Propeller	Pitch-Changing Mechanism

The Convair B-36 was equipped with Curtiss-Wright propellers. Model C636SP-A hubs with 1129-5C6-24 blades, were used with the SAE No. 60-A propeller shafts on R-4360-25 (B-36A), or R-4360-41 and -41A (B-36B, D, E). C736SP-A hubs with 1129-17C6-24 blades were used on R-4360-53 SAE No. 70 propeller shafts (B-36D, E, F and H; RB-36D, E, F and H). These propellers differed considerably from the Curtiss Electric propellers used during WWII. The earlier pitch-changing mechanisms were fully electric, whereas these later propellers used a mechanism driven by the propeller rotation in combination with hydraulic clutches and brakes to change the blade pitch, allowing pitch changes at approximately 2.5° per second during normal propeller operation, or at 45° per second when feathering, reversing, or returning from reverse pitch. Both pitch change rates were based on a 900 rpm propeller speed. This control system was electrically operated. Propeller blade anti-icing was accomplished by conducting heated air from shrouds surrounding the exhaust manifolds through the hollow steel blades, as documented in Tom Fey's article

Starting at the engine and moving aft, each propeller consisted of the following assemblies: the attachment assembly, the pitch-changing mechanism, the hub and blades, miscellaneous parts including the thermal anti-icing components, and the spinner.

The propeller attachment assembly consisted of front and rear cones that, in addition to a snap ring, were standard parts used in conjunction with SAE No. 60A or No. 70 engine noses. A bronze shaft nut was equipped with internal splines that mated with the shaft nut lock, which also served as a heated air conductor and shield for the thermal anti-icing system.

The hub was of conventional design. The blades had a ball bearing preload system. Worm gears were used for pitch-change blade rotation. The hub forward end, with respect to the airplane, was extended and used to mount the pitch-changing mechanism. A propeller blade, blade nut, two blade nut seals, and the blade bearing stack comprised a blade assembly.

Miscellaneous equipment included the power unit and hub assembly hot air manifolds, the spinner-mounting bulkhead, the shaft nut locking sleeve, and the spinner assembly.

Propeller Flight Deck Controls and Indicators

B-36 propeller controls and indicators were duplicated in two flight deck locations: the aircraft commander's station and the flight engineer's station; the flight engineer had the most complete control and indicator set.

Convair B-36 Propeller rpm Limitations

One is struck by how disorganized the propeller controls and indicators are in both locations; cockpit designers still had a lot to learn about human factors. In addition to the baroque control and indicator placement, the flight engineer had to contend with several restricted rpm ranges to avoid resonant propeller, engine and airframe vibration.

Fortunately (or not if you were the poor flight engineer), there were plenty of tests to be run before and during each flight, which gave the flight engineer lots of practice getting familiar with the propeller control and indicator placement.

Ground Operation and Testing (Engine Stopped)

Ground operation required external DC power to be plugged into the airplane.

Ground Operation and Testing (Engines Operating)

After the engines were warmed-up and while they were running, the following tests could be performed.

Due to the possibility of engine damage, a complete feather test could not be made with the engine running. The feather test made with the engines not running and the reverse test made with the engines running proved the circuits and systems used during all phases of propeller feathering operation were functional.

(NOTE. After completing the above tests, the controls were prepared for take-off with the propeller circuit breakers ON, the selector switches in AUTOMATIC, the reverse control switches SAFE, and the feather switches NORMAL.

Simplified Schematic

B-36 propellers were controlled either manually or synchronically by proportional synchronizers manufactured by the Curtiss Wright Corporation. A master motor, together with six contactors and six alternators, provided the means by which the propellers were synchronized. Propeller speeds were matched against the electrically driven master motor whose speed was varied manually by a knob located on the pilots' pedestal and/or by a similar mechanically interconnected knob on the flight engineer's table. Two master tachometers, one located near each of the knobs, indicated propeller speeds during synchronous operation by indicating the master motor speed. Six manual speed control switches, which overrode the synchronizer unit, and six switches for feathering were installed in the flight engineer's table. Three propeller reverse pitch control switches on the pilots' pedestal, when used in conjunction with a single reverse pitch switch, enabled the pilot to reverse any symmetrical propeller pair. A switch for propeller anti-icing heat control was located on the engineer's panel.

The pitch-changing mechanism was driven by the rotating propeller shaft by means of a main drive gear (M-17) keyed to the propeller hub extension. The main drive gear transmitted power to the high-speed drive gear (M-18) of a two-stage planetary reduction system. The high-speed drive gear acted as an idler for the high-speed clutch gears (M-19 and M-28) and through the reduction system drove the low-speed gear (M-22). The low-speed drive gear transmitted power to low-speed clutches (M-23 and M-24); two clutches were for fast rate correction and two were for slow rate correction. One fast rate and one slow rate clutch for increase rpm were mechanically splined by means of plates to an output gear (M-14). Assemblies of the decrease rpm fast rate and slow rate clutches were identically splined to another output gear. The two output gears, stationary except during pitch change, meshed with a large externally-internally toothed gear (M-11), the internal teeth forming-the movable differential inter-gearing assembly ring gear. The inter-gearing assembly enabled energy to be transmitted from the pitch-changing mechanism stationary portion to the rotating section. During pitch change operations, power was directed through the inter-gearing assembly to a large movable ring gear (M-6), which meshed with three hub pinion gears (M-5). The hub pinion gears drove bevel gears (M-1), which drove worms (M-3) that turned blade gears (M-4) splined to each blade shank, thus turning the blades in the direction required.

A brake assembly (M-34) held the blades in fixed pitch when no pitch change was required. Slight differences existed between brake assemblies of the two propeller models. The brake obtained its holding force through the action of oil pressure on a piston forcing brake plates to lock the gear. The brake gear (M-38) meshed with the inter-gearing movable ring gear (M-11) as did the clutch output gears (M-14) and (M-15). Oil pressure acted on the brake piston and plates during fixed pitch, locking the brake gear and the inter-gearing movable ring gear (M-11) but simultaneously the oil pressure was removed from the clutches, thus, releasing the clutches. During pitch change, oil pressure was directed to the desired clutch piston and plates and the pressure to the brake was removed allowing the brake gear and movable ring gear to turn. Emergency braking power in the event of oil pressure failure was obtained from the centrifugal force on steel balls or flyweights rotating at high speed. Steel ball or flyweight centrifugal action forced the piston to clamp brake plates together, locking the brake gear. The brake held the propeller in fixed pitch until the propeller rpm fell below approximately 200 rpm and the balls or flyweight centrifugal force was overcome by the feather motor (E-5).

The feather motor drove the pitch-changing mechanism when the propeller rotation was below approximately 200 rpm, to complete the feathering cycle after the propeller rpm fell below the speed necessary to maintain oil pressure, and to start unfeathering the propeller. When feather motor operation was required the jaw clutch (M-40) was engaged allowing power to be conveyed through the feather motor reduction gearing (M-36), the jaw clutch gear (M-37), and the brake output gear (M-38) to the movable ring gear (M-11). A solenoid-operated brake shaft locked the feather motor when it was not energized.

The high-pressure oil used to operate the propeller hydraulic components was obtained from a self-contained pumping and regulating system. Rotational pressure fed this oil to a high-pressure pump that directed this oil through selector valves to the brake or a clutch, a motor control switch and a reservoir shut-off valve. The high-pressure oil was controlled by a pressure relief valve designed to relieve oil pressure above that necessary to operate the clutches. A centrifugal drain valve also was connected into the high-pressure line to open the high-pressure line to atmospheric pressure at propeller speeds below 150 to 200 rpm.

When the propeller speed was above approximately 200 rpm, a call for blade angle change actuated a selector valve solenoid, which shifted its selector valve to direct high-pressure oil to the corresponding clutch piston. This changeover from feathering motor to clutch operation was effected automatically by movement of the feather motor control switch in the propeller. At low propeller rpm there was no high-pressure oil in the system and a spring held the switch in the feather motor position. At propeller speeds above approximately 200 rpm (approximately 600 engine rpm), high-pressure oil was present in the system. The oil pressure acted on a piston to overcome the spring force and shift the feather motor control switch to its opposite position. This made the feather motor inoperative and at the same time furnished a ground for the selector valve solenoids. A call for blade angle change resulted in clutch operation. The reservoir shut-off valve was located in the oil line between the reservoir and the rotating sump. This valve maintained the proper oil level in the sump during propeller operation by permitting oil to flow to and from the reservoir. Low pressure oil from the rotating sump was available at the solenoid valve cylinders. During fixed pitch, low pressure oil filled all clutch lines furnishing lubrication for clutch plates and back pressure against the normal rate clutch pistons for rapid pick-up.

Although the pitch-changing mechanism was hydro-mechanically operated, the controls were electrical. The electrical impulses were conducted through wiring to the limit switch assembly. When the particular circuit was closed, either a selector valve solenoid or the feather motor circuit could be energized, as required. Limit switches were provided in each operating circuit to prevent blade angle change beyond a predetermined limit of low, feather, reverse, etc. When each limit was reached, the respective cam tripped a switch to open the circuit.

Propeller Controls Operation

The alternator, mounted on each engine's the governor drive pad, was a three-phase alternating current generator whose frequency varied directly with engine rpm. Each alternator was connected to a corresponding contactor unit.

The direct-current amplidyne-type synchronizer motor accurately maintained a selected speed. As flyweights were rotated by the motor armature, their centrifugal force tended to straighten out a mounting spring that moved a rotating contact toward a fixed contact. When the contacts closed they shunted out one of two opposing fields, resulting in a motor speed reduction. When this occurred, the points separated and both fields were again energized, allowing the motor speed to increase. This action was repeated many times each second, causing the synchronizer motor to accurately maintain any speed selected by moving the non-rotating contact (on an adjustable rack) closer to or away from the rotating contact, thus varying the speed at which the governing action took place. This rack was moved by the master rpm control. A single-phase alternating current tachometer generator, which was part of the motor assembly, indicated the synchronizer motor speed in terms of engine rpm on a synchronizer tachometer.

A pulsating voltage developed by the opening and closing of the speed control contacts operated a protective circuit relay whose contacts completed each contactor unit's grounding circuit. If the synchronizer motor did not operate normally at the selected speed, the pulsating voltage ceased and the protective relay points opened. This broke the contactor ground circuits, preventing improper automatic operation and placed the blades at the angle attained when the circuits were opened. Propellers could then be operated by the selective fixed pitch control.

Each contactor contained a rotating stator geared to, and driven by, the synchronizer motor. The corresponding engine-driven propeller alternators were electrically connected to the contactor rotating stator windings. The alternator voltage developed around the stator's rotating magnetic field had a rotation opposite to the stator direction. When the alternator speed equaled the stator speed, the magnetic field rotated at the same speed as the stator rotation, but in the opposite direction, resulting in a stationary magnetic field. A magnetic bell-shaped rotor, which surrounded the stator windings, was driven by this magnetic field. Therefore, when the engine speed differed from the rotating stator speed, the magnetic field rotated, which rotated the bell-shaped rotor at a speed equal to the difference between alternator and stator speeds. The direction of rotation depended on the off-speed condition.

The Master Motor and Contactor was a 52-lb assembly shock-mounted in a bracket fastened to the floor underneath the engineer's table. Driving power of 26 – 29 VDC was furnished through a ground return circuit with the hot load taking power from the bus in the engineer's propeller control panel. A filter condenser located in the propeller filter and junction box was connected between this lead and ground. A single phase AC tachometer generator that served as a power source for the master tachometers was constructed integrally with the master motor with its rotating element mounted on the main shaft. A three-minute warm-up period was required to allow the master motor to stabilize.

Flexible shafts connected the rpm control knobs and the master motor. The shaft from the knob on the engineer's table was interconnected to the shaft from the knob on the pilots' pedestal at a tee junction located on the floor under the engineer's table. An additional shaft extended from the tee junction to the master motor. Shafting was attached to the structure by clips and grommets. The shaft from the pedestal to the tee connection was composed of two sections and two angle drives. The first section extended straight down from the knob and connected to an angle drive under the pilots' floor. From the angle drive the second shafting section ran under the pilots' floor to a second angle drive that was connected to the tee junction. A leg of each angle drive furnished the passages through the floor.

Each of the six contactors employed gear-driven stators that were rotated by the master motor shaft. This shaft was adapted to a special gear driving three idler gears on the gear plate assembly; the idler gears in turn drove the stator shafts in the contactors. Each stator windings received three-phase current from its engine-driven alternator, which was conducted to the contactor by three wires. Contactor disturbances were prevented from entering the DC system by filter condensers installed in the propeller synchronizer filter and junction box located under the engineer's table; a filter inductor located in this junction box was connected in each contactor's power lead. Power for the contactor came from the DC bus in the engineer's propeller control panel, passed through the feather switch normal contacts, the selector switch auto contacts, and then to the contactor via the post on the condenser and inductor coil. Ground wires from each contactor were interconnected at a bus connection in the junction box. A single wire from this bus grounded the bus inside the master motor. The increase and decrease rpm wires were routed from the contactor to its corresponding propeller hub, picking up the filter condenser posts in the junction box en route.

A propeller alternator was mounted on the governor drive pad of each engine and was connected through a splined drive to the nose accessory drive gear. The alternator rotor was driven at crankshaft speed. The alternator was a three-phase alternating current generator that indicated engine speed by the generated frequency, which varied directly with engine rpm. A three-wire system connected the propeller alternator to the corresponding contactor unit.

Normal Propeller Controls

Control switches, warning lamps, and safety devices used during normal propeller operation were located in the flight engineer's table. This control panel contained a row of circuit breakers, a row of control selector switches, and a row of push-to-test type tel-lamps.

The selector switch was of the single-pole triple-throw type with four positions: AUTO.CONSTANT SPEED, FIXED PITCH, INCREASE RPM, and DECREASE RPM. The latter two switch positions were momentary contact. When the selector was placed in the AUTO.CONSTANT SPEED position, the synchronizing system provided automatic propeller operation at the rpm setting selected by the pilots or the flight engineer with the master motor rpm control. When the selector switch was in the FIXED PITCH position, the propeller operated as a fixed pitch propeller and the rpm could be adjusted by momentarily holding the selector switch in either the INCREASE RPM or DECREASE RPM position. If the switch was held in the DECREASE RPM position, the propeller blades would stop at the high blade angle position and would not go all the way to feather.

A tel-lamp was installed in the synchronizer circuit of each propeller. If any one contactor experienced a power failure during synchronous operation its corresponding tel-lamp went out. All six tel-lamp circuits were grounded through the protective relay circuit in the master motor. This circuit was designed so that the relay opened the circuit if the master motor failed to operate normally at the selected speed. Therefore, if the master motor malfunctioned during synchronous operation, all six tel-lamps went out.

Six push-button-reset circuit breakers were installed in the six power leads from the 28 VDC bus to the feather switches. Power for all propeller control action was supplied through these six leads.

Feather and Reverse Circuits. Six feather switches were arranged in a row parallel to the normal propeller controls on the flight engineer's table. Those feather switches were of the double-pole double-throw type. When the feather switch was placed in the FEATHER position, the normal and reverse circuits were broken and the propeller would then change to the feather angle and stop at that setting. If the engine was not turning over and DC power was on, the feather motor in the propeller unit drove the blades to the feather position. When the engine was not operating and the feather switch was returned to NORMAL, the feather motor changed the blade angle from the feather position into the normal range. Propellers were reversed in any combination of symmetrical pairs through the use of propeller reverse control switches, reversing relays, and a momentary-contact reverse pitch switch. Control switches were double-pole double-throw with their two positions labeled READY and SAFE. Each propeller pair had its own control switch and reversing relay. A control switch set up the circuit to its corresponding propeller pair by supplying power circuits and a hold-in circuit to the relay contacts, but it did not actuate the relay when it was placed in the READY position. The reverse pitch switch was used to energize the relay coils but was effective on a relay only when its associated control switch was in the READY position. After the reverse pitch switch had been pushed in and released, the relays held in and furnish the circuits to their related propellers until the propeller blades moved to the reverse position and broke the circuit by means of cut-out switches in the propeller limit switch. The reverse relays remained energized as long as the control switches remained in the READY position. To return from reverse the control switches were placed in the SAFE position and the propeller pitch returns to normal automatically.

Detailed Functional Descriptions

The pitch-changing mechanism hydraulic portion consisted of a high-pressure pump, a rotating separator whirling in a circular sump, a scavenger pump, a low pressure regulator valve, a reservoir shut-off valve, and a rotor shaft on which were mounted the centrifugally-operated high-pressure relief valve and drain valve. a rotating separator or strainer that provided lubricating oil and back pressure to the clutches and other hydraulic units by the centrifugal pressure created by rotation. This pressure was limited 7 to 10 psi by the low pressure regulating valve. A spring-operated pressure-released shut-off valve was located in the line between the storage reservoir and the rotating sump. This valve was held open by high-pressure and supplied fluid to the system. When for any reason the hydraulic pressure dropped, the valve closed by spring action, thereby stopping the oil flow from the reservoir. Solenoid-operated selector valves fed from the high-pressure pump directed hydraulic or lubricating oil to the required clutch or brake by using electrical energy from the control system.

The electrical components consisted of the connector stack and the limit switch, which were located on the power unit housing forward face, the feather motor, the feather motor switch, and the selector valve solenoids. Electrical cables from the feather motor, solenoids, limit switch assembly, and feather motor switch were connected with prongs forming the connector stack, which was bolted to the power unit housing. The airplane propeller cable, which supplied power to the above components, was plugged into the stack and was secured to it with two screws and self-locking nuts. The limit switch housing, cam housing assembly, rotor assembly, and reduction gearing system comprised the limit switch assembly. The limit switch housing was a magnesium casting fitted closely around and protected by the cam housing assembly. The cam housing assembly included a steel housing upon which were mounted 11 small cut-out switches. The limit switch cover was fastened to the limit switch housing and supported one end of the cam housing assembly. The cam rotor assembly comprised 11 cams mounted and keyed to the rotor. The feather, high blade angle, low-low blade angle, high-low blade angle, low reverse, and high reverse cams were locked in place on the rotor by means of small gears. These gears meshed with one large locking gear pinned to the rotor end. The angular cam positions could be adjusted and instructions for this adjustment were printed on the limit switch nameplate. Angles were etched on the end cap rotor; index marks on the cam housing, used in conjunction with the index marks on the cap, permitted accurate blade angle limit setting. Each cam tripped one cut-out switch secured to the cam housing when a blade angle limit was reached. When the switch rider was moved against the cam's rising slope, the switch opened its electrical circuit. The cam lobes were positioned to prevent propeller operation beyond the predetermined blade angle. The switch contacts were connected by wire through the moulded synthetic rubber cable to the rectangular electrical connector. The limit switch drive gear drove the cam rotor through a small spur gears. The drive gear was mounted on a bushing on the rotor shaft end. The first spur gear, meshing with the drive gear, was mounted on small steel pins in the base of the cam housing. The final spur gear was mounted on the rotor shaft and was keyed to the rotor. The drive gear was meshed with the phenolic idler gear of the inter-gearing assembly.

The feather motor was a sealed unit with all components designed to operate for the propeller lifetime. The feather motor consisted of a motor and a motor brake. The complete assembly was mounted on the power unit housing exterior and was secured in place by a bracket and bolts. The motor housing base fitted into a shallow socket in the housing. A large o-ring seal between the motor housing and the power unit housing prevented any oil leakage and cushioned the motor in the housing. Two ball bearings seated in steel bushings at each motor housing end supported the motor armature in the housing. The commutator brushes were installed in the brush tubes and were accessible from the motor exterior. The motor output gear was an integral with the motor shaft and was free to float. This ensured a constant mesh regardless of any feather motor drive gear movement. The motor was connected to the power supply by a molded cable ending in the rectangular molded electrical connector. The motor shaft also was used as a brake shaft. The brake consisted of a small brake facing splined to the brake shaft, a fixed brake plate bolted to the motor frame, and a movable brake plate on the facing opposite side. The movable brake plate was spring-loaded to press against the brake facing. A solenoid seated at the motor housing end and connected in series with the motor windings, pulled the movable brake plate away from the brake facing whenever the motor was energized, releasing the brake. When the motor was not operating, the solenoid was not energized and the movable brake plate was held tightly against the facing; the motor shaft was locked.

The feather motor switch consisted of a plunger, movable contacts, fixed contacts, and cable assembly. The movable contacts were pinned to the plunger, which was held in its normal position by a coil spring and spring guide. The 10 fixed contacts were mounted on a plastic block. In one position, six of the fixed points were in contact with the movable contacts; in the other position four of the fixed and movable points were in contact. These fixed contacts were connected through a molded conduit to the rectangular molded electrical connector. The switch assembly was mounted in the power unit housing with its plunger aligned with and moved by the motor control piston mounted on the brake assembly.

The solenoids were contained in the selector valve housing and were a part of the selector valve assembly. There were four selector valve assemblies, one for each pitch change clutch. The solenoids were seated in the selector valve housing face over the cylinders' mouths. The actuator rod was seated against the piston at one end and against the solenoid armature at the other. The four solenoids were mechanically secured together by brackets and a bolt to permit installation or removal as a unit. Passages in the selector valve housing permitted oil flow to and from the valves, as required. The input and output oil passages were aligned with passages in the power unit cover. The molded cable assembly leads were connected to the four solenoids. This cable assembly, fastened to the power unit cover by several brackets, passed through the power unit cover and ended in the rectangular molded electrical connector. A flat, round, oil seal and tapping plate, installed around the cable where it passed through the cover, preventing oil leakage. The power unit housing was a machined magnesium casting containing the pitch-change mechanism components. The stationary power unit housing and cover were located at propeller installation forward end between the mounting pad on the engine nose and the rotating collector housing. The power unit housing was bolted to the fixed ring gear of the inter-gearing assembly, which was contained in the collector housing. A ring mounted around the casting shoulder was staked in place to provided a seat for the plastic seal ring. This seal, which was secured by a synthetic-rubber o-ring seal, provided an oil-tight fit between the hub assembly rotating collector and the power unit housing. Two spot-faced surfaces on the housing outer part provided a mount for the feathering motor and for the limit switch assembly. Between these two surfaces was a flat surface upon which the cable assemblies' molded connectors were bolted. Screw bushings set in the housing provided a mounting for these external assemblies. Sixteen screw bushings installed in the power unit housing rear face provided a mounting means for the power unit cover. A carbon seal that was placed around the housing outer diameter prevented hot air leakage from the hot air anti-icing system. Two long slots cast in the outer webbing provided a passage for hot air through the power unit housing. Provisions also were made on the housing for mounting the anti-icing hot air manifolds.

A machined magnesium cover casting was attached to the power unit housing by 16 nuts. On the cover inner diameter an adapter was mounted and secured by 8 screws. The pump and valve housing assembly and the brake assembly were mounted on the two spot-faced cover surfaces. Screw bushings set into the housing secured these two assemblies to the cover. Seats to support the two clutch assemblies and the planetary gear reduction assembly were machined in the housing. A hole machined in the casting provided space for the cable to the selector valve solenoids. Oil passages cast in the housing permitted high- and low-pressure oil to pass from the oil pump and pressure regulators to the clutches, the brake, the feathering motor, etc. Holes machined in the housing provided seats for the sump shut-off valve and the low-pressure relief valve. A large hole in one of the spot-faced surfaces provided a space for the high-pressure oil pump assembly. Fastened to the cover rear was a synthetic rubber ring that fitted on the engine propeller shaft reduction gear housing front face.

Propeller pitch-change component functions and interrelationships in each operational phase from ground operation before flight to propeller reversing after flight are explained below.

Propeller ground operation to check pitch change during the mechanic's preflight inspection, or any time it was desired to change blade pitch when an engine was not running, was achieved by actuating the feather motor. The circuits were so arranged so that the feather motor was capable of producing complete blade angle change from feather to high reverse. Since the engine was inoperative the feather motor control switch was not depressed by hydraulic pressure and the motor circuit was subsequently grounded. Consequently, control impulses passing through any of the cut-out switches in the limit switch assembly were conveyed directly to the feather motor. The absence of hydraulic pressure permitted the spring-loaded jaw clutch to be engaged, and feather motor rotation was conveyed through the motor reduction gearing, clutch and brake output gear, to the ring gear in the inter-gearing system. The ring gear, in addition to driving the inter-gearing planetaries and subsequent gears in the pitch change train, also drove the rotating cam in the limit switch through the limit switch drive gear. When the cam tripped the cut-out switch at the pitch change limit the circuit was interrupted and the feather motor ceased operation.

Since the master motor was calling for 2,700 rpm before the engine was started, a decrease pitch impulse was transmitted by the contactor and was passed through the normal rate decrease-pitch cut-out switch in the limit switch assembly to the increase-rpm selector valve solenoid. When the drain valve closed and the hydraulic pressure increased, it was directed to the corresponding pitch change clutch. Since the power settings used during engine run up did not provided sufficient power to permit a speed of 2,700 rpm, a "solid correction" increase rpm impulse was transmitted by the contactor unit, the increase rpm selector valve solenoid continued to be energized, the decrease pitch clutch remained meshed, and the blade angle decreased until the cut-out switch tripped at the low limit. Although the low pitch blade angle limit had been reached, a speed differential still existed between the synchronizer and the propeller and continued to exist until either an increase in engine power or decreased propeller loads during take-off, permitted the propeller speed to match the master motor speed. During take-off the initial "on-speed" condition was of very short duration because of the above two factors, increased power and decreased loads, would have permitted the propeller to overspeed. However, as soon as the engine-driven alternator output indicated excessive speed, the synchronizer action transmitted a decrease rpm impulse and an increase pitch change resulted.

Automatic control in reverse was accomplished through the high and low reverse cut-out switches in the limit switch assembly. After the propeller had been reversed and the low reverse limit was reached, all circuits except feather and high reverse were broken. When a decrease rpm impulse was transmitted by the contactor it passed through the high reverse cut-out switch to the normal-rate decrease pitch selector valve solenoid. Solenoid actuation diverted high-pressure oil to the corresponding clutch and created the subsequent increase pitch change. When an increase rpm impulse was transmitted to the limit switch, it passed through the low-reverse cut-out switch to the normal rate increase pitch selector valve solenoid to create decrease pitch change. (Note: During normal operation an rpm increase was accomplished by decreasing pitch (blade angle), and decreasing speed was accomplished by increasing pitch. But if all propeller blade angle movement for decrease pitch was from feather through zero (flat pitch) to high reverse and all that increase pitch movement was from high reverse back to feather, when using the manual controls with the propellers in reverse, an increase in rpm could only be accomplished by using the DECREASE RPM switch and a decrease in rpm could only be attained by using the INCREASE RPM switch. When reverse pitch operation was conducted in AUTOMATIC the control reversal was automatically accomplished by the propeller limit switch.)

Propeller Anti-Icing and Cooling

This included the power unit and hub assembly hot air manifolds, the spinner-mounting bulkhead, the shaft nut locking sleeve, and the spinner assembly. The two manifolds mounted on the power unit housing and the three mounted on the hub assembly collector housing were constructed of two formed steel sheets welded together. A steel flange spot-welded to each manifold base provided a mount for a heat seal and a means for mounting the manifolds. The rubber seals were installed on the flanges and each manifold was mounted in its respective location with the flange and seal located in the mating slots in the housings. A magnesium clamp, bolted to the housing, also fitted over the flanges and locked the manifolds in place. The larger hub assembly manifolds were also supported by the spinner mounting bulkhead. The spinner bulkhead, bolted to the hub assembly front, had three large round holes that fitted over the three manifolds and provided a rigid support for them. The three bolts securing the bulkhead to the hub assembly also secured the shaft nut locking sleeve. This sleeve locked the propeller shaft nut in place and provided a passage for hot air through the hub center. The three holes in the sleeve were aligned with the three hub preload pad hot air tubes.

The spinner assembly comprised the spinner dome, the spinner mounting ring, and the spinner apron. The spinner, mounting ring mounted on the collector housing bolt bosses, was secured with bolts and nuts. The spinner apron was located on the spinner mounting bulkhead studs and was fastened to the spinner mounting ring with screws. The spinner dome had a steel cone riveted inside to act as a cover for the hot-air compartment formed by the spinner bulkhead and the shaft nut locking sleeve. The complete dome was secured to studs on the spinner bulkhead with nuts, washers, and cotter keys.