Optimus drives winglet optimisation

Wingtip devices are intended to reduce aircraft drag through partial recovery of the tip vortex energy. Fuji Heavy Industries (FHI) used Noesis Solutions' Optimus to automate and couple computational fluid dynamics (CFD) with finite element method (FEM) simulations to identify optimised winglet design configurations offering operational and environmental benefits. Optimus realised a 1.2% reduction in aircraft take-off weight for a typical mission profile at 1G cruise speed by balancing the aerodynamic benefits and weight penalties of a passenger aircraft model integrating wing reinforcement to withstand winglet induced loads. Optimus' design of experiments (DOE) and response surface modelling (RSM) capabilities helped FHI engineers understand the impact of winglet characteristics on aircraft performance up-front. Global design optimisation enabled them to reduce the aircraft's cruise drag by 4% and its take-off weight by 676kg, while respecting the shock induced separation (SIS) airflow constraint. Winglet aerodynamics versus structural penalties FHI Aerospace Company, located in Japan, is heavily involved in aircraft development and production. The company successfully participated in multiple Boeing commercial aircraft programmes. In search of ways to further evolve to a greener environment and more economic air travel, FHI evaluated aircraft wingtip devices of a civil passenger aircraft model. Concretely, winglets increase the lift generated at the wingtip by smoothing the airflow across the upper wing side near the tip. Winglets also reduce the lift-induced drag caused by wingtip vortices, improving the lift-to-drag ratio of the aircraft. Favourable winglet aerodynamics result in higher fuel efficiency and longer aircraft range without materially increasing the wingspan. From a structural standpoint, however, winglets and their attachments increase the weight of the wings. Up in the air, the modified wing shape introduces increased tip bending and twist loads. This changes wing deflection to such an extent that reinforcement of the primary wing structures is required. To make up for these structural penalties and maximise the added value of winglet integration, FHI relied on Optimus for simulation process integration and aircraft design optimisation. In Optimus, FHI engineers set up multi-disciplinary design optimisation (MDO) of a wing equipped with winglets based on computational fluid dynamics (CFD) coupled with finite element method (FEM). FHI engineers modelled the wing structure using FEM software to address the weight impact of the reinforced wing. At the same time, they ran CFD simulations to be able to evaluate the aeroelastic effects of the wing equipped with the winglet. They first sketched the winglet design optimisation process flow in Optimus' graphic drag-and-drop process integration editor. Then they defined the design variable ranges for winglet angles (sweep, cant, tip/root twists), winglet ratios (taper and chord) and one winglet dimension (span) – in addition to wing twist angle distribution. By interfacing directly with the FEM and CFD tools, Optimus is able to automatically parameterise the wing models, execute simulations and extracts results for design optimisation purposes. The objective of the winglet design optimisation is minimising the aircraft take-off weight, while fulfilling preset structural/aerodynamic constraints. The FHI engineers attempted to maximise aerodynamic performance at the cruise condition with minimum structural penalties. The structural sizing of the wing is based on critical static load conditions. Directing simulation toward optimised winglet design The winglet design space is non-linear due to complex flow physics at the wing/winglet junction. Transonic airflows are known to induce shock waves on the upper wing surface. For this reason, winglet optimization targets limited shock wave strength, to avoid excessive interaction between the shock waves and the boundary layer that may cause the airflow to separate. For efficiency and security reasons, FHI uses the shock induced separation (SIS) criterion as an airflow constraint. Following the Latin-Hypercube DOE method, Optimus sampled the design space to acquire the most relevant information with minimum computational effort. Subsequently, highly efficient response surface modelling (RSM) based on the Kriging method reliably determined the multimodality of the objective function. The hills and valleys on the response surface provided valuable insight into the non-linear winglet physics. With this information, FHI pre-evaluated multiple winglet variants and design characteristics in relation to aircraft performance. The automatic search for the optimal winglet design turned out to be straightforward. By formalising the optimisation process flow and interfacing directly with the coupled FEM and CFD packages, Optimus automated the simulation-based design process. A genetic global optimisation algorithm efficiently directed the multi-disciplinary simulations in order to retrieve the best winglet design variants. With Optimus, the FHI specialists identified the optimum wing with winglet design configuration that reduces the aircraft's cruise drag with 4% with minimum weight impacts, while actively avoiding SIS related risks. The structural penalty for this impressive drag reduction is 454kg additional airframe weight. The optimised aerodynamic performance results in a 676kg decrease of the aircraft take-off weight, overall representing a 1.2% reduction. The 1,130kg difference between the increase in gross weight and the decrease in take-off weight is the fuel that is saved with each flight mission. Over the entire lifecycle of passenger aircraft the winglets potentially save airline companies millions on kerosene, contributing to a cleaner environment and favourable air travel economics. The winglet integration project is a typical example of component optimisation that is seen as a subsystem of a complete MDO model. By integrating and automating the simulation process flow, Optimus efficiently directed the simulations toward a global aircraft mission improvement. FHI sees great potential in the achieved results and the applied MDO process that it accomplished with Optimus. www.noesissolutions.com