Aircraft technical Basics: TM 1-412, Aircraft Propellers, 1941: VI. Hamilton Standard Constant Speed Propeller

SECTION VI. HAMILTON STANDARD CONSTANT SPEED PROPELLER

32. Principle of operation.-The Hamilton standard constant speed propeller (fig. 17) utilizes the same hydraulic principle of operation as the two-position controllable propeller, except that the action of a spring return assembly is added. This spring assembly, which is installed inside the propeller piston, is compressed when the propeller shifts to low pitch (high r. p. m.) thus aiding the operating force of the counterweights in returning the propeller to high pitch (low r. p. m.). The spring force is greatest in full low pitch, becoming less as the pitch shifts toward high pitch. About two-thirds of the way from full low pitch (high r. p. m.) to full high pitch (low r. p. m.) the spring force is discontinued and the counterweights alone provide the entire operating force. It is because of the increased angular travel of the counterweights and the slope of the counterweight cams that the spring return is needed with 20° propellers. In low pitch (high r. p. m.), the counter-weights are closer to the axis of rotation of the hub than they are in high pitch (low r. p. m.) position of a two-position controllable (10° pitch range). Consequently for the same r. p. m. the centrifugal force on the 20° counterweights is less. Either heavier counter-weights must be used or additional operating force provided in some other way to obtain responsive pitch changing. It is preferable to utilize the added force of the spring return assembly rather than to make the counterweights heavier. Due to the design requirements of the 20° pitch range propeller, the usual engine oil pressure is not great enough to shift the blades to low pitch (high r. p. m.) position and it is necessary to use a booster pump. This is incorporated in the Hamilton standard constant speed control. The pump takes oil from the engine lubricating system and raises its pressure to 180 to 200 pounds per square inch. A built-in relief valve regulates the pressure and returns all oil to the pump except what is actually required to shift the propeller pitch and take care of leakage at the transfer rings. Only a very small quantity of oil is drawn from the engine, inasmuch as the propeller demands oil only when going to a lower pitch (higher r. p. m.) setting. The constant speed control takes the place of the three-way valve. It regulates the flow of oil to and from the propeller cylinder, automatically shifting the pitch so as to maintain constant r. p. m. The pilot can set the constant speed control to any r. p. m. within the operating range. The constant speed control shifts the propeller pitch as necessary to maintain this r. p. m. Under these conditions, the pilot does not directly control the pitch. However, provision is made so that he may discontinue the constant speed action and shift the propeller to positive high pitch in which case the propeller will remain in full high pitch (low r. p. m.) position. Except where a limiting r. p. m. stop has been installed on the constant speed unit, it is possible to move the controls to obtain positive low pitch. This would permit operation similar to that for a two-position propeller not having a constant speed control.

33. Description.-The description in paragraph 26 applies to the Hamilton standard constant speed propeller except as modified herein. The difference in construction is as follows :

a. Shim plates and laminated shims are used to obtain proper barrel fit of the hub assembly. Shim plates are made of self-lubricating material (oilite). Formica chafing strips are not used as the barrel is supported by micarata blocks. These shim plates are free to turn on the spider, no dowel pins being used. To insure a reasonably tight over-all fit of the blade assembly parts which are held between the spider and the barrel, laminated shims are peeled to the required thickness and placed between the oilite shim plates and the spider.

b. The piston is of steel. Care is taken in turning the threads at its base to insure their proper fit on the engine crankshaft. A shoulder is turned at the outer end of the piston on which gaskets fit. These gaskets form an oil tight surface between the cylinder and the piston and serve as a guide to the cylinder as it moves back and forth. A similar but smaller gasket is provided at the base of the piston to act as a seal between the piston and oil supply pipe. Several holes are bored radially through the base of the piston. These permit crankcase ventilation through the engine shaft. The same piston can be used for engines having other means of crank-case ventilation. Interchangeability is accomplished by the use of the proper engine shaft oil supply pipe adapters. A spring assembly fits inside the piston and is carried on a puller bolt attached to the cylinder head. When the cylinder moves outward to high r. p. m. (low pitch), the springs are compressed, thus introducing additional force to help the counterweights return the pitch to low r. p. m. (high pitch).

c. Each counterweight bracket carries a counterweight bearing assembly which consists of a curved steel counterweight bearing race, a curved bearing retainer, and a circular steel cap race. The counter-weight bearing race fits snugly into the cam slot of the counter-weight bracket. The circular cap race has a ball seat on its outer face into which the ball seat of the bearing shaft fits. The retainer is held in place between the two races by means of the bearing shaft. When the bearing shaft is tightened in place, the fit of the counter-weight bearing assembly should be such as to provide 0.002-inch to 0.003-inch clearance between the oilite thrust washer and the face of the counterweight bracket. To prevent bending of the counterweight bearing retainers and to generally improve the action of the counter-weight bearings, a spacer is added to the assembly. The spacer maintains the alinement of the bearing retainer with respect to its inner and outer races. It provides a guide for the retainer, permitting it to work back and forth freely but preventing it from bending outward under the pull of centrifugal forces.

d. Grease retainers are provided to retain the grease inside the spider arms and blade cavities. They are made of leather and shaped on one side to fit the fillet at the base of the spider arm. The flat side of the retainer rests against the face of the blade bushing. The sides of the retainer are supported by a special coil spring.

34. Installation and removal.-a. Installation of the propeller is accomplished as follows :

(1) Thoroughly clean out the inside of front portion of crank-shaft. Remove screw plug from inside crankshaft.

(2) On Wright engines, install oil supply pipe with gasket in feed line inside crankshaft. On Pratt and Whitney engines, install oil supply pipe and adapter inside end of crankshaft.

(3) Install bronze rear cone on engine shaft against thrust nut and thread guard with sleeve over threaded end having splines of thread guard alined with those of the shaft.

(4) See that the blades of the propeller are set in low r. p. m. (high pitch) position. Remove cylinder head lock wire and clamp nut lock wire installed in hexagon portion of cylinder head. Unscrew clamp nut and remove gasket and vernier lock plate (figs. 18 and 19). Unscrew cylinder head, using wrench furnished with propeller. Remove complete spring return assembly by unscrewing piston gasket nut. Remove piston. To remove piston, the propeller is shifted toward the low pitch position in order to remove front cones from piston.

(5) Oil splines of crankshaft and hub and oil rear cone and cone seat.

(6) Aline spline and slide hub well back on shaft.

(7) Aline cylinder by means of blade beams on blades until proper cylinder alining bushing slides freely over sleeve and into cylinder.

(8) Remove cylinder alining bushing, sleeve, and thread guard.

(9) Check oil supply pipe packing nut for tightness and proper safety.

(10) Coat threads of piston with thread lubricant and insert piston and front cones. It will be necessary to shift the propeller pitch as given in paragraph 34.

(11) Having alined the cylinder, the piston will screw on easily unless the piston or shaft threads are damaged. If piston does not start on easily, check condition of these threads. The use of lapping compound on these threads is considered a dangerous practice.

(12) Insert proper wrench and tighten piston, using a bar 4 feet long. Apply a force at the end of the bar of approximately 175 pounds.

(13) Install complete spring assembly in piston and tighten down against piston gaskets. Use a wrench provided and a bar approximately 2 feet long.

(14) Place cylinder head gasket on cylinder head. A light coating of grease will hold this gasket in place.

(15) Screw cylinder head on cylinder. This should be tightened with wrench and bar used on spring assembly. As cylinder head is tightened, the clamp washer on splined spring puller bolt will enter guide on underside of cylinder head. The purpose of this guide is to help center the spring puller bolt.

(16) Lock cylinder head with its lock ring.

(17) Lock cylinder head to spring puller bolt by means of vernier lock plate. By turning vernier one cog at a time, a combination will be found which will allow the vernier to be pushed in place. The groove and ring on one side of the vernier are to facilitate its removal and should be toward the front.

(18) Place clamp nut gasket in cylinder head face.

(19) Tighten clamp nut on threaded end of spring puller bolt. A relatively short wrench should be used. The object is merely to hold the clamp washer on the spring puller bolt tightly against the cylinder head and provided an oil seal.

(20) Lock clamp nut with its lock wire.

(21) Check all lock wires and cotters

b. The method of removing the propeller from the crankshaft is the reverse of installation procedure as follows :

(1) Move blades toward full low r. p. m. (high pitch) position to remove any compression of spring return assembly. It is important that this is done before removal of clamp nut. This will prevent the threads of the clamp nut from being stripped by the pulling force in the compressed spring return assembly.

(2) Remove clamp nut lock ring and unscrew clamp nut.

(3) Remove vernier lock plate and clamp nut gasket. Failure to remove vernier lock plate before attempting to unscrew cylinder head will result in serious damage to puller bolt.

(4) Remove cylinder head lock ring and unscrew cylinder head.

(5) Unscrew piston gasket nut and pull out with spring assembly.

(6) Remove two piston gaskets.

(7) Unscrew piston. Slide propeller slowly forward on engine shaft and remove. Take care not to damage engine shaft threads.

35. Lubrication.-Before the propeller assembly is installed on the engine, or if the propeller is assembled and placed in storage, the hub spider is filled and the counterweight shaft bearings thoroughly coated with grease.

a. At the specified periodic inspection, the counterweight shaft bearings are coated with grease, the purpose being to prevent corrosion by means of a protective film of grease more than to actually lubricate the moving parts. The counterweight shaft bearings can be lubricated through the slot in the counterweight without removing the cap. If the counterweight cap is removed, care is excercised to insure that the location of the stop nuts on the adjusting screw is not disturbed as any change in the position of the stop nuts will result in a change of the blade angle.

b. Lubricate hub spider through lubricator fittings provided. Since these propellers have grease retainers which prevent the grease inside the spider arms and blade cavities from leaking out past the shim plates, there should be no trace of lubricant at the barrel ends. If grease does appear at this point, it is an indication that a grease retainer is not functioning properly and should be replaced. However a check should be made to insure that the grease is not surplus grease which has been forced into the hub shell during the lubricating periods.

36. Propeller controls.-a. The governor is a device used in conjunction with the Hamilton standard controllable propeller to automatically maintain the engine r. p. m. constant at any speed selected by the pilot (fig. 20). It does this by changing the blade angles to meet new conditions of altitude, airplane attitude, and throttle setting. The control permits the independent setting of engine power and engine speed. This means that with the specified propeller it will permit the engine to develop any selected power at any selected speed and will maintain that engine power and speed throughout all flight conditions until the pilot readjusts the governor controls for some new operating r. p. m.

b. The only limits to this are those imposed by the governing range of the governor and the mechanical stops in the propeller counter-weights. Under conditions demanding a lower or higher blade angle than is possible within the limitation of the propeller itself, the governor, although trying to hold the r. p. m. constant, is unable to do so because the blades are prevented from assuming a lower or higher angle by their counterweight stops. Under these conditions, the propeller will act as a fixed pitch propeller and the throttle will govern r. p. m. as well as manifold pressure.

c. The governor is an independent unit whose only function is to regulate the flow of oil to and from the propeller. Since the magnitude of the blade angles of this type of controllable propeller depends on the position of the cylinder, which in turn depends on the volume of oil in the cylinder, it is apparent that a mechanism which automatically permits the flow of oil under pressure to the propeller cylinder and the drainage of oil from the cylinder in a rapid cycle will enable the blades to assume an indefinite number of angle settings.

d. Horsepower varies with r. p. m. and manifold pressure. The r. p. m. is controlled, within the limits previously mentioned, by the governor; the manifold pressure is regulated by the throttle.

e. Because it is the governor rather than the throttle which controls the engine r. p. m., it is possible to use a larger range between the minimum and maximum angle limits of the propeller blades and consequently it is possible to obtain higher horsepower for take-off and power descent. With the two-position controllable propeller, the minimum blade angle must be high enough to prevent overspeeding of the engine during take-off and climb; the maximum blade angle must be such as to permit cruising at the specified conditions of r. p. m. and manifold pressure at altitude. The factors limiting the blade angle range for a constant speed propeller are, that in case of failure of the mechanism, the minimum angle shall be high enough to permit sustained flight without overspeeding the engine, and the maximum angle shall be low enough to permit flight without increasing the manifold pressure beyond the engine manufacturer's specified limits. The lower minimum angle permits better take-off characteristics since it allows the full horsepower of the engine to be developed early in the takeoff. The higher maximum angle permits power descents from altitude without overspeeding of the engine.

f. The governor is a self-contained assembly which is mounted on one of the engine accessory pads or on a specially built pad on the nose of the engine. The unit's drive is coupled to the engine by a suitable gear ratio to insure that its operating range coincides with the engine r. p. m. range. A small gear booster pump is incorporated in the unit. This pump takes oil from the engine lubricating system and increases its pressure to 180-200 pounds. The oil pressure is regulated by a relief valve through which all oil not actually required to shift the propeller angle is returned to the inlet side of the pump. Only a small quantity of oil is drawn from the engine system, inasmuch as the propeller demands oil only when going to a lower angle or to replace that lost at the shaft transfer rings. Governing action is obtained by flyball forces working against a speeder spring. The metering of oil to and from the propeller cylinder is dependent on the degree of balance which exists between these forces.

g. Engine oil enters the governor unit through the base and is led to the low pressure side of the booster gear pump. As the oil passes through the pump, its pressure is controlled to 180-200 pounds. From the high pressure side of the pump, the oil is led past the relief valve and into the hollow drive gear shaft through ports located in the upper portion of the shaft. Whether this pressure oil is permitted to go through the lower propeller ports of the drive gear shaft and out to the propeller or whether it will circulate through the relief valve, the hollow idler gear shaft, and return to the inlet side of the booster pump, depends upon the position of the pilot valve with relation to the propeller ports. The position of the pilot valve depends in turn upon the relation of the centrifugal force generated by the fly-balls (fig. 21) to the force exerted by the compression of the speeder spring. Under theoretically stable conditions of flight, these two forces are balanced and the pilot valve covers the propeller ports in the drive shaft. If the throttle setting is changed, or if the altitude or attitude of the airplane is altered, or if the pilot desires a different engine speed and changes the constant speed cockpit control setting, the balanced condition which existed between the forces of the flyball and the speeder spring will be disturbed. Any unbalance in the fly-ball speeder spring forces will allow one of the two forces to override the other and cause the pilot valve to open the propeller ports in the drive gear shaft, either to the position which allows the high pressure oil in the shaft to flow to the propeller cylinder or to the position which allows the oil in the propeller cylinder to drain into the engine sump.

h. Figure 22 indicates three states of balance and unbalance for three different operating conditions.

(1) The first, "On speed," is the condition which exists when the factors in flight are constant; that is, the airplane is flying level at a selected r. p. m. and manifold pressure. There is no tendency for any change in the desired engine r. p. m. The flyballs are turning at a constant speed and exerting a constant force against the balancing compression of the speeder spring. The propeller ports in the drive gear shaft are just closed by the pilot valve. Since no oil is flowing either to or from the propeller cylinder, the angle of the blades is being held fixed. The arrows show that oil is passing through the booster pump and being by-passed through the relief valve to the inlet side of the pump.

(2) The second, "Underspeed," shows what happens when the fly-ball speeder spring system is thrown into a state of unbalance caused by a small decrease in engine r. p. m. Such a case occurs momentarily when the airplane is pulled up in a steep climb or when the throttle is suddenly moved toward the closed position. Since the flyballs are driven at a definite speed with relation to the engine r. p. m., their rotation will decrease with the decreasing engine r. p. m. This will cause a lessening of their centrifugal force which will permit the speeder spring to override the flyballs and force the pilot valve downward. This movement of the pilot valve opens the propeller ports to the high pressure oil. This oil enters the propeller cylinder, moving the blades to a lower angle and permitting the engine to regain and hold its r. p. m.'s at the desired value.

(3) The third, "Overspeed," indicates what happens when the engine r. p. m. tend to increase. Such a condition occurs when the airplane is nosed down or when the throttle setting is increased. With the increase in engine speed, there is an increase in the flyball rotational speed and a corresponding increase in centrifugal force. The flyball speeder spring system is thrown out of balance as the force generated by the flyballs overrides the speeder spring load. In moving outward, the flyballs lift the pilot valve and open the propeller ports to the drain position. Oil flows from the propeller cylinder permitting the blades to assume a higher angle and thus bring the engine r. p. m. back to the desired figure.

(4) Under actual operating conditions, the pilot valve speeder spring system is rarely in the position indicated by the "On speed." Due to more or less constant changes in altitude, bumpy air, and variations in engine hp. requirements due to maneuvers of the airplane, the governor is continually operating to make corrections in propeller blade angles necessary to maintain the engine r. p. m. constant. It is rarely possible to have the balanced condition indicated in the "On speed," as there is a continual leakage of oil in the engine transfer rings. Since the angle of the propeller blades is regulated by the volume and pressure of oil in the propeller cylinder, it is necessary to supply an additional amount of oil equal to that lost through the transfers in order to hold the blades at a desired angle. To accomplish this, the propeller ports in the governor drive shaft must be "cracked" open. The extent of this opening depends on the amount of transfer ring leakage.

i. The unit is removed as follows :

(1) Disconnect cockpit control from unit.

(2) On units having external piping, disconnect pipe connections.

(3) Remove mounting stud nuts.

(4) Remove governor.

(5) On some installations, the fit is so close that it is impossible to raise the unit high enough to clear the mounting studs. In these eases, the usual procedure is to remove the cap section of the unit's case. This is done by removing the nuts and palnuts which fasten the cap section to the body section. The cap section may now be lifted from the unit. As the cap section is lifted, the control shaft should be turned counterclockwise looking at the face of the pulley. The purpose of this is to disengage the speeder spring adjusting rack and the control shaft. If the cap section is removed with the pilot valve spring collar assembly, it cannot be raised high enough to pull the pilot valve out of the drive gear shaft and still keep these two parts in alinement. This alinement is kept in order to prevent side loads which might bend the spring collar spindle. If it is necessary to remove a unit between propeller overhaul periods temporarily, the control is moved to full decrease r. p. m. position, the pulley or lever and shaft are marked and removed from control shaft. This will permit reinstallation in exactly the same position and will facilitate the adjustment of the control system.

j. The installation of the governor and cockpit controls will be in accordance with the installation drawings covering the particular air-plane extent that the following precautions are taken :

(1) The propeller governors are checked to insure their proper functioning before installing on an engine.

(2) To install a governor, remove cover from surface on which governor is to be mounted. Then set governor in position, checking fit of governor circular lining boss. In some cases it is found that this boss is slightly larger than the opening on the engine into which it fits. If this condition exists, further investigation is made to determine which part is to be reworked.

(3) The governor securing nuts are then screwed on the mounting studs and run down finger tight. Remove governor head and check backlash and freedom of movement while tightening governor securing nuts. It is essential to tighten these nuts down evenly. The securing nuts are not to be drawn down excessively tight as this may cause displacement of the gasket material in the vicinity of the mounting studs and result in warping the governor base. In some cases it has been noted that the governors are susceptible to binding after having the nuts tightened. This condition can normally be relieved by slacking off slightly one or more nuts. The nut or nuts causing the difficulty can be determined by trial.

(4) Another precaution which is advisable is that during the tightening of the securing nuts the propeller shaft is rotated to at least three positions, checking the governor for backlash and freedom of movement at each point.

(5) It is good practice to check the governors for freedom of movement before installing, as a few cases have been reported where cold temperatures and improper fit of bearings have caused the governors to bind. Therefore, any binding or drag in the governor is thoroughly investigated before attempting to install it on an engine.

(6) If binding or drag is experienced with two or more governors, it is advisable to check the engine drive parts. Engines equipped with the governor drive in the nose sections can be checked by removing the nose section and installing a governor on it. If the governor binds, the vertical drive gear in the nose section is checked for freedom by rotating it back and forth to determine the backlash. The alinement of the governor drive spline in the vertical drive gear is determined by working the vertical drive gear up and down. Both these checks are made at several different points by removing the governor and turning the vertical drive shaft to a new position and then reinstalling the governor.

(7) The primary precaution is to make sure the governor turns freely when assembled to its drive on the engine.

k. The angular range required at the constant speed unit to give rated r. p. m. at one end and positive high angle at the other is only a part of the unit's total angular range. Before flying, it is important that the control system between the governor and the cockpit is adjusted to set the unit for rated r. p. m.

1. For trial setting, place cockpit lever in extreme rear position. Turn pulley or lever attached to governor control shaft in a clock-wise direction until the rack bottoms in the cap. Connect control cable or rod extending from cockpit control lever to pulley or lever attached to governor shaft. Loosen increase r. p. m. stop and shift away from governor. Start engine and operate on ground until desired take-off r. p. m. is obtained, moving governor lever in cockpit forward. Stop engine, exercising care to insure that setting of governor lever in cockpit is not disturbed. Adjust increase r. p. m. stop toward governor until it bottoms against pulley stop or lever. Make flight test to insure that governor is adjusted properly. If during flight, the rated r. p. m. cannot be obtained or if excessive r. p. m. is encountered, readjustment of the controls is necessary. To increase the r. p. m., the governor stop is loosened and shifted away from the governor pulley or lever. Make another trial flight adjusting cockpit control lever until rated r. p. m. is obtained, marking the quadrant at this position. After landing and stopping engine, a-just cockpit control lever to marking on the quadrant and reset governor stop. To decrease the r. p. m., adjust cockpit control during trial flight marking quadrant, and after landing and stopping engine, adjust cockpit control to marking and governor stop until it firmly bottoms against governor control lever or pulley.

37. Operation. - a. Prior to starting an engine equipped with a Hamilton standard constant speed propeller, the propeller control is placed in "positive high pitch" position. After starting, the engine is run for at least 1 minute with the control in this position.

b. The propeller control is shifted to low pitch (high r. p. m.) position for the warm-up. The warm-up must be at the required r. p. m. and to the specified temperature as outlined in operating instructions for the particular type of engine.

c. The propeller cockpit control is moved to the "increase" r. p. m. position for take-off and climb. As the airplane increases from zero speed at the start of the take-off toward flying speed, the engine r. p. m. will increase until it reaches the amount for which the high r. p. m. limit stop has been set. From this point on, the r. p. m. is held constant by the governing action of the unit. Soon after take-off, it is desirable to reduce both the manifold pressure and the r. p. m. The logical sequence is to reduce the manifold pressure first and then the r. p. m. All movements of the throttle and propeller cockpit control are done slowly.

d. Once the r. p. m. has been brought to the desired tachometer reading for normal level flight, it is held constant by the governor. Any change in the cruising r. p. m., or manifold pressure is made by first adjusting the propeller control to the new value and then the manifold pressure. This is especially true of multiengine installations. Once the propellers have been brought into synchronization, the manifold pressure may be altered without causing any change in r. p. m. or necessitating resynchronizing.

e. If the governor is operating satisfactorily, it will permit a power descent without overspeeding the engine. The unit compensates for the increased airspeed of the descent by increasing the propeller blade angles. If the descent is too rapid or is being made from a high altitude, the maximum blade angle limit of the blades is not sufficient to hold the r. p. m. constant. When this happens, the r. p. m. is responsive to any change in the throttle setting.

f. As the manifold pressure and airspeed are reduced in the approach for a landing, the r. p. m. is held constant by the governor and the propeller is moved to a lower blade angle. When conditions of manifold pressure and airspeed are such as to require a blade angle equal to the minimum angle stops in the counterweights, the blades will be against the minimum angle stops in order to maintain the r. p. m. constant, and the governing action will be unable to operate them to a lower angle.

(1) Some pilots consider it advisable to set the governor control for maximum r. p. m. during the approach in order to have full horsepower available in case of emergency. If the governor control is set for this higher r. p. m. early in the approach when the blades have not yet reached their minimum angle stops, the r. p. m. will in-crease to unsafe limits. If, however, the governor control is not readjusted for the take-off r. p. m. until the approach is almost completed, the blades will be against, or very near, their minimum angle stops and there will be little if any change in r. p. m. In case of emergency, both throttle and propeller controls can be moved to take-off positions.

(2) Most pilots prefer to feel the airplane respond immediately when they give short bursts of the throttle during approach. By making the approach under a little power and having the governor control set at or near cruising r. p. m., this result will be obtained.

(3) Although the governor responds quickly to any change in throttle setting, a sudden and large increase in the throttle setting will cause a momentary overspeeding of the engine until the blades become adjusted to absorb the increased power. If during approach an emergency demanding full power should arise, the sudden advancing of the throttle will cause momentary overspeeding of the engine beyond the r. p. m. for which the governor is adjusted. This temporary increase in engine speed acts as an emergency power reserve.

g. For the purpose of preventing exposure and corrosion of the propeller piston on idle engines, the propeller control is shifted to "positive high pitch" prior to stopping the engine.

38. Inspection and inspection maintenance. - a. The range of operation of the propeller is checked as follows :

(1) After the engine is warmed up to specified operating temperature, set throttle to give approximately 1,600 r. p. m. with propeller in low pitch position.

(2) Move propeller control to "positive high pitch" and observe corresponding pitch change and decrease in engine r. p. m. When no further decrease is indicated on the tachometer, the propeller is in full high pitch (low r. p. m.) position.

(3) Move propeller control to "take-off" r. p. m. position and observe the corresponding pitch change and increase in engine r. p. m. When no further increase is indicated on the tachometer, the propeller is in full low pitch (high r. p. m.) position.

(4) Move propeller control in constant speed position. Any movement of the throttle within limitations is not indicated by an increase or decrease in engine r. p. m. The propeller is then functioning in the constant speed position.

b. After the daily or preflight warm-up, an inspection is made of the propeller and governor for oil leaks. Special attention should be paid to the mounting base of the governor. Any leaks noticed are corrected before flight. The more probable cause for oil leaks are

(1) Cylinder head; loose cylinder head or damaged gasket.

(2) Base of cylinder; damaged or worn piston gaskets.

(3) Grease leaking from around blade; grease retainers are worn or improperly installed.

(4) Base of governor; gasket damaged or loose mounting stud nuts.

c. The propeller controls are inspected for full movement from cock-pit control to governor. Cables are inspected for size and tension. Torque rods and bell cranks are inspected for freedom of movement. Any indication of binding or interference is corrected before flight.

d. The procedure for checking the piston of a Hamilton standard constant speed propeller is as follows:

(1) With the aid of blade beams, move propeller into full high pitch (low r. p. m.) position.

(2) Remove clamp nut lock wire and clamp nut.

(3) Remove vernier lock plate.

(4) Remove cylinder head lock wire and cylinder head.

(5) Unscrew piston gasket nut and remove spring return assembly.

(6) Insert proper wrench into piston. With a 4-foot bar, one man weighing approximately 175 pounds pulling on the end of the bar is sufficient to check the piston for looseness. A steady pull on the end of the bar without jerking is recommended to make the check.

(7) Install spring return assembly and tighten piston gasket nut.

(8) Install cylinder head and cylinder head lock wire.

(9) Install vernier lock plate, clamp nut, and clamp nut lock wire.

e. When cleaning and lubricating the exposed portion of the piston of a Hamilton standard constant speed propeller having a spring return assembly, the following procedure applies :

(1) With the aid of blade beams, move propeller into full high pitch (low r. p. m.) position.

(2) Remove clamp nut lock wire, clamp nut, and vernier lock plate.

(3) Remove cylinder head lock wire and cylinder head.

(4) Unscrew piston gasket nut and remove spring return assembly.

(5) By using blade beams, move propeller into full low pitch (high r. p. m.) position.

(6) Clean exposed portion of piston with approved cleaning fluid. Remove all corrosion and defects by careful handstoning and by the use of crocus cloth.

(7) Lubricate piston with clean engine oil.

(8) Move propeller into full high pitch (low r. p. m.) position.

(9) Install spring return and tighten piston gasket nut.

(10) Install cylinder head and cylinder head lock wire.

(11) Install vernier lock plate, clamp nut, and clamp nut lock wire.

f. The problems and corrections as listed in section V are applicable to this type of propeller. Other maintenance problems and their corrections are as follows :

(1) When installing a governor on an engine, trouble may be encountered in synchronizing the propeller with the governor and controls. In such case, Air Corps Technical Orders on this equipment should be consulted.

(2) It must be remembered that the spring return is compressed when the propeller is in low pitch (high r. p. m.) position. In event it is necessary to have the propeller in low pitch so as to be able to perform the required maintenance on the piston, the engine should be stopped with the propeller in high pitch (low r. p. m.) position. Remove the spring return, and with the aid of blade beams, the propeller is placed in low pitch (high r. p. m.) position. Never attempt to remove the spring return unless the propeller is in full high pitch position.

(3) During operation, it is sometimes difficult to maintain a constant r. p. m. This has all the symptoms of the propeller control "creeping" and may be due to the cable "stretching." Readjust the tension on the cable and check all mountings.

(4) Constant speed propellers without the spring return may seem slow and sluggish in return to high pitch (low r. p. m.) position. Probably the engine r. p. m. is not sufficiently high with correspondingly high centrifugal force to return the propeller to high pitch. Excessive blade friction may be the cause. The adjustment in and to the governor should be checked if neither of the above corrections help the condition.

(5) With the propeller control in the take-off position (low pitch), the propeller returns toward the high pitch position at full throttle. Check linkage between governor control shaft and cockpit and ascertain if governor control shaft has its full movement of travel.

FIGURE 17.-Hamilton standard constant speed propeller.

FIGURE 18.-Removal of spring return assembly,

FIGURE 19.-Removal of vernier lock plate and clamp nut.

FIGURE 20,-Governor assembIy.

FIGURE 21.-Oil system of governor and propeller.

FIGURE 22.-Governor operating principle.