A screw propeller wherein the blades are turned about their axes to vary the pitch is provided with means which are a function of the engine speed and which tend to turn the blades against the action of a resisting means such as a spring, these two controls being interconnected so that the resultant moment acting to turn the blades from one set position to another increases as soon as the resultant is sufficient to initiate the movement of the blades, the latter being then definitely moved from one set position to another. A weight 5 is coupled by an arm 4 to the blade 1 and by centrifugal force tends to move this blade. It is connected by a link 7 to a member 9 under spring control 12 in such a manner that when the centrifugal force is sufficient to overcome the spring the blade is moved and the resisting moment diminishes. The member 9 has two collars 10, 11, which contact with stops to determine the high and low pitch positions. The stop positions may be adjustable as by the movement of the block 13 which is screwed into the engine case. The block 13 may be adjusted by hand or it may have a weighted arm 81 whereby centrifugal force acting against a spring automatically sets the stop position. In one form the connection of the mechanism with the blade is by means of an arm 17 secured to a ring 16 that is attached to a block 14 embracing the blade root. The end positions of the stops may be adjusted from the cockpit of the aeroplane or may be modified automatically in accordance with the variation in pressure due to change of altitude. In a modification the mechanism provides for three stop positions and for this purpose in addition to the. main spring a second spring is provided which comes into operation after a predetermined action of the first spring.
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