FLEXOP

FLEXOP stands for "Flutter Free FLight Envelope eXpansion for ecOnomical Performance improvement" and is an H2020 aeronautics European project that started in June 2015 and is expected to conclude by December 2018. 

The TASC group is leader of WP2 “Flutter control and analysis technologies” and technically contributes to the development of on-board flutter analysis and control techniques.
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The webpage of the project is: https://flexop.eu/

Team members:
* Mr. Andrea Iannelli (PhD student: flutter analysis through mu and bifurcation tools )
* Dr. Nandor Terkovics (Post-Doc: LPV modelling and on-board flutter analysis)
* Prof. Mark Lowenberg (co-supervisor to Mr. Iannelli)

State-of-Art
The competitiveness of the European aerospace industry, and the associated employment generated, depends on member states retaining world class engineering capabilities in design, manufacturing, and systems integration. In addition, the European goal of reducing development costs, including the cost of certification by 50% relative to year 2000 levels by 2050, represents a specific challenge and opportunity.

FLEXOP believes that the approach to this challenge involves rapid adaptation of existing wing designs (i.e. derivative wing development process) to meet mission objectives, see Figure. Thus, the project will demonstrate in a quantified manner the potential to mature several technologies and concepts that will make a significant contribution towards these high level goals.

It is well known that aerodynamic efficiency can be improved by increasing the aspect ratio, leading to an increase in span. There are a number of challenges to such a span increase and at least two of these are tackled by FLEXOP. The first is the problem of aeroelastic stability. A pure span increase will tend to lead to a reduction in the flutter speeds, so the development of effective flutter suppression systems is needed to maintain acceptable operating speeds. Secondly, increasing the span has an important effect on wing bending loads, requiring structural reinforcement. To avoid the large penalty from the increase in structural mass some form of load alleviation function must be used. The control surfaces on the wing are typically multi-functional, and could be used for both flutter suppression and load alleviation, but the control algorithms (software) will require separate development. In addition, an opportunity to exchange structural mass for increased payload efficiency could arise through aeroelastic tailoring, and thus FLEXOP will strive to understand the overall aircraft benefit and certification implications. Within this aspect, composite laminates can be designed to increase the coupling between twist and bending, and can introduce a passive load alleviation effect. But despite these advantages, the strength implications could erode some or all of the benefits. Nonetheless, as aforementioned, an active control system may be able to suppress these unwanted side-effects of aeroelastic tailoring, and thereby make it a viable design option for designing lighter structures.

Objectives
The goal of FLEXOP is to develop and flight test multidisciplinary aircraft design capabilities addressing aircraft flutter and flexible effects, in order to increase European competitiveness in terms of aircraft development costs. 

A closer coupling of wing aeroelasticity and flight control systems in the aircraft design process opens new opportunities to explore previously unviable designs. Common methods and tools across the disciplines also provide a way to rapidly adapt existing designs into derivative aircraft, at a reduced technological risk (e.g. using control to solve a flutter problem discovered during development).

FLEXOP goal will be achieved by:
  1. Improving efficiency of currently existing wing, by increased span at no excess structural weight, while establishing modifications by aeroelastic tailoring to carry the redesigned derivative wing;
  2. Developing methods and tools for very accurate flutter modeling, analysis and control synthesis. These methods will enable improved flutter management during development, certification, and operation, enabling to fly with the stretched wing at same airspeed as the baseline aircraft;
  3. Validating the accuracy of developed tools and methods on an affordable experimental platform, followed by a scale-up study, demonstrating the interdisciplinary development cycle. Manufacturers will gain cost efficient methods, tools and demonstrators for enhancing aircraft performance by integrated development of flutter control and aeroelastic-tailoring.

Description of Work
FLEXOP is planned to last for 42 months and the foreseen work plan is structured in 5 work packages:
  • WP1 "Aeroelastic Tailoring and Wing Design" focuses on tools and methods related to aero-elastically tailoring of flexible wings, maturing currently existing tools and combining them into a tool-chain relevant for industrial consideration. WP1 will keep the control design and flutter analysis related modelling requirement in view, to be able to close the design loop feeding inputs to WP2.
  • Research and maturation of active flexible aircraft control and the related robust flutter prediction methods will be the target of WP2 "Flutter Prediction and Control Design Methods", which also requires effort on the model reduction side. The developed wing design together with the advanced control design tools will be evaluated in flight test as a part of WP3.
  • Within WP3 "Flutter Management Demonstrator", an affordable, scaled demonstrator platform with interchangeable wings will be manufactured, assembled and instrumented, comprising of minimum 5 pairs of wings and two air vehicle bodies, – in the interest of minimizing risk during project execution.
  • The evaluation of the tools and methods will be carried out within WP4 "Assessment, Validation and Scale-Up Issues" where the theoretical predictions will be compared with experimental flight test, and the finding will be generalized in a scale-up study applicable for derivative commercial aircraft design.

Expected Results
The project will develop simulation and testing capabilities that provide a legacy for future research activities in Europe. The design tools developed will work across disciplines to produce integrated wing structures with aeroelastic tailoring and control. They will be implemented to reduce the number of design iterations, reducing the design effort for future configurations.

A low-cost flight demonstrator will allow validation of the methods and tools while adding significant value in Europe for future research, by establishing a platform for validation and testing. This flight demonstrator will incorporate three different kinds of replaceable high aspect-ratio wings to validate different aspects of the project. These wings will be equipped with loads and shape monitor-ing for validating the accuracy of the open loop (without aeroelastic controller) and closed loop (with aeroelastic controller) aeroelastic models. At least for the “flutter wing” it is planned to fly beyond critical flutter speed, which is made possible by active flutter control.