Aircraft technical Basics: TM 1-412, Aircraft Propellers, 1941: III. Lycoming-Smith Controlable Propeller

SECTION III. LYCOMING-SMITH CONTROLLABLE PROPELLER

11. Principle of operation. - The Lycoming-Smith controllable propeller (fig. 6) accomplishes a change in blade angle mechanically from the rotation of the propeller shaft. The blades are turned about their longitudinal axis by engaging and disengaging a series of gears in the hub with a stationary worm mounted on the engine. The propeller gears operate only during a change in the blade angle. The high gear ratio affords a sufficiently slow movement of the blades to permit the pilot to select the particular blade angle desired and to make the gears irreversible. The propeller gear mechanism is engaged and disengaged by means of an electric solenoid control. A pitch indicator mounted on the engine instrument panel, indicates the angle of the propeller at all times.

12. Description. - The description of the major assemblies is of a general nature. No attempt is made to describe all of the component parts making up these assemblies nor should the description be construed as instructions for the assembly of the propeller.

a. The hub is a one-piece chrome vanadium steel forging. The blade assemblies are retained in the hub by blade nuts screwed into the end of the blade barrels of the hub and locked to the hub with serrated lock plates. Buttress threads are used in the hub and on the blade nut to take the centrifugal load of the blades. The gear mechanism of the propeller is encased in a gear housing which is assembled over dowels in the rear cavity of the hub and held in position by three bolts.

b. The blades are made of chrome vanadium steel and are of hollow construction. The exterior of the blade is chrome plated to provide a protective coating. This blade is adapted to the propeller design, inasmuch as the blade bearings can be located on the blade shank and threads can be machined directly on the blade shank. To reduce the bending stresses originating from the thrust loads, the blades are tilted forward 1/2°. A counterbalance is used on each blade to neutralize the twisting moment, which is inherently true of all propeller blades.

c. The blades are secured in the hub by a series of ball bearings so arranged and held on the blade shank that the moments and loads are evenly distributed to each bearing. As the centrifugal load on the blades is applied by the rotation of the propeller, the outer races of the blade bearings tend to expand in the hub and the inner races contract around the blade shank to hold the blade assembly rigidly in the hub. A blade end bearing locates the blade radially. A slight preloading of the main blade bearings against the end bearing is provided, insuring a rigid blade assembly at low engine speeds.

d. When the control worm rotates the blade stop gear (fig. 7), the gear rotates the tube on which the gear is mounted through a ratchet clutch on one end. A stationary threaded shaft is mounted inside the tube, carrying two adjusting nuts which are keyed to the tube. When the tube rotates, the nuts rotate and travel together along the stationary shaft until one nut pushes the driving clutch out of engagement with the blade stop gear. The gear continues to turn, but the clutch, tube, and worms mounted on the tube stop rotating. Reversal of the control reverses the direction of rotation of the blade stop gear and drives the tube through a second ratchet clutch of the opposite hand on the other end of the gear. The nuts on the stationary shaft travel together toward the second clutch, eventually pushing it out of engagement and stopping rotation of the tube and worms. By adjusting the distance between outer faces of the adjusting nuts on the blade stop shaft, the rotation of the blade stop mechanism may be limited to a definite number of revolutions before the clutches disengage the blade stop gear. Thus a means is provided to limit the operation of the propeller gear train within definite blade angle limiting stops.

e. A control housing adapter (fig. 8), which locates a stationary control worm concentric with the propeller shaft, is mounted on the engine crankcase front cover. The stationary control worm has two threads on its outer circumference; a rear or right-hand worm thread, and front or left-hand worm thread. When the stationary control worm is in its neutral position, the worm threads are out of mesh with the propeller hub gears. By shifting the control worm forward or rearward, one or the other of the worm threads is engaged with the teeth of the blade stop gear in the propeller hub. The blade stop gear is rotated one tooth space by the stationary control worm with each revolution of the propeller hub. The rotation of the blade stop gear is transmitted through the blade stop worms to the three blade stop worm shafts which mesh with the blade gears attached to the ends of the blades. The movement of the blade gear nut turns the blades in their socket in the hub.

f. The nut on the end of the blade shank has gear teeth cut around its circumference to engage the blade worm shaft in the final stage of the propeller gear train. The blade gear is screwed directly over the buttress threads on the blade shank and is locked securely by a serrated lock plate doweled and screwed to the blade end.

g. To overcome slight variations in the weight of the blades, balance weights are installed in the gear housing. The position of these weights is not disturbed unless it is intended to balance the propeller.

13. Installation and removal. -  a. The instructions for installing propellers as outlined in section IX are followed when installing this type of propeller.

(1) In order to obtain the proper location of the blade stop gear in the propeller hub with respect to the control housing worm in the control housing adapter, four rear cone shims are furnished with each propeller. The shims vary in thickness from 1/32 inch to 3/32 inch by 64ths. Place the thickest shim over propeller shaft sliding it back against thrust bearing nut. Assemble bronze rear cone, sliding cone back against the shim. Install crankshaft thread guard on propeller shaft. The purpose of this tool is to protect the threads on the propeller shaft and to prevent the threads from scraping any plating from the inside of hub splines.

(2) The propeller, from which the retaining nut, front cone, and snap ring have been removed, is raised to the proper height and pushed back over the thread guard and over the splines of the crank-shaft. The thread guard is then removed. If facilities are available, the tail of the airplane is raised to a level position before the propeller is installed or removed. Place the halves of front cone on flange of retaining nut and screw on crankshaft. Firmly tighten nut in place by means of a bar approximately 5 feet in length, passed through the nut so as to extend equally on each side. Two men of average weight should use the bar, one pushing upward, the other pulling downward, and applying an equal force on each end of the bar. For final tightening of the retaining nut, use a 2-pound lead hammer applying about two or three raps on the downward or pulling side of the bar close to the nut. Install snap ring hub nut lock, spacer, and screw. Tighten screw and safety to the hub. The model P-432-2 propeller hub retaining nut is locked to the hub with lock and cotter pin. The head of the lock pin is on the inside of the propeller hub nut.

(3) A check is made as to the location of the propeller hub with respect to the adapter assembly. This is done on the model P-5315 propeller installation by inserting a thickness gage in the clearance between the propeller locating in (on the rear of the propeller) and the front face of the propeller control housing. This installation clearance is 3/16 inch, plus or minus 0.008 inch. The installation clearance for the model P-432-2 propeller is 0.030 inch plus or minus 0.008 inch as measured from the rear face of the hub cover to the front face of the control adapter. If the clearance as measured above is greater or less than specified, the propeller is removed and the rear cone shim replaced with one of such thickness as will provide an installation clearance within the tolerance.

(4) Following installation of the propeller, control, and pitch indicator, the following tests are made : before operating for the first time, turn propeller over by hand in direction of engine rotation to insure clearance of all engine and airplane parts. Check all wiring from switches to propeller. Check stops in pitch indicator to determine if they are set for the minimum and maximum angle of propeller.

b. To remove propeller, remove hub nut lock screw, spacer, and lock from flange in front of the hub. Unscrew hub nut an amount sufficient to loosen hub from rear cone. Before removing propeller from crank-shaft, remove snap ring from hub and unscrew hub nut further so as to remove front cone and nut. Screw crankshaft thread guard on propeller shaft and then remove propeller.

14. Lubrication.-a. The hub of the Lycoming-Smith controllable propeller is to be full of grease during all propeller operations. A grease filler connection is furnished for use in lubrication. The hub of the propeller is lubricated as follows :

(1) Remove plug from one of the cover plates on side of hub and assemble grease connection.

(2) Remove screw from venthole opposite cover plates.

(3) Rotate propeller so venthole is upward.

(4) With clean grease gun, completely fill hub with grease. When filled, grease will overflow from venthole.

(5) Remove grease connection from side of hub. and replace plug and screw. Safety screw to hub cover screws.

b. If propeller hub has been disassembled and filled with grease prior to reinstallation, the grease will pack along the blade bearings during the first few minutes of operation. It is necessary to refill hub after 5 minutes of ground operation. Before installing propeller hub on shaft, care must be taken to clear grease from annual opening in hub cover plate in order to permit entrance of control worm.

15. Propeller controls.-a. General. - Rapid engagement and disengagement of the stationary control worm with propeller hub gears is accomplished by means of an electric solenoid control unit (fig. 9) which is assembled on the control adapter on the engine. The control is operated by relays from switches in the cockpit. These are momentary type switches, and by the correct manipulation of the switch, the pilot can obtain any angle in a predetermined range. The solenoid control assembly is a 12-volt, 2-wire radio shielded unit, consisting of a housing and cap assembly in which are incorporated two solenoids. The two solenoid plungers are assembled on the ends of a rack which meshes with the lower of two gear segments splined to a supporting shaft. An adjustable rack support bearing mounted on the housing base prevents distortion of the plunger and rack assembly and reduces friction on the solenoid plungers. The upper gear segments mesh with the top rack which is riveted to both the bumper plate assembly and operating rod. The bumper plate actuates the pitch indicator contact switches mounted on the housing. A slot provided in the top rack engages with the shift arm in the propeller control housing. The cycle of operation is as follows : Upon closing the propeller control switch in the cockpit, one of the control relays closes thereby energizing one coil in the solenoid control unit. The solenoid actuates the solenoid plunger and rack, longitudinal motion being transmitted by means of gear segments to the operating rod and top rack which engages the control housing shift arm. Operation of the control housing shift arm rotates the shift ring and control worm through a small angle. This movement of the control worm causes the blade stop mechanism to rotate, which in turn will make the gear train operative through a series of gears to the blades which turn the blades in the hub. When the desired blade angle has been reached, the propeller control switch is returned to the neutral position. The springs provided at either end of the operating rod return the control housing shift arm to the neutral position, thereby disengaging the propeller shifting mechanism in the propeller and opening the pitch indicator circuit. The operating rod is held in the neutral position by a ball and spring device when the propeller control is not in use.

b. Installation.-(1) Mount control adapter on front crankcase cover over bolts projecting through the cover so that shift arm is in the middle of its travel and extends vertically downward. Assemble washers and nuts on bolts and tighten evenly. Lock nuts together with safety wire. See that adapter assembly operates freely and without binding after installation by moving shift arm to its limit of travel. Apply clean engine oil to felt seal in groove in adapter.

(2) Remove control housing cap from propeller control and mount cap in groove on upper half of control adapter so that the narrow side section extends to the rear. Oscillate cap to insure that head of pin in control adapter fits into hole in cap. Adjust shift arm in middle of its travel so that it points straight downward. Hold cap in place on control adapter and assemble propeller control housing assembly with flat face forward in groove on lower half of control adapter. When sliding studs through holes in the cap, be sure that shift arm in adapter assembly enters into slot in operating rod of con trol assembly. Assemble nuts and washers on studs in control housing and tighten evenly. Safety nuts with cotter pins. Install supports. Use standard Air Corps drawing to determine the location of supports.

c. Propeller pitch, indicator.-(1) This instrument indicates at all times the exact blade angle at which the propeller is operating. In principle, the indicator consists of a small gear mechanism having exactly the same ratio as the propeller gearing and driven from the gun synchronizer drive connection in the engine. An electrically operated clutch in the pitch indicator engages the indicator gear mechanism simultaneously with the gear mechanism in the propeller thus causing the indicator hand to follow the movement of the blades throughout their range of blade angles.

(2) Before installing pitch indicator assembly, check model designation as being correct for the particular propeller being installed. Also see that pitch indicator dial hand stops are set for maximum and minimum blade angles stamped on propeller blades. These stops or the pitch indicator are adjusted by removing the dial snap ring and glass from the instrument. Attach one end of a standard flexible drive shaft assembly to the pitch indicator drive assembly mounted on the gun synchronizer pad with gasket and cap screws. Attach the other end of flexible drive shaft assembly to fitting on reduction unit which is mounted on pitch indicator assembly. The installation of electrical lines and conduit for the pitch indicator are made in accordance with a standard wiring diagram for the particular airplane on which it is to be installed.

16. Operation. a. Start engine and open throttle until approximately 800 r. p. m. are obtained and then allow engine to warm up. To insure low pitch (high r. p. m.), open throttle to 1,000 r. p. m. and manually operate propeller control to low pitch (high r. p. m.) until there is no further increase of r. p. m. indicated on the tachometer. Do not operate propeller control while engine is operating at less than 1,000 r. p. m.

b. For take-off, climb, and cruising, operate propeller control manually to increase or decrease the r. p. m. in accordance with operation instructions for the particular engine airplane combination to obtain best operating r. p. m. for flight conditions.

c. When gliding in to land, manually operate propeller control to low pitch (high r. p. m.). This position allows maximum r. p. m. to be developed in case of an emergency.

17. Inspection and inspection maintenance.-a. As part of the daily inspection, the range of operation of the propeller is checked as follows :

(1) With the engine warmed up to operating temperature, set throttle to give 1,000 r. p. m.

(2) Move propeller control switch to the "decrease r. p. m." position and hold it until no further decrease in r. p. m. is noted on the tachometer. Check pitch indicator to see that it indicates the maximum angle setting of propeller.

(3) Move propeller control to the "increased r. p. m." position and hold it until no further increase in r. p. m. is noted by the tachometer. Check pitch indicator to see that it indicates minimum angle setting of propeller.

(4) Check each propeller as outlined above.

b. The control switches, relay box, and pitch indicator are inspected for loose screws, bolts, pins, etc.; all conduit, wire, and electrical terminals are inspected for corrosion, accumulation of dirt and oil, and looseness of terminals. All defects are corrected before flight.

c. The procedure below is followed when checking retaining nut for looseness.

(1) Remove locking device.

(2) Insert a 5-foot bar evenly spaced into retaining nut, one man weighing approximately 175 pounds pulling on the end of the bar while another pushes up on the opposite end. The force is applied steadily with no sudden jerks. Final tightening is accomplished by striking the bar next to retaining nut two blows with a 2-pound hammer.

(3) Check installation clearance.

(4) Safety retaining nut.

d. Maintenance problems.-The maintenance problems listed below are most likely to occur during normal operation.

(1) Grease leaking from around rear of hub is a common occurrence and is caused by the felt seal in the control adapter being worn. To correct this requires the removal of the propeller and replacement of the gasket.

(2) Failure of the pitch indicator to register the blade angle may be caused by improper connections or a broken shaft. All wiring should be checked in accordance with Air Corps Technical Orders and the shaft inspected for loose connections and condition.

(3) Failure of the propeller to change pitch will normally be noticed at the preflight inspection. To correct this, remove front plate from solenoid housing and move propeller switch in cockpit to either "increased" or "decreased" r. p. m. position. This closes the circuit to the solenoid, which should operate the rack; if not, check all wiring for proper connections. Check relay for condition of wiring and connections. If these checks prove that all electrical connections are in good condition, the trouble may be in the control adapter or propeller. In either case, the service of a trained propeller mechanic should be secured to correct them.

FIGURE 6.-Lycoming-Smith controlIabIe propelIer

FIGURE 7.-Blade stop mechanism (schematic view).

FIGURE 8.-Gear train.

FIGURE 9.-Propeller controls.