I think some of the early designs with this type of variable pitch had a magnet "hold" from the starting position. Once the forces overcame the magnet force the spring-loaded mechanism, coupled with a shock absorber allowed the blades to rotate. Quirks/Dunlite is one that comes to mind. Anyone recall this, and/or what the benefits might be, if any?
~ks
When I was working at Wind Energy Group of the University of Eindhoven we had a Dunlite wind turbine on the roof of our building. At that time Dunlite was bought by Philips and Philips has given us a wind turbine for testing. The Dunlite had a 3-bladed rotor and a pitch control safety system with an eccentricity in between the blade axis and the rotor axis. A centrifugal weight was mounted directly on the blade shaft of each blade at the back side of the rotor plane, so just as given in figure 7 of KD 437. I don't remember that there was a magnet in this pitch control system. This mechanism wasn't working properly and now I think that I know why.
It appeared that at a certain rotational speed, there was a sudden large shift of the blade angle. This resulted in a strong decrease of the tip speed ratio and so the rotational speed went down. At a certain much lower rotational speed, the blade turned back to its starting position. So there was a strong oscillation of the pitch movement. This oscillation resulted in a large variation of the power at high wind speeds which isn't nice. But the system was good enough to prevent too high rotational speeds at very high wind speeds.
The first reason of the oscillation is that the angle gamma in between the arm of the centrifugal weight and the rotor axis was about zero for the starting position. In figure 8 of KD 437 it can be seen that this results in a strong increase of the centrifugal moment Mc if gamma becomes larger. So once the blade starts moving, it will make a jump. The second reason is that the bearings of a blade are positioned at a small distance of each other. This means that the radial load on these bearings because of the rotor thrust, is large. The centrifugal force because of the blade mass is also large and this causes a substantial bearing friction. Bearing friction causes hysteresis in the blade movement.
If there would have been a magnet which keeps the blade fixed in the starting position, this magnet would have been the third reason of the oscillation as a magnet works in the same way as bearing friction but then only for the outwards movement of the weight. So to my opinion using of a magnet is useless. The system works only fluently if there is almost no friction moment and if the centrifugal moment is mainly steered by the rotational speed.
