Groundbreaking structural dynamics and aeroelastic tests
By Cristina Riso and Rafael Palacios|December 2023
The Structural Dynamics Technical Committee focuses on the interactions among a host of forces on aircraft, rocket and spacecraft structures.
In January, researchers at Sapienza University of Rome completed the experimental validation of sloshing nonlinear reduced-order models for integration into full-scale structural and aeroelastic analysis tools. The combined effects of rotation and vertical tank motion on the dissipative behavior of sloshing were characterized for the first time with Sapienza’s closed-loop control environmental testing device.
Also in January, the Delft University of Technology in the Netherlands and the Israel Institute of Technology conducted the first parametric flutter margin, or PFM, wind tunnel test on a highly flexible wing. PFM is a novel method for safe experimental identification of flutter behavior and prediction of flutter margins via a self-contained measuring wind-mounted device called a flutter pod. The method captured the nonlinear evolution of the flutter boundary as a function of angle of attack of a highly flexible high-aspect ratio wing.
In February, researchers tested a pylon-mounted version of the X-DIA aircraft wind tunnel model at the Politecnico di Milano in Italy. In this FAA-sponsored joint project with the University of Washington, the researchers investigated the impact of free play on aeroelastic stability and active flutter suppression. The test configuration with a pylon allowed researchers to accurately control incidence and sideslip angles, which strongly impact limit-cycle oscillations. Due to the elastic pylon, the model shows two flutter mechanisms — symmetric and antisymmetric wing bending and torsion. By actively suppressing the first mechanism, it was possible to identify the second one. Finally, both flutter mechanisms were controlled, and the model was safely tested at speeds beyond the second flutter point.
In March, NASA finalized the data analysis and correlation for the dynamic rollout test and wet dress rehearsal of the Space Launch System, which in late 2022 launched an unoccupied Orion capsule for the Artemis I mission. In general, rolling out to the launchpad produces relatively small structural loads compared with those experienced during launch and ascent, but these repeated loads can progressively cause structural damage or failure. It is not possible to measure rollout loads while the rocket is at the launchpad, so NASA engineers applied operational modal analysis techniques to identify the modal characteristics of the rollout and wet dress configurations.
Also in March, AIAA members Peretz Friedmann, George Lesieutre and Daning Huang published a graduate-level textbook on structural dynamics as the 50th volume of the Cambridge Aerospace Series. The book covers fundamental material in structural dynamics augmented by in-depth treatment of damping, rotating systems and periodic systems.
In June, the Technical University of Berlin and the German Aerospace Center’s Institute of Aeroelasticity froze the configuration of a radio-controlled demonstrator named TU-Flex to study the coupling among flight mechanics, aeroelasticity and controls for very flexible aircraft. The demonstrator is to have exchangeable wings, permitting tests of different levels of flexibility: a flexible wing with wingtip deflections up to 10% of the wing semi-span, and a very flexible one with wingtip deflections up to 20%, for which nonlinear aeroelastic effects are expected. Construction of the first prototype started in July. Ground vibration and wind tunnel tests are planned for the end of the year, with first flight tests targeted for 2024.
In July, University of Michigan researchers validated their new technique for conducting ground vibration tests for very flexible aircraft. By hanging their flexible aircraft model from a 35-meter-tall crane via bungee cords, they created a suspension system with a very low frequency of 0.15 hertz, sufficiently separated from the aircraft’s natural frequency of 0.95 hertz, to measure its modal parameters.
Contributors: James Akers, Carlos E.S. Cesnik, Giuliano Coppotelli, Daning Huang, Dexter Johnson, Teresa Kinney, Russel Parks, Sergio Ricci, Flávio Silvestre, Jurij Sodja and Francesco Toffol