Optimal Design Of Winglet Modeled By Freestyle Curve

Application of winglet on transport will lead to improvement of aerodynamic characteristics for it is able to reduce induced drag of wing. However, Wing Root Bending Moment (WRBM) will also increase due to the winglet, and then the structure of wing has to be adapted. NURBS curve controlled by a lot of number is able to from various forms, thus it is suit for modeling a novel winglet. Airfoil used in the model of winglet should be selected particularly for it has large effect on the performance of winglet. Besides, the calculating method and strategies of optimization used in optimal design have direct influence on the results.The design target of the optimization of winglet modeled in this paper is to reduce the drag of wing with winglet, which subjects to the constraint that the percent of increase of WRBM should be less than 5%, and the design status is cruise phase of commercial aircraft. Firstly, the criteria of choosing suitable airfoil is setup according to the feature of winglet, and then a good airfoil is carried out by Hicks-Henne function method. Secondly, surface model of winglet is established by couple of freestyle curve based on NURBS and airfoil chose in CATIA. Thirdly, considering that Vortex Lattice Method (VLM) is a fast method, CFX is of high fidelity, global optimization is able to search in large scale; stratified optimization can reduce the amount of calculation, this paper put them into two strategies: Global Optimization based on VLM and Stratified Optimization based on CFX. To make some comparison, stratified optimization base on VLM is also adopted in Optimization of winglet; Global Optimization based on VLM with the constraint of trailing edge in small range is also conducted. Finally, the flow field of winglet and simple extension of wing with same span are analyzed and compared to figure out the mechanism of winglet reducing drag. The results indicate that:Winglets designed in this paper via different methods and strategies achieve large reduction of drag, particularly winglet designed by global optimization with constraint and the one by stratified optimization based on CFX reduce drag by 7.02% and 6.7% respectively. Winglet can reduce drag by scattering the strong tip vortex into several vortex of less strength in the neighborhood of wingtip and moving the tip vortex away while simple extension of wing can only move vortex away. However, in far field, all of winglets and extension of wing have little effect on scattering wingtip vortex.