This paper presents the preliminary results of the recent whirl flutter wind tunnel test campaign performed within the Advanced Testbed for TILtrotor Aeroelastics (ATTILA) project. The Froude-scale ATTILA testbed consists of a semi-span wing with powered tip-mounted proprotor reflecting the proprietary design of the Next Generation Civil TiltRotor (NGCTR). An overview of the ATTILA testbed, wind tunnel test procedures, team organisation and preliminary flutter results are presented. In line with pre-entry dynamic characterization tests, the wind-on test activities in the DNW Large Low-speed Facility (LLF) revealed notable force-dependent nonlinearity in the modal characteristics of, particularly, the wing torsion mode. Further dimensionality was added by early observations that damping in the rotor gimbal degree of freedom, attributed to stiction in the blade pitch mechanism, had the potential to substantially contribute to the damping of the fundamental wing-pylon modes. Nevertheless, the parallel exploitation of multiple monitoring and online modal estimation methodologies enabled a robust identification and safe test progression. The critical flutter mode was found to be configuration dependent, with the wing chord bending mode generally being marginally stable throughout most of the wind speed range, and the wing torsion mode displaying a sharp trend towards negative damping at higher speeds. Despite technical challenges, valuable test data was gathered to advance the experimental methods and support validation of the numerical tools used to obtained clearance for high-speed flight testing of the full-scale NGCTR Technology Demonstrator.

This paper describes the development of a new state-of-the-art large wind tunnel model for active flutter suppression studies as well as the supporting techniques used in tests focused on the effects of uncertainty. Design guidelines and the resulting aeroelastic characteristics of the model are covered together with representative test results. Those would allow other researchers working in this area to develop control laws for the new model and evaluate them. A number of important lessons and insight are reported regarding the design of the model, the level of success of commonly used mathematical modeling techniques to capture its behavior, sources of analysis/test correlation discrepancies, multifunction utilization of control surfaces for both system identification and flutter suppression, active flutter suppression testing safety, and techniques for estimating the robustness of closed-loop active aeroservoelastic systems by tests. The new system has made it possible to repeatedly push the actively controlled model, using various flutter suppression control laws (safely), to the actual flutter limit in tests numerous times. This capability is just one of the new experimental capabilities that the new system brings to the aeroelastic active control community.

The paper summarizes the activity performed in the framework of the EU funded project CS2-AIRGREEN2 aiming at the development and testing of Gust Load Alleviation technologies applied to the so-called Green Regional Aircraft. An extended numerical activity carried out on the reference aircraft established the due knowledge to start designing the experimental model and the systems for the final wind tunnel validation. The wind tunnel model is representative of the half aircraft having a geometrical scale equal to 1:6. The wing is fully aeroelastic, and the model is installed on a dedicated system, named Weight Augmentation System (WAS), allowing for pitch and plunge free motions. The paper describes in the details the wind tunnel model, the adopted controllers and reports the relevant results collected during the experimental campaign carried out at Large POLIMI's wind tunnel.

The paper describes the development of a new state of the art large wind tunnel model for active flutter suppression studies as well as the supporting techniques used in tests focused on the effects of uncertainty. Design guidelines and the resulting aeroelastic characteristics of the model are covered together with representative test results. Those would allow other researchers working in this area to develop control laws for the new model and evaluate them. A number of important lessons and insight are reported regarding the design of the model, the level of success of commonly used mathematical modeling techniques to capture its behavior, sources of analysis / test correlation discrepancies, multi-function utilization of control surfaces for both system identification and flutter suppression, active flutter suppression testing safety, and techniques for estimating by tests of the robustness of closed-loop active aeroservoelastic systems.

Aircraft winglets are well-established devices that improve aircraft fuel efficiency by enabling a higher lift over drag ratios and lower induced drag. Retrofitting winglets to existing aircraft also increases aircraft payload/range by the same order of the fuel burn savings, although the additional loads and moments imparted to the wing may impact structural interfaces, adding more weight to the wing. Winglet installation on aircraft wing influences numerous design parameters and requires a proper balance between aerodynamics and weight efficiency. Advanced dynamic aeroelastic analyses of the wing/winglet structure are also crucial for this assessment. Within the scope of the Clean Sky 2 REG IADP Airgreen 2 project, targeting novel technologies for next-generation regional aircraft, this paper deals with the integrated design of a full-scale morphing winglet for the purpose of improving aircraft aerodynamic efficiency in off-design flight conditions, lowering wing-bending moments due to maneuvers and increasing aircraft flight stability through morphing technology. A fault-tolerant morphing winglet architecture, based on two independent and asynchronous control surfaces with variable camber and differential settings, is presented. The system is designed to face different flight situations by a proper action on the movable control tabs. The potential for reducing wing and winglet loads by means of the winglet control surfaces is numerically assessed, along with the expected aerodynamic performance and the actuation systems’ integration in the winglet surface geometry. Such a device was designed by CIRA for regional aircraft installation, whereas the aerodynamic benefits and performance were estimated by ONERA on the natural laminar flow wing. An active load controller was developed by PoliMI and UniNA performed aeroelastic trade-offs and flutter calculations due to the coupling of winglet movable harmonics and aircraft wing bending and torsion.

This work addresses control law synthesis for active flutter suppression followed by the design and development of an active flexible wind tunnel model representative of high aspect ratio commercial aircraft. An initial early-design math model of the structural dynamics, unsteady aerodynamics, sensing, and actuation of the system was used to synthesize three control laws that would stabilize the system against flutter over a range of speeds below and above the passive open-loop flutter speed. Three methods for establishing closed-loop robustness measures were used to quantify the robustness of the system based on its initial mathematical model. The system was then tested with these control laws, and their robustness to system variations in the wind tunnel was studied. Such variations included speed as well as gain and phase in the control loops, representing gain and phase uncertainties in the system. This was followed by revisiting the robustness of the control laws with mathematical testing, this time with a more accurate mathematical model of the system. The work highlights the importance of (a) working with as high accuracy as possible math models of the aeroservoelastic system, (b) understanding the key sources and types of uncertainties possible, (c) evolving the control laws during the vehicle development process to account for initial uncertainties in the models that can be large, (d) reaching final control laws that demonstrate sufficient robustness (as required by certification agencies) with the most accurate math models available, and (e) confirmed robustness with tests in flight at selected critical conditions subject to selected variations in the system. The work adds insight regarding the kind of model variations that the control designer needs to consider and the way different measures of system robustness compare to one another and relate to the capacity of the control design to permit model uncertainties.

This paper summarizes the activities and results carried out in the framework of the Active Flutter Suppression project in cooperation between University of Washington and Politecnico di Milano, aiming at the investigation and validation of technologies for flutter suppression. The project included both numerical and experimental activities. In particular, the results collected during the last phase of the test campaign on the complete F-XDIA wind tunnel model, in both open and closed loop, are summarized.

The paper presents a new flexible medium fidelity aerodynamic computational tool, developed from the collaboration between Politecnico di Milano and A by Airbus, and tailored to obtain reliable and fast aerodynamic simulations of new aircraft configurations like Vahana, the fully-electric vertical take-off and landing multi-rotor tiltwing aircraft built by A by Airbus. The proposed solution, called DUST, relies on the Helmholtz decomposition of the velocity field to recast the aerodynamic problem as a mixed boundary elements-vortex particles method. In DUST different aerodynamic elements can be combined in a single model to best capture the relevant physical phenomena, while an accelerated vortex particle model of the wakes allows for a numerically stable Lagrangian description of the free vorticity evolution. Pressure field evaluation in a rotational flow relies on an integral boundary problem for the Bernoulli polynomial obtained from the Navier–Stokes equation. The code is validated against numerical and experimental data available for conventional vehicle configurations, like airliner and helicopter models, and more complex architechtures, such as a tiltwing-rotor in hover and forward flight. Finally a comparison between flight test data and DUST computations is shown for Vahana.

Morphing winglets are innovative aircraft devices capable to adaptively enhance aircraft lift distribution throughout the flight mission while providing augmented roll and yaw control capability. Within the scope of the Clean Sky 2 REG IADP, this paper deals with nonlinear simulations of a regional aircraft wing equipped with active morphing winglets in manoeuvring conditions. The fault tolerant morphing winglet architecture is based on two independent and asynchronous control surfaces with variable camber and differential settings capability. The mechanical system is designed to face different flight static and dynamic situations by a proper action on the movable control tabs. The potential for reducing wing and winglet loads by means of the winglet control surfaces is numerically assessed by means of static aeroelastic analyses, using a feedforward manoeuvre load alleviation controller. An electro-mechanical Matlab/Simulink model of the actuation architecture is used as design tool to preliminary evaluate the complete system performance and the ability to cope with the expected morphing aeroshapes. Then, the aeroelastic model of the aircraft is combined with the nonlinear simulator of the response of the winglet actuation system to evaluate a symmetric and asymmetric manoeuvres obtained by a sudden deflection of the main control surfaces. The use of the morphing winglet tabs shows to alleviate the wing loads in such conditions. The introduction of the dynamic actuator model leads to a reduction of the performances with respect to predictions of the static analyses but a reduction of the manoeuvre loads can still be observed.

In the present work the design of a wing tip device is presented along with the definition of the maneuver load controller and the gust load controller.A parametric sensitivity study has been performed to define the aerodynamic shape of the device, considering its effect on the dynamic properties of the aeroelastic system.After defining the aerodynamic shape the maneuver load alleviation controller and the gust load alleviation controller are described, the former is obtained by finding the distribution of surface deflections that can minimize the internal loads in a trim condition, while for the latter two different strategies are compared, one based on a static output feedback controller and the second based on a recurrent neural network controller.The active wing tip extension designed results able to contribute to the alleviation of wing dynamic loads and can compensate the increment of such loads resulting from the span extension.

The paper presents the design of a symmetric, active, gust load alleviation system for a regional transport aircraft, based on a static output feedback with a constrained structure. The design is carried out on a comprehensive finite state aeroservoelastic model, including sensor units and actuator transfer functions, and verified by taking into account saturated control positions, rates, and hinge moments. The controller is designed within a quadratic optimal framework, through a second-order Hessian-based optimization algorithm, exploiting block diagonal Schur transformations of the closed-loop state equations and performance weightings. An accurately chosen worst discrete gust and a reference flight condition provide a baseline design, which is significantly effective in alleviating continuous turbulence loads. Such a reference design proves itself robust enough to alleviate atmospheric loads over the complete flight envelope and is eventually further improved and robustified through a simple bilinear q-M algebraic scheduling.