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Aircraft wings bend while in flight. To see this for yourself, all you have to do is look out of the window during a flight. Beyond a certain limit, these wing deflections become known as Large Deformations. Professor Wolf Krüger of the DLR Institute of Aeroelasticity explains whether these deformations are dangerous, the conditions in which they occur and why modern wings are becoming increasingly elastic.

This report presents comprehensive flutter calculations of the DLR-D150-VAMP configuration with respect to the influence of structural changes on flutter behaviour. The main focus thereby was on the influence of modifications concerning the position of the engines/nacelles. Furthermore the influence of gyroscopic and thrust vector effects as well as the influence of pylon stiffness regarding the flutter behaviour has been investigated. Finally, the influence of an integrated ultra high bypass ratio engine/nacelle (UHBR) instead of a classical standard CFM56 engine on the flutter stability has been investigated. The results are shown in comparison to the aeroelastic flight envelope for this assessed configuration, which is similar to an A320 aircraft.

This paper presents the cybermatrix protocol, a novel approach to multidisciplinary design optimization in the contex of multiple-fidelity disciplinary analyses, many involved disciplines and high use of high-performance computing resources. The approach is presented from its formal mathematical background to actual on-disk implementation of running processes. As the demonstration case, a twin-engine long-range transport aircraft is optimized. Four disciplines are employed: overall aircraft wing planform design, aerodynamic airfoil design using 3D RANS computations, structural wing design using global shell-element FEM model, and loads selection and evaluation process based on low fidelity aerodynamics.

In order to allow for a more economic and environmentally friendly aircraft operation and to fulfill the greener imperative demanded by today's society, fuel savings and cost reduction play a key role in the development of modern aircraft. Besides the efficiency of engines and aerodynamics, the aircraft weight has a major impact on fuel consumption. Reducing structural wing loads caused by atmospheric disturbances (such as gusts, turbulence and wake vortices) thereby became a main research interest of today's aircraft industry. Reducing such loads will allow the aircraft manufacturer building and certifying aircraft to smaller load envelopes, inherently reducing the airframe structural weight and thus reduce fuel, emissions and cost. In this contribution we discuss latest research and development trends in the field of modelling and control for load alleviation on modern, fly-by-wire aircraft. Covering a broad range of relevant topics, the paper is divided into four main subtopics as follows: (i) For analyzing loads on the aircraft in an early design stage with high fidelity simulation techniques, detailed models of the structure and the aerodynamics are generated. Here, we discuss the latest developments on how these complex aeroelastic models (generated by CFD methods, e.g.) are derived and can be approximated by lower order models for facilitating the subsequent controller design. (ii) Directly linked to the modelling in (i) is the feedback control design task. The use of available local measurements as well as virtual sensors for incoming gusts provides a profound basis for alleviating the associated structural loads. This subsection discusses the current trends in control development such as estimator design and robust control techniques to feed back the gust estimates. (iii) Also, in direct link to the modelling (i) but also to the lidar sensors (iv) feedforward control allows for an enhanced load alleviation performance than a pure feedback due to its anticipative behavior (control actions can be taken earlier). In contrast to the feedback approaches (ii), it requires additional forward sensing capabilities, such as lidar for the gust anticipation. Detailed models of these sensors are needed to allow for a dedicated feedforward control design: these lidar sensor models are investigated in great depth within a dedicated activity (iv) and in surrogate models – derived from them – are considered in the feedforward design tool. In this subtopic the surrogate models of the lidar sensors, the wind reconstruction algorithms as well as the feedforward load alleviation design workflow are presented. (iv) The feedforward control approaches in (iii) require measurements of the incoming gust, which shall be provided by advanced lidar measurement systems. The latest development directions for adapting the lidar technology to fulfil the needs of feedforward gust load alleviation applications are discussed in this subtopic. These progresses represent a key enabling technology for viable anticipation of incoming gusts and winds, paving the way for improving the gust load alleviation capabilities for future aircraft generations.

This paper presents an approach to multi-disciplinary optimization (MDO) of transport aircraft developed within the DLR project VicToria that attempts to strike a balance between two broad classes of MDO approaches: those arising from the formal optimization background, and those coming from the aircraft design background. It starts from the observation that any kind of numerical design process can be viewed as an approximation of a formal optimization process, where Jacobians of cost functions may be inexact and are often not explicitly computed. Based on that, a specific MDO problem representation and a highly parallel process assembly and execution protocol (the ``cybermatrix'' protocol) is defined, as well as one possible realization on high-performance computing (HPC) resources. The approach is applied to an optimization of a long-range transport aircraft, employing disciplinary subprocesses for high-fidelity aerodynamic design of wing airfoil shapes, structural sizing of lifting surfaces, and determination and evaluation of design loads.

The DLR project VicToria brings together design methods that necessite use of high-performance computing, for the multi-disciplinary optimization of a long-range transport aircraft. Three different approaches are followed: the integrated aerostructural wing optimization, the multi-fidelity gradient-based approach, and the many-discipline highly-parallel approach. A common test case is used: XRF1, an Airbus provided research configuration representing a typical long-range wide-body aircraft.

The modal identification of large and dynamically complex structures often requires a multi-point excitation. Sine sweep excitation runs are applied when it is necessary to concentrate more energy on each line of the frequency spectrum. The conventional estimation of FRFs from multi-point excitation requires uncorrelated excitation signals. In case of multi-point (correlated) sine sweep excitation, several sweep runs with altered excitation force patterns have to be performed to estimate the FRFs. An alternative way, which offers several advantages, is to process each sine sweep run separately. The paper first describes the conventional method for FRF estimation in case of multi-point excitation, followed by two alternative methods applicable in case of correlated excitation signals. Both methods generate a virtual single-point excitation from a single run with multi-point excitation. In the first method, an arbitrary structural point is defined as a virtual driving point. This approach requires a correction of the modal masses obtained from modal analysis. The second method utilizes the equality of complex power to generate virtual FRFs along with a single virtual driving point. The computation of FRFs and the modal identification using virtual single-point excitation are explained. It is shown that the correct set of modal parameters can be identified. The application of the methods is elucidated by an illustrative analytical example. It could be shown that the separate evaluation of symmetric and anti-symmetric multi-point excitation runs yield obviously better and more reliable results compared to the conventional method. In addition, the modal analysis of the separate symmetric and anti-symmetric excitation runs is easier, since the stabilization diagrams are easier to interpret. The described methods were successfully applied during the Ground Vibration Tests on Airbus A380 and delivered excellent results. The methods are highly advantageous and may thus be established as a new standard procedure for testing aerospace structures.

The flow over an OA209 airfoil subjected to a sinusoidal pitching motion under dynamic stall conditions is investigated experimentally by means of time resolved particle image velocimetry (TR-PIV) and surface pressure measurements. Dynamic stall is distinguished by the formation and convection of large scale coherent structures and a delay in massive flow separation. A vortex detection scheme based on an identification function derived directly from the velocity fields is adopted to identify vortex cores. The combination of global time resolved imaging and an automated vortex identification algorithm allows for the investigation of the spatial and temporal evolution of vortical structures within a single oscillation. Furthermore, the mechanisms associated with the dynamic stall delay are considered.

An ongoing numerical investigation of unsteady shock/boundary layer interaction on a 2-d supercritical airfoil in transonic flow is presented. Initially, the finitevolume URANS solver DLR-TAU is used to simulate self-sustained periodic shock oscillations well known as shock buffet. Next, emphasis is put on the fixed-point stability of the steady flow field below the shock buffet onset. Therefore the flow is perturbed in time with small sinusoidal deflections of the airfoil geometry and random impulses. With increasing angle of attack the mean flow is shown to develop a damped aerodynamic resonance, that degenerates finally towards self-amplification. The occurrence of the aerodynamic resonance is closely related to the development of shock-induced separation, accompanied by quasi-steady inverse shock motion.

Details of unsteady aerodynamics for a two dimensional NLR7301 airfoil with flap were numerically investigated by a Navier-Stokes code in the transonic regime. Unsteady aerodynamic coefficients such as lift and pitching moment show nonlinear aspects in a small amplitude range of forced excitation modes. The pressure distribution of the supersonic region on the upper surface is very sensitive to the incidence angle, which yields the nonlinearity of the unsteady aerodynamics in a small amplitude range of pitching motion. The behaviour of reattachment of the boundary layer behind the shock wave also affects the nonlinearity. Shock wave motions were also examined. Linearized lift and pitching moment coefficients for each amplitude were applied to the Eigenvalue analysis for the flutter boundary. Time domain flutter simulations were also carried out and the result showed that the amplitude of the limit cycle oscillation could be explained by the flutter boundary of the Eigenvalue analysis.

First a brief overview of C. Possio's short but outstanding and fruitful carreer is given. This is followed by an outline of the state of the art in flutter and unsteady aerodynamic research, and of the challenges and problems arising in aircraft development at that time, like high speed flight. His first publications until 1937 about gas dynamic and supersonic problems are reviewed. Main focus is put on his 1938 report about unsteady subsonic compressible 2D flow, which became famous and was named after him, because he was the first one who developed an unsteady compressible aerodynamic theory, which was so urgently needed in those years. The theory, based on Prandtl's acceleration potential is briefly outlined. Some discussions and comments in Germany and other countries at that time highlight the importance of this work for the scientific community. Early solutions of Possio's integral equation by himself and later ones by other researchers are presented, as well as the following approaches to extend the theory to 3 dimensional flows before the war, like Kuessner's theory, which was probably influenced by Possio. Finally C. Possio's later scientific contributions to windtunnel interference and to hydrodynamics are sketched. Summarising some developments of the 2nd half of the 20th century, which are based on Possio’s work, demonstrate, that C. Possio created a milestone for modern aircraft research during his very short carreer.