Aircraft technical Basics: TM 1-412, Aircraft Propellers, 1941: V. Hamilton Standard two-position Propeller

SECTION V. HAMILTON STANDARD TWO-POSITION PROPELLER

25. Principle of operation.-a. An oil pressure line from the main pressure supply in the engine is conducted to a three-way valve, and thence through a collector ring into the interior of the front end of the crankshaft and out into the pitch operating cylinder (fig. 14). The three-way valve is so arranged that rotation of the valve stem will cause the oil line leading to the propeller to be connected with the pressure supply in one position or with a drain directly into the crankcase in the other position. When this oil valve is thrown into the pressure supply position, the oil flows through the collector ring into the crankshaft and out into the cylinder, causing the cylinder to move forward on the piston.

b. As the cylinder moves forward, the counterweight bearing shaft, which is attached to the base of the cylinder, moves in the cam slot of the counterweight bracket. This bracket is attached to the blade bushing by means of index pins. As the bearing shaft moves up the cam, it causes the bracket to turn. This turning of the bracket is transmitted to the blade, rotating it to high r. p. m. (low pitch) position.

c. When the three-way valve is turned to the "off" position, the oil pressure on the cylinder head is released. On the arm of each blade bracket is a counterweight. The centrifugal force generated by these counterweights turns the brackets and rotates the blade to the low r. p. m. (high pitch) position.

d. Most of the later high-powered engines incorporate a built-in valve as standard equipment. Adaptions for other engines have been worked out by the engine manufacturers.

e. The action of the hydraulic and counterweight controls is such that extra force is available for movement into high r. p. m. (low pitch) when the revolutions are below the normal value, and extra control forces are available for going into low r. p. m. (high pitch) when the revolutions are above the normal value. A typical Hamilton standard two-position propeller is shown in figure 13. The oiling system of this propeller is shown in figure 14.

26. Description.-The various units of this propeller are shown in figures 14 and 15. Following is a brief description of the principal units :

a. The spider is made from heat-treated steel forgings. Through the arms of the spider the principal forces are transmitted between the blades and the propeller shaft.

b. The barrel is of steel and is cadmium plated. It takes up the centrifugal force of the blades. A micarta bushing which fits around the bottom of the spider prevents the barrel from chafing the spider.

c. Oilite shim plates protect the spider from galling. On some propellers, a formica strip is attached to their circumference preventing chafing of the barrel. To insure a reasonably tight over-all fit of the parts which are held between the spider and the barrel, laminated shims are peeled to the required thickness and placed between the shim plates and the spider.

d. The piston is of steel. Its base is threaded to fit on the engine crankshaft. A shoulder is turned at the outer end of the piston on which gaskets fit. These gaskets form an oiltight surface between the cylinder and the piston and serve as a guide to the cylinder as it moves back and forth.

e. The cylinder is of aluminum alloy. A steel liner is installed in the cylinder to prevent the piston gaskets from wearing its inner surface. Threaded holes are located in the flanged base of the cylinder, one opposite each spider arm. A stop plug at the base of each hole limits the distance; the bearing shaft can be screwed into the cylinder, thus assuring the proper fit of each bearing assembly. A bronze bushing in the outer end of each hole prevents the bearing shaft from wearing the aluminum cylinder.

f. The counterweight bearing shaft is of steel. One end is threaded to fit the cylinder holes. The other end consists of a cone which fits the seat in the cap race, and a short extension of the shaft which contacts adjusting nuts and limits the travel of the counterweight bearing shaft.

g. The counterweight bearing consists of a curved steel race, a curved bearing retainer, and a circular steel cap race. The curved (inner) race fits snugly into the cam slot of the counterweight bracket. The circular cap race has a seat on its outer side, into which the cone on the bearing shaft fits. The retainer is held in place, between the two races, by means of the bearing shaft.

h. The counterweight bracket is of steel. The outer end contains the cam slot in which the counterweight bearing moves. The other end fits around the blade bushing. The portion of the bracket next to the blade bushing is scalloped with 40 semicircular holes. Four of these semicircles can be matched with four of the 36 semi-circles which compose the base circumference of the blade bushing. Index pins, tapped into the four alined sets of semicircles, assure the translation of any lateral turning movement of the bracket into a rotating motion of the blade.

i. A steel counterweight is fastened to the outer face of the bracket by fillister screws. A slot in the counterweight corresponds to the cam in the bracket. It is up and down this slot that the cylindrical extension of the bearing shaft moves. Along one side of this slot is a scale. It has degree graduations which are stamped during final assembly and correspond with protractor measurements of the blade at the 42-inch station. Toward one end of the slot is a lead fillet on which is stamped the base setting of the blade.

j. The adjusting screw with its nuts fits into the slot in the counter-weight. It is held from turning by a pin. The nuts may be turned up or down the screw, independently of each other, to any desired position indicated on the stamped scale. As the bearing shaft moves in the cam, its extended end contacts these nuts, thus limiting the pitch setting of the blades.

k. A counterweight cap screws on the face of the counterweight. It acts both as a protecting cover for the adjusting screw and as an added weight whose centrifugal force will help pull the blades into the high pitch setting.

1. The blade shank is of the semihollow type. The semihollow construction makes it possible to almost double the bending strength of the inner portion of the blade without excessive increase in weight.

m. The taper bore of the blade end with its tapered bushing, together with the form of the outside of the blade, provides almost complete freedom from any tendency toward localization of stresses which might cause the blade to break at the shank.

n. The thrust bearing assembly consists of two thrust rings which cannot be removed from the blade, and a split thrust bearing retainer. These roller thrusts bearings are designed for a high capacity. In spite of the great centrifugal forces, they permit the blades to rotate with minimum friction.

o. The design of the roller bearing races makes possible the use of extremely large fillets with resulting increase in resistance to fatigue. The location of the roller bearing around the outside of the blade, in conjunction with the centering action of the spider on the inside of the blade, permits a large roller area without excessive weight.

p. Blade bushings are of aluminum bronze. They are pressed tightly into the hollow blade ends and secured by drive pins and lock screws. Care is not only taken to assure correct alinement between the inside of the bushing and the blade face, but also to make certain that the semicircles on the circumference of the bushing base are in proper relationship to the blade pitch.

27. Installation and removal.-Before installing the propeller, the applicable provisions for preparing the crankshaft for installation as listed in section IX are followed.

a. The installation of a Hamilton standard two-position propeller is accomplished in the following order:

(1) Remove plug from crankshaft and clean the inside.

(2) Install bronze rear cone and rear cone spacer (if used) on engine shaft against thrust nut. On direct drive Wright engines, screw oil supply pipe without gasket into oil plug, which is located inside the crankshaft. Do not pull up oil supply pipe connection too tightly, as this is likely to swage the internal oil supply pipe in the crankshaft sufficiently to choke off the oil supply. On geared Wright engines, the oil supply pipe gasket is used with the oil supply pipe.

(3) Remove cylinder head lock wire and unscrew cylinder head. Remove lock ring from hexagon portion of piston. On propellers for Wright engines, remove piston gasket nut, oil pipe packing nut, and packing. The piston gasket will now be loose and is removed in order to prevent damage. The piston gaskets are left in place on other installations.

(4) Place propeller on crankshaft. Make sure that piston and crankshaft threads are in perfect alinement. In no case should force be used to tighten the piston if there is binding or indication that the threads are not properly started, as serious damage may result. Where it is found that due to handling or reassembling without proper bushings the piston and shaft are not in alinement, the counterweights are disassembled and the bearing shafts removed. This frees the cylinder and piston to permit easy starting of the threads and proper tightening of the propeller on the shaft. The counter-weights are then reassembled. Care is taken in tightening the piston to see that the front cone packing washer, when one is required, does not bind but is pulled properly in place. As an aid in installation, it is suggested that the piston be tightened a few turns and then the hub jarred slightly by hand. This will help prevent jamming the washer into the shaft threads.

(5) Insert proper wrench and tighten piston using a bar 4 feet long. Apply a force on the end of the bar of approximately 175 pounds. One man of average weight (175 pounds) using a bar of this length can usually tighten the piston sufficiently without the need of additional leverage or the use of a hammer on the bar. The force is applied steadily on the bar.

(6) Secure piston with lock ring. Safety lock ring, using steel cotter pins. The heads of these pins are toward the crankshaft.

(7) On propellers for Wright engines, install piston gasket nut and safety. On other engine installations, check to see that piston gasket nut is safetied. On propellers for Wright engines, put supply pipe packing and packing nut in place, tighten packing nut and secure with safety wire.

(8) Install cylinder head gasket on cylinder head. In order to avoid possibility of the gasket becoming damaged during assembly, care is taken to insure that it is properly held in place on the pilot of the cylinder head. This is accomplished by either the use of heavy grease or by lightly tapping the gasket with a hammer while it is on the cylinder head.

(9) Before installing the cylinder head, thoroughly coat threads on cylinder head and in cylinder with thread lubricant. Extreme care is exercised when installing the cylinder head to insure that threads are properly started. Tighten cylinder head sufficiently to prevent oil leaks, using the proper wrench with a leverage of not more than 18 inches.

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

(1) Before propeller is removed from the crankshaft, blades are placed in the low r. p. m. (high pitch) position. Use blade beams to change blade angle. Never use a hammer or mallet on blades, cylinder head, or counterweights when changing position of the blades.

(2) Disengage cylinder head lock wire and remove cylinder head. A container is used to catch the oil from the cylinder when cylinder head is removed. On Wright engines, loosen supply pipe packing nut and unscrew piston gasket nut.

(3) Disengage piston lock ring by removing cotter pins. It is good practice to slide the lock ring up on the piston. The propeller should be in the high r. p. m. (low pitch) position to remove the cotter pins.

(4) Unscrew piston. This will start the propeller off the engine shaft.

(5) Slide propeller slowly forward on engine shaft and remove. Take care not to damage the threads of engine shaft. On Wright engines, care is taken not to damage the oil supply pipe.

28. 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.

29. Propeller control.-a. Engines are adapted for the two-position propeller by extending the engine oil pressure system through the crankshaft or propeller shaft to operate the propeller mechanism. This pressure is controlled by a three-way valve located in most cases on the nose of the engine. This valve is operated from the cockpit by a push-pull or quadrant type of control. The control is either a pulley and cable or torque rod arrangement, depending on the type of airplane on which it is to be installed. Suitable stops in the cockpit prevent unnecessary strains on the control valve unit. The travel of the control lever on the three-way valve is limited by projections of the valve mounting studs. In operation, the cockpit control is moved forward (if mounted horizontally, and down if mounted vertically) to open the three-way valve which allows oil to flow into propeller cylinder, forcing the propeller into low pitch (high r. p. m.) position. A movement of the control to the rear (if mounted horizontally, and up if mounted vertically) will cause the valve to be moved to the drain position, which allows the oil to flow back to the crankcase, and the propeller goes into high pitch (low r. p. m.) position. Figure 16 shows a typical single-engine installation of the controls.

b. The controls are installed by the airplane manufacturer. However, in event it is desired to install propeller controls, a standard Air Corps drawing should be consulted and the directions followed.

30. Operation.-General operating instructions are as follows :

a. The test of operation is judged by observing the changing of pitch during the process of running up. This is observed by watching the travel of the cylinder on the front of the propeller or by watching the change in r. p. m. on the tachometer. In this connection, it should be borne in mind that hydraulic pressure changes the blade angles from low r. p. m. (high pitch) setting to high r. p. m. (low pitch) whereas centrifugal force acting on the counterweights changes the blade angles from high r. p. m. (low pitch) to low r. p. m. (high pitch). Therefore, the shift from high r. p. m. (low pitch) to low r. p. m. (high pitch) is accomplished more rapidly at high engine r. p. m. because of the greater centrifugal force generated.

Conversely, shifting from low r. p. m. (high pitch) to high r. p. m. (low pitch) is more rapid at lower engine speeds when centrifugal forces are of lower value.

b. Prior to starting the engine equipped with a two-position Hamilton standard controllable propeller, the control is placed in the "de-creased" r. p. m. position in order to avoid possible lack of oil to master rod bearings.

c. After starting, the engine will be run for approximately 1 minute in the "decreased" r. p. m. position, after which the propeller is shifted into the "increased" r. p. m. (low pitch) position and maintained in this position during the warm-up. With the propeller in the "increased" r. p. m. position check engine r. p. m. at full throttle.

d. For take-off and climb, the propeller control is placed in the "increased" r. p. m. (low pitch) position until the altitude is reached at which level flight is intended. Overspeeding of the engine is avoided by use of the throttle. As maximum efficiency is essential when in a climb near the ground after take-off, propeller's should not be shifted to low r. p. m. (high pitch) position until an altitude of at least 1,000 feet has been obtained, unless level flight is to be assumed at a lower altitude. Where the rate of climb is below normal, the engine is throttled slightly to prevent excessive r. p. m.

e. The propeller control is placed on the "decreased" r. p. in. (high pitch) position for normal level flight.

f. During glides and landing, the propeller control is placed in the high r. p. m. (low pitch) position for the most effective use should an emergency take-off become necessary.

g. For the purpose of preventing exposure and corrosion of the propeller piston on an idle engine, the propeller cylinder is with-drawn by shifting the propeller to the "decreased" r. p. m. position prior to stopping the engine. To inspect the propeller cylinder for galling and wear, the engine is stopped with the propeller control in the "increased" r. p. m. position.

31. Inspection and inspection maintenance.-a. As part of a periodic inspection, the range of operation is checked as follows :

(1) When the engine has been warmed up to the specified operating temperature, set throttle to give approximately 1,400 r p. m. with propeller control in low pitch (high r. p. m.) position.

(2) Shift propeller control into high pitch (low r. p. m.) position and note decrease in engine r. p. m. on tachometer. This decrease in engine r p. m denotes propeller is functioning correctly, the amount of change depending upon the range of the propeller.

(3) Without changing position of the throttle, move propeller control to low pitch (high r. p. m.) position and note increase in engine r. p. m. This change signifies propeller is going into low pitch (high r. p. m.) position.

b. A loose cylinder head or a damaged cylinder head gasket will allow oil to leak from around the cylinder head. This leakage will be noticed during the warm-up and is corrected before flight. A damaged piston gasket or loose piston gasket nut will allow oil to leak from around the base of the cylinder.

c. An inspection is made of all barrel bolts, piston lock ring, cylinder head, counterweight bearing shaft, and counterweight cap to determine the condition of the safetying devices. Replace all worn or damaged cotter pins and lock wires. Position heads of pins so that centrifugal force will tend to hold pin in place in case of a failure.

d. At the specified periodic inspection, remove cotter pins from piston lock ring and disengage the ring; the propeller should be in the low pitch position. With the aid of blade beams, turn blades to high pitch position and remove cylinder head lock wire and cylinder head. Insert proper wrench into head of piston. Use a bar approximately 4 feet in length and weight of an average man on the end of the bar to check piston for looseness. No extension nor hammering on end of bar is permitted. Replace cylinder head and cylinder head lock wire. Turn blades to low pitch position and reposition piston lock ring and safety. On Wright engines, the supply pipe packing nut and piston gasket nut are removed before attempting to check the piston for looseness.

e. The propeller control in the cockpit is checked for security of mounting and freedom of movement. All bell cranks, rods, tubes, and cables are inspected for defects and corrosion. Determine by movement of cockpit control that operating arm of the three-way valve has full range of movement. Coat all controls with clean engine oil.

f. With propeller in full low pitch (high r. p. m.) position, clean exposed portion of piston with kerosene, gasoline, or some approved cleaning fluid. Inspect piston for wear, corrosion, galling, etc. ; remove any defect. by careful handstoning with fine sandpaper or crocus cloth. Coat surface of piston with clean engine oil. Place propeller in full high pitch position.

g. During normal operation, maintenance problems may develop which can be corrected by the airplane mechanic. A number of such problems are given below. In event the correction requires the disassembly of the propeller, the services of a trained propeller mechanic should be obtained.

(1) A slight trace of grease between the barrel and blades after flight is considered normal, but when it becomes excessive the condition is corrected. First ascertain from where the grease is coming. It may be that during the lubrication of the hub spider, grease has leaked from around the grease fitting. This will collect in the cavity between the barrel and blades and during engine operation will be thrown out. If this is not. the case, the propeller should be removed from the shaft and the grease retainers replaced, which would require disassembly of the propeller.

(2) During warm-up, the cylinder runs eccentric when in low pitch (high r. p. m.) position. This is usually due to the fact that the projection of the counterweight bearing shaft is not striking the upper adjusting nuts simultaneously. This may be corrected by stopping the engine in full low pitch (high r. p. m.) position and removing the counterweight caps. Check counterweight adjusting screws to see if they are held tightly by counterweight bearing shaft. bearing against adjusting nuts. If one or more counterweight adjusting screws are found loose, make corrections by adjusting nuts until they all bear against counterweight bearing shaft at the same time. If any adjustment is made, the blade angle of the propeller is checked.

(3) Engine oil leaking from around the cylinder head is easily corrected. First check cylinder head for tightness,

FIGURE 13.-Hamilton standard two-position propeller

FIGURE 14.-Cross section of propeller hub.

FIGURE 15. - Counterweight assembly.

FIGURE 16.-Propeller control.