The Hydromatic CSU

The constant-speed units mentioned above are all of the single-action type. The oil pressure acts on only one side of the piston with the opposing force provided by counterweights,

compressed air and aerodynamic or centrifugal turning moments. In some units, a spring may also be used to assist the centrifugal turning moment.

For transport aircraft, Hamilton Standard introduced the Hydromatic double-action CSU, where oil pressure is directed to either side of the CSU as required. Normal engine oil pressure is fed to the upper side, or front of the piston to turn the blades to fine/flat pitch, assisted by the centrifugal turning moment. Engine oil pressure boosted to 450 PSI by the governor boost pump is directed to the lower side or back of the piston to turn the blades to coarse pitch. An increase of oil flowing to either side of the piston changes the prop blade angle via the cam connected to the piston. The advantage of having a double-action CSU is the need and weight of the spring and counterweight are excluded.