1 5 4 2 6 6 3 1 2 3 4 5 6 Counterweight action Hydraulic force Aerodynamic force Nitrogen pressure or spring force, and counterweight action High-pressure oil enters the cylinder through the center of the propeller shaft and piston rod. The propeller control regulates the flow of high-pressure oil from a governor. A hydraulic piston in the hub of the propeller is connected to each blade by a piston rod. This rod is attached to forks that slide over the pitch-change pin mounted in the root of each blade. The oil pressure moves the piston toward the front of the cylinder, moving the piston rod and forks forward. The forks push the pitch-change pin of each blade toward  the front of the hub causing the blades to twist toward the  low-pitch position. A nitrogen pressure charge or mechanical spring in the  front of the hub opposes the oil pressure and causes the  propeller to move toward high-pitch. Counterweights also cause the blades to move toward the  high-pitch and feather positions. The counterweights counteract the aerodynamic twisting force that tries to  move the blades toward a low-pitch angle. Figure 12-4.  Pitch change forces. from the feathered position. The ignition is turned on prior to  engine rotation with the throttle at low idle and the mixture  rich. With the propeller control in a high rpm position, the  starter is engaged. The engine begins to windmill, start,  and run as oil pressure moves the blades out of feather. As  the engine starts, the propeller rpm should be immediately  reduced until the engine has had several minutes to warm up;  the pilot should monitor cylinder head and oil temperatures. pin senses a lack of centrifugal force from propeller rotation  and falls into place, preventing the blades from feathering.  Therefore, if a propeller is to be feathered, it must be done  before engine rpm decays below approximately 800. On one  popular model of turboprop engine, the propeller blades do,  in fact, feather with each shutdown. This propeller is not  equipped with such centrifugally-operated pins due to a  unique engine design. In any event, the AFM/POH procedures should be followed  for the exact unfeathering procedure. Both feathering and  starting a feathered reciprocating engine on the ground are  strongly discouraged by manufacturers due to the excessive  stress and vibrations generated. An unfeathering accumulator is a device that permits starting  a feathered engine in flight without the use of the electric  starter. An accumulator is any device that stores a reserve  of high pressure. On multiengine airplanes, the unfeathering  accumulator stores a small reserve of engine oil under  pressure from compressed air or nitrogen. To start a feathered  engine in flight, the pilot moves the propeller control out of  the feather position to release the accumulator pressure. The  oil flows under pressure to the propeller hub and drives the  blades toward the high rpm, low pitch position, whereupon  the propeller usually begins to windmill. (On some airplanes,  an assist from the electric starter may be necessary to initiate  rotation and completely unfeather the propeller.) If fuel and  ignition are present, the engine starts and runs. For airplanes  As just described, a loss of oil pressure from the propeller  governor allows the counterweights, spring, and/or dome  charge to drive the blades to feather. Logically then, the  propeller blades should feather every time an engine is shut  down as oil pressure falls to zero. Yet, this does not occur.  Preventing this is a small pin in the pitch changing mechanism  of the propeller hub that does not allow the propeller blades  to feather once rpm drops below approximately 800. The  12-5