Morphing Overview

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 uninhabited 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 a seamless flight control in their first flying machine. Morphing wings for flight control bring new challenges to the design of control laws for flight. Because configuration changes move the aerodynamic centre, control of the aircraft during planform morphing requires attention.

One primary advantage of a morphing platform would be the increased cost effectiveness of aircraft through eliminating the need for multiple, expensive, mission specific aircraft. However, from current trends in this research area, it is clearly evident that the practical realization of a morphing structure is a particularly demanding goal with substantial effort still required. This is primarily due to the need of any proposed morphing airframe to possess conflicting abilities to be both structurally compliant to allow configuration changes but also be sufficiently rigid to limit aeroelastic divergence. There are typically four applications of morphing:

These different applications are all regarded as morphing, however each is very different in terms of the magnitude of the shape changes required and time constants necessary for these changes. Fortunately large changes for improved performance are only required at low frequency, and very fast changes for vibration control only need to be small amplitude. This does mean that there is never going to be a single solution for a morphing aircraft, and the technology employed will be vastly different depending on the application required. However all applications require that morphing achieves the objective of improved performance and/or functionality. Often this improvement will be at the expense of increased weight and complexity, and the performance improvement must account for this.

The structural technologies available to achieve a shape changes in a morphing aircraft fall into two major categories, namely planform changes using rigid mechanisms, and compliance (for example wing twist or compliant mechanisms). Vibration control systems are usually based on directly applying a force to the structure. For shape control, actuators are required to effect the shape change, and sensors are required to measure the actual deflection. Large scale morphing motions for configuration morphing (that is significant planform changes) include wing extension, wing folding, and wing sweep. Significant aerodynamic performance gains are only really achievable through large overall changes in the aircraft geometry via wing sweep, area and/or span. The application of morphing to flight control usually involves small geometric wing changes such as the use of deployable slats and flaps as well as wing warping techniques to enhance the control authority of the aircraft. At present, in both of these categories, such medium to large scale changes are obtained with complex and sophisticated mechanical devices significantly increasing the installation and maintenance costs as well as the structural weight of the airframe. It is clear therefore, that substantial gains in these areas could be made if alternative methods to enact these changes were found. Basic morphing motions for seamless flight control include wing twist, wing chamber change, and asymmetric wing extension. The use of winglets as control effectors may also yield substantial benefits.

There are many challenges in the design of morphing aircraft: the integrity of compliant structures needs to be ensured, the system should be designed so the required actuation force is realisable, the skin has to be designed to give a smooth aerodynamic surface yet support the aerodynamic loads, the design process should be extended to encompass multiple flight regimes, engines need to be designed for efficient low and high speed operation, and control systems will have to cope with highly coupled control effectors. While many questions remain unanswered regarding the utility of morphing air vehicles, enough evidence of improved performance and new abilities has been established to warrant further consideration of the prospects of morphing aircraft, both for multiple flight regimes and for flight control.

Selected References