General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. A recent consideration in aircraft design is the use of folding wing-tips with the aim of enabling higher aspect ratio aircraft with less induced drag, but also meeting airport gate limitations. This study investigates the effect of exploiting folding wing-tips in-flight, as device to reduce both static and dynamic loads. A representative civil jet aircraft aeroelastic model was used to explore the effect of introducing a wing-tip device, connected to the wings with an elastic hinge, on the loads behavior. For the dynamic cases, vertical discrete gusts and continuous turbulence were considered. The effect of hinge orientation, stiffness, damping and wing-tip weight on the static and dynamic response was investigated. It was found that significant reductions in both the static and dynamic loads were possible. For the case considered, a 25% increase in span using folding wing-tips resulted in almost no increase in loads.

A recent consideration in aircraft design is the use of folding wing-tips with the aim of enabling higher aspect ratio aircraft with less induced drag, but also meeting airport gate limitations. This study builds on previous work investigating the effect of exploiting folding wing-tips in-flight as a device to reduce dynamic gust loads, but now with the introduction of a passive nonlinear hinge spring to allow wing-tip deflections only for larger load cases. A representative civil jet aircraft aeroelastic model is used in a multi-body simulation code to explore the effect of introducing such a hinged wing-tip device on the loads behavior. It was found that significant reductions in the dynamic loads were possible. Nomenclature

It has previously been shown that, at high Strouhal numbers, oscillating airfoils can produce deflected jets that can create very high lift-coefficients for otherwise symmetric scenarios. These deflected jets form through pairing of the trailing-edge vortices to create asymmetric vortex couples that self-propel at an angle to the freestream, resulting in an asymmetric flow field and non-zero lift. In this paper results are presented that indicate these high-lift deflected jets cannot form for finite wings. Instead of the straight vortex tubes that pair and convect at an angle to the freestream observed for effectively infinite wings, finite wings exhibit vortex tubes that break into two branches near the tip forming double helix structures. One branch connects with the last vortex; one branch connects with the next vortex. This creates a long “daisy chain” of interconnected trailing edge vortices forming a long series of vortex loops. These symmetric flow fields are shown to persist for finite wings even to Strouhal numbers more than twice those required to produce asymmetric wakes on plunging airfoils. Two contributing reasons are discussed for why deflected jets are not observed. First the tip vortex creates three-dimensionality that discourages vortex coupling. Second, the symmetry of the circulation of the interconnected vortex loops, which has been confirmed by the experiments, is a natural consequence of the vortex topology. Therefore, the asymmetry in trailing edge vortex strength previously observed as characteristic of deflected jets cannot be supported for finite wings.

Volumetric three-component velocimetry measurements have been performed on low aspect ratio wings undergoing a small amplitude pure plunging motion. This study focuses on the vortex flows generated by rectangular and elliptical wings set to a fixed geometric angle of attack of α = 20°. An investigation into the effect of Strouhal number illustrates the highly three-dimensional nature of the leading edge vortex as well as its inherent ability to improve lift performance. Computational simulations show good agreement with experimental results, both demonstrating the complex interaction between leading, trailing, and tip vortices generated in each cycle. The leading edge vortex, in particular, may deform significantly throughout the cycle, in some cases developing strong spanwise undulations. These are at least both Strouhal number and planform dependent. One or two arch-type vortical structures may develop, depending on the aspect ratio and Strouhal number. At sufficiently high Strouhal numbers, a tip vortex ring may also develop, propelling itself away from the wing in the spanwise direction due to self-induced velocity.

This paper details five aeroelastic modelling methods applied to the study of an example high aspect ratio wing subject to high loads resulting in large structural deformations. Each method is discussed in turn and example static results from each are compared. Overall agreement is illustrated between the methods for key quantities of interest although aerodynamic modelling choices regarding the orientation of aero forces is observed to play a significant role in the agreement between predicted distributed loads and deflections. Quantitative differences resulting from linearisation of the wing model are also presented and discussed. It is found that by linearising the problem, wing deflection, aerodynamic forces and root bending are all over-estimated. Large differences are also observed between linear and nonlinear predictions of root twist, however the modelling of drag effects is deemed important to the exact nature of the observed discrepancy. Altogether, linearised assumptions are shown to have a noticeable impact on the accuracy of predicted results for the considered wing test case and are deemed unsuitable in isolation for the analysis of this class of flexible problem.