Structural and aerodynamic performance of a three-dimensional compliance-based composite camber morphing wing

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)

Abstract

Fixed-wing aircraft are traditionally controlled using deflectable trailing edge rigid flaps, commonly known as control surfaces. When deflected, these flaps modify the camber distribution of the aerofoil, which changes the aerodynamic pressure distribution over the wing. These changes in aerodynamic pressure result in net aerodynamic forces and moments that can be used to control the lift generation and orientation of the aircraft. However, flaps change aerofoil shape in a sharp and discontinuous way, resulting in surface discontinuities and gaps. These discontinuities induce flow separation, which leads to a significant increase in drag. Alternatively, if these control surfaces could vary camber distribution in a smooth and continuous way, similar control authority can be achieved with a significantly reduced drag penalty. This alternative approach is known as camber morphing, and its implementation on fixed-wing aircraft could lead to a reduction in fuel consumption and noise.

One of these promising camber morphing concepts is the Fish Bone Active Camber (FishBAC) device, a compliance-based design capable of achieving large, smooth and continuous changes in camber. A preliminary 3D printed prototype of this concept was wind tunnel tested, and results showed a 25% drag reduction at the 2D aerofoil level when compared to a flap. However, this first-generation of FishBAC devices were designed using low-fidelity structural and aerodynamic models and manufactured using 3D printed plastic. To implement this technology in real aerospace structures, it is necessary to manufacture this morphing device using aerospace-graded materials. Also, it is crucial to develop modelling tools that can fully capture the complex coupled three-dimensional structural and aerodynamic behaviour of a 3D morphing FishBAC wing. These modelling techniques must be physically rich enough to accurately capture the detailed response of the morphing device while also being computationally efficient to allow for rapid design iterations and optimisation that results in better performing devices.

To address the modelling requirements, two discontinuous structural models based on composite plate model theories (i.e. Kirchhoff-Love and Mindlin-Reissner) and an aerodynamic model based Weissinger’s Lifting Line Theory with viscous 2D panel method corrections were developed. Additionally, the large changes in shape that the FishBAC produces are associated with large changes in aerodynamic pressure (and vice-versa), resulting in a strong coupling between aerodynamics and structural loads. Consequently, to accurately capture both structural and aerodynamic behaviour of these morphing wings, a Fluid-Structure Interaction (FSI) analysis that couples the two different physics was developed. These structural, aerodynamic and FSI modelling techniques capture the highly orthotropic structure of the composite FishBAC, the 3D aerodynamics of the morphing wing and the interaction between structural and aerodynamic loads. Moreover, these models have a useful and appropriate level of fidelity for design and optimisation tasks: they converge using one to two orders of magnitude fewer degrees of freedom than fully coupled Computational Fluid Dynamics (CFD)/Finite Element Method (FEM)-based routines and all structural and material properties are parametrically defined and can be easily modified, allowing for wide-ranging explorations of the design space.

The development of these novel modelling techniques is complemented and validated by the design, manufacture and test of a composite FishBAC wind tunnel wing model. This prototype was manufactured using a combination of manufacturing techniques, including autoclave curing of carbon fibre prepreg, additive manufacturing (3D printing), and traditional metal machining. The composite FishBAC wing was then tested under static actuation loads, and these results were used to validate structural models. Additionally a 2D wind tunnel test was performed, where force balance, wake rake and Particle Image Velocimetry data were collected and analysed to further explore the aerodynamic behaviour of the FishBAC, and to benchmark it against both rigid (non-morphing) and flapped aerofoils.

Results presented in this thesis show that the discontinuous Mindlin-Reissner plate-based model predicts the structural behaviour of the FishBAC using 99% fewer degrees of freedom than FEM, whereas the aerodynamic viscous corrected Lifting-Line model is suitable to analyse the 3D aerodynamics of the FishBAC morphing wing at low Mach numbers and at attached flow regimes. Additionally, the FSI results showed that the 3D FishBAC wing can achieve a lift control authority (i.e. change in lift coefficient) between 0.5 and 0.63 for a wide range of angles of attack. In terms of aerodynamic efficiency, the FishBAC wing showed a 44% increase in lift-to-drag ratios at low lift coefficients, when compared to a flap. Lastly, the 2D wind tunnel test results showed efficiency gains over flaps of between 16% and 50% at the 2D aerofoil level. In summary, these results highlight the potential aerodynamic benefits that a FishBAC morphing wing can bring to a full-size aeroplane and also suggest that the developed modelling tools are suitable for future design iteration and optimisation studies of composite morphing aerostructures
Date of Award24 Mar 2020
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorBen K S Woods (Supervisor), Paul M Weaver (Supervisor) & Jonathan E Cooper (Supervisor)

Keywords

  • Camber Morphing
  • Composite materials
  • FishBAC
  • Aerodynamics
  • Structural Modelling

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