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INTRODUCTION Since the first powered flight, propellers have been used to convert aircraft engine power into thrust. Although many modern transport category aircraft are powered by turbojet or turbofan engines, most of the aircraft in use today are propelled by one or more propellers that are driven by either a turbine or reciprocating engine Regardless of the engine type, the primary purpose of a propeller is to convert engine power to thrust. Therefore, as an aircraft maintenance technician, you must have a thorough understanding of the basic principles, maintenance, and repair of propeller systems.
BASIC PROPELLER PRINCIPLES The aircraft propeller consists of two or more blades and a central hub to which the blades are attached. Each blade of an aircraft propeller is essentially a rotating wing. As a result of their construction, propeller blades produce forces that create thrust to pull or push the airplane through the air. Power is furnished by the engine. On low-horsepower engines, the propeller is mounted on a shaft that is usually an extension of the crankshaft. On highhorsepower engines, the propeller is mounted on a propeller shaft that is geared to the engine crankshaft. In either case, the engine rotates the airfoils of the blades through the air at high speeds, and the propeller transforms the rotary power of the engine into thrust.
NOMENCLATURE You must be familiar with some basic terms and component names All modern propellers consist of at least two blades that are connected to centre of hub.
Nearest the hub is the blade shank
hub. or hub assembly. is bored out to create a hub bore which permits a propeller to be mounted
furthest from the hub is called the blade tip
The blades of a single-piece propeller extend from the hub assembly. Blades have a shank and a tip. while the hub assembly has a h\lb pore and bolt holes that facilitate propeller mounting.
The nomenclature for an adjustable propeller, or ground-adjustable propeller, is in figure
The variation in airfoil shape and blade angle along the length of a propeller blade compensates for differences in rotational speed and allows for a more even distribution of thrust along the blade.
Some large turboprop propeller blades are fitted with blade cuffs to improve the airflow around the blade shanks.
The Blade-Element Theory The first satisfactory theory for the design of aircraft propellers was known as the blade-element theory. This theory was evolved in 1909 by a Polish scientist named Dryewiecki; therefore, it is sometimes referred to as the Dryewiecki theory
According to the blade-element theory, the many airfoil sections, or elements, being joined together side by side, unite to form an airfoil (the blade) that can create thrust when revolving in a plane around a central axis. Each element must be designed as part of the blade to operate at its own best angle of attack to create thrust when revolving at its best design speed
All propeller blades have a leading edge. a trailing edge. and a chord line A propeller 's blade angle is the acute angle formed by a propeller's plane of rotation and the blade 's chord line The curved, or' cambered side of a propeller blade is called the blade back The flat side is called the blade face.
Blade root has a flanged butt. or shoulder. which mates with groo in the hub assembly.
All propeller blades have a leading edge, a trailing edge, and a chord line. In addition, all propeller blades are set at a specific angle that is defined by the acute angle formed by the propeller's plane of rotation and the chord line.
PROPELLER THEORY Like the wing's curvature creates a low pressure area above the wing The amount of thrust produced depends on several factors Including: -the angle of attack -the speed the blades -the shape of the airfoil The angle of attack of a propelle r blade is the angle formed by the chord line of the blade and the relative wind. The direction of the relative wind is determined by the speed an aircraft moves through the air and the rotational motion of the propeller.
In fnrward flight, the airplane moves from point A to point B while the propeller moves from point C to point D. In thl. case, the propeller's trailing edge follows the path from C to D which represents the resultant relative wind. This results in an angle of attack that is less than the blade angle.
With no forward velocity, the relative wind is directly opposite the movement of the propeller blade. In this case, a propeller's angle of attack is the same as its blade angle.
If the forward velocity of the aircraft remains constant, but a propeller's rotational speed increases. the propeller's trailing edge will move a greater distance for a given amount of forward movement. This increases the angle at which the relative wind strikes the propeller blade which, in turn, increases the angle of attack.
The most effective angle of attack for a propeller blade is between 2 and 4 degrees Any angle of attack exceeding 15 degrees is ineffective because of the possibility of a propeller blade stall. The circumference of a circle is calculated with the formula: 2π r to find rotational velocity is circumference multiplied by engine rpm To compensate for the difference in velocity along a propeller blade, each small section of the propeller blade is set at a different angle Decrease in blade angle from the hub to the tip is called pitch distribution Blade twist allows the propeller to provide a fairly constant angle of attack along most of the length of the blade. A thicker, low speed airfoil near the blade hub and a thinner, high speed airfoil near the tip. Combined with blade twist, permits propeller to produce relatively constant amount of thrust along a propeller blade’s entire length.
To aid in identifying specific points along the length of a propeller blade, most blades have several defined blade
stations
As a propeller blade rotates at a fixed rpm, each blade segment moves through the air at a different velocity.
FORCES ACTING ON A PROPELLER Centrifugal force can best be described as the force which tries to pull the blades out of the hub-greater than 7,500 times the weight of the propeller blade Thrust bending force attempts to bend the propeller blades forward at the tips Torque bending forces occur as air resistance opposes the rotational motion of the propeller Blades tends to bend the blades opposite the direction of rotation Aerodynamic twisting force tends to increase a propeller's blade Angle Centrifugal twisting force opposes aerodynamic twisting force in that it attempts to decrease a propeller's blade angle.
When a propeller is rotating, centrifugal force tries to pull propeller blades away from the hub. Torque bending forces exert a pressure that tends to bend the blades opposite the direction of rotation Thrust bending forces exert a pressure that tends to bend the propeller blade tips forward. The majority of thrust produced by a propellers exerted ahead of the blade's axis of rotation. This produces an aerodynamic twisting' force that attempts to increase a propeller's blade angle. Centrifugal twisting force attempts to decrease blade angle by aligning a propeller's center of mass with its center of rotation. The majority of thrust produced by a propeller is exerted ahead of the blade's axis of rotation. This produces an aerodynamic twisting' force that attempts to increase a propeller's blade angle.
PROPELLER PITCH Propeller pitch is the theoretical distance a propeller advances longitudinally in one revolution. A geometric pitch is defined as the distance , in inches, that a propeller will move forward in one revolution if it were moving through a solid medium and did not encounter any loss of efficiency. Effective pitch is the actual amount a propeller moves forward in one revolution . The difference between geometric pitch and effective pitch is called slip
Geometric pitch is the theoretical distance a propeller would move forward if it were 100% efficient. Effective pitch, on the other hand, is the actual distance a propeller moves forward in one revolution. Slip is the difference between geometric and effective pitch.
Tip Speed Flutter or vibration may be caused by the tip of the propeller blade traveling at a rate of speed approaching the speed of sound, thus causing excessive stresses to develop. This condition can be overcome by operating at a lower speed or by telescoping the propeller blades-that is, reducing the propeller diameter without changing the blade profile. Tip speed is actually the principal factor determining the efficiency of high-performance airplane propellers of conventional two- or three-blade design. It has been found by experience that it is essential to keep the tip speed below the speed of sound, which is about 1116.4 ft/s [340.28 m/s] at standard sea-level pressure and temperature and varies with temperature and altitude. At sea level, the speed of sound is generally taken to be about 1120 ft/s, but it decreases about 5 ft/s [152.4 cm/s] for each increase in altitude of 1000 ft [304.80 m].
The efficiency of high-performance airplane propellers of conventional two- or three-blade design may be expressed in terms of the ratio of the tip speed to the speed of sound. For example, at sea level, when the tip speed is 900 ft/s [274.32 m/s], the maximum efficiency is about 86 percent, but when the tip speed reaches 1200 ft/s [365.76 m/s], the maximum efficiency is only about 72 percent. It is often necessary to gear the engine so that the propeller will turn at a lower rate of speed in order to obtain tip ratios below the speed of sound. For example, if the engine is geared in a 3:2 ratio, the propeller will turn at two-thirds the speed of the engine. When the propeller turns at a lower rate of speed, the airfoil sections of the blades strike the air at a lower speed, and they therefore do not do as much work in a geared propeller as they would do in one with a direct drive. It is necessary in this case to increase the blade area by using larger-diameter or additional blades.
Ratio of Forward Velocity to Rotational Velocity The efficiency of a propeller is also influenced by the ratio of the forward velocity of the airplane in feet per second to the rotational velocity of the propeller. This ratio can be expressed by a quantity called the V-over-nD ratio (or the slip function, as discussed previously), which is sometimes expressed as a formula, V/(nD), where V is the forward velocity of the airplane in feet per second, n is the number of revolutions per second of the propeller, and D is the diameter in feet of the propeller. Any fixed-pitch propeller is designed to give its maximum efficiency at a particular aircraft speed, which is usually the cruising speed in level flight, and at a particular engine speed, which is usually the speed employed for cruising. At any other condition of flight where a different value of the V/(nD) ratio exists, the propeller efficiency will be lower.
Propeller Load A propeller being driven at a given speed will absorb a specific amount of power. It requires more power to drive a propeller at high speeds than at low speeds. Actually, the power required to drive a propeller varies as the cube of the rpm. This is expressed by the formula hp = K x rpm³ where K is a constant whose value depends on the propeller type, size, pitch, and number of blades. Another formula that can be used to express the same principle is
It requires eight times as much power to drive a propeller at a given speed than to drive it at half that speed. If the speed of a propeller is tripled, it will require 27 times as much power to drive it as it did at the original speed. .
Propeller Efficiency Some of the work performed by the engine is lost in the slipstream of the propeller, and some is lost in the production of noise. The lost work cannot be converted to horsepower for turning the propeller. The effect of tip speed on propeller efficiency has already been examined. In addition, the maximum propeller efficiency that has been obtained in practice under the most ideal conditions, using conventional engines and propellers, has been only about 92 percent, and in order to obtain this efficiency it has been necessary to use thin airfoil sections near the tips of the propeller blades and very sharp leading and trailing edges. Such airfoil sections are not practical where there is the slightest danger of the propeller picking up rocks, gravel, water spray, or similar substances that might damage the blades.
The thrust horsepower is the actual amount of horsepower that an engine-propeller unit transforms into thrust. This is less than the brake horsepower developed by the engine, since propellers are never 100 percent efficient. In the study of propellers, two forces must be considered: thrust and torque. The thrust force acts perpendicular to the plane of rotation of the propeller, and the torque force acts parallel to the plane of rotation of the propeller. The thrust horsepower is Jess than the torque horsepower. The efficiency of the propeller is the ratio of the thrust horsepower to the torque horsepower:
Propeller Torque Reaction Torque reaction involves Newton's third law of physics: for every action there is an equal and opposite reaction. As applied to the airplane, this means that as the internal engine parts and propeller are revolving in one direction, an equal force is trying to rotate the airplane in the opposite direction. Asymmetric Loading (P Factor
Asymmetric Loading (P Factor When an airplane is flying at a higl\ angle of attack (climbing), as illustrated in Fig. 19- 20, the load of the downwardmoving propeller blade is greater than the load on the upward-moving propeller blade. This moves the center of thrust to the right of the propeller center, thus causing a yawing moment toward the left around the vertical axis of the aircraft. With the airplane being flown at positive angles of attack, the right or down-swinging propeller blade is passing through an area of resultant velocity which is greater than that affecting the left or upswinging blade. Since the propeller blade is an airfoil, increased velocity means increased lift. Therefore, the down-swinging blade, having more lift, tends to pull the aircraft's nose to the left (all directional references are from the pilot's seat looking forward).
PROPELLER CONTROLS AND INSTRUMENTS PROPELLER CLEARANCES • • •
Ground Clearances Water Clearance Structural Clearances
PROPELLER CLASSIFICATIONS Classified according to their position on the aircraft -Tractor propellers are mounted on the front of an engine -Pusher-type propellers are mounted on the aft end of an aircraft Propellers also are classified by the method used to establish pitch fixed pitch. ground adjustable. controllable pitch. constant speed. reversible. And feathering.
fixed-pitch propeller. Propeller with a low blade angle. often called a climb propeller. Propeller with a high blade angle. often called a cruise propeller. Ground-adjustable cannot be changed in flight
Controllable-pitch may be adjusted to any angle between a minimum and maximum pitch setting. Constant-speed automatic propellers, controlling device known as a
governor.
Provide maximum efficiency by allowing the pilot to control the propeller blade angle for most conditions encountered in flight. Reversible-pitch blades can be rotated to a negative angle to produce reverse thrust Most multi-engine aircraft equipped with a featherable propeller. The feather position eliminate a great deal of the Drag.
Propeller construction Wood, steel, aluminum or some type of composite material. Some composite materials are now being utilized because of their light weight and flexibility. Wood used on low horsepower engines of small aircraft. Aluminum alloy More desirable than wood because it allows thinner, more efficient airfoils to be constructed without sacrificing structural strength Aluminum Alloy More desirable than wood because it allows thinner More efficient airfoils to be constructed without sacrificing structural strength
Better airflow for engine cooling Less maintenance than wood propellers, thereby reducing the operating cost.
Steel Antique and older generation transport aircraft. heavy metal, steel blades are normally hollow consisting of steel sheets attached to a rib structure. filled with a foam material to help absorb vibration and maintain a rigid structure. Composite Slowly gaining in popularity. Lightweight. Extremely durable Absorb vibration and are resilient Resistant to damage and corrosion.
FIXED-PITCH PROPELLERS FIXED-PITCH CLASSIFICATIONS Propeller with the lower blade angle provides the best performance for takeoff and climb is often called climb propeller However, once the aircraft reaches its cruising altitude and begins to accelerate the low blade angle becomes inefficient. Slightly higher blade angle is called a cruise propeller. Efficient at cruising speed and high altitude flight Inefficient during take off and climb out. A standard propeller compromise between a climb propeller and cruise propeller. Operates from short runways, or high field elevations with a climb propeller Operated at sea level from airports with long runways equipped with a cruise propeller.
PROPELLER CONSTRUCTION Fixed-pitch propellers wood or aluminum Experimental aircraft that utilize wood propellers. WOODEN PROPELLERS A majority of fixed-pitch propellers were made from wood until World War II Hardwoods ash and birch used to build a wooden propeller. Include mahogany, maple, cherry, oak, and black wall nut must be free of grain irregularities, knots, pitch pockets, and insect damage constructed minimum of five layers of wood that are kiln-dried and laminated together with a waterproof resin glue. Once the layers of wood ore laminated together, they form what is called a propeller blank. Rough-shaped blank as a white the center bore and bolt holes drilled and a metal hub assembly is inserted through the hub bore to accommodate the mounting bolts and face plate.
(A)- The first step In the manufacture of a wood propeller is to laminate planks together to form a propeller blank
.(B)- The propeller blank is shaped and its hub is drilled to produce a "white."
(C)- Once sanded smooth, a fabric sheathing and varnish coating are applied for reinforcement and protection.
finished with a black or gray plastic coating that provides additional protection against chipping. Is said to be armor coated.
Meta{ tipping is applied to propeller blade tips and leading edges to help prevent erosion damage. Three small holes drilled in the tip of each blade release moisture and allow the wood to breathe
Monel, brass, or stainless steel applied to the leading edge . And tip of most wooden prop will to prevent damage from small stones. To permit the metal edging to conform to the contour of the leading edge, the metal must be notched. To attach the edging to the blade, countersunk screws thick blade while
copper rivets are used in the thin sections near the tip. Three small holes are then drilled 3/16 inch deep into the tip of each blade Allow moisture to drain allow the wood to breathe.
METAL PROPELLER Propeller blades can be made thinner and more efficient without sacrificing structural strength when using aluminum instead of wood. Has the strength and flexibility to accommodate the high horsepower engines available in today's small aircraft aluminum alloy blades are less susceptible to damage from gravel and debris normally incurred during ground operations. infrequent damage such as small nicks and upsets are easily dressed out with special files , easier to repair than wooden blades. aluminum propellers are much more susceptible to damage caused- by resonant vibrations. Aluminum propellers must be vibrationally tested during the certification process. Once the blade angle is set. the propeller is heat treated to relieve internal stresses. Prevent excessive vibration, all new propellers are balanced both horizontally and vertically. Horizontal balance : By removing metal from the blade tip while vertical balance is achieved by removing metal from a blade's leading and trailing edges. Once the propeller is balanced. the surfaces are finished by anodizing and
On McCauley fixed-pitch propellers. the builders name. model designation. serial number. type certificate number. and production certificate number are stamped around the propeller hub.
A Sensenich aluminum propeller has information stamped on the hub which identifies its hub design blade design, blade length, and pitch. In addition, the number1 stamped on one of the blade roots identifies that blade as blade number one.
Advantages. The advantages of a single-piece fixed pitch metal propeller are (1)simplicity of maintenance, (2)durability, (3)resistance to weathering, (4) light weight, (5) low drag, and (6) minimum service requirements. Such a propeller is efficient for a particular set of operating conditions.
GROUND-ADJUSTABLE PROPELLERS The blade angle to be changed when the aircraft is on the ground and the engine is shut down. Found on older aircraft equipped with radial engines. Ground adjustable propeller consists of two aluminum or steel halves that are machined to form a matched pair. Each hub half is machined out so that the shank of two propeller blades can be held between the two hub halves To prevent centrifugal force from pulling the blades out of the hub, the base, or butt, of each metal blade is machined with shoulders which fit into grooves that are machined into each hub half. If wooden the shoulders are cast or machined into a metal sleeve that is fastened to the blade shank by lag screws
To help ensure that the propeller blades are not pulled out of the hub on a groundadjustable propeller, shoulders are machined into the base of each blade shank. These shoulders fit into grooves that are machined into each hub half.
Ground adjustable propellers utilize either clamp rings or bolts to secure the hub halves and hold the blades tightly.
CONTROLLABLE-PITCH PROPELLERS
Advantage over ground adjustable propellers in that the blade angle may be changed while the propeller is rotating. two-position controllable propeller: or the pitch may be adjusted to any angle between a minimum and maximum pitch setting.
TWO·POSITION PROPELLERS Developed in thtl1930's and permitted the pilot to select one of two positions: low pitch or high pitch. The low pitch setting was used during takeoff and climb. The high pitch setting was used during the cruise phase of flight to permit more efficient high speed flight while increasing fuel economy. Primary components of a two-position propeller include the propeller hub. propeller blades. and a piston assembly. Hamilton-Standard two-position propeller hub is the spider. A typical spider consists of two or three arms on which the blades are attached. The blades are inserted on the hub. Counterweight brackets are attached to the base of each blade
The blade angle on the Hamilton-Standard propeller is changed by using a combination of hydraulic and centrifugal forces. Hydraulic force is used to decrease blade angle while centrifugal force acting on a set of counterweights is used to increase blade angle. Engine oil into the piston assembly is controlled by a three-way selector valve that is mounted in the engine and controlled from the cockpit. When this valve is moved forward to decrease propeller blade angle, engine oil is routed into the piston assembly to force the piston outward. The piston pulls the counter weights in and decreases the blade angle The blades reach their low pitch stop in the counterweight assembly, oil pressure holds the blades in this position. To move the blades to a high pitch position, the propeller control lever is moved aft, rotating the selector valve to release oil pressure in the propeller hub. Rotating the blades to their high pitch position. The blades stop rotating when they contact their high pitch stops located in the counterweight assembly.
When low pitch is selected, engine oil pressure forces the cylinder forward. This motion moves the counterweights and blades to the low pitch position.
When high pitch is selected, engine oil pressure is removed from the piston assembly allowing centrifugal force to move the counterweights outward, This rotates the blades to the high pitch position.
MULTIPLE-POSITION PROPELLERS Two-position propeller was improved to allow the operator to select any blade angle between the high and low pitch stops. For example, during takeoff, the propeller blade angle is set at its lowest blade angle so the engine can generate its maximum power output In a climb, the blade angle can be increased slightly to provide the best climb performance. In cruise flight, the blade angle is further increased to obtain the best cruise performance.
CONSTANT·SPEED PROPELLERS A constant-speed propeller, often called variable-pitch or controllablepitch propeller, is the most common type of adjustable-pitch propeller used on aircraft today. Advantage of a constant speed propeller is that it converts a high percentage of the engine's power into thrust over a wide range of rpm and airspeed combinations. More efficient than other propellers is because it allows the operator to select the most efficient engine rpm for the given conditions. Governor automatically adjusts the propeller blade angle as necessary to maintain the selected rpm. The range of possible blade angles for a constant-speed propeller is called the propeller's constant-speed range and is defined by the high and low pitch stops. As long as the propeller blade angle is within the constant-speed range and not against either pitch stop, a constant engine rpm will be maintained.
OPERATING PRINCIPLES Most constant-speed, non-feathering propellers rely on a combination of hydraulic and centrifugal forces to change the propeller blade angle. Feathering propellers utilize counterweights and centrifugal force to pull the blades to high pitch and oil pressure to force the blades to low pitch. The device that is responsible for regulating the flow of high-pressure oil to the propeller is called the governor. A typical governor does three things; it boosts the engine oil pressure before it enters the propeller hub, it controls the amount of oil that flows to the propeller, and it senses the rotational speed of the engine. Governors consist of three basic components; a gear-type boost pump, a pilot valve, and a speed sensitive flyweight assembly.
A typical propeller governor consists of a gear type boost pump that increases the pressure of the oil before it enters the propeller hub, a pilot valve that controls the amount of oil flowing into and out of the propeller hub, and a flyweight assembly that senses engine speed and positions the pilot valve as needed to maintain a constant rpm
The valve that is responsible for routing oil into and out of the propeller hub is called a pilot valve. The portion of a governor that sense engine speed is referred to as the flyweight assembly. The pilot valve is located inside the drive shaft and extends into the flyweight assembly where it rests on the toe of each flyweight. To allow the operator to select, or set, a desired blade angle, a speeder spring.
The flyweight assembly in a typical governor consists of a set of flyweights that are mounted to a flyweight head that is driven by the governor drive shaft. The pilot valve extends up through the drive shaft and rests on the toe of each flyweight so that, as the flyweights move, the pilot valve also moves. In addition, to allow the operator to select a blade angle, a speeder spring is provided so the amount of force acting on the flyweights and pilot valve can be adjusted.
When a governor is in an underspeed condition, speeder spring pressure is greater than the centrifugal force and the flyweights tilt inward.
When a governor is in an overspeed condition, the centrifugal force acting on the flyweights overcomes the force of the speeder spring to tilt the flyweights outward.
McCAULEY CONSTANT-SPEED PROPELLERS The McCauley series of constant-speed propellers is comprised of both nonfeathering and feathering types
HARTZELL CONSTANT•SPEED PROPELLERS There are two major ways to categorize Hartzell constantspeed propellers. One is by the type of blade angle: nonfeathering, feathering, or reversing. The other is by the typeof metal used in the propeller's construction: steel hub or compact hub (aluminum hub).
Steel Hub Propellers
Figure 12-27. (A)- McCauley threaded blades use retention nuts and threaded ferrule to secure the propeller blades to the hub. (B) - The threadless design is the more modern of the two types of propeller blades and Incorporates a split retainer ring to hold each propeiler blade in place.
Figure 12-28. McCauley constant-speed propellers use oil pressure to increase the blade angle and a combination of centrifugal twisting force and spring pressure to decrease the blade angle
McCAULEY GOVERNORS The governor directs high pressure oil to the propeller hub to increase the propeller blade angle. Difference between the McCauley and Hamilton Standard systems is that the McCauley governor produces an oil pressure of approximately 290 psi instead of the 180 to 200 psi Hamilton Standard.
Figure 12-30. A McCauley non·feathering governor ports high pressure oil to the propeller hub to increase blade angle and releases oil pressure to decrease blade angle.
Governors use a control arm instead of a pulley to adjust the speeder spring pressure acting on the flyweights and pilot valve. The end of the control arm typically has between one and four holes that permit either a rigid control shaft, a flexible control cable, or a combination of the two to be connected to the arm.
For safety purposes. the governor control lever is spring-loaded to the high rpm setting. This way, if the propeller control cable breaks, the propeller blades will automatically go to low pitch allowing the engine to develop its maximum power output. As another safety feature, all McCauley governors incorporate a high rpm stop to prevent the engine and propeller from over-speeding In some cases, a McCauley may also have an adjustable low rpm stop .
Some McCauley governors incorporate both a high and a low rpm stop. The high rpm stop screw Is adjusted to set a minimum blade pitch that allows the engine to turn at its rated takeoff rpm at sea level when the throttle Is opened to allowable takeoff manifold pressure.
Hamilton-standard constant speed propellers The two-position Hamilton-Standard counterweight propeller system was transformed into a constant-speed propeller system. The propeller used with the Hamilton-Standard constant-speed system is essentially the same counterweight propeller used as the two-position propeller. Oil pressure provides the force required to decrease blade angle while centrifugal force acting on the counterweights if used to increase the blade angle. The governor used with a Hamilton-Standard constant-speed propeller is divided into three parts the head. the body, and the base.
(A)- Hartzell steel hub propeller has an exposed pitch-changing mechanism. (B) - On the other hand, the pitch changing mechanism on a Hartzell compact propeller is contained entirely within the hub.
HARTZELL CONSTANT-SPEED PROPELLERS Hartzell produces two types of constant-speed propellers, a steel hub propeller and a Compact model. The pitch change mechanism on Hartzell compact propeller is housed inside the propeller head.
In a Hartzell constant speed, Counterweighted steel hub propeller, oil pressure forces an aluminum piston forward. This motion is then transmitted to the propeller blades through a sliding rod and fork system.
STEEL HUB PROPELLERS The central component of a Hartzell steel hub propeller is a steel spider. A typical spider consists of central hub and two arms. The two arms provide an attachment point for each propeller blade and house a bearing assembly that allows the blades to rotate. The piston is connected to the blade clamps on each blade by a sliding rod and fork system. This way, as oil is directed into and out of the propeller hub, the piston moves in and out and the propeller blades rotate as appropriate.
The aluminum hub of a compact propeller houses the entire pitch-change mechanism. Depending on the model. governor oil pressure may be used to increase or decrease blade angle.
COMPACT PROPELLER Hartzell compact propellers are more modern than steel hub propellers and incorporate several features that make the compact propeller hub smaller, lighter, and more dependable. Hartzell constant-speed propeller systems typically utilize either a Woodward governor or a modified Hamilton-Standard governor.
When a propeller is feathered, the blades are rotated beyond their normal high angle to an angle that is approximately 90 degrees to the plane of propeller rotation, This presents the smallest possible blade profile to t he airstream and decreases aerodynamic drag.
FEATHERING PROPELLERS When an engine fails in flight. the propeller continues to windmill or turn. slowly as air flows over the blades. To help eliminate the drag created by a windmilling propeller. design engineers developed a way to rotate the propeller blades to a 90 degree angle. Today. all modern multiengine propeller-driven aircraft are equipped with feathering propellers. If a propeller is feathered from a low blade angle, the blades will move from a low angel through a high angel before they reach their feathered position. In some systems the propeller can be feathered without engine rotation. To help simplify this discussion only the Hartzell compact feathering propeller and the Hamilton Standard hydromatic propeller will be discussed in detail.
The Hartzell compact feathering propeller depicted above relies on governor oil pressure to decrease blade angle and a combination of blade mounted counterweights and compressed nitrogen in the propeller cylinder to increase blade angle.
Some Hartzell and McCauley propellers use a hydraulic accumulator to unfeather the propeller in flight. The accumulator enables the governor to provide pressurized oil for unfeathering the propeller blades enough so the propeller can windmill
Hamilton Standard Hydromatic propeller is made up of three major assemblies; the hub assembly, the dome assembly. and the distributor valve.
In an overspeed condition, governor oil pressure forces the piston in the dome assembly forward. When this happens, the cam rollers also move forward causing the rotating cam to turn. As the rotating cam turns, the beveled gear attached to the back of the cam engages the sector gears on the propeller blades and rotates the blades to a higher angle.
In an underspeed condition, governor oil pressure is ported back to the engine through the governor. Once the pressure behind the piston decreases below the engine oil pressure, the piston moves aft. As the piston moves aft, the rotating cam turns and drives the propeller blades to a lower blade angle.
To unfeather a Hydromatic propeller, high pressure oil bypasses the governor and forces the distributor valve over so the oil is directed to the outboard side of the piston. At the same time, a passage is provided for oil to drain from the inboard side of the piston.
POWERPLANT - BASIC AIRCRAFT MECHANIC COURSE
CHAPTER 12 Propellers Section D
Turboprop Propellers
Merpati Training Centre
One of the most popular turboprop engines in use today is a free turbine design that houses the reduction gear assembly in the front section of the engine.
The Allied-Signal TPE-331 is a fixed shaft turbine engine with a two-stage centrifugal compressor and a three-stage axial turbine. Depending on the airframe/engine combination, the propeller reduction gearing may be installed either above or below the engine drive shaft
A Hartzell HC-B3TN-5 three-bladed propeller is often used on TPE-331 engines. This particular propeller relies on high pressure oil to move the blades to low and reverse pitch, while spring pressure and centrifugal force acting on counterweights increase propeller blade angle.
TPE-331 turboprop controls include a power lever and a speed. or condition, lever.
The schematic in this figure shows how each of the components in a TPE-331 turboprop propeller system operate to control the propeller
When the power lever is moved toward the REVERSE position, the follower sleeve in the propellerPitch control moves forward exposing an oil port in the oil transfer (Beta) tube. This allows high pressure governor oil to flow to the propeller hub and decrease the
When the propeller blades reach the Beta angle selected by the power lever, the port in the Beta tube becomes blocked by the follower sleeve. This traps high pressure oil In the propeller hub and holds the blades In a set position.
When the power lever is moved forward in the Beta mode, the follower sleeve in the propeller pitch control unports the Beta tube allowing oil to flow out of the propeller hub. This allows the combination of spring tension and centrifugal force acting on the counterweights to rotate the blades to a higher pitch.
To feather a propeller on a TPE-331 engine, the feathering valve must be shifted so the oil within the propeller hull can drain back to the engine case. Once the oil pressure is relieved, spring tension and centrifugal force acting on the blade counterweights can rotate the blades to the feathered position.
The primary governor used on a PT6 engine is similar to most governors in that a set of flyweights are used to sense engine speed and move a pilot valve as necessary to maintain a preset engine speed. However to permit the governor to control the blade angle in the Beta range, a separate Beta valve is also incorporated
As a backup to the primary governor, many PT6 engines incorporate an overspeed governor that prevents the propeller and engine from exceeding their maximum rpm.
The figure above illustrates the location of the components on a PT6 engine that are used to control the engine speed and propeller pitch.
When the power lever is moved aft into the Beta range. the Beta valve is forced inward so that governor oil is directed to the propeller. As the propeller piston moves outward to decrease the propeller blade angle. the feedback ring moves forward and returns the Beta valve to its neutral position.
POWERPLANT - BASIC AIRCRAFT MECHANIC COURSE
CHAPTER 12 Propellers Section E
Auxiliary Propeller Systems
Merpati Training Centre
The basic units of a light twin propeller synchronization system are a control box, master governor, slave governor, and actuator.
This view of a propeller synchronizer system shows the location of the various components when Installed in a light twinengine aircraft.
Synchrophasing allows a pilot to adjust the phase angle between the propellers for minimum noise and vibration levels.
The synchrophasing control panel is mounted in the cockpit so cockpit crew members can adjust propeller blade phase angles in flight
A typical propeller anti-icing system consists of a fluid tank, a rheostat control, a slinger ring for each propeller, and a fluid pump.
Aircraft electrical power is used to operate this propeller de·icing system. When the timer closes the relay, electrical current flows to the carbon brushes which, in turn, pass the current to the rotating slip rings on the propeller hub. Flexible connectors carry the current from the slip rings to each heating element.
POWERPLANT - BASIC AIRCRAFT MECHANIC COURSE
CHAPTER 12 Propellers Section F
Propeller Inspection, Maintenance, and Installation Merpati Training Centre
The metal tipping on a wood propeller should be inspected for cracks in the leading edge and in the solder covering the retaining screws. If a crack is found. a close inspection should be made of the entire area near the crack.
When making a repair to either the leading or trailing edge of a propeller, ensure that the repair will not exceed the propeller blade's minimum dimensions or change the profile of its leading edge.
Repairs on the blade face and leading edge should blend into the blade profile to maintain smooth airflow over the propeller
A protractor is used to determine the amount of bend in a propeller blade. Place the hinge over the center of the bend, set the protractor legs tangent to the blade one inch on each side of the bend centerline, and read the amount of bend in degrees on the protractor
This chart shows the amount of bend damage at given blade station that can be repaired by cold bending. Values that fall beneath the curve are repairable. For example, a 10 degree bend at blade station 22 can be repaired by cold bending. However, a 15 degree bend at station 22 cannot be repaired.
Cracks In the blade retention area of the hub of a groundadjustable propeller are critical defects. Dye-penetrant inspection methods are used to detect such cracks.
In a vertical balance check, the propeller blades are aligned vertically and an imbalance condition causes them to move to a horizontal position.
To check horizontal balance, the propeller is positioned horizontally. Any rotation from this position indicates a heavy blade.
A three bladed propeller is properly balanced when each blade can be placed in the six o’clock position with no tendency to rotate.
A universal propeller protractor such as this one is used to check propeller blade angle
Before measuring propeller blade angle, the protractor must be "zeroed" or adjusted to a reference. A common reference is the propeller hub.
To measure the blade angle, the protractor is held against the blade face and the disk adjuster is rotated until the spirit level centers
To compensate for blade curvature, small pieces of 1/4 inch drill rod are attached to the propeller blade to provide a level surface for the protractor.
A propeller blade paddle makes the task of changing blade angles much easier.
The propeller blade angle of a counterweight propeller is adjusted by positioning the stop nuts located on the index pins inside each of the counterweights
The low pitch setting for Hartzell compact propellers is adjusted with the adjusting screw on the hub cylinder.
A propeller can be tracked by placing a board within 1/4 inch of the propeller arc. Rotate the propeller and mark the path each blade tip follows as it passes the board.
(A) -- On a flanged crankshaft with dowel pin hates, the propeller is mounted to the crankshaft using bolts and nuts. The dowel pin holes are often arranged so the propeller can mount in only one position. (B) – Most installations utilize threaded inserts which are pressed into the crankshaft to eliminate the use of nuts
When Installing a propeller on a four-cylinder opposed engine, one of the blades should come to rest at the ten o'clock position to help reduce vibration and facilitate hand propping.
When Installing a propeller on a flanged crank-shaft, It Is Important to follow the propeller manufacturer‘s recommended sequence to avoid inducing stress in the propeller hub.
Some small training aircraft with flange mounted propeller utilize a skull cap spinner that requires a mounting bracket to be installed using two of the propeller mounting bolts.
A full spinner mounts to forward and rear bulkheads with machine screws. On some smaller aircraft. the spinner may mount to only a rear bulkhead.
McCauley constant-speed propellers are installed on flanged crankshafts with an Oring seal to prevent oil leakage
On some low horsepower engines, the crankshaft is tapered and Is threaded on the end for propeller mounting.
A typical mounting hub for a wood propeller uses a face plate to distribute the compression load evenly over the boss of the propeller hub.
A snap ring is installed inside a propeller hub mounted on a tapered or splined shaft to aid in removing the propeller from the shaft.
The master spline on a splined propeller shaft ensures that the propeller is correctly positioned when installed on the engine
Splined propeller shafts require front and rear mounting cones to ensure that the propeller is properly aligned on the shaft.
With a splined shaft installation, the propeller is held in placeby a retaining nut that threads onto the end of the splined shaft and presses against the front cone.
Rear cone bottoming occurs when the tip of the rear cone contacts the land on the rear seat of the hub before the hub seats on the cone. Removal of a specified amount of material from the cone's apex corrects this problem
Front cone bottoming occurs when the apex of the front cone bottoms on the crankshaft splines. This prevents a good seat between the front cone and propeller hub. As a result. the propeller hub is loose, lacking a tight fit to the crankshaft.
Oh propellers secured to a flanged hub with bolts. pairs of bolt heads are safety wired together to keep the bolts from loosening
If the propeller Installation uses castellated nuts with drilled bolts. safety the nuts with cotter pins.
To safety the retaining nut on a tapered shaft propeller installation, a clevis pin is installed through the safety holes in the retaining nut and crankshaft. The clevis pin must be installed with its flanged head toward the center of the crankshaft