In this article, we develop a nonlinear data-driven model of the aerodynamic force on an aerofoil made tooscillate in pitch in a wind tunnel under constant flow conditions. The proposed model structure is a polynomial nonlinear state-space model (PNLSS) which is an extension of the classical linear state-space modelwith nonlinear functions. In this work, we use experimental data of an aerofoil onto which a pitching motionis imposed, using a dedicated cyber-physical fluid dynamic (CPFD) setup. The pitch angle is consideredas the input to the system, while the aerodynamic force is considered as the output of the system. Different input signals (single sines, multisines) are considered at different amplitudes and offset angles for thepitching aerofoil. Results show that the PNLSS model is three times more accurate than the linear model inminimising the least squares cost function. To the author’s knowledge, this constitutes the first instance of afully nonlinear state space model of an oscillating aerofoil developed from experimental measurements.