IFASD2024 Paper Submission & Registration
17 – 21 June 2024, The Hague, The Netherlands





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11:00   Aeroelastic testing 1
Chair: Alessandro Scotti
11:00
30 mins
Wind tunnel testing and modal validation of TU-Flex's high aspect-ratio wings
Pedro J. González, Guilherme C. Barbosa, Álvaro A. G. Quesada, Gerrit Stavorinus, Flávio J. Silvestre, Jonathan Hilger, Charlotte Hanke, Arne Voß, Wolf R. Krüger
Abstract: High aspect ratio flexible wing aircraft present very complex and coupled structural and flight dynamics. This research describes a bottom-to-top validation process for this type of wing. This procedure starts with static, followed by ground vibration and wind tunnel tests. The concept of this approach is to first validate the structural and aeroelastic models before addressing the full flying vehicle. The experimental data was used to tune the structural model of a flexible flying demonstrator called TU-Flex. This aircraft was designed as a flying lab capable of recording coupled flight dynamic data of a flexible aircraft with a transport/airliner configuration. Three software were used to design the wing and to define the experimental test cases: NASTRAN, Loads Kernel, and ModSiG. The gathered data permitted tuning and showing the accuracy of the structural model. It also allows for finding inaccuracies in the aerodynamic and aeroelastic models for further tuning. The models are capable of capturing the overall aeroelastic trend nevertheless, fine tuning is now necessary. Therefore, the proposed process seems adequate to collect all necessary data to tune aeroelastic models within the process to prepare the models for the full flying vehicle
11:30
30 mins
Finite element model update of a very flexible wing using ground vibration testing data
Bilal Sharqi, Carlos E. S. Cesnik
Abstract: Ground vibration testing (GVT) is typically conducted on an aircraft where the structure is supported using a suspension setup that emulates the free-flying aircraft. When the structure is very flexible, it is challenging to find a suspension system that can support the structure without influencing its dynamic response. This study investigates the computational and experimental techniques required to conduct such a GVT on a very flexible aircraft (VFA) and update its finite element model (FEM). There are two main challenges associated with conducting GVT on a VFA. The first is the measurement of the low frequencies (<1.0 Hz) related to the test structure. The second is the practical challenge of obtaining a soft enough suspension to minimize the interaction between the suspension-related modes and the aircraft’s elastic modes. For VFA, decoupling of the suspension and airframe modes is typically not feasible. This requires accounting for the suspension in the FEM, which can be achieved by characterizing the suspension upfront as an isolated component and accounting for it in the FEM. GVT was conducted on a VFA using the methodology developed by Sharqi and Cesnik1, which is based on performing GVT in various deformed shapes representative of the vehicle’s free-flight behavior. A FEM updating methodology developed by the authors2 was then demonstrated numerically and on simple slender beams. A validation of the methodology was conducted on a scale model with the representative dynamics of a solar-powered flying wing. The final paper will provide details on the GVT and FEM updating conducted on the aircraft and discuss the importance of the suspension setup in such an experiment. A free-free GVT on a VFA is a substantial effort that requires more resources than a conventional GVT on a relatively rigid (or moderately flexible) structure. This work will detail the process of performing GVT on a VFA with the suspension model included and removing it once the FEM is successfully updated. This results in a FEM that is dynamically representative of the actual test structure, bypassing the need for an expensive, if even practical, free-free GVT.
12:00
30 mins
Experimental investigation into the dynamic characteristics of a tilting multirotor system
Tanuj Sharma, Djamel Rezgui, Branislav Titurus
Abstract: Urban Air Mobility (UAM) promises a transformative leap in urban transportation, with electric vertical take-off and landing (eVTOL) aircraft at the forefront. Pivotal in eVTOL flexibility, especially during transitional flight phases, are tilting rotors. However, these configurations pose challenges in maintaining dynamic stability and controlling vibration, leading to potential resonance issues during transition phases. This study aims to explore experimentally the dynamic interplay between modal characteristics of a tilting multirotor system with operational parameters such as tilt angle and rotational speed of propellers. The key focus is placed on understanding the system-wide implications induced due to changes in tilt angle and rotor speed. This research particularly focuses on how these operation parameters influence resonance, leading to shift in shifts in modal frequencies and damping variations, which are critical in the design and operation of tilting rotors. To scrutinise these characteristics, a dynamically scaled experimental rig is developed. This multirotor test rig facilitates investigations into how variations in rotor tilt angles and rotor speed impact the system’s modal characteristics, including natural frequencies, mode shapes, and damping characteristics. A special emphasis is placed on exploring resonance and its implications under different tilt scenarios and operational speeds. Experimental exploration revealed that natural frequencies decrease with an increase in rotor speed, particularly for higher modes. Whereas, increasing the tilt angle from 0◦ to 90◦ results in a substantial increase in frequency and a reduction in amplitude, especially in the first torsional mode. Furthermore, it is found that the system experiences strong resonance at 3120 RPM, where the second-out-plane bending mode is excited by the first rotor harmonics.


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