In order to gain more insight into the effects of propeller ducts on quadrotors in forward flight, this research analyses flight data obtained from free flight tests, performed in the Cyberzoo and the Open Jet Facility of the TU Delft. Using a quadrotor platform with detachable propeller ducts, both trimmed forward flight tests and flights exciting the F_x, F_z forces and the M_y moment, using both ducted and unducted configurations. From the analysis of the steady flight data, the ducted configuration flies at a significantly higher pitch angle compared to the unducted configuration. However, for the power consumption and the rotor speeds, a crossover point is present around an airspeed of 6 ms^-1. Before this crossover point the power consumption and total rotor speeds of the ducted configuration are lower than its unducted counterpart. The flights containing the excitations of the forces and the moment are used to identify an aerodynamic model of the quadrotor using stepwise regression. Using this technique a model of the ducts themselves is identified which can be added onto the unducted model in order to predict the response of the ducted quadrotor. The regressors of this model were also used to gain extra insight into the terms at play in the effects of the ducts.