Structural and Aeroelastic Design, Analysis, and Experiments of Inflatable Airborne Wings

Abstract

A scaled, tethered inflatable wing wind tunnel model is studied to investigate its aeroelastic response as a function of the tether orientations and the internal air pressure. Additionally, experimental test are conducted to validate the developed program in conducting inflatable wing's structural and aeroelastic analyses, which will be used to design a large-scale, tethered wing flying at 10 kilometers or higher above the ground. The tethers connecting wingtip leading and trailing edge points, only carry the tensile loads not compression, restrain the wing's torsion and bending motions. The tension in the tethers may lead to compression, depending on their orientations, in the main wing and lower the wing's effective bending stiffness. All of these result in a change in the wing's effective stiffness, which subsequently influences its aeroelastic stability. The static aeroelastic equilibrium state of the inflatable wing is first determined about which the flutter and divergence speeds are computed. Using high-fidelity detailed finite element model for the wing structural model, computational fluid dynamics (CFD) based un/steady aerodynamics, and their coupling to study aeroelastic response are very expensive; thus, medium-fidelity tools are employed for studying the wing's aeroelastic responses. A six degrees-of-freedom Timoshenko beam model is employed for modeling the large aspect ratio inflatable wing. This model includes internal pressure-induced differential stiffness, compression-induced geometric stiffness, cross-sectional properties of an inflated wing, and added masses due to the external surrounding air and the compressed internal air. The tethers are modeled as 1D spring elements, subjected to tension only. Vortex lattice method, corrected using CFD analysis of a thick airfoil, is used for studying the steady aerodynamics. A tightly coupled aeroelastic analysis is employed for an efficient design analysis. Strip theory is used to compute the wing flutter speed. The developed program has been validated against experimental and numerical results available in the literature, and detailed NASTRAN results. Static testing of the fabricated inflatable wing have been conducted, and the bending and torsional stiffnesses obtained from these tests are compared with the numerical results. The effects of the thick airfoil aerodynamics on the divergence speed and internal pressure and tether orientation on the wing's divergence and flutter speeds are investigated.