Power output during flight operation of multi-megawatt airborne wind energy systems is substantially affected by the mass of the airborne subsystem, resulting in power fluctuations. In this paper, an approach to control the tether force using the airborne subsystem is presented that improves the quality of the power output. This kite tether force control concept is implemented on the 3DOF dynamic simulation of the MegAWES reference model. First, the winch of MegAWES is resized because an analysis of winch inertia and radius shows its effect on power output and tether force overshoot. Second, the power consuming sections during the traction phase are eliminated by using a feedforward winch controller. Finally, the peak power is substantially reduced by implementing the kite tether force controller which uses a measurement of the tether force, angle of attack, and airspeed to keep the tether force constant when the system is at its power limit. This reduces the range between minimum and maximum power output by 75%.

Incremental Nonlinear Dynamics Inversion (INDI) flight controllers are sensor-based control systems, that are robust towards model uncertainty and with good disturbance rejection characteristics. These controllers show coupling effects in structural modes when implemented in specific flying vehicles with low-frequency structural motions. This paper investigates different INDI implementations, standard INDI, hybrid INDI, and notch filter placement in the INDI loop via simulation and flight tests on the Nederdrone. System identification of the structural characteristics of the vehicle and the system’s yaw dynamics are executed via ground vibration and hover flight tests. Closed-loop behaviour of theINDI inner-loop, disturbance rejection performance, and outer loop step-tracking performance was assessed with dedicated flight tests. The investigated INDI solutions show similar disturbance rejection and outer-loop step tracking performance, while the hybrid INDI solution performs a better nonlinear dynamic inversion.
Index Terms—INDI, complementary filter, unmanned vehicle, flight control system structural motion coupling

Incremental Nonlinear Dynamic Inversion (INDI) is a sensor-based control strategy, which has shown robustness against model uncertainties on various aerospace vehicles. The sensor-based nature of the method brings attractive properties which has made it popular in the last decade. INDI globally linearizes the system by making use of control input and state derivative feedback. The time lag between control input and state derivative is seen as one of the main challenges of the method. This has been partially solved by synchronization of actuator outputs with measurement, although the method is still vulnerable to unexpected measurement delays. This paper proposes Hybrid INDI approach to alleviate measurement delays and to reduce sensor dependence of INDI. The approach fuses the system model and sensor measurement via a complementary filter that produces angular acceleration estimation. The estimation responds fast to the system input thanks to the on-board model. It has also a low-frequency accuracy because of the sensor measurement. The method is found to retain good performance in case of model mismatches and measurement delays. An attitude controller is designed for the F-16 aircraft model and findings are verified with time-domain simulations.