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Control and Alleviation of Loads in Advanced Regional Turbo Fan Configurations

Final Report Summary - CLARET (Control and Alleviation of Loads in Advanced Regional Turbo Fan Configurations)

Executive Summary:

The feasibility of using a morphing wing-tip device to improve the aerodynamic performance of a generic regional turbo-fan jet has been investigated.
Initial studies using CFD based aerodynamics were performed to determine the effect of changing the cant, twist and camber of the morphing wing tip on lift and drag, and also the bending moments acting on the wing-tip and the wing. The trade-off between the aerodynamic performance and the extra weight that would be incurred by an increase in bending moment was encapsulated via the classic range equation. Neural network models were developed to enable computation of the performance characteristics relating to any of the wing-tip parameters, Mach number and Angle of Attack. From these surrogate models it was possible to define the range of wing-tip parameters that were required.
A chiral structure based solution was designed to enable the required wing-tip deflections. It was found that the device for the cant deflections had to be separate from that required for camber and twist change. The moment requirements for actuation were computed.
A range of different actuation devices were considered, and it was found that most of the requirements could be met using conventional off-the-shelf actuation.
A complete series of static and dynamic gust loads analyses were performed on an aeroelastic finite element model in order to evaluate the device. It was found that it was possible to impart some static gust loads alleviation through stiffening of some parts of the wing-tip.
Inclusion of the device showed that it is possible to achieve gains of up to 5% in aircraft range compared to the baseline structure due to changes in the morphing winglet shape, and obtain a reduction in gust load of 2% - 4%.
Further work is required to validate the concept and to test its feasibility experimentally.

Project Context and Objectives:
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. Obviously, 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 few 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 does not 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 aerofoils. The adoption of extendable and retractable, slats and flaps has been the best choice to obtain a compromise and guarantee aircraft a certain degree of adaptiveness to conflicting requirements at different flight conditions.
Similarly, there has been a recent surge in the use of winglets on newer aircraft designs, or even fitted retrospectively. Although there is an improvement in the aerodynamic performance this is still aimed at a single point in the flight envelope (cruise) and the winglet can be significantly away from the optimal shape elsewhere in flight.
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. 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 in a controlled manner, 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.
What can be concluded from a review of the literature relating to morphing aircraft 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 the best concepts. Most of the concepts have been applied to either small wind tunnel models or UAVs, in particular to structures that do not have stressed skins. Little work has been devoted to morphing winglets, and even less to the use of morphing structures that enable loads alleviation such as that developed in the SMorph project which achieved a 22% mass reduction in a sensorcraft structure. Gust and manoeuvre loads are often the key design cases for civil aircraft, and if the device is able to reduce the gust loading, then this can be transformed into a mass reduction, thus saving fuel requirements in addition to the benefits of the drag reductions. Chiral structures have also received some attention in recent years, initially as a means to achieve zero Poisson’s Ratio structures, but there have been a few attempts to employ them to morph wing type structures.

The CLAReT project aims to extend the state-of-the-art in a number of ways:
• Development of a novel adaptive winglet concept based upon chiral structures that combines a drag minimization capability with the capacity to provide passive gust loads alleviation
• Parametric design and capability assessment of the device for use on a full scale advanced regional turbo-fan aircraft, including design of the required structure and actuator
• Critical assessment of the total system requirements for the device (assess power and weight requirements vs. drag and loads alleviation (and hence weight loss) gains.

The CLAReT project will develop a novel winglet device that is able to facilitate optimal cant angle and twist throughout the flight envelope whilst also providing an improved passive gust Loads Control and Alleviation (LC&A) capability. Specifically, the aims are to:
1. Perform a CFD based aerodynamic design of an innovative Loads Control & Alleviation wing device
2. Undertake a preliminary lay-out definition of respective structure and actuation system for the device
3. Perform a parametric analysis of the device performance and settings for a transonic wing configuration (cruise M=0.78) with engine–nacelle under wing installation for a future Turbo Fan Aircraft configuration

These aims will be met by focusing on the following six primary objectives:
• Definition of a baseline wing model with associated FE and CFD mesh
• Definition of a baseline LC&A winglet device configuration following consideration of the design requirements
• Preliminary aerodynamic sizing of the winglet device making use of CFD analysis
• Definition of the structures and systems related to the winglet device leading to a structural design and actuator design
• Parametric evaluation of the deployed concept and evaluation compared to the baseline wing
• Parametric evaluation of the concept used in combination with conventional control actuators

Project Results:
• Development of CAD models and CFD aerodynamic meshes for Regional TurboJet.
• Development of software to generate the CAD model and aerodynamic meshes for any combination of wingtip cant, twist and camber
• Establishment of database of aerodynamic and bending moment parameters (lift, drag, pressure distribution, wing root bending moment, wing-tip bending moment) for range of 48 combinations of wingtip shape at different angles of attack and Mach number (0.48 0.60 and 0.74)
• Development of Neural Network surrogate models to enable prediction of aerodynamic forces and bending moments at any combination of parameters, including ignoring those cases where the flow separates.
• Development of a Neural Network model to determine a trade-off between the aerodynamic performance improvement (Lift / Drag) vs. an increase in weight relating to bending moments, based upon the Bregeut range equation.
• Definition of the optimal shape to take at any point in the flight envelope and flight condition
• Development of a wing tip design based upon an internal Chiral structure that enables the desired wingtip shape at any flight point to be achieved.
• Development of a finite element model of the wing, benchmark wingtip and chiral wing tip structures. Development of equivalent beam FE model to enable couple CFD/FE studies
• Evaluation of the required actuation requirements to achieve the desired wingtip shape changes
• Assessment and selection of available “off the shelf” actuators to almost meet all of the desired moments.
• Evaluation of the static and dynamic loads that the wing with chiral wingtip will experience – comparison with baseline model
• Parameter variation of position and leading edge stiffness of the winglet and investigation of effect this has on gust loading.
• Able to achieve up to a 5% increase in range due to aerodynamic shape of wingtip, and 2% – 4% reduction in wing root bending moment due to gust loading.

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. Much work remains to be done to ensure that such a design is feasible, but the results are encouraging; and this work has considered a real commercial aircraft scenario with full scale loads and also evaluation of the actuation systems that would be required and the extra weight and power that they would need.
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. The researchers that have been employed on the CLAReT 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 most of the related activities are in the future.
A conference paper has been accepted at the AIAA SCITech meeting in January 2014 which is one of the major worldwide aerospace conferences, and a further abstract has been submitted to the CEAS Greener Aviation conference to be held in Brussels in March 2014. A presentation about the CLAReT project will be made at the forthcoming 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.
It is intended to complete and submit two referred academic journal papers relating to this project. The first being an overview of the entire project, and the second, a more detailed look at the chiral structure and its design.

List of Websites:
Coordinator. Prof Jonathan Cooper, Dept of Aerospace Engineering, University of Bristol, UK,