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The design of morphing aircraft

Final Activity Report Summary - MORPHING AIRCRAFT (The design of morphing aircraft)

The design of conventional fixed wing aircraft is constrained by the conflicting requirements of multiple objectives. Mechanisms such as deployable flaps provide the current standard of adaptive aerofoil geometry, although this solution places limitations on manoeuvrability and efficiency, and produces a design that is non-optimal in many flight regimes. The development of new smart materials together with the always present need for better performance is increasingly prompting designers towards the concept of morphing aircraft. These aircraft possess the ability to adapt and optimise their shape to achieve dissimilar, multi-objective mission roles efficiently and effectively. One motivation for such aircraft are birds that morph between cruise and attack missions by changing their wing configuration accordingly. Birds also use camber and twist for flight control. The Wright brothers used wing warping as seamless flight control in their first flying machine.

Morphing may be used to improve aircraft performance, to replace conventional control surfaces to improve performance and stealth, or to reduce vibration or control flutter. These different applications are all regarded as morphing; however, each is very different in terms of the magnitude and speed of the shape changes required. Thus there will never be a single solution for a morphing aircraft, and the technology employed will be vastly different depending on the application required. However, morphing must achieve improved overall performance and / or functionality, despite the expense of increased complexity and often increased weight.

The practical realisation of a morphing structure is a particularly demanding goal with substantial effort still required. This is primarily because any proposed morphing airframe has to possess conflicting abilities to be both structurally compliant to allow configuration changes but also be sufficiently rigid to withstand the aerodynamic loads. The morphing aircraft project at the University of Bristol has investigated fundamental issues preventing the widespread application of morphing technology. Although the project considered structural or aerodynamic concepts in depth, systems issues required a multidisciplinary approach. For example designing how the structure changes shape is critically dependent on the aerodynamic loads and the required flight control. In detail, the following topics were investigated, usually via both simulation and experiments.

The elastic coupling properties of composite materials may be used to twist a wing as the lift increases. If correctly designed, this enables the wing to optimally configure itself, without any external control, depending on the flight conditions. In particular, the drag may be reduced for a given flight, therefore reducing fuel consumption.

For large deformations, composite structures may be designed that are able to snap between two configurations. After snapping no energy is required to maintain the configuration. Examples demonstrated include a swept wing, wing-tip devices and a variable camber wing profile.

Winglets were used for flight control, motivated by bird flight, to enable a flying wing to be designed without conventional control surfaces. The aerodynamics of a dragonfly motivated the design of a low speed airfoil by using a virtual shape definition.

The flight control of flexible, morphing aircraft provides significant challenges such highly coupled flight dynamics and the presence of many actuators. Discrete winglets were initially modelled, followed by wings that morph continuously and seamlessly using a distribution of actuators.

Morphing will often require a more complex and heavier vehicle and the benefits must be quantified to identify the aircraft concepts where the technology is best applied. A demonstrator was investigated where a moving winglet was used to increase aircraft range.