In-Flight Estimation of the Aerodynamics of Tethered Wings for Airborne Wind Energy

A new filtering approach is presented, able to estimate online the aerodynamic characteristics of tethered wings used in airborne wind energy systems. The approach is based on the extended Kalman filter (EKF) and on a representative model of the flight dynamics. The filter is fed with measurements available at the ground station, namely, the line angles and their rates, the traction force on the tether, and the wind speed and direction a few meters above the ground. Convergence and accuracy of the estimates are first evaluated through simulations. Then, the approach is demonstrated using extensive experimental data, collected during field tests with two small-scale prototypes built independently by different research groups and four different flexible airfoils (power kites), of both ram-air type and of leading-edge-inflated type. The experimental results show that the algorithm can effectively estimate both the state and the aerodynamic parameters of the wings, as well as the wind speed and direction at the wing's altitude, and makes it possible to analyze the kite aerodynamics during highly dynamical tethered flight.