Hamilton Standard Hydromatic Propellers (Part One)

in Propellers

Many of the hydromatic propellers are used with older type aircraft involved in cargo operations. A hydromatic propeller has a double-acting governor that uses oil pressure on both sides of the propeller piston. Many larger turboprop systems also use this type of system. The governors are similar in construction and principle of operation in normal constantspeed systems. The major difference is in the pitch-changing mechanism. In the hydromatic propeller, no counterweights are used, and the moving parts of the mechanism are completely enclosed. Oil pressure and the centrifugal twisting moment of the blades are used together to turn the blades to a lower angle. The main advantages of the hydromatic propeller are the large blade angle range and the feathering and reversing features.

This propeller system is a double-acting hydraulic propeller system in which the hydraulic pressure (engine oil pressure) on one piston dome is used against governor oil pressure on the other side of the piston. These two opposing hydraulic forces are used to control and change blade angle or pitch. Although hydromatic propeller systems are very old, some are still used on radial engines. Larger new turboprop systems also use this opposing hydraulic force and double-acting governor systems.


The distributor valve or engine-shaft-extension assembly provides oil passages for governor or auxiliary oil to the inboard side of the piston and for engine oil to the outboard side. During unfeathering operation, the distributor shifts under auxiliary pressure and reverses these passages so that oil from the auxiliary pump flows to the outboard side of the piston and oil on the inboard side flows back to the engine. The engine-shaft-extension assembly is used with propellers that do not have feathering capabilities.

The hydromatic propeller [Figure 7-55] is composed of four major components:

  1. The hub assembly,
  2. The dome assembly,
  3. The distributor valve assembly (for feathering on single-acting propellers) or engine-shaft-extension assembly (for nonfeathering or double-acting propellers), and
  4. The anti-icing assembly.

Figure 7-55. Typical hydromatic propeller installation.

Figure 7-55. Typical hydromatic propeller installation. [click on image to enlarge]

The hub assembly is the basic propeller mechanism. It contains both the blades and the mechanical means for holding them in position. The blades are supported by the spider and retained by the barrel. Each blade is free to turn about its axis under the control of the dome assembly.

The dome assembly contains the pitch-changing mechanism for the blades. Its major components are the:

  1. Rotating cam,
  2. Fixed cam,
  3. Piston, and
  4. Dome shell.

When the dome assembly is installed in the propeller hub, the fixed cam remains stationary with respect to the hub. The rotating cam, which can turn inside the fixed cam, meshes with gear segments on the blades. The piston operates inside the dome shell and is the mechanism that converts engine and governor oil pressure into forces that act through the cams to turn propeller blades.

Principles of Operation

The pitch-changing mechanism of hydromatic propellers is a mechanical-hydraulic system in which hydraulic forces acting on a piston are transformed into mechanical twisting forces acting on the blades. Linear movement of the piston is converted to rotary motion by a cylindrical cam. A bevel gear on the base of the cam mates with bevel gear segments attached to the butt ends of the blades, thereby turning the blades. This blade pitch-changing action can be understood by studying the schematic in Figure 7-56.

Figure 7-56. Diagram of hydromatic propeller operational forces.

Figure 7-56. Diagram of hydromatic propeller operational forces.

The centrifugal force acting on a rotating blade includes a component force that tends to move the blade toward low pitch. As shown in Figure 7-56, a second force, engine oil pressure, is supplied to the outboard side of the propeller piston to assist in moving the blade toward low pitch. Propeller governor oil, taken from the engine oil supply and boosted in pressure by the engine-driven propeller governor, is directed against the inboard side of the propeller piston. It acts as the counterforce, which can move the blades toward higher pitch. By metering this high-pressure oil to, or draining it from, the inboard side of the propeller piston by means of the constant-speed control unit, the force toward high pitch can balance and control the two forces toward low pitch. In this way, the propeller blade angle is regulated to maintain a selected rpm.

The basic propeller control forces acting on the Hamilton Standard propeller are centrifugal twisting force and high pressure oil from the governor. The centrifugal force acting on each blade of a rotating propeller includes a component force that results in a twisting moment about the blade center line that tends, at all times, to move the blade toward low pitch. Governor pump output oil is directed by the governor to either side of the propeller piston. The oil on the side of the piston opposite this high-pressure oil returns to the intake side of the governor pump and is used over again. Engine oil at engine supply pressure does not enter the propeller directly but is supplied only to the governor. During constant-speed operations, the double-acting governor mechanism sends oil to one side or the other of the piston as needed to keep the speed at a specified setting.

Figure 7-57. Typical feathering installation.

Figure 7-57. Typical feathering installation. [click image to enlarge]

Feathering Operation

A typical hydromatic propeller feathering installation is shown in Figure 7-57. When the feathering push-button switch is depressed, the low current circuit is established from the battery through the push-button holding coil and from the battery through the solenoid relay. As long as the circuit remains closed, the holding coil keeps the push button in the depressed position. Closing the solenoid establishes the high current circuit from the battery to the feathering motor pump unit. The feathering pump picks up engine oil from the oil supply tank, boosts its pressure, if necessary, to the relief valve setting of the pump, and supplies it to the governor high-pressure transfer valve connection. Auxiliary oil entering the high-pressure transfer valve connection shifts the governor transfer valve, which hydraulically disconnects the governor from the propeller and at the same time opens the propeller governor oil line to auxiliary oil. The oil flows through the engine transfer rings, through the propeller shaft governor oil passage, through the distributor valve port, between lands, and finally to the inboard piston end by way of the valve inboard outlet.