European Research Council Morphing Project at Swansea (2010-2015)

Professor Friswell was awarded a prestigious 2.5M euro grant from the European Research Council to continue to develop in flight morphing of aircraft wings. The project title was the Optimisation of Multi-scale Structures with Applications to Morphing Aircraft and started on 1 May 2010 for 5 years.

Flight, and the design of aircraft, has been a great success story. Early pioneers were inspired by the natural world, and early aircraft, such as the Wright Flyer, twisted a compliant wing for roll control. The demand for higher payloads and faster cruise speeds required stiffer wing structures. Controlling the aircraft by deforming the wings was replaced by discrete aerodynamic surfaces. However, modern aircraft wings are a compromise that allows the aircraft to fly at a range of flight conditions, but the performance at each condition is inevitably sub-optimal. The ability of a wing surface to change its geometry during flight has interested researchers and designers over the years as this reduces the design compromises required. Concepts based on mechanisms, for example variable sweep, have been successful, but concepts based on the compliance of the structure have met with limited success. Such compliant concepts, often termed morphing aircraft, were the subject of this ERC project.

The design of efficient morphing aircraft requires advances in system level modelling and performance assessment, innovative and novel structural concepts, and improved components such as compliant skins. All of these aspects were considered in the project and some highlights are summarised below.

The key requirement for the adoption of morphing technology is the demonstration of the potential performance benefits. This requires a system level analysis that considers not only the aerodynamics of the aircraft, but also the sizing and weight estimation for both the structure and actuators, and how the vehicle will be operated. For example, detailed analysis has been performed on aircraft with variable span for combined dash and loiter missions, to determine if the improved aerodynamic performance is able to offset the weight penalty.

Novel configurations investigated include the biologically inspired FishBAC compliant variable camber device, consisting of a chordwise spine with stringers to support a pre-tensioned elastomeric skin. A significant improvement in aerodynamic efficiency (the ratio of lift to drag) has been demonstrated in simulation and wind tunnel testing, for a relatively small increase in weight. A novel span extension concept with a compliant skin has been designed and extensively modelled.

The structures that comprise a morphing aircraft must be understood to enable the system level performance optimisation. Equivalent models are used to capture their characteristics at the system level and must allow for changes in dimensions and geometry. Skins are vital components, and are typically anisotropic to combine stiffness to support the aerodynamic loads, and flexibility to enable deformation. Corrugated and reinforced elastomer skins have been a particular focus within the project.

The design of morphing aircraft continues to be a huge challenge, and this project has made significant advances in understanding the key requirements in the design process. However, modern aircraft are lightweight and highly optimised, and more research is required before morphing can be routinely considered as a design option for commercial aircraft.

The efficient design of morphing aircraft requires the integration of many disciplines, and the introduction of new structural concepts. Hence the project has made progress in a number of directions that will now be outlined briefly. Only an outline can be provided here, and the published papers give further details.

System Level Optimization

Multi-objective system level optimization of morphing aircraft. Modern aircraft are lightweight and highly optimized; morphing concepts usually add weight and hence the performance benefits must be sufficient to counteract this penalty. Conventional aircraft have the advantage of significant experience to enable low fidelity conceptual design; for morphing aircraft this knowledge is limited and hence low fidelity prediction tools must be used. One of the core objectives of the project was to improve, integrate and validate these low fidelity tools for morphing solutions. The concepts considered included variable span, active winglets, adaptive torsion and variable camber. Objectives included global metrics such as range and endurance, as well as more local metrics such as the lift-to-drag ratio.

Novel Morphing Concepts

The FishBAC compliant variable camber device is a biological inspired concept consisting of a chordwise spine with stringers to support a pre-tensioned elastomeric skin. This concept was extensively modelled and optimized, and the models were validated using wind tunnel and bench top tests. The FishBAC provides a lightweight solution that is aerodynamically efficient, and has generated significant interest in industry for both fixed wing and rotary wing applications. The AdAR (Adaptive Aspect Ratio) concept uses a compliant skin and allows span extensions of 100%. The novel internal structure comprises a telescopic spar, sliding ribs, an elastomer skin and a tensioned strap drive. The key problem is the weight of the actuation mechanism, and this was modelled extensively.

FishBAC Active Camber

Morphing Transition

Active Camber using Piezoelectric Actuators

Span Extension

Adaptive Torsion

Homogenization Methods

As part of the analysis equivalent models of structural components, such as skins, must be obtained. These models must retain their dependence on the physical geometry of the structure so that they may be integrated into the low fidelity tools for system level optimization. Corrugated panels used for skins on morphing aircraft were a particular focus, because their anisotropic properties which are stiff in one direction to support the aerodynamic loads and flexible in the orthogonal direction to allow deformation. Other aspects considered were bio-inspired materials, honeycombs and fibre reinforced elastomers.

Corrugated Structures

General Homogenization and Bio-inspiration

Component Analysis

Skins are a key component that were modelled and analyzed extensively. Bistable structures were also considered for load alleviation and also for large geometry changes. A novel control technique was developed using the structural resonance to activate the snap through with low power requirements. Uncertainty analysis was also performed for composite structures, for example corrugated panels, to ensure their robust performance.

Actuation for Morphing

Actuation of Bistable Structures

Corrugated Skins