oswald conducted his thesis in collaboration with the Institute of Aerodynamics and Gas Dynamics at the University of Stuttgart and the von Karman Institude for Fluid Dynamics. Read more about this research project here.

Johannes Oswald

Ice build-up at the leading-edges of unmanned aerial vehicle (UAV) wings has been identified to severely limit the aerodynamic performance of mid-sized fixed-wing UAVs like the Maritime Robotics “Falk”. This UAV type with wing spans up to several meters usually flies long-range and long-endurance reconnaissance missions. When the flight path of the UAV is passing through clouds or precipitations, the UAV can encounter icing conditions and is prone to ice shapes accretion on its fuselage. This is why the aerodynamic performance losses caused by three different ice shapes has been investigated experimentally and numerically.

Icing on a fixed-wing UAV. Artistic illustration: UBIQ Aerospace.

The experimental campaign was conducted in Belgium at the von Karman Institute for Fluid Dynamics. The institute provided its largest wind tunnel facility to perform the aerodynamic performance experiments on a RG-15 airfoil. To determine the deteriorations from ice build-up at the leading-edge of an UAV wing, a clean airfoil reference case was compared to an iced airfoil case. Therefore, the clean airfoil tests served as the baseline comparison case for the iced airfoil tests. The RG-15 airfoil was artificially iced by taping 3D-printed ice shapes to the wing’s leading edge. Throughout the experimental campaign, the baseline clean airfoil was tested at multiple angles of attack and wind speeds. Due to some experimental challenges, only the glaze ice shape could be tested at multiple angles of attack and two different wind speeds. The other two considered ice shape geometries (rime and mixed) were only tested at one angle of attack at multiple wind speeds. The comparison of the experimental data revealed that the glaze ice shape introduces severe aerodynamic penalties compared to the clean RG-15 airfoil.

RG-15 airfoil mounted inside the wind tunnel with artifical ice shapes attached. Photo: Johannes Oswald.

The minimum aerodynamic drag of the glazed RG-15 airfoil was increased with up to 130% compared to the clean airfoil. The lift coefficient was slightly decreased by 0.03-0.07. Furthermore, the ice shape introduced a destabilizing “nose-down” moment to the baseline RG-15 airfoil. Since experimental campaigns are in general expensive and time consuming, it is also of great interest to virtually simulate these aerodynamic performance penalties. These numerical computational fluid dynamic (CFD) simulations also require validation experiments.

Different ice shapes on the RG-15 airfoil. Figure: Johannes Oswald.

Consequently, the data of the conducted experiments was used to compare to CFD simulations with FENSAP-ICE. The CFD simulations were generally in-line with the measured aerodynamic drag propagation. Limitations occurred in the prediction of the maximum aerodynamic lift of the iced RG-15 airfoil. This led to the conclusion, that the chosen CFD simulation approach of the iced RG-15 airfoil may be adequate to estimate general aerodynamic penalties but may be limited in the flight-stability critical onset of stall region.

Reference: Johannes Oswald, “UAV Icing: Numerical and Experimental Study of Performance Penalties on an RG-15 Airfoil“, master thesis, NTNU/Uni Stuttgart/VKI, 2021.

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