Within the FLEXHAT program a rotor has been developed and build (the PRT1 configuration), equipped with passive controlled blade tips. The rotor was tested by means of the 25m HAT rotor test facility at the Netherlands Energy Research Foundation (ECN) in Petten during the period July 1st 1989 - November 6th 1990. As a support for the assessment of the measurements with regard to the characteristics of the PRT1 configuration computer calculations were carried out by means of the program PHATAS-II. These calculations concern the rotor performance for a slowly increasing wind speed and different generator capacities. An important parameter in the rotor performance as a function of the wind speed is the spring tension of the tip mechanism which determines the relation between the tip angles and the speed. The calculated results show strong fluctuations in some cases, which can be attributed to negative aerodynamic cushioning, caused by the stall performance at negative angles of descent of the tip profile. 19 figs., 8 tabs., 8 refs., 1 app.