Teeter rotor operating limits

This paper addresses the effects of the loss of aerodynamic damping as a two-bladed teetered wind turbine rotor enters the stall regime. A distinction is drawn between teeter instability due to loss of damping and teeter 'stop-pounding' (i.e. hitting the teeter stop limits causing large and destructive blade loads) due to operation that results in large, but stable, teeter excursions. A theoretical development shows that in the first order approximation, the teeter damping coefficient is directly proportional to the lift coefficient versus angle of attack derivative and that the damping increases with the cube of the rotor radius. The code TEETER, with various lift coefficient stall models, indicates that the rotor reaches operating limits (stop-pounding) as the 70% spanwise blade station enters the stall regime and that the limits widen with increasing yaw angle.