Double acting propellers:
This type of propeller is normally fitted to larger engine and, because of engine requirements, is more complicated than the propellers fitted to smaller engines. Construction is similar to that of the single acting propeller, the hub supporting the blades, and the cylinder housing the operation piston. In this case, however, the cylinder is closed at both ends, and the piston is moved in both direction by oil pressure. In one type of mechanism links from the annular piston pass through seals in the rear end of the cylinder, and are connected to a pin at the base of each blade. In another type of mechanism, the piston is connected by means of pins and rollers to a cam track and bevel gear, the bevel gear matching with a bevel gear segment at the base of each blade; axial movement of the piston causes rotation of the bevel gear, and alteration of the blade piston causes rotation of the bevel gear, and alteration of the blade angle. Operating oil is conveyed to the propeller mechanism through concentric tubes in the bore of the engine reduction gear shaft. And another type Fig, has the blades connected by links to the cylinder which moves with the piston held stationary.
The piston may move in the cylinder or the cylinder may move over the piston, the linear movement being transmitted to the blades by various linkages to convert it to rotary motion.
The geared, or hydromatic, pitch change mechanism utilises a piston inside a stationary cylinder. The piston is 'U' shaped to contain a pair of co-axial cylindrical cams. The outer cam is fixed and the inner is free to turn. This carriers a bevel gear which meshes with bevel gear segments on the blade roots.