13:30
Wind tunnel testing 1
Chair: Anders Karlsson
13:30
30 mins
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Assessment of Active Load Control Approaches for Transport Aircraft – Simulation and Wind Tunnel Test
Wolf R. Krueger, Holger Mai, Thiemo Kier, Lars Reimer
Abstract: Active flight load alleviation is an important contribution towards lighter wing structures and wings of higher aspect ratios, both important measures to increase the efficiency of transport aircraft. In the DLR project oLAF (optimal load adaptive aircraft), strategies for active and passive load alleviation are developed and validated. In the project, different lines of investi-gation are followed in parallel – first, a reference configuration of a long-range aircraft is de-signed on a preliminary design basis, and both aerodynamics and structure of the wing are further optimized using coupled CFD- and finite-element-based design methods. Second, various aspects of load control technologies (aeroelastic properties of wings and control devic-es, structural wing design, design of load control algorithms with and without LIDAR) are studied independently, and the results are applied using the design process of the reference aircraft. Third, a closer look it taken at the aerodynamics of spoilers and control surfaces. While spoilers are regularly used as secondary control surfaces, the numerical analysis of de-ployed spoilers and of the efficiency of control surfaces downstream of the spoilers is still challenging. To assess the applicability of spoilers directly for load control as well as the influ-ence of spoilers on the control surfaces, both numerical and experimental investigations are performed.
Outputs of the aforementioned activities form the basis for the definition of a wind tunnel experiment for the validation of active load alleviation approaches in the DNW-NWB low speed wind tunnel. The planform of the wind tunnel wing is derived from the overall aircraft design. Aeroelastic tailoring approaches are used for the design of the wing structure, but whereas for a full aircraft, the objective function is optimal performance, i.e. a combination of minimum structural mass and minimum drag, the wind tunnel wing is optimized for maximum deflection and minimum natural frequency to enable real-time testing of active load control approaches. Another important objective of the wind tunnel test is the application of the nu-merical descriptions of spoilers and control surfaces to be used in control design. The active control laws applied in the experiment are derived from the control design approaches devel-oped for the complete aircraft. The results of the wind tunnel experiment are used for the val-idation of the numerical approaches developed in the project.
This paper focusses on the specifications of the wind tunnel experiment resulting from the overall project investigations and the contributions of the experiment to the project goals. Fur-ther papers will provide detailed descriptions of the wing design and the wind tunnel experi-ment.
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14:00
30 mins
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Design, manufacturing and identification of an active controlled flexible wing for subsonic wind tunnel testing
Johannes Dillinger, Holger Mai, Wolf R. Krüger, Thomas Schmidt, Felix Stalla
Abstract: To minimize the climate impact of commercial flight, aircraft emissions have to be significantly reduced. Important contributions towards this goal are the reduction both of aircraft drag and of structural mass. An increase of wing aspect ratio is a well-known design measure to reduce induced drag, however, a higher wing span usually leads to higher wing mass because of increased structural loads. Thus, load alleviation is essential for the realization of high aspect ratio wings, active gust load alleviation being a promising step to further decrease sizing loads.
The paper presents the design, manufacturing and testing of an actively controlled wing in the German Aerospace Center (DLR) project oLAF (optimized load adaptive wing). A wing of 1.75m semi-span is designed and built, equipped with five trailing edge devices (flaperons/ailerons) and two spoilers. The sensors built into in the wing include 12 accelerometers, 10 pressure sensors and a fiber-optical sensor for strain measurements. Furthermore, the forces and moments of a piezo-balance in the wind tunnel mounting are available for feedback, and a marker-based optical measurement system is used for the high-speed tracking of wing and control surface deflections. The wind tunnel campaign takes place at the DNW-NWB subsonic wind tunnel in Braunschweig, a tunnel belonging to the German and Dutch Wind Tunnel organization. The wind tunnel has a cross section of 3.25m x 2.8m and is operating at a maximum flow speed of 90m/s.
For the experiment, a gust generator is specifically designed, based on four stationary airfoil vanes, each followed by a downstream rotating, slotted cylinder (RSC) mounted vertically. Load control is implemented on a real-time environment based on control laws being developed in MATLAB/Simulink. The design of the wing, as well as that of the gust generator, is supported by comprehensive numerical studies.
The wind tunnel model structure is designed, manufactured and equipped using our in-house aeroelastic model design process. The transfer functions of the actuators, required for the control design, are identified in a specific set-up on the model. The dynamic properties of the model structure are identified both wind-off in a standard ground vibration test (GVT), and wind-on at specific test points for identification. The paper focuses on the design and construction of the wing.
Separate papers will give a specific view on design and analysis of the gust generator, as well as on the design of the control laws, and an overview of the test matrix, the data acquisition systems and control hardware, and finally of the experimental results.
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14:30
30 mins
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Design and Experimental Characterization of a Gust-Generator Concept with Rotating-Slotted Cylinders in the Low-Speed Wind Tunnel DNW-NWB
Thomas G. Schmidt, Johannes Dillinger, Markus Ritter, Anna Altkuckatz, Charlotte Hanke, Marc Braune, Holger Mai, Wolf R. Krüger
Abstract: The present study focuses on the design of a gust generator based on stationary airfoils coupled with downstream rotating, slotted cylinders, specifically configured for the low-speed wind tunnel DNW-NWB. Owing to the technical requirements imposed by the test facility and the desired gust characteristics, this particular gust-generator concept is chosen. Numerical computations for both wake-flow characteristics and structural properties are performed, so as to explore various design parameters. In a subsequent step, an experimental test campaign is conducted, in which the gust generator's wake flow is characterized using an unsteady fast-response 5-hole probe. The results reveal that highly periodic gust flows are induced by the gust generator, with gust amplitudes that are sufficiently large for investigating the gust response of flexible aircraft structures, e.g., wings. Besides, the gust frequency is directly proportional to the cylinders rotational speed, whether a continuous or discrete gust is generated. Future work is dedicated towards further enhancing the present gust generator, including - but not limited to - the motor's controls that drive the rotating cylinders.
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15:00
30 mins
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Wind Tunnel Testing Active Gust Load Alleviation of a Flexible Wing
Felix Stalla, Thiemo M. Kier, Gertjan Looye, Kolja Michel, Thomas G. Schmidt, Charlotte Hanke, Johannes Dillinger, Markus Ritter, Martin Tang
Abstract: Increasing efficiency is at the core of next generation aircraft development. Aerodynamics can be improved by high aspect ratio wings, but as these wings are more susceptible to loads from gusts and maneuvers, structural weight might increase. Active load control enables the use of high aspect ratio wings while maintaining a relatively low structural weight. To enhance the technology readiness of such secondary flight control functions, experiments are indispensable. This paper describes the wind tunnel testing of a gust load alleviation controller on a flexible swept wing, equipped with multiple trailing edge flaps and acceleration sensors. Disturbances are injected by a gust generator located upstream of the model. The complete process leading up to the active control experiment is outlined: the simulation model used for controller development, the experimental identification of the system and updating of said model, the controller design, and the validation in the wind tunnel. µ-synthesis robust control is used, achieving the best possible performance with respect to the uncertainties in the system. The robust controller achieves a significant load reduction of up to 80% in the wind tunnel tests.
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