Aerodynamic Comparison of Freezing Rain and Freezing Drizzle Conditions at the RTA Icing Wind Tunnel

The simulation of icing conditions in icing wind tunnels (IWTs) is a significant element in the certification of aircraft components and offers unique possibilities for research purposes. Up to 2014 only the conditions defined in Appendix C of the EASA Certification Specification 25, respectively the FAA Code of Federal Regulations Title 14 Part 25 were used for the certification processes in IWTs. In addition, Appendix O was introduced in 2014 to cover the supercooled large droplet (SLD) icing conditions of freezing drizzle and freezing rain, which pose a potential risk for flight safety. The simulation of SLD icing in IWTs is, due to the different behavior of the large droplets, very challenging and not all required conditions have successfully been recreated yet. RTA Rail Tec Arsenal Fahrzeugversuchsanlage GmbH has focused on the simulation of in-flight icing conditions since 2012 and increased effort was put in the simulation of SLDs in recent years. During several research projects funded by the Austrian government it was shown that droplet size distributions for freezing drizzle MVD > 40 μm and freezing rain MVD > 40 μm can be recreated close to the requirements. With further improvements to the spray system, a cloud uniformity within certain tolerances also has been achieved over areas large enough to cover an unscaled wing section. In order to determine the influence of the different droplet sizes on the aerodynamic performance, a NACA0012 wing section with a wingspan of 1.8 m and a chord of 1 m was exposed to different icing clouds including Appendix C conditions, freezing drizzle and freezing rain. The aerodynamic lift coefficient was recorded over a large angle of attack range and the characteristics of the different ice accretions were analyzed and documented using high resolution 3D scan technology. Furthermore, the drag increase during the icing process was measured. It was the first time that an unscaled wing was iced under artificial freezing rain conditions.