Experimental Investigation and Aerodynamic Analysis of Supercooled Large Droplets

Until to 2014 only the Appendix C (FAA CFR Part 25) conditions were used for the certification processes in icing wind tunnel (IWT). Appendix O was introduced in 2014 to cover the supercooled large droplet (SLD) icing conditions of Freezing Drizzle and Freezing Rain, which result in a worse endangerment for flight safety. AVIC ARI FL-61 IWT has focused on the simulation of in-flight icing conditions since 2016 and increased effort was put in the simulation of SLD. The cloud parameters with different LWC and MVD characteristics were combined to achieve a SLD icing cloud with a bimodal droplet distribution. The mass distribution is achieved through modification of the existing spray system of the FL-61 IWT to allow two spray profiles to be simultaneously injected into the flow. Simulations of SLD icing environments in FL-61 IWT have been performed in which broad band mass distribution spectra are achieved that include a distinct pattern of LWC over a range of droplet sizes. Results of spray profile distributions measured in the test section have demonstrated the Freezing Drizzle conditions (FZDZ). It was shown that droplet size distributions for FZDZ MVD > 40 μm and MVD > 40 μm can be recreated droplet size distributions close to the requirements defined in Appendix O requirements, and the large droplet sizes up to 520 μm. The construction of SLD icing conditions in FL-61 IWT can be well predicted and guided through generating large and small droplet clouds with specified droplet mass distributions and combined spraying, and the main feature of SLD icing is having icing limits that extended further back on the chord of the model and the generation of relatively independent icicle. In order to determine the influence of the different droplet sizes on the aerodynamic performance, a NACA0012 airfoil was exposed to different icing clouds including Appendix C and Appendix O FZDZ. Aerodynamic of the iced NACA0012 airfoil was been analyzed by CFD. The ice accretion reduced the maximum lift and the stall angle, the drag increase was the most significant and a rapid increase. The Appendix O impact on the aerodynamics also were more severe compared to the Appendix C. The investigated FZDZ condition further reduced the maximum lift, had the most severe impact on the aerodynamics with a maximum lift reduction and the stall angle.