Aircraft technical Basics: Aircraft Propellers - Navy Training Courses Edition of 1945: Chapter 5: The Electric Propeller

FACTS AND FIGURES

The electric propeller depends upon electricity for changing its pitch. It can operate either as an automatic-controllable OR as a fixed-pitch propeller. Steel-bladed propellers can be equipped with a type of cuff that gives additional airblast for cooling radial air-cooled engines. Dural-bladed propellers, being of different design, do not require cuffs.

Furthermore, some electric propellers have a reversible pitch, for greater maneuverability. On flying boats equipped with electric propellers, the direction can be reversed on the water for getting in or out of tight spaces.

One of the most prominent features on an electric propeller like the one in figure 48 is the motor and speed reducer at the front-and-center of the hub. Inside that housing is the heart of the electric pitch-changing mechanism.

The POWER UNIT contains the adapter plate, speed reducer, motor, and brake assembly. At the inboard side of the propeller you sometimes find a part known as the SLINGER ASSEMBLY. This is part of the de-icing unit, and is not found on all electric propellers. The pitch-changing mechanism is also known as the PROPORTIONAL GOVERNOR CONTROL SYSTEM.

The purpose of the governor is to maintain the engine at a set, constant speed by changing the propeller-blade angle. This control system consists of two major assemblies designated respectively as the UPPER CASE and the LOWER CASE. The upper case incorporates a RACK, PINION, and LEVER which function together to control the force of the flyweight spring.. The lower case contains an OIL PUMP, a RELIEF VALVE, a PRESSURE SWITCH, a FLYWEIGHT SPINDLE ASSEMBLY, and a CONTACT MECHANISM.

Figure 48.-Electric propeller assembly.

Where a faster rate of pitch change for feathering or reversing is desired, a VOLTAGE BOOSTER may he added to the train of controls.

The HUB used on an electric propeller is machined from a single solid forging of alloy steel. On the rear of the hub, you'll find four bronze slip rings, insulated from the hub and each other.

Figure 49.-Blade and gear assemblies (aluminum).

Four insulated brass connector rods carry the electrical circuits through passages from the slip rings to contacts at the front face of the hub. The blade assemblies are inserted into the hub barrels and are held in place by retaining nuts.

Figure 49 shows the BLADE ASSEMBLY for the aluminum propeller, and figure 50 shows the blade assembly for the steel propeller. Steel blades are formed by welding together two formed sheets of electric-furnace-processed alloy steel. The shank and back of the blade are formed from one sheet, and the face from the other. The atomic-hydrogen welding process, which gives a high-strength and uniform weld, is used to join the two plates along the leading and trailing edges in a seam that converges at the shank and extends to the tip end of the blade.

The shank end is internally threaded to receive a spiral bevel gear, which is pinned in place. A stack of angular-contact-type, anti-friction bearings is placed on the blade shank together with a retaining nut. The bearing stack permits free rotation of the blade in the hub under high centrifugal loads.

Figure 50.-Blade and cuff assemblies (steel).

Blade shank cuffs, which assist in the distribution of air to radial-engine cylinders, may also be placed on the blade. The cuff consists of a cast magnesium support and a stiffener to which a formed sheet (cover assembly) is attached by screws. The entire cuff assembly is held in position by a shoulder on the blade.

Aluminum blades are made by casting and forging to shape. The main difference over steel is the blade retention in the hub. Naval aircraft use both aluminum and steel blades.

Figure 51 shows the POWER GEAR ASSEMBLY. The power gear is a bevel gear which meshes with the blade gears. It is internally splined to engage with the low-speed splined drive of the speed reducer. This power gear is equipped with an angular-contact type thrust bearing, which absorbs the power-gear thrust and is mounted in a steel adapter plate. The adapter plate also serves as a mounting for the power-unit cover or propeller spinner, if either part is used.

Figure 51.-Power gear assembly.

The SPEED REDUCER assembly is seen in figure 52.

It consists of two stages of planetary-type reduction gearing, contained within an aluminum alloy housing. The rotating parts are fitted with ball hearings to provide maximum efficiency and also to facilitate assembly. The gear teeth are surface hardened to insure long service life. Gaskets between the front and rear housings, and seals at each end, make the unit oil-tight and eliminate the necessity for frequent lubrication of the enclosed parts. The unit is partially filled with an oil having an extremely low pour-point, thus providing a continuous oil bath for the speed reducer gears.

The low pour-point insures unrestricted operation at the low operating temperatures encountered at high altitudes.

At the hub of the speed reducer are located blade angle LIMIT SWITCHES which are operated by pivot arms riding on a cam attached to the low speed bell gear of the speed reducer. The limit switches, shown in figure 53, are connected in the electric motor leads.

Figure 53.-Limit switch assembly.

They are spring-loaded electrical contacts which, upon installation of the power unit, mate with the fixed contacts on the front face of the hub. As the switch arms ride on their respective cam lobes, the contacts are retracted and the circuits are opened. By their location, the cam lobes accurately control the low, high, and feather blade-angle settings. Reverse thrust propellers have an ADDITIONAL limit switch replacing the common return contact, to halt the pitch change at the NEGATIVE angle setting.

A magnetic brake assembly, as in figure 54, is mounted on the front of the pitch-change motor. It consists of a brake hub, keyed to the armature shaft, a brake lining splined to the hub, and a steel brake plate supported by a diaphragm and held against the brake disk by coil springs. A solenoid coil, connected in series with the electric motor, is located behind the steel plate. When the motor is operated, the solenoid is energized, thereby releasing the brake. When the motor is not operating, the solenoid is not energized, and the brake is applied by the spring forces.

Figure 54.-Brake assembly.

The pitch change motor assembly, seen in figure 55, is attached to the front housing of the speed reducer, and the armature is fitted to the driving pinion of the high-speed stage of the speed-reducer unit. The motor is of the series type, and has two field windings which provide for rotation in either direction.

An aluminum alloy housing bolted to the nose section of the engine incorporates a mounting for the slip ring brush holder assembly, also called the brush cap. This assembly is shown in figure 56.

Quick removal of the brush holder from the housing is made possible by trunk type latches.

Figure 56.—Brush holder and housing.

An electrical connector permits complete removal of the assembly from the airplane. An early-type holder is mounted to the cap with vibration absorbing bushings. A later-type holder, of molded composition, may be attached directly to the cap without shock mountings.

HOW IT WORKS

Suppose your airplane is equipped with an electric propeller, and you have set your rpm control for 2,000. When your engine is started and gets warmed up, assume that it's actually turning at 1,800 rpm. The governor will now go to work. Current from the airplane's electrical system is fed into the SLIP RINGS at the inboard end of the hub by the BRUSHES. The slip rings seen at the right in figure 48, are made of bronze and are carefully insulated from the hub and from each other. As the propeller rotates, the rings rub against the brushes and pick up the current from them.

The current is carried inside the hub through connector leads from the slip rings. It flows through leads in the speed-reducer assembly and on into the motor—the simple armature motor you saw in figure 55. This motor, you'll recall, has two field windings so that it will run in either direction, and is of the series (not parallel) type. The inboard end of the armature is fitted to the driving pinion of the high-speed stage of the SPEED REDUCER unit.

The speed reducer unit stands between the motor and the outboard end of the hub. A torque has been set up in the speed reducer by the current turning the armature shaft. There are two FIXED RING GEARS—low and high speed—and two planetary gears, also high and low speed, in the speed reducer. The high-speed gears take the torque from the motor and pass it along to the low-speed gears. These gears, in turn, pass the torque—now reduced in speed—along to an outer bell gear.

The speed reducer has a two-piece housing and oil-sealing gaskets front and rear. Its gears work in a continuous oil bath, and, since the assembly is oiltight, they do not need additional lubrication very often. Oil with an extremely low-pour point it is used in it to assure dependable lubrication at high altitudes and low temperatures.

The POWER GEAR, which can be seen by referring to figure 51, lies between the speed reducer and the blade-shank: gears. The job of the power gear is to drive the bevel gears located on the blade shanks.

Movement of the blade-shank gears, as you have seen, is caused by the torque originally set up by the motor. The final result is that these gears turn the blades to the proper pitch for bringing the propeller onspeed—that is, to 2,000 rpm as you indicated in setting the control in the first place.

The brake assembly has also been functioning It has two jobs—stopping the rotation of the motor when the current is cut off, and acting as lock to hold the blades in fixed position when no angle change is being called for. The brake disk is keyed to the armature shaft, and a steel brake plate is supported by the diaphragm and held against the brake disk by coil springs. The coil of the solenoid, connected in series with the motor, is located between the steel plate and the motor.

When the motor is operating, the solenoid is energized and acts against the spring forces. This releases the brake so that the motor is free to turn over. When the motor isn't in action, neither is the solenoid—so the spring forces hold the brake "on."

The entire power unit of the electric pitch-changing mechanism is interchangeable, and can he removed or replaced as one piece. The electrical contacts inside the power unit are spring loaded, so they automatically complete the electrical connections between the motor and the hub as soon as the power unit is attached to the hub.

There are, of course, controls that "decide" when it's time for the current to flow through the brushes and slip rings and activate the motor. They are the real brains of the pitch-changing equipment. The following are its important parts

The GOVERNOR, a schematic drawing of which you see in figure 57, is usually mounted on the nose section of the engine. In principle it is very similar to the constant-speed governor with which you are already well acquainted. As a matter of fact, oil is also used in the electric governor, but for a different purpose.

The FLYWEIGHTS, driven by the engine, serve somewhat the same purpose as in hydraulic constant-speed propellers. The governor is, basically, a single-pole, double-throw switch that is opened or closed by the flyweights.

More accurately, the flyweights don't do the actual opening and closing of the switch, but rather they actuate an OIL SERVO MECHANISM, which

Figure 57.-Schematic drawing of proportional governor.

does the job for them. This mechanism moves the center contact of the switch either up or down. If it goes UP, it moves against a contact which closes the circuit to the motor, and makes the motor operate in ONE direction. If the center switch moves DOWN, it touches a contact that closes the circuit to the motor for operation in the OTHER direction. Thus, depending on which way the center contact moves, the blade angles are in-creased or decreased.

The pressure to operate this oil mechanism comes from the engine oil system, and is regulated by a valve which is linked to the flyweights. The standard governor has am integral OIL PUMP and RELIEF VALVE which keep a constant pressure within the governor, regardless of any variations in engine oil pressure. A helical spring counter-balances the flyweight forces so that, at governing speed, the flyweight valve will supply only enough oil pressure to balance the piston of the servo unit against its spring. This holds the center switch contact in the neutral or "off" position.

With this arrangement, as soon as the flyweights move faster or slower, in accordance with engine revs, the servo piston is thrown out of balance. When the engine speed increases, the flyweights move outward because of the additional centrifugal force. This force causes the valve to increase the oil pressure to the servo. As soon as the pressure is increased, the center switch contact moves up and connects with the switch contact above it.

When the engine speed decreases, the flyweights move inward. The valve is thereby caused to de-crease the oil pressure to the servo, and the center switch contact moves down to connect with the switch contact below it.

The next unit in the control group is the RELAY. You see the two types in figure 58. The relay is a heavy-duty switching mechanism that operates on low amperage current controlled by the governor contacts. One relay is required for each propeller and is in use during the time the propeller is operating in automatic control.

When the governor points make contact, a lowamperage current flows to either one of the relay solenoid coils. This energized coil acts on the armature of the relay, which in turn causes the heavy relay points to make or break the heavy amperage propeller circuit.

Usually the relay will be mounted in the engine compartment in an accessible location, preferably the firewall. If it is located on the engine mount, it is generally provided with a support to minimize vibration. Under usual conditions the relay will be mounted in a ,junction box, provided for the connection of all propeller wiring in the vicinity of the engine compartment.

There are two types of` relays in Navy use, the clapper type and the plunger type. The clapper relay assembly consists of fixed and movable contact assemblies, an armature, an equalizer arm, a condenser assembly, a solenoid coil assembly, and a resistor. A base, with the necessary springs, wiring, connectors, and pins, completes the unit.

The armature consists of a T-shaped iron casting. Two transverse arms (located by four pins) are held against the head of the assembly by individual preloading springs, which bear against the arms through four self-aligning bushings. The equalizer arm centers the armature between the two solenoid coils. The upper arm carries the two movable contact points which mate with the fixed contacts, mounted on L-shaped supports, to complete the circuit from the electrical power supply to the electric motor,

The armature assembly and transverse arms are mounted on a floating pin which in turn is linked, top and bottom, to a fixed pin on the coil assembly support. As the resistor across the coil in the relay circuit cannot function satisfactorily unless the proper coil and armature alinement is obtained, the flexibility afforded by this type of linkage is highly important,

A RESISTOR is connected across the coil of the relay to damp out small fluctuating current impulses from the governor. This damping action increases the life of the points by preventing chattering and enabling the points to make and break cleanly.

CONDENSERS are connected between the propeller wiring and ground, for the purpose of smoothing out sharp electrical pulsations which in some cases produce radio interference. These pulsations are voltage surges, arising from the operation of such electrical equipment as the propeller motor, governor, and relay contact assemblies. Each condenser is equipped with an integral fuse for circuit protection, should a condenser develop a short circuit.

The points used in these relays are made of Elkonite, a very hard material, which takes on a charred gray appearance after continued service, and develops very fine pit marks. These are normal characteristics and do not affect the operation of the relay as long as satisfactory contact pressure is maintained. The Elkonite points should not be dressed or polished, as this only decreases the life of the contact and does not improve operation. The contact point material is 1/8  inch thick when new. If this material becomes less than 3/32 inch thick at any point due to wear or burning, the contact should be replaced. The surface of the new movable contact is slightly ground, to facilitate the seating of the two contacts when first put into operation.

So far, you have been considering the clapper-type relay. A newer development is the plunger-type, with which you also will want to be familiar. The purpose of the new PLUNGER-TYPE RELAY is to provide a means of making and breaking the heavier electrical currents required to operate the new larger-model propellers. It is also of considerably more durable construction and is better able to withstand vibration than the clapper design. Another distinct advantage is that its service and maintenance are more simple than for the standard type.

The plunger-type relay is designed to be installed in the same size relay box now used for the clapper relays. The box is equipped with four hinged clips which are permanently fixed to it and which snap over the top to hold the cover firmly in place. The various parts of this relay are mounted on a molded base of synthetic material of good heat-resistant and dielectric electrical insulating qualities. The assembly is arranged in such a manner that all movable parts are located above the panel, while all wiring is below the panel. This eliminates the possibility of the wires interfering with moving parts. The electrical terminals are located along one edge of the panel, instead of being staggered as on older units.

All wiring between the panel and the box is combined in a harness arrangement, with sufficient allowance to permit the complete panel assembly to be partially lifted out of the box for examination of work.

The actual movable parts of the relay are two iron plungers, each carrying two contacts. Each of the plungers is actuated by a solenoid coil, mounted directly under the panel below the contacts. The movable contacts are mounted on two special alloy disks, which provide a large area for heat dissipation, and in turn allow for the passage of larger currents without the likelihood of electrical failure. The stationary contacts are mounted on the relay base.

Each solenoid coil has a shunt resistance wound inside the housing to dampen small fluctuating currents. This design eliminates a separate part. in the form of an individual resistor, which is more subject to breakage when handling.

A condenser of special construction composed of three sections is employed for reducing radio interference. This condenser is connected to the three wires from the switch panel in such a way that radio interference is reduced to a minimum on these wires.

Figure 59.-Switch installation.

The condenser is designed to give improved filtering action at higher radio frequencies, as compared to the old type.

The next unit in the control group is the pilot's control panel. A single engine installation will have a circuit breaker of either toggle or push-button type, and a selector switch which has four positions.

These positions are AUTOMATIC, MANUAL INCREASE, MANUAL DECREASE, and OFF. Figure 59 shows a twin engine switch installation. You will notice that it has circuit breakers and selector switches, one set for each engine. But, in addition, feather switches are there, too, since on a multi-engine airplane a propeller can he feathered to prevent windmilling.

On airplanes incorporating reverse pitch control, there will he a reverse-return switch for each propeller and one reverse safety switch.

Figure 60.-Voltage booster.

On airplanes incorporating synchronizer control, there will be added a master motor switch and a master motor control knob.

You'll recall that sometimes a voltage booster is used to give a faster rate of pitch change for feathering or reversing. This voltage booster, shown in figure 60 is a single DYNAMOTOR unit, placed in the propeller circuit to increase the normal voltage. A dynamotor functions both as a DYNAMO to generate current and as a MOTOR to operate the mechanism. By increasing the normal voltage, a rapid rate of pitch change for feathering and reversing operations can be secured.

A solenoid switch on the booster works in conjunction with the feather switch—or the reverse switch in the cockpit, to control the operating current. When the feather or reverse circuits are closed, the solenoid switch which is connected in series with these circuits puts the booster into operation automatically. Upon reaching the feather or reverse pitch setting, the limit switches in the circuits are opened.

The use of the reversible type of propeller has proved to be of great value as an aid in the water handling of large multi-engine flying boats. On a four-engine boat, for instance, the pitch of the two inboard propellers may he reversed shortly after landing on the water. This makes a substantial negative thrust available. By operation of the throttles this negative thrust can be applied in combination with the positive thrust of the out-board propellers as required, making it possible to stop or turn in a very short space—even in the presence of adverse current and winds. Reversible propellers used in this way not only make it possible to operate with safety in restricted areas but also reduce the time required to approach buoys and moorings.

To the normally used low, high. and feather angle limit-switches, a fourth is added for reverse pitch operation—to stop the pitch change when the desired angle is reached. Normal propeller operation between high and low pitch utilizes the plane's battery voltage directly.

To speed up the normal rate or pitch change for emergency feathering, an auxiliary voltage booster has been developed which provides approximately three to four times battery voltage. - This increases the rate of pitch change three to fourfold, and has become standard equipment on many multi-engine aircraft using electric propellers. The presence of this unit also makes it possible to provide an accelerated pitch change during reversal and return to low pitch. The action during reversal is much the same during fast feathering.

Switches, operated by the pilot, act to start the booster and pitch-change motor simultaneously. When the predetermined negative angle is reached, a limit switch in the propeller automatically opens, stopping both the pitch change motor and the booster. When you want to return to normal flight pitch, operation of another switch starts the voltage booster and causes the pitch-change motor to run in the opposite direction. A system of relays is provided which brings operation to a stop when the low pitch limit switch closes. It is then merely necessary to return the switches to the normal operating position for conventional constant speed or selective fixed-pitch propeller operations. An auxiliary switch that must be operated to reverse the propellers has been incorporated in order to prevent reversing of the propeller accidentally.

At the present time, experiments are being conducted on a DUAL-ROTATION propeller. This is really two propellers, one mounted ahead of the other. They rotate in OPPOSITE DIRECTIONS. If successful, this type of unit will have several advantages. With the horsepower of aircraft engines steadily increasing, designing propellers to absorb their power has become a complex problem. The blades have been made longer, but the limit has been reached in length because a certain tip clearance must be maintained. They have been made wider, but it has been discovered that beyond a certain limit the efficiency of the propeller is considerably reduced and the stress on the hub is greatly increased. More blades have been added—some engines now using four-bladed propellers—but it has been found that these added blades cause interference, and reduce the efficiency of the propeller. For this reason it would not be practical to have a propeller with more than four blades turning in one direction.

One method of avoiding these difficulties is to mount two propellers, one behind the other, rotating in opposite directions driven by two telescoped propeller shafts. This type is the dual rotation or co-axial propeller.

The dual-rotation propeller has the advantage of not having torque reaction. Thus, when the throttle is opened or closed suddenly, the airplane does not have a tendency to roll. Such propellers require relatively heavy and complicated gearing in the engine, but for heavy, high-powered engines, the present one is a solution to the problem of designing propellers that will absorb the power.

Before you can consider yourself a propeller expert, you'll have to know something about this new 4-way electric propeller.

It has four hollow steel blades and a propeller control system incorporating the proportional type of governor. Look at figure 61 for an illustration of the 4-way propeller assembly.

One of the most important things to keep in mind about these 4-way propellers are their MAGNETIC BRAKE ASSEMBLIES.

The brake assembly, mounted on the front of the electric motor, consists of an outer brake assembly and an inner brake assembly.

In the outer brake assembly you'll find a cage, a forward brake plate fastened in the cage, an internally splined brake facing, an externally splined hub keyed to the motor shaft, a rear brake plate and its supporting diaphragm, four coil springs and a solenoid.

Figure 61.-4-way propeller assembly.

The inner brake assembly consists of an externally splined brake facing an internally-splined brake plate attached to the rear outer brake plate, a large coil spring, and a solenoid.

Glance at figure 62 to see just  what the brake assembly looks like.

Figure 62.-Brake assembly.

Next, you'll have to know about the DYNAMOTOR (fast feathering voltage booster) used with 4-way propellers. It is a single dynamotor unit placed in the propeller circuit to increase the normal voltage supply for rapid feathering operation. Only one voltage booster dynamotor unit is required for every two propellers.

The voltage booster dynamotor consists of an armature supported by ball bearings located at each end of the input and output heads. The heads which contain the brush rigging are doweled to the yoke and held in place by four through bolts.

The input head also contains a solenoid switch, terminal posts, and two conduit outlets. Brushes and commutators are accessible through a removable inspection strap clamped to each head. The field coils are shunt-connected to insure a constant voltage output When the unit is in operation, and are assembled in the yoke by means of four pole shoes.

Four-way propellers are shipped with propeller shaft nut, front cone, hub puller snap ring, grease seal, spreader, and power unit removed. To save space, the propeller is usually shipped with three blades removed from the hub, and must be reassembled.

REMEMBER, when you're installing this type of propeller, that if the slip ring housing is of the bonding type, you must remove the bonding brushes.

Here's how you'll measure the location of the slip ring brush contact on the slip rings

Install the brush assembly in the slip ring housing and measure the distance between the face of the propeller shaft thrust nut and the center of the forward brush.

Measure the distance between the center of the front slip ring and the base of the rear hub cone, while the cone is held firmly in the hub.

If the difference between the two measurements exceeds 0.40 inch, you should use a shim.

More slip ring brushes are damaged by forgetting to remove the brush assemblies from the slip ring housing before installing (or removing) propellers, than from any other cause. Always keep brush assemblies out of the slip ring housing of ummounted propellers except when measuring for brush location as described.

Install the hub rear cone on the propeller shaft. Be sure the cone is clean and dry!

Remember this when you're installing the POWER UNIT. Lubricate the hub with grease—Specification AN-G-4, Grade AA.

You can do that by using a pressure gun on each of the three grease lubricators until the grease flows from the relief fitting. Be sure to place the relief fitting uppermost. If there is no relief fitting, remove the top grease fitting.

Check the oil level. With the airplane leveled, rotate the propeller until the filler plug is approximately 12 degrees below horizontal. Remove the plug. I f there is no oil at the plug opening, add speed reducer oil AAF Specification :1600, until the oil is at this point. Then, install the plug.

'The blades of the four-way- propeller are held at the selected angle by the brake assembly.

The outer brake is released by the solenoid connected in series with both the increase and de-cease rpm windings of the motor and is applied by spring forces acting against the rear outer brake plates when the motor is not operating. The inner brake is released by a solenoid connected in series with the decrease rpm winding of the motor and is applied by spring forces acting against the inner brake plate. Therefore, the inner brake drags during increase rpm operation. This helps to counteract the centrifugal twist which tends to move the blades to a low pitch position and makes the rate of pitch change equal for increased and decreased rpm.

The cut-out (limit) switches at the hub end of the speed reducer limit the high and low blade angles to the flight range, and also stop the blade angle change at the feather position. The high and low angle cut-out switches are effective while operating in automatic constant speed. The feather and low angle cut-out switches are effective in selective fixed pitch.

As you know by now, the term "feathering" designates the operation of rotating the propeller blades beyond the highest angle required in normal flying to an edge-to-the-wind position.

Full feathering is used when mechanical trouble develops in one of the engines, thus preventing the propeller blades from "windmilling" in flight and causing further damage to the engine.

Fast feathering under boosted voltage is accomplished by means of the feather switch in the cockpit. The feather switch breaks the normal propeller circuit and at the same time completes the feather circuit, causing the propeller blade angle to increase to the feather position.

Feathering also can be accomplished at the normal rate of pitch change by holding the selector switch in the DECREASE RPM position until the blades reach the feather position.

You're going to have to know the way to check inner brake clearance. As far as adjustment goes, you first remove the nut and duplex brake locking bolt.

Next, remove the duplex brake cages, using a wrench and a light hammer. Remove the splined disk brake facing, then the cotter-key and nut from the brake shaft. Use a holder to prevent the hub from turning.

Use a puller and remove the brake hub from the shaft. Then, remove the duplex brake cover facing assembly and the brake shaft key.

Add or remove shims as necessary, between the spacers around the shaft.

Install the inner facing assembly, brake hub and shaft key. Then, install the washer and shaft nut and tighten the nut.

Check the inner brake clearance. If the clearance is as specified, cotter-key the shaft nut.

Install the outer brake facing and brake cage. Use a wrench and light hammer to tighten the brake cage.

Be sure you tighten the cage until the fine segment shoulders on the cage are in contact with the shoulder on the outer coil housing. Use a 0.001-inch feeler gage to make certain that the shoulders are in contact.

Further. tighten the cage a minimum of 9 degrees or approximately 7/16 inch as measured at the circumference of the threads.

Check for insertion of locking bolt. If necessary, tighten until a cage locking bolt hole lines up.

Under no circumstance should the cage he backed off to line up the locking bolt holes or to adjust the bake gap.

Lock the cage with bolt and nut.

Check the outer brake clearance by inserting a feeler gage between the rear outer brake plate and the inner ring of the outer solenoid housing.

The outer brake clearance should be 0.010 to 0.020 inch.

Now, if you are going to adjust the outer brake clearance-

Remove the brake cage and the four nuts and screws from the brake cage. Add or remove shims, as required, between the brake cage and the forward brake plate.

Reinstall, tighten and lock the cage. Check the motor. Inspect the general condition of the motor terminals and leads and tightness of brush rigging.

If you find worn or damaged parts, repair the motor in an overhaul shop.

THE ELECTRIC-AUTOMATIC SYNCHRONIZER

The automatic synchronizer is a propeller control system used to synchronize the engines of a multi-engined airplane at any desired speed. This control system consists of several major parts. The MASTER UNIT consists of a manually adjustable, constant speed d-c motor operated from the electrical system of the airplane. Mounted on, and partially driven by the MASTER MOTOR are individual CONTRACTOR UNITS, one of which is required for each propeller. Each engine is equipped with a small three-phase ALTERNATOR mounted on the governor drive pad. A master unit tachometer, and necessary control switches complete the synchronizer control system. The conventional governor is not used with this system.

The value of the automatic synchronizer lies in the elimination of "beat", or harmonic vibration, which is the source of annoyance during flight, if not a dangerous hazard due to failure of parts caused by metal fatigue. It has been found that even when engines are only slightly off speed, for instance, 20 rpm between engines, a distinct beat will occur every three seconds. It is difficult to read an engine tachometer to within 20 rpm during flight and therefore adjusting engine speeds manually until beat has disappeared is pretty much guesswork. This is particularly true in a four-engine airplane and synchronization has often been a long and tedious process. Moreover, even after engines are once synchronized, rough air or variations in operating

Figure 63.-Electric automatic propeller synchronizer.

conditions may throw them out of phase and the process of synchronization must be repeated all over again.

With the electric synchronizer, these difficulties are eliminated. All that need be done on the part of the pilot is to flip the master motor switch ON, throw the selector switches of each engine into "Automatic," and select the rpm desired by turning a single control knob until the required speed is registered on a master tachometer. Immediately, blade angles of the individual propellers will change to bring the engines to the selected rpm, and keep them there regardless of attitude of the airplane or throttle settings, within the range of the propellers. If a change in rpm is desired, the control knob is turned until the new desired rpm is read on the master tachometer. Readjustment of throttles is then necessary to maintain required manifold pressures but not rpm, since that will be controlled by the synchronizer.

If so desired, one or any number of engines may be cut off synchronization by merely throwing the selector switch for those particular engines off. If it is necessary to feather a propeller the pilot need only throw the proper switch starting the feathering operation. Automatically, that engine will be cut from the synchronizer.

The principle of operation of the electric synchronizer depends on the balancing of the speed of each engine against that of an adjustable constant speed unit, THE MASTER -MOTOR. The essential mechanism in this balancing is the CONTACTOR.

Three phase alternating current, supplied by the engine-driven ALTERNATOR is impressed across a three phase armature in the CONTACTOR. This current creates a magnetic field which rotates around the armature at the same speed as that of the engine driven ALTERNATOR. However, the units must be wired so that the rotating magnetic field will always turn in a counterclockwise direction regardless of the direction of rotation of the ALTERNATOR. Therefore, right- and left-hand governor drives will necessarily call for different electrical connections.

This armature, which is really the stator of a small hysteresis motor, located in the contactor, is geared to and rotated by the MASTER MOTOR in a clockwise direction at the speed selected by the pilot.

Since these two rotations are opposite in direction to each other, they tend to cancel each other out so far as actual speed of rotation is concerned. It is as though you substituted for the stator armature a merry-go-round constantly rotated in a clockwise direction at a selected speed; and for the magnetic field, substituted a man walking around the outer edge of the merry-go-round in the opposite direction. With that picture in mind, imagine the man and the merry-go-round traveling at the same rate of speed. In relation to the ground the man will be stationary. However, if the man either slows down or speeds up, he will either be losing ground or gaining it, despite the fact that he is constantly walking in a counter-clockwise direction.

Now, examine the condition set up in the stator. The magnetic field set up by the ALTERNATOR will be the constant one of the two speeds, while ALTERNATOR AND MASTER MOTOR are equal. That is on onspeed condition. If the ALTERNATOR speed is increased due to flight conditions, the counter-clockwise rotation of the magnetic field in the stator will overbalance the mechanical rotation imposed by the MASTER MOTOR and the field will then rotate counterclockwise at the difference in the two speeds. That is an overspeed condition.

If an underspeed condition exists, the same will occur except that the field will be carried backward, i. e., in a counterclockwise direction.

These conditions are translated into mechanical rotation by means of a bell-shaped rotor set over the stator. This bell, magnetized and therefore gripped by the magnetic field. will remain stationary in onspeed, turn clockwise in underspeed, and counterclockwise in overspeed.. Always, when it does rotate, it spins at a rate of speed determined by the amount of offspeed.

The direction of rotation and its speed determines, through a directional contact mechanism mounted on the shaft of the rotor, the need for increasing or decreasing the rpm at the engine. Through a system of relays, pulses of current are sent to the particular propeller needing correction and blade angles change so as to control the engine.

Actually, the synchronizer has much more detail than has been described thus far. By having the directional contact mechanism mounted on a small commutator with live and dead segments, and by having the current carry through brushes mounted on this commutator, the flow of current to the relays is broken into pulsations. By careful timing of the propeller relays plus control of those relays by an interrupter relay, the pulses are timed so as to have the pitch-change motor operate in running periods each lasting approximately  1/10 of a second. If the amount of offspeed is small there will he a few pulsations in a given period of time. But if the offspeed is large there will be many pulsations in the same period of time. This is the proportional feature.

Also incorporated in the unit is the feature permitting solid correction if the amount of offspeed is as great as 58 to 64 rpm. This is accomplished by the assistance of a condenser and by timing the interrupter relay so that it will no longer be effective at these higher rpm.

The installation facilitates maintenance and also avoids complications arising from operating difficulties by having only the ALTERNATORS mounted on the engines. The MASTER MOTOR and CONTACTORS are located in the pilot's compartment and are readily accessible if service or changes are required. Furthermore, since the more delicate parts which actually control pitch change are located in the pilots compartment, vibration is kept to a minimum. This is a factor often causing trouble in delicate mechanisms.

A protective feature is also included in the form of a relay mounted in the master motor. As long as everything functions properly in that unit, the points of the relay remain closed. But if anything should happen to cause the master motor to run too slowly or too fast, the points will open automatically, disconnecting the complete synchronizer from the propellers. This is important because of the master motor being the central unit against which the speeds of all the engines are matched. Any variation in the speed of that unit is reflected by changes in engine rpm. If a fault such as described should occur, all propellers will be held in fixed pitch from the moment the trouble occurs. Thereafter, the pilot, having been warned by an indicating light going out, will make changes as needed by means of direct-control selector switches. The individual engine tachometers will then be used as indicators.

However, the master motor, being the amplidyne type with a built-in governor, is constructed so as to practically cancel out any tendency to run off set speed. Actually the motor, when operating, is continually being corrected in its rpm by the governor contact., which switch electricity through control field windings. These corrections occur 400 times per second.

REMOVAL

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DISASSEMBLY

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INSPECTION AND REPAIR

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ASSEMBLY

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BALANCING

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INSTALLATION

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