Patent US 5,214,596

System for determining the airspeed of helicopters, having a coupled measuring device to which is supplied by the cyclic and collective control signals, the attitude angles and the rotational velocities about the axes of the helicopter. The measuring device contains two models, namely a first model of the input behaviour of the helicopter motion, on which the cyclic and collective control signals act as input, and a second model of the system behaviour of the helicopter, on which the condition parameters act as input parameters. The estimated state parameters are obtained in an integration stage by integration of the algebraic sum of the outputs of the two models and the output signal of the correction arrangement, from which the values of the the velocity components are supplied to an indication. The integration stage is connected in front of a correction arrangement for the measured parameters of the rotational velocity and the attitude angle. There is further provided an arrangement for the compensation of steady state model errors.