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Gust Load Alleviation techniques assessment on wind tUnnel MOdel of advanced Regional aircraft

Final Report Summary - GLAMOUR (Gust Load Alleviation techniques assessment on wind tUnnel MOdel of advanced Regional aircraft)

Executive Summary:
The need for more efficient aircraft able to meet the new challenging requirements defined by ACARE with its strategic road map stated in the Vision 2020 forces the researchers to look for more advanced aircraft configurations, based on more efficient aerodynamics and structures together with more sophisticated flight control systems. Aircraft industry has to be able to deliver new significantly greener aircraft with a substantial reduction of fuel consumption, emissions and perceived noise levels. One of the most promising concept regarding more environmentally friendly aircraft concerns Natural Laminar Flow (NLF) wing. This concept requires a fully multidisciplinary approach due to the counteracting role of main aircraft parameters such as wing sweep, wing thickness and cruise speed, so to minimize the potential structural wing penalty. Looking for this global target, a possible solution is based on combining Natural Laminar Flow (NLF) with an aggressive use of maneuver (MLA) and gust load alleviation (GLA) technologies that offer the potential to greatly improve both the weight and aerodynamic terms in the classical Breguet equation. For all these reasons the interest in the development, implementation and experimental verification of MLA and GLA technologies is becoming a key topic in the development of the next generation transport aircraft.
The GLAMOUR project, in response to the SP1-JTI-CS-2013-01-GRA-02-022 call under the JTI-Cleansky 1 initiative, tackles this topic in two different ways. At first, having the Green Regional Aircraft as reference, starting from the active control laws proposed by a ITD member of Cleansky program, it tries to deeply explore their validity over the entire flight envelope, and to extend them with new control strategies based on alternative approaches, such as the Neural Networks, as well as based on Robust Model Predictive Control techniques.
Finally, an extended experimental validation campaign was conducted at Wind Tunnel available at Politecnico di Milano. To this scope, a large wind tunnel half aircraft model properly scaled was manufactured and tested in a free-free configuration allowing to validate the most promising GLA techniques. Aiming at this goal, a dedicated gust generator has been designed, installed and tested into the wind tunnel.

Project Context and Objectives:
The main goals of GLAMOUR project have been pursued along two different research lines: optimization of Loads Alleviation COntrol System and Experimental Validation.

Optimisation of Loads Alleviation Control System
The GLAMOUR project tackled this topic in two different ways. Firstly, using the Green Regional Aircraft as a simulated reference model, the validity of the active control lows proposed by ITD member over the entire flight envelope were explored. Then, in a second stage, the GLAMOUR consortium explored new and alternate control architectures and compared them with the ITD proposed methods. The control laws were then applied to a simulated mathematical model of the wind tunnel model with two objectives, firstly to provide a comparison of the effectiveness of the control laws on both the full-scale and scaled models, and also to determine the experimental gains that were required for application in the wind tunnel experiments.
The results from the simulation of the full-scale GRA model show that it is possible to achieve a significant amount of loads alleviation using the relatively simple control laws considered in this project. Very similar results were obtained for the aeroelastically scaled model that was developed to steer the design of the wind tunnel model.

Experimental Validation
The above loads alleviation activities were then validated by an experimental campaign. A major part of the project involved the design, manufacture and test of a scaled half model version of the GRA aircraft and the associated sensors to measure the static and dynamic response of the model. A series of static and dynamic tests were performed in order to validate the experimental model, both wind-off (dry) and wind-on (wet).
A further essential part of the test set-up was the design and manufacture of gust vanes that were able to simulate different gust inputs in the tunnel. A significant effort was devoted towards simulating various configurations of the gust vanes in order to ensure that the capability of achieving “1 – cosine” (1MC) was feasible, and this was demonstrated experimentally using aerodynamic flow measurements.

Although there was not an exact match between the experimental model and the simulation (this would be extremely hard to achieve), the experimental results do provide enough evidence to enable a validation of the control laws that were considered in this study. The correlation between the general behaviour of the aircraft model to the gust excitation, and the amount of loads alleviation that was achievable, was good. Experimental validation of technologies is always important to increase the technology readiness levels (TRLs) and this project has achieved this at a number of different levels.

The gains of the control laws that were optimised on the scaled simulated model were different compared to the full-scale model, but this would be expected due the scaling that was used and there was the anticipation that this would be the case. The transfer of the simulated control laws on the scaled model to the experimental version was extremely easy, and demonstrated the good degree to which the dynamic and aerodynamic simulated models mimicked the experimental test set-up. Such findings indicate that a great deal can be determined from the use of low speed scaled wind tunnel models for loads applications.

Pressure of time meant that it was not possible to consider a wider range of test cases – in particular it would have been desirable to try out more gust lengths, and also the response to a turbulent gust field. It was not possible to experimentally test the fuel weighted cases due to manufacturing considerations; although this would have been desirable, it does not affect the validity of the results. The closeness of the zero fuel cases for both simulations and experiments shows that there is a good correlation of the structural and aerodynamic modelling; the addition of extra masses representing the fuel should not lead to major differences between simulation and experiment as the aeroelastic modelling has been validated.

Given the vast amount of data that has been obtained, and the ability of the various control algorithms to reduce the gust loads, then it is reasonably safe to conclude that it is possible to achieve gust loads alleviation for flexible aircraft using conventional control surfaces and sensor technologies.

Assessment in Terms of Current State of the Art and Clean Sky Call

Current State of the Art
The GLAMOUR project has provided, certainly in terms of the open literature, a number of firsts in terms of the application of active gust loads alleviation technologies to flexible aircraft structures. The novel gust alleviation control laws that have been developed and compared all demonstrate that it is possible to achieve a significant amount of loads alleviation using relatively simple approaches.
The development of the scaled flexible half aircraft model combined with the gust generation system is unique for such a large wind tunnel model. Of further particular note are the rigid body motion system and the amount of sensing that can be measured simultaneously. The comparison of the methods using the full scale aircraft model and its scaled counterpart, both simulated and experimental, are unique. The approach shows that it is possible to validate control laws developed on simulated models using scaled wind tunnel test, and then to transfer the findings back to the full scale version.

Clean Sky Call
With regards to the Clean Sky initiative, the GLAMOUR project has made a substantial step towards the inclusion of loads alleviation technologies as part of the next-generation Green Regional rear-fuselage engine A/C configuration. In particular, the development of Load alleviation technologies enables the avoidance of any possible loads that exceed given structural limits thus facilitating the optimisation of the wing structural design for weight savings.
Such reductions in weight will enable more efficient aircraft structures to be designed, stretching beyond the GRA configuration considered in this project. The use of gust loads alleviation technologies should be extended to manoeuvre load alleviation as well. The outcomes of this project will help towards meeting the performance and environmental goals defined by the 2020 Vision and Flightpath 2050 initiatives.

Project Results:
To synthesize, the main goals of GLAMOUR projects are the following:
- Validate the Load Alleviation techniques based on control architectures defined by ITD member
- Develop of alternative control schemes
- Design and manufacturing of a wind tunnel model representing half GRA aircraft
- Perform wind tunnel test under gust excitation
- Draw a final assessment on the global benefits achievable using technologies in both design and off-design flight conditions
The GLAMOUR project appears as especially challenging for two main reasons. At first for the complexity of the model to be tested, i.e. a complete aeroelastic model, fully instrumented so to measure the relevant aeroelastic responses (accelerations, displacements, section loads, actuation forces) by means of embedded sensors as well as external optical measurement systems, and equipped with different control surfaces. On the other hand, for the test conditions to be investigated, allowing for a deep validation of proposed MLS and GLA concepts in a very realistic flight environment.

The activity performed during the whole project aimed at the following targets:
- Finalize the design and manufacturing of the wind tunnel model and of the Gust Generator
- Complete the ground test activity for model validation, including static test and GVT in different configurations
- Finalize the measurement and control system to be used during the wind tunnel test campaign
- Perform the wind tunnel test campaign to validate the different gust load alleviation strategies developed and implemented during the project activity
- Assess the obtained results and the possible future implementation of such as techniques on future aircraft.

In conclusion, a range of control laws have been developed and compared with previous laws, developed for the ITD, for the gust loads alleviation of a GRA simulated aeroelastic model. It was shown that all of the approaches were capable of achieving good loads reduction based upon the wing root bending moment. A range of different weight cases, flight conditions and gust lengths were considered. In some cases the loads alleviation was significant. No one control law could be considered to be better than the others.
A scaled wind tunnel model was designed, and the control laws applied to a simulated version of this model. Similar loads alleviation to the full scale model was achieved by all of the control laws (Figures 1 and 2).
A unique half-aircraft model was designed and manufactured (Figure 3), in combination with heave and pitch motions and also gust generation vanes (Figure 4). A sophisticated sensor set-up was implemented in order to be able to simultaneously measure a wide range of displacement, accelerations, loads and control surface rotations (Figure 5). Apart from a minor issue with friction, and the inability to be able to put representative fuel weight into the model, the wind tunnel model is a great achievement and is a valuable resource that should be used in future research programmes in this technical area.
The experimental test campaign validated all of the findings from the simulations (Figure 6).

The skills that have been developed by the partners in GLAMOUR are going to be used in other projects involving the European aerospace industry. It is also hoped that the loads alleviation technologies will find their way into Clean Sky 2. As much exploitation has been achieved as would be expected at this stage of a short term research application project; future years will tell whether the projected exploitation is successful. There have already been a number of inquiries about the possibility of using the GLAMOUR wind tunnel model for other EU funded projects.

Over 13 papers have been presented at technical conferences or appear in journal publications, and there are more to follow. The conference papers have been targeted at the major meetings in this technical field (IFASD and SCITECH) and also of note are the two presentations to the Greener Aviation Conferences, highlighting this important activity within Clean Sky.
Dissemination to the general public relating to loads alleviation and the GLAMOUR project has occurred through several Cool Aeronautics outreach days held at the University of Bristol and also a public event held in Bristol (Sept 2015) to showcase EU funded research.

Potential Impact:
GLAMOUR is addressing the validation of Load Alleviation technologies by means a dedicated design activity coupled to an extensive experimental activity. The project objective is directly related to aircraft performances in terms of load alleviation, reductions of maximum stress with and a potential weight saving and consequently fuel saving, so impacting the quality of future aircraft design and the global impact on the environmental. The impact of GLAMOUR will be felt through a comprehensive design and evaluation of the proposed LA concepts by means of numerical tools and analysis together with dedicated experimental validation. The potential of new control strategies proposed into GLAMOUR will be assessed by comparing the obtained performances with ones achieved adopting the already available approaches proposed by ITD member and to other related design requirements. A complete model representing half aircraft equipped with a aeroelastically scaled wing the requested control systems and measurement devices will be tested in POLIMI’s wind tunnel with different gust excitations so to cover the whole flight envelope, as requested by ITD member.

GLAMOUR is oriented towards the objective of the industrial development of the European regional green aircraft. This kind of transport system would directly derive benefit from the LA solutions offered by the project in terms of new technologies for active aeroelastic control, response alleviation, sizing loads mitigation and hence contributes to reduced emissions towards the challenging target set up by ACARE. Furthermore, GLAMOUR combines advanced control, aero-servo-elastic and structural design and optimization which is, up to now, rarely seen in the European research programs. Hence, GLAMOUR will influence and encourage future research programs to focus more on multiple disciplines not only in the analysis of the performance but also in the design of new innovative concepts. GLAMOUR will thus provide an impact on the European aerospace research and industry.

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