Aircraft technical Basics: Aircraft Propellers - Navy Training Courses Edition of 1945: Chapter 4: Hydromatic Quick-Feathering Propeller

WHY'S AND WHEREFORE'S

You remember from boat drill what is meant by FEATHERING an oar in rowing. The reason for feathering, as you know, is to reduce the air resistance, or DRAG, against the flat side of the blade as it moves through the air. And by reducing air drag, of course, you increase your speed.

Feathering an airplane propeller blade is exactly like feathering an oar. Where the average normal blade angle may be anywhere from 16° to 55°, the pitch of a feathered blade may be about 90°. Thus, instead of offering the broad side of the blade, so that its flat surface lies in the direction of flight, a feathered propeller offers the leading edge of the blade, so that its flat surface lies in the direction of flight. This reduces the air resistance against the blades, and cuts down drag.

(the paragraph above is garbeled in the original form)

As you can guess, a dragging propeller is a headache for any pilot. Not only does it slow down the airplane, but it causes vibration that can do a lot of damage. 

Another thing. A feathered propeller will prevent windmilling, which is harmful to the engine and interferes with maneuvering the airplane. Feathering keeps a propeller from turning the engine over as the air smashes against the blades in flight. Figure 28 shows you how feathering can mean the difference between safety and a washout when flying a multi-engined airplane with a conked powerplant. With a feathered propeller, even that mythical Chinese flyer, Wun Wing Lo, might make a three-point landing.

In an emergency, feathering the blades makes it possible to stop engine rotation almost instantly. If a broken movable part, or some similar serious condition, is involved, the pilot can feather his hydromatic propeller without first having to reduce engine power or speed. You can see how this may mean the difference between a wrecked powerplant and a minor repair job on one part.

This ability of a feathered propeller to stop drag, vibration, and windmilling means an increase of ceiling, rate of climb, and speed for a multi-engined ship with a dead engine.

So, all in all, if you're in an airplane with a quick-feathering propeller, you have the edge over an airplane that doesn't have one.

On the other hand, if there is no need for feathering, the hydromatic propeller can be made into a nonfeathering propeller simply by adjusting the high-pitch stop to a specified top limit.

You'll discover as you go along that many of the principles of construction and operation of the hydromatic quick-feathering propeller are very similar to those of the counter weight-type constant-speed propeller. A hydromatic propeller could even be called a NON-COUNTERWEIGHT CONSTANT SPEED WITH A FULL-FEATHERING ADJUSTMENT. What takes the place of the counterweights? The answer is-OIL. But the oil is under HIGHER PRESSURE now. How does it get that way?

The "brains" of this feathering outfit is a unit much like the counterweight constant-speed control. The hydromatic propeller retains the basic structural advantages of constant-speed propellers and, in addition, provides a means of feathering and unfeathering the propeller blades in flight.

The size, weight, and general shape of the hydromatic control are the same as that of the constant speed, as is the method by which the oil enters and passes through the gear pump and is delivered into the hollow portion of the drive gear.

BUT-the feathering hydromatic control has still another advantage. It has a high-pressure oil supply to use for feathering and unfeathering the propeller blades. A study of figure 29 will make it clear to you. The propeller is not equipped with blade counterweights or a cylinder spring assembly. The CYLINDER is movable on the controllable and constant pitch propellers. On the hydromatic, however, the cylinder DOES NOT move, but the PISTON is movable.

The control unit consists of the same gear-type booster pump to raise the engine oil pressure to the pressure needed for the pitch-changing mechanism. There's the same pilot valve to control the flow of oil to and from the propeller, and the same spring-balanced flyballs that operate from the engine driveshaft.

The relief valve plunger has been modified to permit the force of the relief valve to be supplemented by the force of engine oil pressure. This permits the relief valve to operate at a pressure equal to engine oil pressure plus the spring pressure. And a transfer valve has been provided in the base of the unit to provide for passage of the special high-pressure oil that does the feathering and unfeathering.

The UNDERSPEED condition, as shown in diagram "A," exists when the speed of the flyballs has been reduced, and the spring force overcomes the force of the flyballs. In this condition the spring forces the pilot valve DOWN. The UPPER land of the valve moves below the metering port in the drive gear and cuts off the high-pressure oil. The LOWER land moves into the recess in the gear and opens the propeller line to drain.

When oil drains from the rear of the hydromatic propeller piston, the blades assume a lower angle and permit the engine speed to return to the original value. The flyballs and speeder spring in the control unit return to a balanced state as shown in the onspeed condition.

In the OVERSPEED condition, as shown in diagram "B," the flyball speed has increased; their forces have exceeded the force of the speeder spring, and the pilot valve is RAISED. The upper land on the valve then opens the ports through which the high-pressure oil flows, and the lower land closes the drain. Since oil to the rear of the propeller piston increases the blade angle, the engine speed is thus reduced, and the flyball spring forces a return again to a balanced state, as shown in the onspeed condition.

Figure 29.-Diagram of underspeed, overspeed, and onspeed.

The ONSPEED condition, as illustrated in diagram "C," exists when the flyball and spring forces are in balance. The pilot valve closes the line to the propeller and maintains a given blade angle. Both the pressure and drain ports are closed during this condition. All the oil from the gear pump is being by-passed through the relief valve back to the inlet side of the pump.

So far, the operation of this control unit closely parallels that of the constant-speed unit except for the movement of the pilot valve. Notice what happens at "D." During the feathering and un-feathering operations of the propeller, high-pressure oil from an AUXILIARY source is supplied to the propeller through a TRANSFER VALVE. in the base of the constant-speed unit. The function of this valve is to cut off oil from the unit to the propeller and open the passages through the engine nose to the high-pressure feathering oil.

The VALVE ASSEMBLY consists of a PLUNGER, a RETURN SPRING, and a BALL CHECK. The auxiliary high-pressure oil forces the plunger against the spring, as shown in diagram "D." When either operation is completed, and the pressure at the source of the auxiliary oil supply is reduced, the spring returns the ball to its seat and reopens the propeller line to the governor oil.

The minimum limit of the governing range is set by an adjustment of the minimum rpm ADJUSTING SCREW located within the speed-adjusting rack. The smaller end of the speeder spring is seated against the low rpm adjustment.

As shown by the illustration, the low rpm adjustment is threaded into the speeder spring rack. With this type of construction it is possible to set the minimum rpm adjustment in an infinite number of positions in relation to the speed-adjusting rack.

'The minimum rpm is set by adjusting the low rpm adjusting screw in relation to the rack, so that when the rack is raised until it strikes the inside of the cover and is at its extreme position toward low rpm, there remains a minimum spring force which the flyballs will balance at some minimum rpm. If the flyball speed drops below the minimum, the action is the same as that already described under the underspeed condition.

The relief valve design interconnects the valve with the engine oil system. This valve is held closed by the force of the by-pass valve spring plus the force of the engine oil pressure on the area of the valve plunger. Such arrangement provides for a maximum pressure differential across the propeller piston equal to the relief valve spring setting, and thus the effects on the operation of the propeller due to variations in engine oil pressure in any one engine, or between engine types, are eliminated.

For blade designs of basic diameters over 12 feet 6 inches, it has been found necessary to increase the pressure differential across the propeller piston in which case a special relief valve spring is required.

Now a word about the construction of the hydromatic propeller itself. Like all the others, its foundation is the spider, barrel, and blades. Blades used with the hydromatic propeller are identical in basic design with those in the two-position controllable and the constant-speed propellers. They differ slightly detail at the inner end, however, so they can't be interchanged. The blades are manufactured from high-strength aluminum alloy forgings and are of semi-hollow construction. This design allows the use of thin solid tips of high efficiency and a strong hollow shank for attaching the blade to the hub.

There's all internal aluminum-bronze bushing incorporated in the shank. This bushing supports the blade on the spider arm and transmits the blade thrust and torque loads to the spider. The centrifugal blade load is also taken by a part of this shank. To transmit the centrifugal load to the barrel, a ROLLER BEARING ASSEMBLY is used. This assembly is made up of two steel races, not removable from the blade, and a split type bearing retainer. To carry large forces with a minimum of friction, especially designed bearings are used. Between the inner bearing race and the blade butt you'll find a phenolic (plastic) sleeve. The purpose of this sleeve is to reduce the concentration of stress in the blade and also to eliminate chafing between the aluminum blade material and the steel bearing race.

The DOME ASSEMBLY is shown extended in figure 31. As you can see, it is a nesting of more or less intricate parts inside a bell-type cover. The main parts are the ROTATING CAM, the STATIONARY CAM, PISTON, and the SHELL. In addition, it houses the DISTRIBUTOR VALVE.

When the dome unit is installed in the hub assembly, the stationary cam is rigidly fixed in the barrel and provides support for the remaining parts of the dome unit. The rotating cam is supported by ball bearings which take the gear reactions and piston oil forces. The piston motion is transmitted to the rotating cam by means of four sets of cam rollers carried on shafts supported by the inner and outer walls of the piston.

In figures :31 and 32, you have a cutaway view of the DISTRIBUTOR VALVE. This part acts as a guide for the oil under two different pressures.

During constant-speed operation of the propeller, the distributor valve provides a passage through which engine oil, under BOOSTED pressure from the governor, runs to or from the INBOARD side of the propeller piston. Also through this valve, the oil under NORMAL. engine pressure is conducted to or from the OUTBOARD end of the cylinder.

Figures 31 and 32.-Cutaway views of the distributor valve.

During feathering, the same two passages provide a channel for the high-pressure oil (from the auxiliary pressure system) to get to the INBOARD side of the piston, and a route for the oil from the OUTBOARD end of the cylinder to return to the engine lubrication system.

In (A), the distributor valve is in an unfeathering position. In (B), it is in a feathering and constant speed position.

Before you move along to further details, have a look at the construction of the governor unit. In figure 33, you see a cutaway of the governor unit with the main parts labeled.

The drive-gear shaft is steel and has the job of driving the gear pump and the flyball assembly. One end of this shaft has 12 splines to fit the engine drive coupling. This governor assembly operates whenever the engine runs. Above and below the spur gear portion of the shaft there are bearing surfaces. At each bearing there is an oil PORT. The upper ports permit the high-pressure oil to enter the drive-gear shaft. The lower ports open to the propeller oil line. The steel idler gear is supported by the idler gear shaft, which is hollow and is made from cast iron. Cast iron is used to provide a satisfactory bearing surface for the idler gear. The pump gear wear is evenly distributed by using 13 teeth on the drive gear and 14 teeth on the idler gear.

The RELIEF VALVE ASSEMBLY consists of a bushing, a plunger, and a spring and plug with locking washer. The bushing is made from hardened carbon steel and is a press fit in the body. It acts as a guide for the plunger. The plunger is also made from hardened carbon steel, and has six small holes in the wall behind the web. These permit the force of engine oil pressure on the area of the plunger to supplement the force of the relief valve spring. The vent into the flyball chamber from the rear of the relief valve is plugged to trap the engine pressure behind the valve.

Then there's the PILOT VALVE, which is made from case-hardened carbon steel. The upper and lower bearing surfaces are held to close tolerance limits. The upper end of the pilot valve is threaded to fit the spring collar. Between the threaded portion and the upper bearing surface is the pilot valve ball bearing. The pilot valve is designed to meter oil to the pitch-changing mechanism as demanded by the governor.

Figure 33..—Governor unit.

Attached to the pilot valve is a SPRING COLLAR, made from a carbon steel which is not hardened. It is threaded at the lower end to fit the pilot valve stem. The flange at the base of the spring collar supports the base of the speeder spring and retains it in its seat.

A minimum rpm adjusting screw is threaded into the rack. This adjustment is, in turn, threaded into the speed-adjusting rack. After the relative positions of the three pieces have been adjusted to give the desired minimum rpm, the whole assembly is locked by tightening a small wedging screw in the upper end of the adjusting screw.

A stop for the high limit of the governing range of the unit is provided on the CONTROL PULLEY. It consists of a pin that can be located in any one of 18 holes in the pulley. The motion of the pulley is stopped when the pin comes to rest against an adjustable screw, threaded through a boss cast integral with the cover of the unit.

The FLYBALL ASSEMBLY consists of a flyball head, flyball-head cup, two flyballs, and two flyball-head hinge pins. The flyball head is designed to fit on the upper end of the drive-gear shaft. On the upper face of the flyball head are hinge-brackettype flyballs attached by suitable hinge pins. A flyball head cup is spun on the flyball head and secured by two spot welds.

This cup has a mighty important job. It prevents any oil which might get into the head from exerting a pressure on the flyballs. This elimates excessive side loads and prevents any turbulence of the oil from interfering with the action of the flyball. The flyball assembly is cadmium-plated, and is held in place by a wire-snap-ring which fits in a groove at the top of the drive-gear shaft.

All constant-speed controls require a gasket between the cover and body section and another between the body and base sections. The body-cover gasket is of the conventional type. The oil seal gasket, between the body and base, is a rubber composition ring which fits in the groove milled in the lower surface of the body. This type of gasket is used because it is necessary to hold the pump housing and clearances to tolerances which preclude the use of a standard type of gasket. The ring gasket permits a surface-to-surface fit of the base and body and at the same time furnishes a seal which is oil tight.

A governor of increased output was designed to meet the requirements for higher rate of pitch changes on some installations.

The capacity of the double-capacity governor-pump is nominally 16 quarts per minute, and the spring loaded valves are designed to handle this high capacity with no delivery to the propeller on low power input.

Basic model numbers are

The general operating principles remain the same. The body and base assemblies have been redesigned to incorporate the higher capacity booster pump. The high pressure feathering, fitting and transfer valves are incorporated in the body rather than in the base, and the high pressure relief valve system has been modified.

No maintenance or operating difficulties should be experienced by anyone familiar with the single capacity pump.

The booster pump output is delivered into the chamber surrounding the pilot valve. When the pilot valve is in the "overspeed" condition the output from the pump is delivered to the propeller and also against the relief valve. In this ease. the pressure on each side of the relief valve is approximately equal, and the spring force equivalent to about 50 psi holds the relief valve closed.

If, during the overspeed condition, the piston reaches the end of its travel, the pressure will increase. Then the pump valve will open and let the oil on the spring side of the relief valve bypass into the engine oil pressure line. The passage from the pilot valve to the spring side of the relief valve is small, and will cause a pressure drop of

Figure 34.-Schematic double capacity governor-pump.

about 30 pounds at a flow of 2 1/2 quarts per minute through this passage. This will cause a pressure differential across the relief valve sufficient to cause it to open and bypass the remaining booster pump output into the engine oil pressure system.

In the "onspeed," "underspeed," and "in and out feathering" positions, the pilot valve drops down and no oil is delivered to the spring side of the relief valve. Look at figure 34. Thus, with no oil pressure, the valve opens at approximately 50 pounds or at a lower power input pressure. This saves wear on the gears and shafts.

The only oil pressure adjustment on the governor is the screw adjusting the dump valve. This screw is found is the base of the governor. It is necessary to lock the adjustment with sealing wax or stick-shellac melted onto the adjusting screw.

ELECTRIC HEAD

The electric type head, shown in figure 35, used on a constant speed control accomplishes the same results as the mechanical head, namely it adjusts the compression in the governor speeder spring to regulate the rpm of the propeller. One advantage of the electric head is that governor settings are not affected by relative movement between the governor and the cockpit control brought about by structural deflections, cable slack and flexible engine mounts.

The advantages of this electric control are essentially the same as the advantages of any remotely controlled device compared with mechanical control. It eliminates the need for mechanical connections from the cockpit to the constant speed controls. Electric wiring is comparatively simple to install and maintain. Also it represents a saving in weight over mechanical connections.

Figure 35. - Electric head and cover.

Furthermore, the brake included in the electric control eliminates any creeping to which the mechanical control may he subject. It has been found that mechanical controls must be installed very carefully to prevent creeping and at the same time to provide precise adjustment needed for synchronizing. With the electric control, it is comparatively easy to synchronize, and once synchronized, the constant speed controls are not liable to change their setting and require readjustment.

The electric governor head consists of a small direct current, series-wound, reversible motor which (through a reduction gear train) drives the screw shaft. Movement of the screw shaft in an up or down direction varies the compression of the governor speeder spring which in turn regulates the rpm setting of the governor.

A spring loaded solenoid clutch is incorporated in the assembly to permit accurate and positive control of the position of the screw jack.

When the solenoid is deenergized, the clutch is held open by a spring and a brake is applied to the gearing so that the screw jack cannot move. At the same time as the motor starts running, the solenoid is energized releasing the spring loaded brake and clutching the motor to the reduction gearing.  When the circuit is again opened, the clutch releases, applying the brake to the gearing and allowing the motor to coast to a stop.

When the solenoid is energized, the clutch, which slides on the armature shaft, engages with the motor clutch driving member splined to the armature shaft. With the sliding clutch in this position, motor rotation is transmitted through a spur gear incorporated in the sliding clutch to an intermediate gear.

With the solenoid engaged, the rotation of the armature is transmitted through the sliding clutch to the intermediate gear, and the intermediate gear, in turn, meshes with a system of planetary reduction gears. The planetary pinions mesh with the screw shaft gear causing it to rotate.

Rotation of the screw shaft gear causes the screw shaft, which is threaded into the gear, to move up or down depending on the direction of the gear rotation. Rotation of the screw shaft is prevented by a stop guide plate which slides on a post forged in the sub-base plate.

Two levers, hinged to the sub-base plate, are actuated at the limit of screw shaft travel by the stop guide attached to the screw shaft. These levers, as shown in figure 35, operate the micro switches connected into the motor and solenoid circuit, and in this manner, limit the travel of the screw shaft. Rough adjustment of the screw shaft travel is provided by the adjusting screws located near the fulcrum of each lever. Fine adjustment is provided by the screws threaded into the end of each lever which press on the micro switch push pins.

The following procedure is recommended for disassembly of the electric head.

Remove all external safety wire from the cover, take out the five screws and the included washers, and then remove the cover.

Remove the resistor from the resistor clip. Remove the two screws which fasten the resistor block to the sub-base.

Unless replacement is required, it is not necessary to unsolder the leads. When removing the micro switch, note the position of the insulating fibre under each switch.

On electric heads which incorporate an external micro switch cam adjustment, the two cams should be removed at this point. An electric head, Type 3A, with these cams is shown in figure 35.

Remove the screw holding the stop guide plate to the screw shaft and then detach the stop guide plate.

Take out the screws which hold the sub-base to the housing and lift off the sub-base.

Remove the spring retainer located at the bottom of the screw shaft, and then the screw shaft and the screw shaft gear.

Lift out the sun gear, but do not disassemble the sun gear and its accompanying parts. If a part must be replaced, replace the sun gear as an assembly.

Loosen the brake posts evenly to avoid jamming, and remove posts and brake assembly together.

All shims under these posts should be retained and wired to their respective posts to prevent loss. See that no shims stick to the housing.

The brake assembly may be disassembled by loosening the two screws which serve as spring retainers.

Remove the clevis pivot post. As noted above, all shims which are under the post should be kept and wired to the post. See that no shims stick to the housing.

The sliding clutch member and the intermediate gear should now be removed simultaneously, and then remove the lock ring and two ball bearings from the intermediate gear.

Do not disassemble the sliding clutch. If replacement of any of the parts of the sliding clutch is required, the sliding clutch should be replaced as an assembly.

Unscrew the two knurled knobs, shown in figure 35, on either side of the motor head and lift out the brushes. The motor bearing cap is removed by unscrewing the four screws which fasten it to the motor head.

Remove the nut from the commutator end of the armature shaft. When loosening this nut, the sliding clutch member may be slipped over the drive end of the shaft and engaged with the motor clutch driving member in order to provide a lever on the shaft.

Remove the motor head and the brush box leads.

The spacer and shims on the end of the armature shaft should be carefully kept with the armature shaft. Next take out the brush box leads but be careful to note the way in which the leads go around the armature.

Take out the ball bearing from motor head, and then the armature shaft may be removed by tapping lightly on the drive end of the shaft. The motor clutch need not be removed from the armature shaft unless replacement of the shaft, ball bearing, or motor clutch is required.

To remove the solenoid, loosen the two screws which hold it to the housing, and unsolder the black solenoid leads at the terminal block and at the motor brush box. (On Model 3A, this lead was loosened when the jack plate was removed.) Lift out the solenoid and clevis together.

To facilitate disassembly of the clevis, solenoid and plunger, cut off the spun-over parts of the trunnion shaft which secure the plunger trunnion. Next remove the screw shaft oil seal. The mounting studs should be taken out only if replacement is necessary.

Do not remove the field coils unless replacement is required. Should replacement be required, re-move the studs holding the field coils and draw out the assembly.

After disassembling the unit, wash all parts thoroughly (except completely sealed ball bearings) in carbon tetrachloride or unleaded gasoline.

Then inspect all parts for excessive wear using the clearances listed in the latest Technical Order or Service Bulletin.

The armature should be dipped in unleaded gasoline and then scrubbed with a brush. If the ball bearing is of the completely sealed type, do not clean or lubricate it as sufficient lubrication for the life of the bearing was included by the manufacturer. Early models embodied a bearing with a seal on only one side, and these bearings should be completely washed and lubricated.

ACCUMULATOR TYPE GOVERNORS

In order to prevent the overspeeding of engines in fighter and dive bomber airplanes, it has been found necessary to provide an oil accumulator system in connection with the governor. The purpose of this installation is to provide an alternate supply of oil at sufficient pressure to change the pitch of the propeller whenever there is a loss of pressure in the governor for any reason. A schematic accumulator governor is shown in figure 36.

The operation of the governor in controlling the pitch of the propeller is similar to the Double Capacity Hydromatic Governor. In the Accumulator type, oil is allowed to flow from the pressure side of the pump to the accumulator tank. This oil compresses an air-filled neoprene rubber bag in the accumulator tank until the air pressure is equal to the governor pressure.

The oil is then held in the accumulator tank until such time as the oil pressure in the governor drops below that in the accumulator tank. When this occurs the air in the bag expands, pushing the oil into the pressure side of the governor pump. In this way a large volume of oil at governor pressure is available at all times for changing the pitch of the propeller.

To guard against the loss of oil in case the accumulator line should leak or break, an automatic shut-off valve is incorporated in the governor. If there is a serious loss of pressure in the accumulator line, the shut-off valve will close and prevent the governor oil from being lost. But, since the accumulated pressure in the accumulator always bleeds out when the engine is stopped for any length of time, a way is provided to build up the pressure in the accumulator tank. This is done by means of bleed holes in the shut-off valve, which allows from 2 to 5 quarts of oil

Figure 36.-Schematic accumulator governor.

per hour to flow through the valve into the accumulator tank. This small volume of oil is sufficient to increase the pressure in the line until it  is enough to open the shut-off valve. As soon as the valve opens, the governor oil will flow into the tank and equalize the tank pressure with the governor pressure.

There are two types of accumulator governors, the Single Capacity Accumulator, and the Double Capacity Accumulator.

THE ACCUMULATOR

*The accumulator governor used on the SBD—5 and 5A airplanes is being reworked (as of October 1943) to pro-vide a relief valve pressure of 350 psi. C. D. P., and an Accumulator bag Static Charge of 175 psi. The new governor model designation incorporating this change is 4G10—G41C.

The single capacity accumulator governor is used on the SBD-4, the double capacity on the 6F-3, and the F4U-1, mid the SBD-5.

THE ACCUMULATOR TANK

The accumulator or pressure storage tank can be likened to the storage battery in electric circuits. It is a chamber for storing power to be called on as an auxiliary to hasten or aid the action of the governor, or to act as an emergency source of power in case the pump is starved for a short period.

The accumulator tank consists of a steel shell or housing, a fully enclosed synthetic rubber bag molded to a high-pressure type air valve, and oil grate, and the necessary sealing and connecting parts.

In REMOVAL of the tank, release the air pressure. And a word of caution here! The pressure should be released gradually to prevent any sudden exhaust of air resulting in possible injury or damage.

Then break the oil line connection to the accumulator.

Loosen brackets and remove the accumulator. That's all there is to it.

In DISASSEMBLY, you first remove the core of the valve with a core tool.

Remove solder plug in the spanner wrench hole of the closure cap and remove the cap. Pull the plug (strainer) and sealing gasket from the mouth of the shell.

Unscrew the two nuts from the valve stem and push the valve stem into the tank after attaching a wire to the stem.

Push the other end of the wire through the tank and out the mouth of the shell, being careful not to puncture the bag. Pull the wire completely out of the tank until the valve stern appears in the mouth of the tank.

Grease the mouth of the tank thoroughly and then slowly pull the bag from the tank. Do not use the wire for this operation, but grasp the rubber in the hands and gently pull it out.

For INSPECTION of the tank, a 30-pound load should be applied to pull the stem from the bladder. This load must have no effect on the bond.

Inflate the bag to 200 percent its normal size and inspect the surface carefully for weak spots, flaws, or breaks. The bag should then be completely immersed in water to detect leakage. Then bag should remain inflated for 24 hours without loss in size or any permanent stretch.

The inside of the shell should be cleaned thoroughly and air blasted. Examine the inside surface for sharp edges, pits, and protrusions.

When inspecting the plug, see that there are no sharp corners on the grate-plate holes.

The sealing gasket must be clean and free from cuts and abrasions.

In ASSEMBLY, remove the valve core. Thoroughly wet the inside of the bladder with hydraulic fluid by inserting a small quantity of the fluid into the bag and washing it around the whole interior. The excess oil is then ejected.

Attach a wire to the valve stem and thread the opposite end of the wire through the accumulator tank and out the air end. Thoroughly grease the mouth of the shell, wrap the bladder longitudinally to expell the air, and pull the wire gently until the bladder is in the tank. Remove the wire and replace the washer. Replace the two nuts, locking the second down on the first. Replace the valve core.

Replace the synthetic rubber sealing gasket against the shoulder of the plug. Then insert the plug in the mouth of the housing and draw the nut down tightly on the assembly with a spanner wrench.

For BENCH TEST of the assembly, inflate the bladder to its operating pressure.

Check for leakage by immersing in water. If air bubbles come out of the oil end, it indicates a leaky bladder, or if they come out the air end, a new valve core is required.

If time is available, it is recommended that the accumulator be allowed to stand for one hour and that the pressure again be checked. No pressure drop whatsoever should be detected.

For INSTALLATION, place the accumulator (with the air pressure in it) loosely in its supporting brackets.

Disconnect the accumulator to governor oil line at the governor end and prime the line with engine oil to insure that no air bubbles are trapped in the line.

Connect the line at both ends in such a manner as to prevent air from getting into the system.

Care should be taken to avoid twisting or distorting the lines and fittings.

Tighten accumulator support brackets. (Check safety wiring, bonding, and oil line supporting clips for security and tightness.

Warm up engine thoroughly.

A DAILY INSPECTION should be made of all joints and connections in the oil line for signs of leakage. Also check the air pressure in the bladder.

Low pressure installations should have an initial charge of 175 psi air pressure in the bladder. Minimum pressure for good operation is 150 psi.

High pressure installations should have an initial charge of 300 psi air pressure in the bladder. Minimum pressure for good operation is 250 psi.

AUXILIARY PRESSURE SYSTEMS

Where does that extra-high-pressure oil used for feathering and unfeathering come from ? There are several types of auxiliary pressure systems that can be used for the purpose.

The usual system has an lNDIVIDUAL electricmotor-driven pump for each engine of the air-plane. Each pump supplies the high-pressure oil for the propeller attached to that engine.

Another type has a CENTRAL electric motor pump which sends the oil, through a selector valve, to any individual propeller the pilot wants to feather.

A third method uses the REGULAR HIGH PRESSURE OIL SYSTEM of the airplane powerplant, and sends a special low viscosity oil through individual valves, to the different propellers. You'll find out all you need to know about each of these systems by consulting the service literature for the particular system on which you may be working.

HOW IT WORKS

Regardless of which of these three oil-booster systems furnishes the governor oil pressure, the story of what happens after the oil starts on its journey to the propeller is the same. You're already familiar with the operation of the control unit. The blade and cam arrangement (which takes the place of the blade counterweight system on the constant-speed propeller) is quite different.

Figure 37.-Propeller control forces.

In the counterweight type propeller, you recall, the cylinder moved back and forth on the piston to change the blade pitch. In the hydromatic propeller, the cylinder is stationary and the piston does the moving. As it moves inside the cylinder, it pulls or pushes a cam roller which turns the inside cam. The outer cam remains stationary. As the cam turns, the beveled gears at the end of the cam operate in mesh with the gears at the base of the blade to change the pitch.

You can see from figure 37 that if oil from the ENGINE pressure lines enters the cylinder, it will force the piston BACKWARD (inboard) and move the cam in one direction. If the GOVERNOR pressure oil enters the cylinder, the piston will move FORWARD ( outboard).

Between these two pressures, the piston keeps the propeller balanced and the constant-speed system in an ONSPEED condition. The entire propeller-governor system is so sensitive that a deviation of even 2 or 3 rpm from the speed for which the governor is set is sufficient to send the control into action.

The engine oil pressure is used only to assist the blade twisting moment in moving the blades toward low pitch. The entire control of the propeller during constant-speed operation is accomplished by means of a single oil passage between the governor base and the propeller. Variations in pressure and volume of the oil flowing in this passage will control the propeller.

So far, all this has nothing to do with feathering. It's simply the application to the hydromatic propeller of the constant-speed principle you already know about. What happens when the pilot decides to feather his propeller is something else again.

In figures 38, 39, 40, 41, and 42 you'll notice the changing positions of the distributor valve in the dome assembly. When only the constant-speed mechanism is operating, this valve just acts as a passage for the flow of oil to and from the propeller cylinder.

Also, when the feathering control button is operated by the pilot, this valve continues to carry the high pressure to the inboard side of the piston. First, the auxiliary pressure oil system is instantly brought into circulation. It comes into the system through the external high-pressure oil line.

 Figure 38.-Underspeed

Figure 39.-On-Speed

Figure 40.-Overspeed.

Figure 41.- Feathering Condition

Figure 42.- Unfeathering Condition

KEY-FIGURES 38 TO 42

The oil in the distributor valve is then under a much higher pressure than it was for constant-speed operation.

During the constant-speed period, the only portion of the cam slot that's used is that part from the inboard end to where the slot takes a sudden drop. This portion of the slot—roughly, the in-board half—provides a blade angle range of 35°. Normally, this amounts to from 10° to 45° at the 42-inch radius.

When the higher oil pressure is admitted into the distributor valve, the pressure on the inboard section of the piston moves the piston outward into the cylinder. This moves the cam roller down into the outboard portion of the cam slot and rotates the cam accordingly. As the cam turns, it shifts the propeller blade to the feathering angle.

The displaced portion of the oil that was in the outboard end of the cylinder runs back into the engine oil system through the same passages as during constant-speed operation.

Having reached the full-feathered position, further movement of the mechanism is prevented by the positive high-pitch stops on the rotating cams. The propeller blades are now full-feathered, and remain there through balanced forces on the blades, until the pilot wishes to unfeather them.

UNFEATHERING

Unfeathering the propeller causes the distributor valve to reverse the flow of oil.

When the pilot closes the control switch for unfeathering, the auxiliary oil pressure is brought to bear on the outboard end of the piston. As the outboard portion of the cylinder fills with oil, the piston moves back (inboard) and the cam roller and cams and blade gears act exactly in reverse of feathering. When the propeller is unfeathered to the desired pitch, the pilot permits the control switch to open, and the system stabilizes itself. The oil that is displaced from the inboard section of the cylinder as the piston is pushed inward runs hack into the engine supply, and the constant-speed control again takes charge.

Now to go into a little more detail as to the whys and wherefores of this process. Assume that your airplane is equipped with an individual electric motor-driven pump for supplying the auxiliary oil pressure supply.

Figure 43.--Diagram of electric motor-driven pump.

The diagram in figure 43 shows the parts of such a system. The pilot depresses the propeller control switch C. This closes the electric circuit from ground G through battery B, switch C, and solenoid switch S to ground F. The circuit G–B–D–D–M–L is now closed and the motor M starts the pump. At the same time, holding coil H is energized through the circuit G–B–D–H–E-R. This keeps switch C closed without further attention from the pilot.

The pump, as soon as it starts operating, sends a supply of oil to the governor transfer valve through the external line 28. At a pressure of approximately 150 psi, the valve disconnects the governor from the system by closing the governor port. Simultaneously, the valve connects the pump with the inboard end of the propeller cylinder by means of the identical passages which formerly were used to conduct governor oil to the propeller for constant-speed operation. The piston moves toward the outboard end of the cylinder and the blades are feathered at a speed proportional to the rate at which oil is supplied to the cylinder.

The pressure in the system during the feathering stroke differs according to the mechanical friction and the blade twisting moment. When the blades reach the full-feathered position, the pressure in the inboard cylinder end, and in the passages connecting it with the pump, now increases rapidly. When the pressure reaches the opening pressure of the pressure cutout switch E, it opens. This de-energizes the holding coil H and allows the control switch C to return to the OFF position. This, in turn, de-energizes the coil K, breaking the motor circuit and stopping the pump. The pressure in both ends of the cylinder is equalized, and the propeller remains in the feathered position because of the balanced forces on the blades.

When the pilot desires to unfeather the propeller, he holds the control switch C in the on position. This starts the pump and increases the pressure until the distributor pilot valve moves.

With the inboard end of the cylinder connected with the engine lubricating system, and the high pressure on the outboard end of the piston, the latter moves inward, unfeathering the blades and forcing the oil on its inboard end into the enginesystem. As the blades are unfeathered, they begin to windmill.

When the engine has reached a reasonable rpm (depending on the blade design, engine-propeller gear ratio, and air speed) the control switch C is released. This discontinues the high-pressure oil from the pump and allows the propeller distributor pilot valve and the governor cut-off valve to return to their normal positions. The governor is again connected with the inboard end of the cylinder and constant-speed operation is automatically resumed at the rpm for which the governor is set.

The dome pressure relief valve prevents excessive pressures in the outboard cylinder end, should the propeller be unfeathered, until the mechanism reaches the positive low-pitch stops. In this ease the pressure tends to reach the maximum value capable of being supplied by the pump. The dome relief valve is set to limit this pressure to 300 psi, which is adequate to unfeather the propeller under any set of conditions.

This valve, like the distributor valve and the governor relief valve, is balanced so that a maximum pressure difference of 300 psi is available in the outboard cylinder end for unfeathering, regardless of the lack of pressure which may be encountered by the piston in displacing the oil on its inboard side into the engine lubricating system. This is accomplished by allowing the back pressure to assist the relief-valve spring.

This relief valve normally would not be called on to function in flight because, in unfeathering the propeller, excessive wind-milling speeds normally would exist before the blades reached the low-pitch stops and the pilot would have to shut off the auxiliary pump.

The distributor valve remains connected with the outboard cylinder to act also on the outboard end of the distributor valve, aiding the spring, and increasing by an equal amount the pressure required at the inboard end of the valve to close the port. In that way, a maximum pressure difference equal to the distributor valve spring tension will be available for moving the piston in the feathering direction, regardless of the back pressure it may encounter in forcing the oil from the outboard end of the cylinder into the engine lubrication system.

You close the propeller feathering switch and hold it closed until the propeller is rotating at the desired rpm. In this case the pump motor circuit is completed in exactly the same manner as during the feathering operations.

As this pressure increases to over 400 psi, the pressure cut-out switch opens and de-energizes holding coil H. This, however, does not break the pump circuit, because the pilot still is holding the control switch C closed. Also, as the pressure in the distributor valve passages increases, the distributor valve moves outward against the spring under the action of the pressure on its inboard end. When this pressure increases through 400 psi, the land on the distributor valve passes the port, shutting off the connection between the pump and the inboard cylinder end.

At the same time, this end of the cylinder is connected with the engine oil system by spring tension of distributor valve spring. Then another port admits the high-pressure oil from the pump to the outboard end of the cylinder.

Another important feature of hydromatic propellers, on installations using the individual pump system, is the DIFFERENTIAL PRESSURE CUTOUT SWITCH. Have a look at figure 44.

The cutout switch is designed to mount on the governor and, at a specified differential in oil pressure, to break the flow of electrical current to the cutout switch. This in turn de-energizes the solenoid holding coil in the cockpit and stops the high pressure pump when the blades reach the feathered position.

Figure 44.-Differential cutout switch.

When the oil pressure the propeller builds up to the cutout valve after the feathered position has been reached, this switch stops the electrically-driven, independent pump motor supplying the auxiliary pressure. The switch itself is of the single-wire, pressure-operated. single-pole, single-throw type. Normally, it is in the closed position. When connected into the propeller control system, this switch opens the feathering-button holding-coil circuit when the auxiliary oil pressure reaches the cutout valve. This pressure is supplied by the feathering pump.

Auxiliary oil pressure is applied to the center port, and is greater than the engine oil pressure that is applied to the ring port. When this difference in pressure overcomes the spring tension, the switch opens. As soon as the pressure difference drops below the spring tension, the switch closes, and stays closed until the auxiliary oil pressure builds up again.

What goes on inside the switch? The auxiliary high-pressure oil supplied to the center port, moves the plunger assembly away from the fixed contact point, working against and overcoming the normal engine oil pressure that comes through the ring port, and against the force of the switch spring. The movement of the plunger breaks the cutout switch circuit. This de-energizes the feathering-button holding coil, releases the button, and shuts off the auxiliary oil pump.

Wondering about keeping the cutout switch in the pink of condition? Well, between overhaul periods the feathering test will usually suffice.

If a failure occurs in an engine, especially a bearing failure, the cutout switch should be disassembled, and blown clean with air. This will prevent small chips from clogging up the moving parts or grounding the insulated parts.

Better check the gaskets for oil leaks, and if necessary, tighten the screws or replace the gaskets.

If the spring doesn't measure up to specifications, replace it.

You can add shims if the switch opens at any pressure before the blades are full feathered.

REMOVAL

(....)

DISASSEMBLY OF HUB

(....)

DOME ASSEMBLY

(....)

DISTRIBUTOR VALVE DISASSEMBLY

(....)

GOVERNOR DISASSEMBLY

(....)

INSPECTION AND REPAIR

(....)

HUB ASSEMBLY

(....)

BLADE AND GEAR SEGMEnT ASSEMBLY

(....)

ASSEMBLY OF HUB AND BLADES

(....)

DOME ASSEMBLY

(....)

VALVE ASSEMBLY

(....)

DE-ICING EQUIPMENT

(....)

BALANCING

(....)

INSTALLATION

(....)