Aeroelastic tailoring of strut-braced wings

Aeroelastic tailoring of strut-braced wings

Ran, J.

De Breuker, R. (mentor)

Aerospace Engineering

Aerospace Structures & Materials

Aerospace Structures & Computational Mechanics

2017-05-29

Strut-braced wing aircraft are believed to have great potential for the future of large passenger air transport with regard to fuel burn reductions due to the combination of high structural and aerodynamic efficiency. Strut-braced wings deviate from conventional wings in the fact that they employ additional bracing at certain span location away fromfuselage to relieve the bending moment at the root of thewing. The large aspect ratio wings that are thereby enabled, experience less induced drag, and reductions in wave drag and parasitic drag compared to conventional wings. The downside of a high degree of wing slenderness is the inherent wing flexibility which in turn makes strut-braced wings very prone to aeroelastic (flutter) instabilities. In the last few years a lot of work has been done to get a better understanding of aeroelasticity of strutbraced wing and truss-braced wing aircraft in light of the Boeing Subsonic Ultra Green Aircraft Research (SUGAR) (Bradley et al., 2015). Nonetheless, flutter constraints still cause a significant weight penalty for these configurations. At the same time, passive load alleviation by the application of aeroelastic tailoring for the purpose resolving these flutter instabilities has not been exploited fully yet. To investigate whether aeroelastic tailoring can indeed resolve aeroelastic instabilities in—and increase the potential of strut-braced wings, the existing Proteus framework was modified and verified to enable design of these configurations. Fourteen strut-braced wing models with varying strut configurations and a reference model of a clean wing, all with an aspect ratio of 19.4, have been optimized for a minimum weight objective and the corresponding results have been analyzed. It was found that configuration parameters such as the axial stiffness of the strut, and the spanwise and chordwise location of the strut-wing connection had a significant influence on the aeroelastic vertical wing deflections and wing twist, respectively. Results of aeroelastic tailoring clearly showed that the spars were tailored for the inboard normal force caused by the strut. Stiffness results also implied that the wings were optimized for a wash-out effect at the root and a wash-in effect at the tip. As this result is essentially the exact opposite of what would be seen for conventional wing design, static aeroelastic deformations, active design constraints and critical aeroelastic eigenmodes of the strut-braced wings were further investigated. The stiffness distribution throughout the wing had a smaller influence on the wing deformations than the configuration parameters, and mainly affected the rate of wing twist along the span of the wing. The wash-out effect at the root indeed resulted in an increase in wing twist over the first section of the wing, but the supposed wash-in effect only decreased this twist angle slightly, resulting in a small decrease in wash-out instead of actual wash-in. Nevertheless, the wash-in effect could be explained by the flutter instabilities thatwere found to be the key design driver for tailoring of all strut-braced wing designs. Moreover, because of the extreme susceptibility of strut-braced wings to flutter, the bands of aeroelastic stability showed to be very narrow. As a consequence, laminate design was driven towards the boundaries posed by laminate feasibility constraints. The deformations corresponding to the most critical eigenmodes showed direct links to the tailored stiffness distribution. The set of aeroelastically tailored strut-braced wing designs shows that the isolated wing mass decreases for increasing values of axial stiffness of the strut. At the same time the stiffest struts make a larger contribution to the total mass of the strut-braced wing. The configuration with the strut at 60% of the span, 30% of the chord length and with the second lowest axial stiffness was the most optimumdesign, resulting in amass decrease of 20.9% with respect to the optimized clean wing. From the presented results it can be concluded that aeroelastic tailoring is very well capable of avoiding flutter without the addition of any weight penalties. Aside from modifications aimed at increasing the accuracy of the current work, it is recommended that this research is continued on a full scale Boeing SUGAR strut-braced wing. In case aeroelastic tailoring on such a full scale model indeed resembles the promising results that were found here, the future of aviation will finally be associated withmore disruptive developments and larger steps in fuel burn reductions alike.

http://resolver.tudelft.nl/uuid:525970df-c593-40f2-9c59-fb771a774459

Student theses

master thesis

(c) 2017 Ran, J.