General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/pure/about/ebr-terms A B S T R A C TFused filament fabrication (FFF) is a 3D printing technique which allows layer-by-layer build-up of a part by the deposition of thermoplastic material through a nozzle. The technique allows for complex shapes to be made with a degree of design freedom unachievable with traditional manufacturing methods. However, the mechanical properties of the thermoplastic materials used are low compared to common engineering materials. In this work, composite 3D printing feedstocks for FFF are investigated, wherein carbon fibres are embedded into a thermoplastic matrix to increase strength and stiffness. First, the key processing parameters for FFF are reviewed, showing how fibres alter the printing dynamics by changing the viscosity and the thermal profile of the printed material. The state-of-the-art in composite 3D printing is presented, showing a distinction between short fibre feedstocks versus continuous fibre feedstocks. An experimental study was performed to benchmark these two methods. It is found that printing of continuous carbon fibres using the MarkOne printer gives significant increases in performance over unreinforced thermoplastics, with mechanical properties in the same order of magnitude of typical unidirectional epoxy matrix composites. The method, however, is limited in design freedom as the brittle continuous carbon fibres cannot be deposited freely through small steering radii and sharp angles. Filaments with embedded short carbon microfibres (∼100 μm) show better print capabilities and are suitable for use with standard printing methods, but only offer a slight increase in mechanical properties over the pure thermoplastic properties. It is hypothesized that increasing the fibre length in short fibre filament is expected to lead to increased mechanical properties, potentially approaching those of continuous fibre composites, whilst keeping the high degree of design freedom of the FFF process.

Broad-scale area change of a non-porous surface while maintaining resistance to aerodynamic loading was demonstrated through the development of a passive elastomeric matrix composite morphing skin. The combined system includes an elastomer-fiber-composite surface layer that is supported by a flexible honeycomb structure, each of which exhibit a near-zero in-plane Poisson’s ratio. A number of elastomers, composite arrangements, and substructure configurations were evaluated and characterization testing led to the selection of the most appropriate components for prototype development. The complete prototype morphing skin demonstrated 100% uniaxial extension accompanied by a 100% increase in surface area. Results from out-of-plane pressure loading showed that out-of-plane deflection of less than 0.1 in. (2.5 mm) can be maintained at various levels of area change under pressures of up to 200 psf (9.58 kPa). Applications to wing span morphing UAVs are also discussed.

This work presents comparative experimental investigations into the aerodynamics of the recently proposed Fish Bone Active Camber morphing structure. This novel, biologically inspired concept consists of four main elements: a compliant skeletal core, a pre-tensioned elastomeric matrix composite skin, an antagonistic pair of tendons coupled to a non-backdriveable spooling pulley as the driving mechanism, and a non-morphing main spar. The Fish Bone Active Camber concept is capable of generating large changes in airfoil camber and is therefore proposed as a high-authority morphing solution for fixed-wing aircraft, helicopters, wind turbines, tidal stream turbines, and tilt-rotors. This testing compares a baseline airfoil employing a conventional trailing edge flap to a continuous morphing trailing edge using the Fish Bone Active Camber concept. Testing is performed in the low-speed wind tunnel at Swansea University over a range of camber deformations and angles of attack. Both approaches are capable of generating similar levels of lift coefficient; however, comparison of the drag results shows a significant reduction for the Fish Bone Active Camber geometry. While purely two-dimensional flow was not achieved due to restrictions of the tunnel, the two airfoils operated in similar flow environments, allowing for a direct comparison between the two. Over the range of angles of attack typically used in fixed and rotary wing applications, improvements in the maximum obtainable lift-to-drag ratio on the order of 20%–25% are shown.

This paper introduces a novel airfoil morphing structure known as the Fishbone Active Camber (FishBAC). This design employs a biologically inspired compliant structure to create large, continuous changes in airfoil camber and section aerodynamic properties. The structure consists of a thin chordwise bending beam spine with stringers branching off to connect it to a pre-tensioned Elastomeric Matrix Composite (EMC) skin surface. Actuators mounted in the D-spar induce bending moments on the spine through an antagonistic pair of tendons in a manner similar to natural musculature systems. Several potential morphing configurations using this concept are introduced. The paper then focuses on a trailing edge morph wherein the compliant spine connects a rigid leading edge D-spar to a solid trailing edge strip. The motivation for exploring this novel morphing architecture is established through analytical aerodynamic comparison to the NACA 0012 airfoil with and without a discrete trailing edge flap. A prototype device is built to explore various aspects of manufacturing this concept, and to prove the large deflection capability of the FishBAC.

Recent developments in morphing aircraft research have motivated investigation into conformal morphing systems, that is, shape change without discrete moving parts or abrupt changes in the airfoil profile. In this study, implementation of a continuous span morphing wing is described. The system consists of two primary components: (1) zero-Poisson ratio morphing core and (2) fiber-reinforced elastomeric matrix composite skin with a nearly zero-Poisson ratio in-plane. The main goal for improved air vehicle efficiency was a nominal 100% change in area of the active wing section with less than 2.54 mm out-of-plane deflection under representative aerodynamic loading. Objectives of this study included exploring fabrication techniques for advanced morphing core shapes (i.e., having airfoil-shaped cross-section), exploiting customizable design parameters of in-house fabricated skin and core material, designing a prototype wing structure such that integration with a candidate UAV was feasible, and experimentally evaluating a laboratory prototype. As a result of this study, the ability to physically build and test a viable airfoil structure capable of increasing its planform area by 100% (doubling span with constant chord) was demonstrated on a prototype hardware demonstration article. Satisfying objectives of designing, fabricating, and testing a prototype morphing wing section capable of 100% span extension, while maintaining constant chord, a wind tunnel test highlighted the resulting viable aerodynamic surface in a wind tunnel test up to 130 km/h wind speeds. The prototype wing in its resting condition had a span of 61.0 cm, which could be extended to 122.0 cm, with less than 2.54 mm out-of-plane deflection in dynamic pressures consistent with the maximum speed, 130 km/h, of a candidate unmanned aerial vehicle platform. In meeting these goals, the morphing core was successfully transitioned from a simple 1D concept into a complex, cambered airfoil with sufficient free volume to house an actuation system. A refined elastomer matrix composite skin fabrication technique was also devised and experimentally validated on skins of various thicknesses and overall dimensions.