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Modelling of Adaptive Wing Structures

Final Report Summary - MAWS (Modelling of Adaptive Wing Structures.)

Executive Summary:
Adaptive wing tip concepts have been considered to enable controlled wash-in and wash-out throughout the entire flight envelope. The concepts, based upon adaptive stiffness structures, enable control of the aeroelastic deflections to optimise the aerodynamic performance along with the use of changing the leading edge and trailing edge camber shape. A mathematically based design tool was developed, making use of Reduced Order Models (ROMs) of the wing structure and aerodynamics, so that an inverse approach can be employed to determine the required sizing of the internal aircraft structure, and also the position and sizing of any adaptive structural components and actuators. The adaptive structural concept and the inverse ROM approach will be demonstrated on a regional jet aircraft test case.
Initial studies using panel based aerodynamics were performed to determine the desired aerodynamic shape of the wing at different points in the flight envelope, and these were then used to optimise the internal structure so that the desired aeroelastic deflections were achieved subject to a number of structural constraints; further work considered the use of Euler based CFD.
Three types of adaptive structure based solution were considered: rotating spars, moving spars and moving spar caps designed to enable the required wing-tip deflections through changes in the bending and torsional stiffness along with the position of the shear centre. The rotating spar concept had the greatest effect upon the stiffness characteristics of the wing cross-section.
Inclusion of the device showed that it is possible to achieve the required aerodynamic shapes at all parts of the flight envelope including the extreme low speed Mach 0.25 case; however the least amount of morphing required was when both the adaptive stiffness and leading/trailing edge camber stiffness concepts were used together.
The effect of uncertainty in both the aerodynamics and structural models was assessed using both Polynomial Chaos Expansion and Bayesian methods, and it was found that the morphing performance was most sensitive to the structural stiffness and aerodynamic parameters (speed and air temperature).
Further work is required to validate the concept using a more detailed aerodynamic and structural model.

Project Context and Objectives:
Air transport is increasingly becoming more accessible to a greater number of people who can afford travelling by air, both inside and outside Europe, for leisure and business purposes. This is evidenced by the fact that last year the European air transport system moved more than 1 billion passengers and 14 million metric tonnes of freight through its airports whilst handling more than 12 million movements over the same period. Despite the effects of 9/11, SARS, the IRAQ war and even the recent volcano problems, the sector forecasts that over the next decade, both passenger and freight traffic is expected to increase at an average 4-5% p.a. (with freight slightly higher) both significantly above global GDP growth: in air transport terms, this implies a doubling of traffic about every 16 years. It is evident that environmental requirements, such as noise impact and emissions, will play a dominant role in future transport aircraft development, becoming a driving force for aircraft design. This is the main reason for which ACARE, in the so-called Strategic Research Agenda 2, established the so-called greening aircraft as the first objective of future research activities related to Aeronautics. The adoption of this kind of global requirement has two main consequences: firstly, the greening level becomes one of the criteria for which a new aircraft has to be judged or selected; and secondly, the aircraft configuration itself must be defined to fulfil the greening requirements.
The direct greening design criteria, as formulated into Vision 2020 ACARE Agenda, are represented by: 80% cut in NOx emissions, halving perceived aircraft noise, 50% cut in CO2 emissions per pass-Km and green design, manufacturing and maintenance. Other indirect greening requirements must also be considered, for instance, drag reduction and weight savings. It is obvious that by reducing a small amount of fuel burn and multiplying this by the total number of transport aircraft leads to a remarkable reduction of emissions into the atmosphere. Looking at actual transport aircraft it is very easy to identify many similarities in shape and configurations of different airplanes, even if during the last decades great technological improvements have been reached, for example concerning engine emissions and noise reduction, high-lift device configurations and advanced materials. One of the reasons is related to the fact that configuration and performance of commercial aircraft, especially fixed-wing aircraft, have been optimized within a limited range of conditions, especially cruise conditions, in terms of speed and altitude. Outside this range, aircraft behaviour is less than optimal. If, for example, a single wing is designed for a broad performance spectrum, this doesn’t guarantee that optimal performance may be reached in any specific area. High speeds require thin low cambered airfoils, whilst take-off and landing requires thick and highly curved airfoils. The adoption of extendable and retractable slats and flaps have been the best choice to obtain a compromise and guarantee aircraft a certain degree of adaptiveness to conflicting requirements at different flight conditions.
Looking at the medium and long term period it is evident that significant steps forward to more efficient aircraft, able to meet direct and indirect greening requirements, will be achievable only by enhancing the aircraft capability to adapt its configuration to different flight conditions so as to be always in the optimal configuration. All aircraft adapts their shape in flight when subject to external loads, such as lift and inertial loads, and assume a deformed shape that is substantially different from the unloaded one due to the structural deformability; this effect is due to static aeroelasticity.

Although this self-adaptation is taken into account from the conceptual/preliminary design as part of the “jig shape” design, this is traditionally only aimed at one point in the flight envelope, and a general loss of performance from both static and dynamic points of view (different sub-optimal aerodynamic load distribution, less control effectiveness, etc.) are found at all other points in the flight envelope. Traditional aircraft design is based upon the premise that all aeroelastic effects are undesirable, and thus aircraft structures are built to be stiff and heavy in order to avoid them. Since conventional aircraft are unable to alter their structural shape in flight, these structures have to be designed to have the optimal aerodynamic shape at a single point in the flight envelope. From this point of view, aeroelasticity has been considered for many years as a necessary evil, causing potentially catastrophic problems such as divergence, aileron reversal and flutter.
The last few years have shown an increasing interest in the development of aircraft structures that allow aeroelastic deflections to be exploited in a beneficial manner. There has been a great deal of recent activity in this area in USA through the Active Flexible Wing wind tunnel test programme, the US Air Force / NASA Active Aeroelastic Wing flight test programme. By turning the wing into an entire deformable control surface and dispensing with ailerons and flaps, Boeing expects to save 20% in fuel costs. During Framework Five, the 3AS project aimed to make some headway towards reducing the 10 year headstart that the US has in this important field. The SMorph project has continued the work of 3AS, including the development of a design environment for morphing. There is a major need for the EU to expand its research capabilities in this rapidly developing area, in order to compete with future fuel efficient and environmentally friendly US designs.
From a practical point of view, it is commonly accepted that morphing can be categorised between local morphing (where the change in configuration is limited only to some part of aircraft but the external shape substantially is kept unchanged) and global morphing (where global aircraft characteristics like wingspan, planform, sweep angle, are changed). Research and development in the field of morphing aircraft aims to change continuously wing shape, instead of using conventionally actuated wing technology, where usage of discrete control surfaces is adopted to realise a morphing effect. Among the previously wing morphing parameters that were considered, one can point to variable twist, camber, sweep, dihedral and span. This work concentrates upon a local morphing concept based upon the adaptive aeroelastic structures approaches developed by the lead investigator.
Many morphing concepts rely on the usage of hinges, screws, hydraulic actuators, and other similar devices to change aircraft structure geometry; other morphing systems are based on piezo-ceramic actuators or shape memory alloys, to deform the structure from its initial condition to a final configuration. Recent developments in morphing structures can be found in composite laminates controlled with piezoelectric actuators, as well as in morphing hyper-elliptic cambered span wing, where a single-degree-of-freedom mechanism is used to continuously morph the wing from a planar to a non-planar configuration.
DARPA is playing a leading role in developing morphing aircraft technologies through the Morphing Aircraft Structures (MAS) program; in this program, an aircraft with retractable wings has been developed. This aircraft is expected to morph from an extended wing configuration at low speeds to a retracted configuration at high speeds. The aircraft uses two morphing mechanisms, a first morphing mechanism to extend and retract the wing and a second mechanism to deform the leading edge so that at the retracted position it conforms with the fuselage of the aircraft. The former mechanism is actuated by a thermo-polymer actuator driving a helical spline. The latter is actuated by a high frequency material, such as a piezoelectric device, driving a screw jack.
Further notable morphing aircraft concepts include the NextGen aircraft enables dramatic planform changes in a UAV structure, and also the Morphlet project that used mechanical devices to change the cant and twist angle of winglet.
What can be concluded from the above is that there has been a large amount of activity investigating different morphing concepts, but that this has been rather haphazard and there is no clear way to determine which the best concepts are. Also, most of the concepts have been applied to either small wind tunnel models or UAVs, in particular to structures that don’t have stressed skins. What is needed is an approach to decide the best way to apply local morphing concepts, in particular applied to more realistic sized civil aircraft with full size construction and operating conditions.

The primary aim of the MAWS project is to develop an inverse design technique to determine the internal stiffness distribution in the wing-tip in order to meet an in-flight defined aerodynamic shape as well as providing inherent gust loads alleviation.

This aim was met by focusing on the four following primary objectives:

• Development of a Reduced Order Model (ROM) inverse approach to determine the stiffness requirements of aircraft wings, and hence the internal structure, in order to meet defined aerodynamic shapes (e.g. giving maximum L/D ratio) including wash-in, wash-out and neutral aerodynamic lift.
• Definition of an adaptive spar/rib concept to enable the variation in stiffness requirements to be met.
• Evaluation of the selective shape capability of the concept and optimisation of the design for a range of flight conditions.
• Definition of the limitations of the concept and ROM approach, including power requirements, structural and aerodynamic non-linearities.

Project Results:
• Development of an aerodynamic shape optimisation approach based upon Lamar’s method applicable for any point in the flight envelope.
• Development of beam FE model and aerodynamic panels for a Regional TurboJet wing.
• Development of an optimisation approach to obtain the minimum mass wing that achieves the required structural deflections at any point on the flight envelope subject to various structural constraints.
• Investigation into three different approaches of adaptive stiffness morphing : rotating spars, moving spars and moving spar caps to achieve required changes in bending and torsional stiffness along with moving the leading edge and trailing edge shape.
• Development of optimisation approach to determined required amount of morphing needed to achieve required aerodynamic shape throughout the flight envelope
• Demonstration of approach on Regional TurboJet wing model
• Evaluation of the effects of uncertainty on the morphing optimisation process using Polynomial Chaos Expansion and Bayesian methods.

Potential Impact:
The main impact from this work is that it illustrates the potential to make use of a morphing wing tip device to improve the performance and reduce the environmental effect of future aircraft. Such a device would help towards meeting the goals of the ACARE 20-20 Vision and FLIGHTPATH 2050 initiatives. The key finding is an approach to define the required amount of morphing needed to achieve the desired flight shape throughout the flight envelope.
As well as benefiting the environment, the use of such a morphing device could lead to more competitive aircraft designs, helping to strengthen the European aerospace industry which employs many thousands of people across the continent. Some of the researchers that have been involved on the MAWS project will continue on other projects related to aeroelasticity, loads and morphing, and therefore the expertise that has been gained here will be likely to propagate into future technologies and products.
As this has been a relatively short project, it has not been possible to undertake a great deal of dissemination, so some of the related activities are in the future.
A presentation about the MAWS project was made at the Airbus sponsored DiPART meeting in Bristol in November 2013 which is an annual meeting of researchers and industry with an interest in aerodynamics, aeroelasticity and loads.
Conference papers were presented at the CEAS Greener Aviation conference held in Brussels in March 2014, the AIAA AVIATION meeting in July 2014 which is one of the major worldwide aerospace conferences, and also the RAeS Applied Aerodynamics meeting held in Bristol in July 2014.
It is intended to complete and submit two referred academic journal papers relating to this project. The first being a combination of the above three conference papers, and the second, an investigation into the effects on uncertainty on the entire morphing optimisation approach.

Coordinator. Prof Jonathan Cooper, Dept of Aerospace Engineering, University of Bristol, UK, j.e.cooper@bristol.ac.uk