The Hamilton Standard Hydromatic propeller is a classic mechanical-hydraulic design widely used on legacy cargo aircraft and radial engines. Unlike older counterweight systems, it uses a completely enclosed pitch-changing mechanism driven by a balance of engine oil pressure, boosted governor pressure, and centrifugal force. For students and technicians, mastering this system is key to understanding the principles of constant-speed, feathering, and double-acting propeller operation.
Many Hydromatic propellers remain in service on older cargo aircraft. A Hydromatic propeller uses a double-acting hydraulic system in which oil pressure acts on opposite sides of a single propeller piston to control blade pitch. Many larger modern turboprop systems also use variations of this opposing hydraulic force design.
The governors are similar in construction and principle of operation to normal constant-speed systems, utilizing internal flyweights to sense engine speed. The major difference lies in the pitch-changing mechanism within the hub itself.
In the Hydromatic propeller hub, no external blade 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 its large blade angle range and its comprehensive feathering and unfeathering features.
This system is a double-acting hydraulic propeller design in which engine oil pressure on the outboard side of the piston balances against governor oil pressure on the inboard side of the piston. These opposing hydraulic forces are used to control and change blade angle or pitch.
The distributor valve 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 an unfeathering operation, the distributor valve shifts under auxiliary pump pressure and reverses these passages so that oil from the auxiliary pump flows to the outboard side of the piston, while oil on the inboard side flows back to the engine.
An engine-shaft-extension assembly is used instead of a distributor valve on specific propeller models that do not have feathering capabilities.
The Hydromatic propeller [Figure 1] is composed of four major components:
- The hub assembly,
- The dome assembly,
- The distributor valve assembly (for feathering on standard propellers) or an engine-shaft-extension assembly (for non-feathering configurations), and
- The anti-icing assembly.
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| Figure 1. Typical hydromatic propeller installation |
The hub assembly is the structural foundation of the propeller mechanism. It contains both the blades and the mechanical means for holding them securely 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:
- rotating cam,
- fixed cam,
- piston, and
- 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 turns inside the fixed cam, meshes with gear segments on the butt ends of the blades.
The single piston operates inside the dome shell and is the mechanism that converts engine and governor oil pressures into forces that act through the cams to turn the 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 system. A bevel gear on the base of the rotating cam mates with bevel gear segments attached to the butt ends of the blades, thereby turning the blades.
This blade pitch-changing action is illustrated in Figure 2.
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| Figure 2. Diagram of hydromatic propeller operational forces |
The centrifugal force acting on a rotating blade includes a component force that results in a twisting moment tending to move the blade toward low pitch.
As shown in Figure 2, a second force, engine oil pressure, is supplied directly to the outboard side of the propeller piston to assist the centrifugal twisting moment 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 primary counterforce to move the blades toward a 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 precisely balance and control the two combined forces working 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 the centrifugal twisting force, engine oil pressure, and high-pressure oil from the governor.
The centrifugal force acting on each blade of a rotating propeller results in a twisting moment about the blade centerline that tends, at all times, to move the blade toward low pitch.
Governor pump output oil is directed by the governor to the inboard side of the propeller piston during constant-speed operation to increase pitch, while the engine oil on the outboard side opposite this high-pressure oil returns to the engine lubricating system.
Engine oil at regular supply pressure enters the outboard side of the dome directly to assist in low-pitch operations, while a separate supply feeds the governor intake.
During constant-speed operations, the governor mechanism regulates oil delivery to the inboard side of the piston as needed to balance engine oil and centrifugal forces, keeping the speed at a specified setting.
Feathering Operation
A typical Hydromatic propeller feathering installation is shown in Figure 3.
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| Figure 3. Typical feathering installation |
When the feathering push-button switch is depressed, a low-current circuit is established from the battery through the push-button holding coil and through a solenoid relay. As long as the circuit remains closed, the holding coil keeps the push button in the depressed position.
Closing the solenoid relay establishes the high-current circuit from the battery to the electric auxiliary feathering motor-pump unit.
The feathering pump picks up engine oil from the oil supply tank, boosts its pressure 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 line to pure auxiliary oil.
This high-pressure oil flows through the engine transfer rings, through the propeller shaft governor oil passage, through the distributor valve ports, and finally to the inboard piston chamber by way of the valve inboard outlet.
The distributor valve does not shift during the feathering operation. It merely provides an open oil passageway to the inboard piston chamber for auxiliary oil and allows oil on the outboard piston side to drain.
The same positional conditions described for a normal underspeed operation exist in the distributor valve, except that oil at high auxiliary pressure replaces normal governor oil at the inboard side of the piston.
The distributor-valve spring is backed up by engine oil pressure, meaning that the structural pressure differential required to move the piston acts uniformly across the mechanism.
The propeller piston moves outboard under this high auxiliary oil pressure at a speed proportional to the rate at which the oil is supplied.
This linear piston motion is transmitted through the piston rollers operating in the oppositely inclined cam tracks of the fixed cam and the rotating cam, and is converted by the bevel gears into a blade-twisting moment.
Only during feathering or unfeathering is the low mechanical advantage portion of the cam tracks used. (The low mechanical advantage portion lies between the track break and the outboard end of the track profile.)
Oil at engine pressure, displaced from the outboard piston chamber, flows through the distributor valve outboard inlet, past the valve land, through the valve port, into the propeller shaft engine oil passage, and is finally delivered back into the engine lubricating system.
Thus, the blades move smoothly toward the full high-pitch (feathered) angle.
Having reached the full-feathered position, further movement of the mechanism is prevented by physical contact between the high-angle stop ring in the base of the fixed cam and the stop lugs set in the teeth of the rotating cam.
The pressure in the inboard piston end now increases rapidly, and upon reaching a set cutout pressure, the electric pressure switch automatically opens.
This cutout pressure is calibrated to be less than the pressure required to shift the distributor valve.
Opening the switch de-energizes the holding coil and releases the cockpit feathering push-button control switch. The release of this switch breaks the solenoid relay circuit, which shuts off the auxiliary feathering pump motor.
The pressures in both the inboard and outboard chambers of the piston drop, and, since the aerodynamic forces are balanced, the propeller blades remain securely in the feathered position.
Meanwhile, the governor high-pressure transfer valve shifts back to its normal position as soon as the auxiliary pressure drops.
Unfeathering Operation
To unfeather a Hydromatic propeller, the pilot must manually depress and hold in the feathering switch push-button control switch.
As in the case of feathering a propeller, the low-current control circuits from the battery through the holding coil and through the solenoid relay are completed when the solenoid closes.
The high-current circuit from the battery starts the motor-pump unit, and auxiliary oil is supplied at high pressure to the governor transfer valve.
Auxiliary oil entering through the high-pressure transfer valve connection shifts the governor transfer valve, disconnecting the governor from the propeller line and admitting high-pressure auxiliary oil.
This oil flows through the engine oil transfer rings, through the propeller shaft governor oil passage, and into the distributor valve assembly.
When the unfeathering operation begins, the piston is in its extreme outboard position. The oil enters the inboard piston end of the cylinder by way of the distributor valve inboard outlet.
Because the piston cannot move any further outboard, the pressure against the inboard end of the distributor valve land builds up rapidly. When this pressure becomes greater than the combined opposing force of the distributor valve spring and the engine oil pressure behind this spring, the distributor valve shifts.
Once the valve shifts, the internal passages through the distributor valve assembly to the propeller are reversed. A passage is opened through the valve ports directly to the outboard piston chamber.
As high-pressure auxiliary pump oil is directed to the outboard side, the piston is forced inboard, and oil is safely displaced from the inboard piston chamber back through the valve lands and into the engine lubricating system.
At the same time, the pressure at the cutout switch increases and the switch opens; however, the electrical circuit to the feathering pump and motor unit remains complete as long as the pilot manually holds the switch in.
With the inboard end of the propeller piston connected to the drain line and auxiliary pressure flowing directly to the outboard end of the piston, the piston moves inboard, forcing the blades out of the feathered angle.
As the blades unfeather, the oncoming airflow causes them to windmill, assisting the unfeathering operation with the added force toward low pitch brought about by the blade's centrifugal twisting moment.
When the engine speed has increased to approximately 1,000 rpm, the operator releases the feathering pump motor switch.
The pressure in the distributor valve decreases, allowing the distributor valve to shift back to its normal state under the action of its spring.
This action completely reconnects the governor with the propeller and re-establishes the standard oil passages used during normal constant-speed operations.
Setting the Propeller Governor
The propeller governor incorporates an adjustable stop that limits the maximum governed engine speed. As soon as the target takeoff rpm is reached, the propeller moves off its low-pitch mechanical stops.
The resulting larger propeller blade angle increases the load on the engine, thus maintaining the prescribed maximum engine speed and preventing an overspeed condition.
At the time of a propeller, propeller governor, or engine installation, specific steps are taken to ensure that the powerplant correctly obtains takeoff rpm.
During a ground run-up, the technician moves the throttle to the takeoff position and notes the resulting rpm and manifold pressure.
If the rpm obtained is higher or lower than the takeoff rpm prescribed in the manufacturer’s instructions, the technician resets the adjustable mechanical stop on the governor housing until the exact prescribed rpm is obtained.