Hybrid airfoil design methods for full-scale ice accretion simulation

The objective of this thesis is to develop a design method together with a design philosophy that allows the design of “subscale” or “hybrid” airfoils that simulate fullscale ice accretions. These subscale or hybrid airfoils have full-scale leading edges and redesigned aft-sections. A preliminary study to help develop a design philosophy for the design of hybrid airfoils showed that hybrid airfoils could be designed to simulate full-scale airfoil droplet-impingement characteristics and, therefore, ice accretion. The study showed that the primary objective in such a design should be to determine the aft section profile that provides the circulation necessary for simulating full-scale airfoil droplet-impingement characteristics. The outcome of the study, therefore, reveals circulation control as the main design variable. To best utilize this fact, this thesis describes two innovative airfoil design methods for the design of hybrid airfoils. Of the two design methods, one uses a conventional flap system while the other only suggests the use of boundary-layer control through slot-suction on the airfoil upper surface as a possible alternative for circulation control. The formulation of each of the two design methods is described in detail, and the results from each method are validated using wind-tunnel test data. The thesis demonstrates the capabilities of each method with the help of specific design examples highlighting their application potential. In particular, the flap-system based hybrid airfoil design method is used to demonstrate the design of a half-scale hybrid model of a full-scale airfoil that simulates full-scale ice accretion at both the design and off-design conditions. The full-scale airfoil used is representative of a scaled modern business-jet main wing section. The study suggests some useful advantages of using hybrid airfoils as opposed to full-scale airfoils for a better understanding of the ice accretion process and the related issues. Results from the icing tests on both the full-scale and the hybrid model, performed at the NASA Lewis Research Center's Icing Research Tunnel (IRT), indicate that hybrid airfoils designed to simulate full-scale droplet impingement characteristics can be used to simulate full-scale ice accretions and, therefore, show that the hybrid airfoil design method has great application potential. The results also indicate important limitations of the flap-based hybrid airfoil design that arise due to the onset of flow separation. To alleviate this problem, the study demonstrates the advantages of using boundary-layer control through slot-suction as an alternative to using the flap. The study also revisits some of the old ideas for the design of slot-suction airfoils to show the usefulness of the new design tool in many of the applications related to the advanced-concept wings. Finally, the study suggests some useful recommendations to further enhance the scope of the two design methods.