System identification and experimental investigation of mechanism-free ornithopter wings

• Mechanism-free ornithopters offer reduced actuation complexity, enhancing durability, efficiency, and reliability. • A computationally efficient state-space ornithopter model capturing fluid-structure interactions is introduced. • A mechanism-free ornithopter is fabricated with piezocomposite actuators to enable active flapping motion. • Experimental wind tunnel tests are conducted to validate the aerodynamic and structural model predictions. • System identification using empirical data enhances the accuracy of the state-space model. A piezoelectric material-based mechanism-free ornithopter wing has the potential to generate heaving and pitching motions to achieve flapping-based flight without the need for electromagnetic motors and conventional power transmission mechanisms. This approach potentially saves weight and energy consumption, and reduces mechanical complexity enhancing robustness. The mechanism-free, or so-called solid-state, ornithopter utilizes flexible piezocomposite wings to generate the aerodynamic forces needed for flight. Due to the complex nature of the interactions between the aerodynamic loads and structural dynamics, it is recognized that an accurate control-oriented model is needed to represent the interaction between the fluidic, structural, and electrical domains. In previous research by the authors, the structural and fluid domains of mechanism-free ornithopters were modeled using the Rayleigh-Ritz method and the empirical state space Theodorsen function, respectively. The resulting fully coupled system model, presented in state space form, facilitated parametric analyses for preliminary design, multidisciplinary design optimization, and analyses of stability and controllability. This paper focuses on the experimental validation of the state-space model and identification of critical system parameters of the model. To validate the theoretical model, a prototype mechanism-free ornithopter wing is fabricated using Macro-Fiber Composite actuators bonded to a thin carbon fiber composite substrate. Experiments are conducted in a low-speed wind tunnel where the structural and aerodynamic responses of the wing are measured as a function of critical operating conditions. Aerodynamic loads are measured using a six-degree-of-freedom load cell, while the dynamic structural response is measured using a three-dimensional motion capture camera system. Tests are conducted for a range of key parameters such as harmonic excitation voltage magnitude and frequency. A key novelty of this research lies in the systematic system identification process, which enables the experimental determination of nonlinear structural and aerodynamic parameters, including variation of damping, voltage-force coupling effects, and leading-edge vortex contributions. These factors are often neglected in simplified models. The results demonstrate strong agreement between experimental data and the identified model, confirming its accuracy in predicting the dynamic response of a mechanism-free ornithopter.