Maximizing aircraft economy through multidisciplinary winglet optimization

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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.

Executive summary—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 FEM and CFD simulations to identify optimized winglet design configurations offering operational and environmental benefits. Optimus realized a 1.2% reduction in aircraft takeoff weight for a typical mission profile at 1G cruise speed. It achieved this 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 modeling (RSM) capabilities helped FHI engineers understand the impact of winglet characteristics on aircraft performance up-front. Global design optimization enabled them to reduce the aircraft’s cruise drag by 4% and its takeoff weight by 676 kilograms, while respecting the shock induced separation (SIS) airflow constraint.

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Winglets introduce increased wing tip bending and twist loads, which require reinforcement of the primary wing structure. Source: FHI

Winglet aerodynamics versus structural penalties—Fuji Heavy Industries (FHI) Aerospace Company, located in Japan, is heavily involved in aircraft development and production. The company successfully participated in multiple Boeing commercial aircraft programs. 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. Favorable 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 maximize the added value of winglet integration, FHI relied on Optimus for simulation process integration and aircraft design optimization. In Optimus, FHI engineers set up multidisciplinary design optimization (MDO) of a wing equipped with winglets based on computational fluid dynamics (CFD) coupled with finite element method (FEM).

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Optimus automated the simulation-based design process by interfacing directly with the coupled FEM and CFD packages. Source: FHI

FHI engineers modeled 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 optimization 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.

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FHI defined design variable ranges for winglet angles, ratios and span in addition to wing twist angle distribution. Source: FHI

By interfacing directly with the FEM and CFD tools, Optimus is able to automatically parameterize the wing models, execute simulations and extract results for design optimization purposes. The objective of the winglet design optimization is minimizing the aircraft takeoff weight, while fulfilling preset structural/aerodynamic constraints. The FHI engineers attempted to maximize 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 optimized winglet design—The winglet design space is nonlinear 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 modeling (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 nonlinear winglet physics. With this information, FHI pre-evaluated multiple winglet variants and design characteristics in relation to aircraft performance.

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grafiek2 Response surface charts highlight that low aircraft drag and takeoff weight requires relatively high winglet span and cant angle. Source: FHI

The automatic search for the optimal winglet design turned out to be straightforward. By formalizing the optimization process flow and interfacing directly with the coupled FEM and CFD packages, Optimus automated the simulation-based design process. A genetic global optimization algorithm efficiently directed the multidisciplinary 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.

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Optimus efficiently directed simulations to identify the optimum winglet design configuration, reducing the cruise drag of the aircraft by 4% with minimum structural penalties. Source: FHI

The structural penalty for this impressive drag reduction is 454 kilograms additional airframe weight. The optimized aerodynamic performance results in a 676-kilogram decrease of the aircraft takeoff weight, overall representing a 1.2% reduction. The 1,130-kilogram difference between the increase in gross weight and the decrease in takeoff 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 favorable air travel economics.

The winglet integration project is a typical example of component optimization 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.

Conclusion—By integrating and automating the simulation process flow, Optimus efficiently directed the simulations toward a global aircraft mission improvement:

Process optimization

  • Formalize the wing/winglet simulation process—FHI engineers defined the wing/winglet simulation workflow in Optimus’ graphic process editor, to establish direct and automatic interfacing with the simulation tools.
  • Automate the iterative simulation-based design process—The automated execution of the simulation workflow enables Optimus to eliminate weeks of manual data processing.

Design optimization

  • Execute DOE and RSM to explore the winglet design space up-front—FHI executed Optimus design of experiments (DOE) and response surface modeling (RSM) to quickly and reliably determine the multimodality of the objective function.
  • Optimize winglet design while fulfilling specific design constraints—During winglet optimization, FHI avoided airflow separation at the wing/winglet junction by using the shock induced separation (SIS) criterion as an airflow constraint.
  • Achieve further aircraft mission improvement through the integrated MDO process—By integrating and automating the wing/winglet simulation process flow, Optimus efficiently directed the simulations toward a further aircraft mission improvement.