Aircraft technical Basics: TM 1-412, Aircraft Propellers, 1941: IV. Curtiss Controlable Propeller

SECTION IV. CURTISS CONTROLLABLE PROPELLER

18. Models and principle of operation.-a. Models.

The code used to designate these various propeller models may be interpreted as follows:

(1) The prefix letter "C" indicates Curtiss as the manufacturer.

(2) The first digit indicates the S. A. E. standard shaft upon which the hub will fit. For example, the digit "4" indicates a number 40 shaft size and "5" indicates a number 50 shaft size.

(3) The second digit indicates the number of blades of the propeller.

(4) The third or third and fourth digits indicate the blade shank size; for example, the digit "2" indicates a standard No. 2 blade shank or "15" indicates a standard No. 1 1/2 blade shank.

(5) The suffix letter "S" or "D" indicates whether the blades are made of steel or aluminum alloy.

b. Principle of operation.-The Curtiss controllable propeller (fig. 10) is operated electrically from the airplane power supply. The electrical energy for changing the propeller blade angle passes through brushes, mounted in a housing attached to the engine nose, to slip rings mounted in the rear boss of the propeller hub, and thence to the pitch changing motor through connector leads passing through the hub. The electric pitch changing motor controls the angle of blade setting through a two-stage planetary gear speed reducer, which drives a single bevel gear. This gear meshes with a mating bevel gear attached to the shank of each blade. The two motor fields provide a means for opposite directions of rotation of the motor, thereby providing for an increase or decrease in the angle of the blades. The governor is designed to open and close two different electrical circuits. One circuit contains the field which causes right-hand rotation of the motor, and the other contains the field causing left-hand rotation of the motor. Any variation of engine speed will cause the contact points of the governor to make contact in either the increase or decrease pitch circuits, depending on the position of the flyballs in the governor. The angle change will correct the engine speed to the desired value. Upon opening the motor circuit, the brake magnet which is connected in the circuit of both fields releases the brake disc, the lining of which is brought into contact with that of the brake plate by action of springs placed behind the disc. This action prevents the rotating inertia of the armature and the speed reducer gears from carrying the blades beyond the desired angle. This contact is maintained until current is again applied, at which time the two plates are drawn apart and the motor is free to turn. Automatic electric cut-out switches limit the angle range for ordinary operation and give high and low angle settings. When the switch is closed and the automatic control engaged, the governor causes one side of the relay to be energized. The contact arm of the relay is drawn against the stop by action of the energized magnet and closes the circuit of the electric motor. The motor drives the power gear through the speed reducer and causes the blades to rotate in their socket. In feathering, the propeller may be rotated up to approximately 90°, at which point a third automatic electric cut-out switch stops the change of blade angle. Two types of controls (manual and automatic) are available for selection by the pilot, the change from one to the other being made by toggle switches located on the propeller control panel. When on automatic control, a selected engine speed is held constant by an engine-driven governor. Speed selection is accomplished by turning the propeller governor control handle located in the cockpit. When on manual control, the propeller acts as a fixed pitch propeller, the blade angle being varied by operation of the "increase r. p. m." or "decrease r. p. m." switch.

19. Description.-a. The power unit consists of the motor, brake, speed reducer, and drive gear (fig. 11).

(1) The pitch control motor is a 12-volt, series wound, direct current motor, and has a double field to provide for rotation in either direction. The aft end of the armature shaft is machined to fit into the drive shaft of the first stage of the speed reducer. The forward end of the armature shaft extends outside the motor case and is keyed to the magnetic brake.

(2) An aluminum alloy plate is keyed to the armature shaft and brake lining is fastened to the inner face of this plate. The brake magnet, consisting of a winding and a steel housing, is fastened to the motor case. A hole in the center of the core allows the armature shaft and brake plate hub to extend through the magnet. A steel brake disc, which has a leather clutch facing on its front surface, is mounted between the brake plate and the magnet. Studs are riveted to the brake disc. These studs, with a spring on the end of each, fit into corresponding holes in the magnet housing and insure a locked mechanism at all times when current is not flowing.

(3) The speed reducer is built in two stages of planetary type gearing. The low speed stage gives a reduction of 86: 1, the high speed stage 171: 1. This gives a total speed reduction of 14,706:1 for the two stages from motor shaft to drive gear. The unit is mounted in an aluminum alloy housing which is fastened to the propeller hub by bolts. The hub of the movable ring gear in the low speed stage is splined to receive the bevel drive gear. Electrical leads to the motor are carried through holes in the speed reducer housing. The automatic limit switches are located in the lower rear part of the speed reducer housing. These are held in their normal position of contact against the contacts in the hub by springs. The switches cut out by means of a cut-out arm which is actuated by a cam located on the gear hub. A mechanical stop is also employed and consists of a plug extending through the rear speed reducer housing and a segment which is attached to the hub of the ring gear. It is adjusted to engage approximately 1° after the low limit switch has operated.

(4) The drive gear is a bevel gear made of steel with a splined hub. It is mounted in an adapter plate by means of a ball bearing, and, when assembled on the power unit, the splines of the power gear fit over those on the ring gear of the reducer. The adapter fits against the face of the reducer housing.

b. The hub assembly is made up of the hub and the slip ring assembly.

(1) The hub is a steel forging with sockets for three blades and is splined to fit a standard crankshaft. The front face of the hub is machined to fit the power unit housing.

(2) The slip ring assembly is attached to the rear part of the hub by screws which pass through the assembly and screw into holes in the face of the hub. The assembly consists of four brass rings separated by bakelite spacers and a case to hold them. Four electrical brass rod connectors are fastened to the slip rings and pass through holes in the hub to its front face where they make contact with the respective motor leads in the rear face of the power unit.

c. The blade assembly consists of the blade, blade nut, bearing stack, and blade gear.

(1) The blades are of hollow steel. The outside of the shank is machined to fit the stack of bearings and the inside of the blade is threaded to fit the blade gear. The blade gear is pinned to the blade to always insure correct relative position.

(2) A steel nut is screwed into the hub socket, thereby holding the blade assembly in place. Slots are provided for locking the nut to the hub socket. These slots also provide a means of correcting for "out of balance" by the addition of small weights in the slots.

(3) The blade bearings come in stacks of seven. They are ground to divide the load equally between them. A slight preload of the bearings is made possible by reversing the last bearing. This is to insure a rigid assembly at low engine speeds.

(4) The blade gear is a steel bevel gear with hub threaded to screw into the blade root. A small portion of the teeth of some assemblies are milled away to provide clearance for the split cone during installation of the propeller on the crankshaft.

d. The brush assembly consists of the housing and the brush cap assembly.

(1) The housing is an aluminum alloy casting, to one side of which is attached the brush cap assembly. It forms a cover over the slip rings when the propeller is installed.

(2) The brush cap assembly consists of an aluminum alloy cap to which bakelite holders and spacers are fastened. Duplicate carbon brushes are held against the slip rings by means of springs placed in the holder behind the brushes. Three wires are carried in the conduit which is fastened to the top of the cap. These wires are attached to three of the terminals, the fourth brush (feathering) not being used in a single-engine airplane.

20. Installation and removal. - a. Every propeller is completely assembled at the factory where proper shims and locking device are fitted. The propeller control unit is run in for a short time to assure proper functioning, after which it is disassembled into shipping units and packed. The instructions outlined in section IX will be followed when installing the propeller.

b. (1) Before installing the propeller, all component parts and controls are examined for defects and damage and checked for proper fitting. All corrosion and raised points of nicks, burs, galls, scores, etc., on joining surfaces of the attaching parts, hubs, and crankshaft end are carefully dressed off and the parts thoroughly cleaned before installation. In addition, the splines, cones, cone seats, etc., are coated with clean engine oil to provide lubrication and prevent corrosion. Cup grease or semifluid greases are not used for this purpose. The threads on the shafts are thoroughly coated with lubricant.

(2) Remove nuts (and spacer ring if provided) from thrust bearing cover studs on nose of engine and place brush housing on studs. Replace nuts, tightening and securing them. Leave brush assembly out of housing until propeller has been installed. When removing or installing propeller, the brush assembly is removed from the housing to eliminate possibility of damage to the brushes.

(3) Clean and lubricate cone. Place on shaft.

(4) Place propeller shaft locking adapter in end of shaft.

(5) Apply a coating of thread lubricant to threads on shaft and in nut, and a light coating of engine lubricating oil on the splines.

(6) On propeller hub designs 55012-2 and 55003-D, rotate blade assemblies (decrease pitch) until cut-away portion of blade gear is forward and install shaft nut, front cone, and snap ring. On designs 55014-B-4 and 55014-B-6, shaft nut, front cone, and snap ring are installed before shipping.

(7) Rotate blade assemblies back into normal flight range.

(8) Place propeller on shaft, being careful not to damage shaft threads or cone seats in the hub. Slide propeller on shaft until nut touches end of shaft, then carefully start nut on shaft threads and tighten by hand.

(9) With a 3- or 3 1/2-foot bar through the nut, apply force of 250 to 300 pounds at end of bar to tighten nut.

(10) Apply a light coating of Prussian blue on ends of slip ring brushes and place them in the housing. Rotate propeller back and forth slightly. Remove brush assembly and check location of brush contact on slip rings as indicated by the Prussian blue. The brush track should be in the approximate center of the slip rings and not closer than .020 inch to the slip ring separators. If brushes are not correctly alined, remove propeller and place one or more stainless steel shims (furnished with propellers) between rear cone and thrust nut, or shims between engine nose and brush housing. When alinement is satisfactory, clean brushes and slip ring and install brush assembly.

(11) Fit locking tube to adapter inside propeller shaft nut so that a clevis pin hole lines up with the hole in the nut. Specific instructions on the safetying for each type of installation may be found in Air Corps Technical Orders.

c. Two types of blades and power unit indexing systems are used; the first type being used on the 55012-2 and 55003-D-2 propellers, and the second on the 55014-B-4 and 55014-B-6 propellers. They are as follows :

(1) Index marks on power gear and steel adapter plate and index marks on blade gears which aline with corresponding marks in the hub.

(2) Index marks indicating a series of blade angles on blade shanks just outside of the hub, and a series of marks inside the power gear.

(3) To index propellers 55012-2 and 55003-D-2, check alinement of index marks on power gear and on steel adapter plate. To aline these index marks, it will be necessary to remove the steel mechanical stop which is held in place by bolts just forward of the steel adapter plate. This is done to eliminate possibility of damaging the speed reducer when attempting to aline the two index marks. After the mechanical low angle stop has been removed, the power gear is rotated by applying 12-volt current to the pitch change motor through the increase or decrease pitch contacts and allowing the power unit to run until the desired position is reached. Turn blades in hub until index tooth on each blade gear lines up with the mark scribed on inside of hub.

(4) To index propellers 55014-B 4 and 55014-B-6. On the blade shanks are acid stamped lines which indicate blade angles when alined with the index line on the front of each blade socket. The power unit splined shaft has a master or index spline which is indicated by a radial mark across its end. The power gear has marked spline spaces in steps of 1°. The angle indicated on the power gear nearest the master spline is the low pitch for which the propeller is adjusted. To install the power unit with this system, the following procedure is used :

(a) Remove power gear and adapter plate from power unit and see that the low limit cut-out switch is just riding on the low limit cam lobe. If it is necessary to run the power unit to properly locate cam, proceed as outlined in (3) above. Then replace power gear, having master spline in line with the mark indicating the desired low blade angle.

(b) Aline marks on hub and marks on blade shanks, indicating the desired low blade angle (same angles as power gear).

(c) Install felt grease seal over propeller shaft.

(d) Clean contacts on face of hub and their mates in the power unit. Then place power unit on hub, alining contact points and bolt holes of power unit and hub. Push firmly against hub so that power gear meshes with blade gear. Secure unit tightly to hub with the six attaching bolts and safety.

(e) Replace mechanical low stop in proper hole alining the mark "0" stamped on stop and housing and check blade angles.

(f) Install speed reducer cover and safety screws.

(5) After the propeller has been completely installed, the dial on propeller control is adjusted to agree with the tachometer. While the engine is running, throw master switch to "automatic" and adjust crank until tachometer reading is approximately in the middle of its normal flight range. Disconnect flexible control shaft at control unit and adjust dial to correspond to tachometer reading, then replace shaft.

Note. - On some installations, the reading of the dial on the control unit will vary approxImately 50 r. p. m. from the tachometer reading, therefore for the final adjustment of the governor, the tachometer reading is the criterion.

d. The method of removing propeller from crankshaft is the reverse from installing it; however, the procedure below is followed:

(1) Operate power unit until minimum blade angle is obtained. This can be determined by observing the lines scribed into the paint on hub and blades, or by checking with universal propeller protractor. If not already accomplished, paint a small area of the hub and blades and scribe a reference line into the paint for adjusting on reinstallation. On some contracts, this marking on the hub and blades is performed at the factory.

(2) Remove brush assembly from housing.

(3) Remove motor and power unit covers.

(4) Remove power unit.

(5) Remove locking pin and tube from crankshaft.

(6) Unscrew retaining nut. This will draw the propeller from shaft. Care is exercised in removing propeller from shaft to prevent damage to threads of crankshaft. The power unit with the covers removed will not be left exposed for any length of time.

21. Lubrication.-The following lubrication periods are observed and the lubricants specified are used :

a. Before installing propeller on crankshaft, completely fill hub through cavity showing blade an gears with specified lubricant. Propellers assembled for stock purposes are lubricated only sufficiently to prevent corrosion.

b. Fill power unit through filler hole with specified lubricant.

c. Pack governor drive shaft with specified lubricant.

d. After power unit is installed on propeller, check level of oil in speed reducer by removing filler plug and rotating propeller until filler hole is approximately 20° below horizontal. The oil should then be level with filler hole.

e. Fill hub with lubricant. This is accomplished by filling through one of the fittings located on the speed reducer housing, with one blade down. Use bottom fitting to fill and remove top fitting for vent. The hub will be filled when lubricant appears from top hole.

22. Propeller controls.-a. The constant speed control consists of a governor which is regulated to keep the engine at a constant speed by regulating the pitch of the propeller to absorb more or less power, thereby decreasing or increasing the engine speed to bring it to the value set on the governor. Proper switches for operating the system either automatically or manually, working in conjunction with a relay and all necessary conduit, wire, etc., are a part of the propeller controls.

b. The governor consists of a revolving spindle upon which are pivoted spring loaded flyballs, a means of changing the spring pressure, and a switch arrangement. The forces created by the spring load and the centrifugal force of the flyballs oppose each other and jointly control the position of a contact operating rod. The speed of the spindle depends on the type of governor and type of engine on which it is installed. The contact operating rod operates a spring loaded movable contact point which moves between the two fixed contactpoints. When flyball forces overbalance the spring load, contact is made with the governor switch fixed contact point, closing the increase pitch or decrease r. p. m. circuit. When the spring load overbalances the centrifugal force on the flyballs, contact is made with the opposite circuit which decreases the pitch and increases the r. p. m. Spring loads are adjusted by turning a screw which causes a threaded adjusting block to move back and forth along its length, thereby decreasing or increasing the spring load. This screw is turned by a flexible shaft from the pilot's cockpit. The governor is driven by a tachometer shaft attached to one of the engine tachometer drives. A 1:2 adapter is inserted in the drive line so that the flyballs turn at the same speed as the engine.

c. The governor control (fig. 12) is attached to the face of the engine control quadrant in the pilot's cockpit. The housing supports a small crank which turns a worm gear meshing with a gear on the drive shaft. A movement of this crank increases or decreases the compression on the governor spring, with a resultant change in the governor speed, at which sufficient centrifugal force is developed to move the plunger. An extension of the drive shaft carries another worm which turns a gear attached to an indicator dial. The dial is graduated in engine r. p. m. and has a stop to limit its travel on the inside of the dial. The calibrations on the dial are for approximate adjustment only. The engine tachometer is the primary indicator of the governor adjustment.

d. The relay consists of a switch arm pivoted between two fixed contacts and controlled by two magnetic coils mounted on opposite sides of the arm. The magnetic coils are energized by either the constant speed governor or the manual control switch. When the coil on one side of the switch arm is energized, the switch arm leaves its normal neutral position and closes the propeller pitch change circuit which flows through the switch arm and fixed contact point to the electric motor. The two fixed contact points of the relay are connected to oppositely wound fields of the electric motor. The motor rotates in either direction depending on which contact has been closed.

e. (1) The safety switch is a snap switch equipped with a thermal overload relay which automatically throws the switch to an "off" position, breaking the circuit after a predetermined period of overload. After the overload relay has cooled sufficiently, the switch is placed in the extreme "off" position after which it can be moved back to the "on" position, thereby closing the circuit again. Because of this feature, the switch is considered superior to a fuze.

(2) The manual control switch is a standard momentary contact snap switch having two throws and an intermediate "off" position. The switch is held in the "off" position by a spring. When it is desired to close the increase or decrease r. p. m. circuit, the switch must be held in the correct contact position. When it is released, it snaps to the "off" position. The function of this switch is to provide a manual control for increasing or decreasing the r. p. m.

(3) The selector switch is a standard switch having three positions. It is used to switch the power supply into either the manual control circuit or the constant speed control circuit.

(4) The "feathering" switch is a two-position snap switch. When it is thrown into the feather position, the increase pitch circuit is closed and remains closed until it is broken by the feathering cut-out switch (on the power unit). When the switch is in this position, the manual and automatic control circuits are broken. When the feathering switch is thrown to the normal position, the manual and automatic control circuits are closed and the feathering circuit is broken. The function of this switch is to feather the blades independently of the other control circuits.

f. When installing the controls, it is essential that the instructions outlined below be carefully carried out because a slight deviation from the method given may result in very serious consequences.

(1) The control is installed on the front face of the standard engine control quadrant. The control housing is fastened to the engine quadrant in place of the cover, which is removed from the quadrant by removing screws and the control knob. Before assembling the control instrument to the quadrant, it is necessary to remove the gear housing cover. This is accomplished by slipping off the pointer and dial, by removing the drive pin holding the crank handle on its shaft, and by unscrewing the screws holding the cover to the housing. The housing is then fastened to the quadrant and the control unit reassembled.

(2) The governor is normally mounted in the engine compartment. Details of mounting are to be worked out at engine installation. Things to be considered are accessibility of the inspection plate covering the contact points and the fact that the cover on the control side of the governor must be removed for adjustment purposes. It is recommended that the governor be mounted in a horizontal position. The tachometer shaft with a 1 to 2 adapter is connected to the governor drive which operates at engine speed. The speed of this drive is twice that of the tachometer drive on the engine. The flexible control shaft is connected to the governor and the control unit in the cockpit.

(3) The relay may be mounted in any convenient place in the engine compartment. The main considerations are shortest possible leads to the battery and propeller, and accessibility of points. Care should be taken that no part of the container is allowed to short the relay and that the base is kept flat to prevent warping.

(4) The control switches are installed in the pilot's cockpit in such a location and position that they can easily be reached by the pilot. Ample space should be allowed for making all electrical connections.

23. Operation.-a. When starting the engine, set safety switch to "on" position. This switch is on at all times. Set selector switch to the "automatic" position. Start the engine. If propeller is not already at low pitch (high r. p. m.) position, the electric motor will decrease the angle until the minimum angle is reached.

b. For take-off, set selector switch to "automatic" and propeller control for "take-off" r. p. m. It should be remembered that the calibration of the control dial is only approximate, as the engine tachometer is the primary indicator for close adjustment of the governor. Adjust throttle to obtain desired manifold pressure.

c. During climb and level flight, with the selector switch in the automatic position, the governor holds the engine speed constant by varying the angle of the propeller to suit different engine speeds or flight conditions. If a different engine speed is desired at any time during flight, it is necessary only to turn the control handle slowly until the tachometer registers the proper speed. Any combination of engine r. p. m. and manifold pressure is obtained within the operating limitations of the engine by independent adjustment of the engine throttle and propeller control. When the cruising altitude has been reached, turn propeller control handle to cruising r. p. m. and throttle to its cruising position. Switch propeller to "manual" position. By watching the r. p. in., the best setting of the mixture control is obtained, after which the propeller may be switched back to "automatic" or the flight may be continued with the switch in the "manual" position. It will be found that manual operation during prolonged cruising does not require any undue amount of attention, and the saving in battery current and wear on the propeller control mechanism is appreciable. The manual control also has the function of an auxiliary control. In case of failure of the governor, satisfactory performance may still be maintained.

d. When gliding in to land, set selector switch to the "automatic" position. Adjust propeller control to cruising r. p. m. As the throttle is closed, the propeller automatically returns to the low blade angle. A setting for cruising r. p. m. will prevent overspeeding of the engine if throttle is opened in case of an emergency.

e. When "feathering" the propeller as an emergency measure, set feathering switch to feather position. Close throttle. Move mixture control to idle cut-off position. Turn off fuel supply. Leave ignition switch on until propeller stops rotating.

f. As part of routine training or for practice, the propeller is feathered in a slightly different manner. Close throttle. Move mixture control into idle cut-out position. Turn off supply of fuel to engine. Set feathering switch to "feather" position. Leave ignition switch on until propeller stops rotating.

g. To return from feathering (unfeathering), turn ignition switch "on" with the throttle closed. Set propeller control to high pitch (low r. p. m.) position. Turn on fuel supply. Move mixture control to full rich position. Set feathering switch to normal position and selector switch to manual. Hold momentary switch (increase and decrease r. p. m.) in the increase r. p. m. position until tachometer reading reaches 800 r. p. m.; then release switch. Allow engine to operate at this r. p. m. until the required temperature is obtained. Set selector switch to automatic position and open throttle gradually to speed for which governor is set. Adjust mixture throttle and governor to desired power and engine r. p. m. It is important when unfeathering a propeller after the engine has cooled to run engine at a slow speed until its operating temperatures are reached, other-wise serious damage may result. The windmilling action of unfeathering is a very powerful cranking force and will overspeed the engine unless care is taken to stop the propeller at approximately 800 r. p. m. After unfeathering to about 800 r. p. m. at an airspeed of approximately 125 m. p. h., the engine may be permitted to warm up as long as desired without overspeeding.

24. Inspection and inspection maintenance.-a. At the specified periodic inspection, the range of operation of the Curtiss controllable propeller is checked as follows :

(1) Set throttle to give an engine r. p. m. within operating range of propeller governor.

(2) Move propeller switch to the "manual" position.

(3) Hold momentary switch in the "increased" r. p. m. position. Note increase in engine r. p. m. When no further change is indicated on the tachometer, the propeller is in full low pitch position.

(4) Position and hold momentary switch in the "decreased" r. p. m. position, and note decrease in engine r. p. m. When no further change is indicated on the tachometer, the propeller is in high pitch position.

Notice.-During the above checks the throttle is not moved.

(5) Move propeller switch to the "automatic" position.

(6) Adjust r. p. m. selector or control to give maximum r. p. m. Note increase in engine r. p. m., and when no further increase is indicated by the tachometer, the governor is functioning and the propeller is in low pitch position.

(7) Adjust r. p. m. selector or control to give minimum engine r. p. m. Note decrease in engine r. p. m., and when no further change is indicated by the tachometer, the propeller is in full high pitch position.

(8) Return propeller to low pitch position for take-off.

b. The cockpit control, switches, relay, and governor are inspected for security of mounting. Brackets are inspected for cracks and other damage; for loose or damaged screws, bolts, and pins; and for freedom of movement. Any damaged part will be replaced with a serviceable one before flight.

c. Remove brush holder cover plate by taking out screws and inspect for evidence of carbon dust, oil, dirt, etc., particularly at the bottom of the housing. Remove such substances when found. Remove brush holder cap by loosening flexible conduit joint and removing wing nuts. Check brushes for excessive wear and accumulated carbon dust, oil, and dirt. Clean thoroughly by using compressed air and some noninflammable cleaning fluid. Check brushes to make certain they are working free and make good contact.

d. Inspect relay for broken terminals, free action of the arm, and smoothness of contact points. The middle contact point should rest squarely against the fixed ones when in either of the two contact positions. In case of a failure to function, the relay is removed from the box by removal of screws. If necessary, contact points are replaced, observing the caution that contact over the whole surface of the points is to be obtained. Remove relay box cover and seal and check for accumulated carbon, oil, dirt, etc., particularly at bottom of box and behind bakelite panel. A flashlight will probably be necessary to determine whether there is any inflammable material near the contacts.

e. Procedure for checking retaining nut for looseness is as follows:

(1) Remove motor and power unit cover.

(2) Remove power unit. The power unit is secured to the hub by bolts.

(3) Remove locking device.

(4) Insert a 3- to 3 1/2-foot bar into retaining nut. Apply a weight of 250 to 300 pounds on end of bar. The bar is applied steadily; avoid jerking it.

(5) Install locking device.

(6) Turn blades (if they are not marked) until indexed tooth of blade gear alines with reference lines in hub.

(7) Remove mechanical stop from power unit and operate electric motor until indexed tooth on power gear alines with reference line on adapter.

(8) Install mechanical stop and resafety.

(9) Install power unit and secure it to the hub with bolts. Safety all bolts with wire. Do not disturb setting of blades when installing power unit.

(10) Install power unit and motor covers and safety all screws with wire.

f. The brush holder is removed by disconnecting flexible conduit, removing wing nuts or clamps and disconnecting wire terminals under cover plate. The brush holder assembly is then thoroughly washed with cleaning fluid, preferably noninflammable. Dry with compressed air. Replace all worn brushes or other damaged parts. The slip rings and the inside of brush housing are thoroughly cleaned with a cleaning fluid and dried with compressed air.

g. Remove inspection plate and examine contact points for pitting and proper clearance. When the engine is not running, the middle contact should rest against the decrease pitch point and the gap between the middle and the adjustable contact should be 0.025 to 0.030 inch.

h. When checking the magnetic brake, the nose cap of the power unit must be removed. Determine if the brake disc is engaged against the lining of the brake plate when the current is off, and upon application of the current, if the disengagement is effected immediately. Glazing of the brake lining is removed with a coarse file. Any looseness of the brake plate with respect to the motor shaft is noted and the lining checked for excess wear. The gap between the brake lining on brake plate and the clutch facing on brake disc is checked for proper clearance. This gap should be 0.015 to 0.020 inch. Use a gage to make the check. Care is exercised that the gage is inserted between the brake lining and the clutch facing in the direction of rotation of the motor shaft.

i. The relay box cover and seal are removed and relay disconnected and removed from box so that contact points can be inspected.

(1) On some relays, conventional material is used for the contact points. If these points show excessive pitting or piling up of contact material, they will be dressed down with a fine-cut file, such as an ignition contact point file. If an excessive amount of metal is removed, the relay is replaced.

(2) On other relays, a special Elkonite point material is used. Discoloration and fine pitting of these points are normal and will not increase the resistance or their tendency to stick in operation. The Elkonite points are not filed or stoned since this only shortens the life of the points.

(3) Both types of relay points are checked for proper alinement of the contact surfaces. If there is sufficient wear of the center switch contact arm bearing and centering spring to allow the contacts to strike due to jar, without electrical operation, the relay is replaced.

(4) Thoroughly clean relay box and relay since any inflammable substance may be ignited in operation by arcing of relay points.

(5) Install relay panel in box, reconnect wires, seal conduits, attach cover, and safety.

j. In this particular type of propeller, which depends on the electrical system for power to change the blade angle, most of the maintenance problems are electrical. Checking all terminals, connections, brushes, etc., is the first step in locating the source of trouble. If this fails, the trouble may be in the power unit or propeller, which would require the services of trained propeller mechanic to locate and correct the condition. Exceptions as follows:

(1) Occasionally the flexible shaft from the r. p. m. control to the governor is not properly seated or in mesh. This will be noticed during operation in the "automatic" position. Any change of the r. p. m. control will have no effect on the engine r. p. in. This is corrected by removing the control and properly seating it and making sure the control is in mesh at both ends.

(2) If at any time the propeller will not change angle in either the "manual" or "automatic" position and all wiring has been checked and found correct, then check magnetic brake for clearance.

FIGURE 10.-Curtiss controllable propeller

FIGURE 11.-Power umit and brake.

FIGURE 12.-Cockpit control crank and diaI.