High aspect ratio wings on commercial aircraft: a numerical and experimental approach

Abstract

The aim of this work is to assess the aeroelastic response to gust of a flexible high aspect ratio wing (HARW) single-aisle commercial aircraft and to design a viable open-loop Gust Load Alleviation (GLA) system. Aeroservoelastic assessment was carried out by adopting the low-fidelity Cranfield Accelerated Aircraft Load Model (CA2LM ) aeroelastic framework. Wind tunnel testing of two flexible wing models was carried out to assess the limitations of low-fidelity numerical frameworks in modelling highly flexible structures. The numerical work firstly focused on upgrading the CA2LM framework by including the non-linear aerodynamic effects of spoiler deflection into the low-fidelity model. The novel method was able to locally change the wing lift distribution evaluated with strip theory by combining ESDU 14004 experimental data with the numerical estimation. Finally, the aeroelastic response of the High Aspect Ratio Technology ENabler (HARTEN) concept aircraft to gust input was carried out for a single flight condition (h=26000 ft and v=200 m/s) and for two different structural configurations: rigid wing and flexible wing structure. Tuned discrete gust analysis, as specified in CS-25, was adopted in this analysis. Results showed that tuned gust is able to excite flexible wing dynamics along with the rigid-body dynamics, having a detrimental impact on aircraft performance. Finally, an open-loop GLA system was designed to alleviate Wing Root Bending Moment (WRBM) increment due to gust load. The GLA deflected spoilers and ailerons for a fixed amount of time (hold time) once a specific vertical load factor was crossed. An optimization algorithm was used to optimize parameters such as control surfaces deflection, hold time and load factor threshold. Several configurations of the GLA were evaluated. The optimal GLA configuration was able to alleviate WRBM from a minimum of 2.4% to a maximum of 8.1% with respect to the non-alleviated scenario. Two wind tunnel models were built with the common spar and skin configuration, while a novel approach for the skin manufacturing was introduced: the skin was 3D printed with PolyJet technology which allowed to provide a continuous aerodynamic shape removing the typical gaps necessary for flexible models to allow wing bending, limiting the impact of the skin to less than 12.5% of the overall model stiffness. The first model was tested in the Cranfield Weybridge wind tunnel at 27 m/s (Re = 3.5e5) and α = 6 ◦ . The model span was 0.840 m and Aspect Ratio AR = 12. The model was successfully tested to prove the ability of the skin to retain the aerodynamic shape and sustain the load under large deformation, reaching a max wingtip displacement of 32% of the model span. The second model was tested in the Cranfield 8x6 ft wind tunnel in the speed range of 20 m/s to 40 m/s (3.1e5 < Re < 6.2e5) at −2 ◦ < α < 8 ◦ . The model span was 1.5 m and AR = 18.8. The main result showed that in the most severe aerodynamic load scenario (v = 40 m/s and α = 8 ◦ ), the spanwise force coefficient accounted for 10% of the wing overall CL and was 2.5 times higher than CD . The overall damping was also estimated for different velocities at α = 6 ◦ , reaching a maximum of 26.9% at 35 m/s and a minimum of 17.8% at 20 m/s, with aerodynamic damping accounting for a minimum of 61% to a maximum of 74% of the overall damping. Maximum displacement of the wing tip was 13.7% of the model span (0.21 m). In both tests a low-cost acquisition system built with off-the-shelf components was used. The system was based on Raspberry Pi board able to acquire accelerations and rotations from four MPU6050 IMU boards, with the main benefit being the small size of the sensors, which were able to fit within tiny volumes typical of HARW wind tunnel models.