PROPELLERS
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, mainte-nance, and repair of propeller systems.
PROPELLER PRINCIPLES
With few exceptions, nearly all early aircraft designs used propellers to create thrust. During the latter part of the 19th century, many unusual and innovative propeller designs were tried on early fly-ing machines. These early propeller designs ranged from simple fabric covered wooden paddles to elab-orate multi-bladed wire-braced designs. As the sci-ence of aeronautics progressed, propeller designs evolved from flat boards which merely pushed air backward, to airfoils that produce lift to pull an air-craft forward. At the time the Wright brothers began their first powered flights, propeller design had evolved into the standard two-bladed style.
Development of propeller design with new materi-als has produced thinner airfoil sections and greater strength. Because of their structural strength, alu-minum alloys are predominantly used as the struc-tural material in modern aircraft propellers. However, you can still find several propellers that are constructed of wood.
Today, propeller designs continue to be improved through the use of new airfoil shapes, composite materials, and multi-blade configurations. Recent improvements include the use of composite materi-als to produce laminar flow symmetrical airfoils and gull wing propeller designs.
NOMENCLATURE
Before you can fully understand the principles of how a propeller produces thrust, you must be famil-iar with some basic terms and component names.
All modern propellers consist of at least two blades that are connected to a central hub. The portion of a propeller blade that is nearest the hub is referred to as the blade shank whereas the portion furthest from the hub is called the blade tip. The propeller hub, or hub assembly, is bored out to create a hub bore which permits a propeller to be mounted on the engine crankshaft or to a reduction gear assem-bly. [Figure 12-1]
Each blade on a propeller acts as a rotating wing to produce lift and pull an aircraft through the air. Therefore, in addition to the basic nomenclature just discussed, propeller blades share much of the same nomenclature as aircraft wings. For example, all propeller blades have a leading edge, a trailing edge, and a chord line. If you recall from your study of airfoils, a chord line is an imaginary line drawn through an airfoil from the leading edge to the trail-ing edge. The curved, or cambered side of a pro-peller blade is called the blade back and the flat side is called the blade face. A propeller's blade angle is the acute angle formed by a propeller's plane of rotation and the blade's chord line. A pro-peller's plane of rotation is always perpendicular to the engine crankshaft. [Figure 12-2]
Propellers which allow changes in blade angle have removable blades that are secured to a hub assembly
by a set of clamping rings. Each blade root has a
flanged butt, or shoulder, which mates with grooves in the hub assembly. The blade shank on this type of blade is typically round and extends out to at least the end of the hub assembly; however, in some
Figure 12-1. 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 hub bore and bolt holes that facilitate propeller mounting.
Figure 12-2. 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.
cases, the shank may extend beyond the hub assem-bly and into the airstream. When this is the case, blade cuffs may be installed to improve air flow around the blade shank. A blade cuff is an airfoil-shaped attachment made of thin sheets of metal, plastic, or composite material. Blade cuffs mount on the blade shanks and are primarily used to
Figure 12-3. Some large turboprop propeller blades are fit-ted with blade cuffs to improve the airflow around the blade shanks.
increase the flow of cooling air to the engine nacelle. Mechanical clamping devices and bonding agents such as a rubber-base adhesive or epoxy adhesive are utilized to attach the cuffs to the blades. [Figure 12-3]
To aid in identifying specific points along the length of a propeller blade, most blades have several defined blade stations. A blade station is simply a reference position on a propeller blade that is a specified distance from the center of the hub.
PROPELLER THEORY
When the propeller rotates through the air, a low pressure area is created in front of the blade, much like the wing's curvature creates a low pressure area above the wing. This low pressure area, combined with the constant, or high pressure area behind the blade allow a propeller to produce thrust. The amount of thrust produced depends on several fac-tors including, the angle of attack of the propeller blades, the speed the blades move through the air, and the shape of the airfoil. The angle of attack of a propeller 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 air-craft moves through the air and the rotational motion of the propeller. For example, when a pro-peller rotates on a stationary aircraft, the direction of the relative wind is exactly opposite to the rota-tional movement of the propeller. Therefore, the propeller blade's angle of attack is the same as the propeller blade angle. [Figure 12-4]
Figure 12-4. 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.
Figure 12-5. In forward flight, the airplane moves from point A to point B while the propeller moves from point C to point D. In this 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.
Figure 12-6. 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.
When the same aircraft begins moving forward, the relative wind changes direction. The reason for this is that, in addition to rotating, the propeller is also moving forward. The combination of the rotating and forward motion produce a resultant relative wind that is not directly opposite the movement of the pro-peller blade. In this case, the angle of attack will always be less than the blade angle. [Figure 12-5] Based on how forward motion affects the relative wind acting on a propeller blade, it can be deter-mined that for a given propeller speed, the faster an aircraft moves through the air, the smaller the angle of attack on the propeller blade. However, if pro-peller speed is increased, the trailing edge of the propeller blade travels a greater distance for a given amount of forward movement. Therefore, as pro-peller speed increases, the relative wind strikes the propeller blade at a greater angle and the angle of attack increases. [Figure 12-6]
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. Typically, propellers with a fixed blade angle are designed to produce an angle of attack between 2 and 4 degrees at either a climb or cruise airspeed with a specific rpm setting.
Unlike a wing which moves through the air at a uni-form rate, the propeller sections near the tip rotate at a much greater speed than those near the hub. The difference in rotational velocity along a pro-peller blade segment can be found by first calculat-ing the circumference of the arc traveled by a point on that segment. If you recall from your general studies, the circumference of a circle is calculated with the formula:
2irr
The circumference is then multiplied by engine rpm to find rotational velocity. For example, to deter-mine the blade velocity at a point 18 inches from the hub that is rotating at 1,800 rpm use the follow-ing formula:
Velocity = 2TTr x rpm
= 2 XTT x 18 x 1,800 = 203,575
At a point 18 inches from the hub the blade travels 203,575 inches per minute. To convert this to miles per hour, divide 203,575 by 63,360, the number of inches in one mile, and multiply the product by 60, the number of minutes in one hour.
Propellers
Figure 12-7. As a propeller blade rotates at a fixed rpm, each blade segment moves through the air at a different velocity.
Figure 12-8. The variation in airfoil shape and blade angle along the length of a propeller blade compensates for dif-ferences in rotational speed and allows for a more even dis-tribution of thrust along the blade.
allows the propeller to provide a fairly constant angle of attack along most of the length of the blade.
The speed of the propeller at station 18 is 192.7 miles per hour. You can now compare this to the speed of the propeller at station 48. By applying the formulas just discussed, you can determine that, at station 48, the propeller is moving at a speed of 514 miles per hour. [Figure 12-7]
In addition to blade twist, most propellers are built with a thicker, low speed airfoil near the blade hub and a thinner, high speed airfoil near the tip. This, combined with blade twist, permits a propeller to produce a relatively constant amount of thrust along a propeller blade's entire length. [Figure 12-8]
To compensate for the difference in velocity along a propeller blade, each small section of the propeller blade is set at a different angle. The gradual decrease in blade angle from the hub to the tip is called pitch distribution. This is what gives a pro-peller blade its twisted appearance. Blade twist
FORCES ACTING ON A PROPELLER
A rotating propeller is subjected to many forces that cause tension, twisting, and bending stresses within the propeller. Of the forces that act on a propeller,
centrifugal force causes the greatest stress.
Figure 12-9. When a propeller is rotating, centrifugal force tries to pull propeller blades away from the hub.
Figure 12-11. Torque bending forces exert a pressure that tends to bend the blades opposite the direction of rotation.
which tries to pull the blades out of the hub. The amount of stress created by centrifugal force can be greater than 7,500 times the weight of the propeller blade, [Figure 12-9]
Thrust bending force, on the other hand, attempts to bend the propeller blades forward at the tips. This occurs because propeller blades are typically thin-ner near the tip and this allows the thrust produced at the tip to flex the blade forward. Thrust bending force opposes centrifugal force to some degree. [Figure 12-10]
Figure 12-10. Thrust bending forces exert a pressure that tends to bend the propeller blade tips forward.
Torque bending forces occur as air resistance opposes the rotational motion of the propeller blades. This force tends to bend the blades opposite the direction of rotation. [Figure 12-11]
Aerodynamic twisting force results from the fact that, when a propeller blade produces thrust, the majority of the thrust produced is exerted ahead of the blade's axis of rotation. Therefore, aerodynamic twisting force tends to increase a propeller's blade angle. In some cases, aerodynamic twisting force is used to help change the blade angle on a propeller. [Figure 12-12]
Figure 12-12. The majority of thrust produced by a propeller To t^vrted a'nead ot tne'blade's axis o'r rotation.This pro-duces an aerodynamic twisting force that attempts to increase a propeller's blade angle.
Figure 12-13. Centrifugal twisting force attempts to decrease blade angle by aligning a propeller's center of mass with its center of rotation.
Centrifugal twisting force opposes aerodynamic twisting force in that it attempts to decrease a pro-peller's blade angle. When a propeller rotates, cen-trifugal force tries to align the propeller's center of mass with its center of rotation. A propeller's center of mass is typically ahead of its center of rotation; therefore, when a propeller rotates, centrifugal force tries to decrease its blade angle. At operational speeds, centrifugal twisting force is greater than aerodynamic twisting force and is used in some propeller designs to decrease the blade angle. [Figure 12-13]
The final force that is exerted on a spinning peller is blade vibration. When a propeller pro-duces thrust, blade vibration occurs due to the aero-dynamic and mechanical forces that are present. For example, aerodynamic forces tend to bend the pro-peller blades forward at the tips producing buffeting and vibration. On the other hand, mechanical vibra-tions are caused by the power pulses in a piston engine. Of the two, mechanical vibrations are con-sidered to be more destructive than aerodynamic vibrations. The reason for this is that engine power pulses tend to create standing wave patterns in a propeller blade that can lead to metal fatigue and structural failure.
The location and number of stress points in a blade depend on the characteristics of the individual pro-peller/engine combination. While concentrations of vibrational stress are detrimental at any point on a
blade, the most critical location is about six inches from the blade tips.
Most airframe-engine-propeller combinations have eliminated the detrimental effects of vibra-tional stresses by careful design. Nevertheless, some engine/propeller combinations do have a critical range where severe propeller vibration can occur. In this case, the critical range is indi-cated on the tachometer by a red arc. Engine oper-ation in the critical range should be limited to a brief passage from one rpm setting to another. Engine operation in the critical range for extended periods can lead to structural failure of the pro-peller or aircraft.
Propeller design typically allows for some degree of vibrational stress. However, in situations where a propeller has been improperly altered, vibration may cause excessive flexing and work hardening of the metal to the extent that sections of the propeller blade could break off in flight.
PROPELLER PITCH
In the strictest sense, propeller pitch is the theoret-ical distance a propeller advances longitudinally in one revolution. Pitch and blade angle describe two different concepts, however, they are closely related and the two terms are often used inter-changeably. For example, when a propeller is said to have a fixed pitch, what is actually meant is that the blades on the propeller are set at a fixed blade angle.
A propeller's geometric pitch is defined as the dis-tance, 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 effi-ciency. Measurement of geometric pitch is based on the propeller blade angle at a point out from the pro-peller hub that is equal to 75 percent of the blade length.
When traveling through air, inefficiencies prevent a propeller from moving forward at a rate equal to its geometric pitch. Therefore, effective pitch is the actual amount a propeller moves forward in one revolution. Effective pitch varies from zero when the aircraft is stationary on the ground, to about 90 percent of the geometric pitch during the most effi-cient flight conditions. The difference between geo-metric pitch and effective pitch is called slip.
Figure 12-14. 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.
Propeller slip represents the total losses caused by inefficiencies. [Figure 12-14]
If a propeller has a geometric pitch of 50 inches, in theory it should move forward 50 inches in one rev-olution. However, if the aircraft actually moves for-ward only 35 inches in one revolution, the effective pitch is 35 inches and the propeller is 70 percent efficient. In this case, slip represents 15 inches or a 30 percent loss of efficiency. In practice, most pro-pellers are 75 to 85 percent efficient.
PROPELLER CLASSIFICATIONS
Propellers are typically classified according to their position on the aircraft. For example, tractor propellers are mounted on the front of an engine and pull an aircraft through the air. On the other hand, pusher-type propellers are mounted on the aft end of an aircraft and push an airplane through the air. Most aircraft are equipped with tractor-type propellers; however, there are several seaplanes and amphibious aircraft that are equipped with pusher propellers. A major advantage of the trac-tor-type propeller is that lower stresses are induced in the propeller as it rotates in relatively undisturbed air.
Both tractor- and pusher-type propellers effectively propel an aircraft through the air. However, in some instances one type of propeller may be better suited for a given airplane. For example, on land planes that have little propeller-to-ground clearance, pusher-type propellers are subject to more damage
than tractor-type propellers. The reason for this is that rocks, gravel, and small objects that are dis-lodged by wheels are frequently thrown or drawn into a pusher-type propeller. On the other hand, it ■would be very difficult to install a tractor-type propeller on some amphibious aircraft.
Propellers also are classified by the method used to establish pitch. Typical classifications that are used here include fixed pitch, ground adjustable, con-trollable pitch, constant speed, reversible, and feathering.
The simplest type of propeller is a fixed-pitch pro-peller. Fixed-pitch propellers are designed for a par-ticular aircraft to produce optimum efficiency at a specific rotational and forward speed. A fixed-pitch propeller with a low blade angle, often called a climb propeller, provides the best performance for takeoff and climb. On the other hand, a fixed-pitch propeller with a high blade angle, often called a cruise propeller, is more adapted to high speed cruise and high altitude flight. It is important to note that with this type of propeller, any change from the optimum rpm or airspeed reduces the effi-ciency of the propeller.
Ground-adjustable propellers are similar to fixed-pitch propellers in that their blade angles cannot be changed in flight. However, the propeller is constructed in a way that allows the blade angle to be changed on the ground. This type of propeller is found mostly on aircraft built between the 1920s and 1940s.
Controllable-pitch propellers have an advantage over ground adjustable propellers in that the blade angle may be changed while the propeller is rotat-ing. This allows the propeller to assume a blade angle that provides the best performance for a par-ticular flight condition. The number of pitch posi-tions may be limited, as with a two-position con-trollable propeller; or the pitch may be adjusted to any angle between a minimum and maximum pitch setting.
Constant-speed propellers, sometimes referred to as automatic propellers, are unique in that once a pilot selects an operating rpm, the propeller blades auto-matically adjust to maintain the selected rpm. With this type of propeller, pitch control is provided by a controlling device known as a governor. A typical governor utilizes oil pressure to control blade pitch. Constant speed propeller systems provide maxi-mum efficiency by allowing the pilot to control the propeller blade angle for most conditions encoun-tered in flight.
Reversible-pitch propellers are a refinement of the constant-speed propeller. On aircraft equipped with a reversible propeller, the propeller blades can be rotated to a negative angle to produce reverse thrust. This forces air forward instead of backward and per-mits a shorter landing roll and improved ground maneuvering.
Most multi-engine aircraft are equipped with a featherable propeller. A feathering propeller is a type of constant-speed propeller that has the ability to rotate the propeller blades so that the leading edge of each blade is pointed straight forward into the wind. The only time a pilot selects the feather position is if an engine fails. Placing the blades in the feather position eliminates a great deal of the drag associated with a windmilling propeller.
PROPELLER CONSTRUCTION
Almost all propellers produced are made of wood, steel, aluminum, or some type of composite mater-ial. In the early years of aircraft development all propellers were made of wood. However, since wood is fairly susceptible to damage, steel pro-pellers quickly found their way into aviation. Today, aluminum alloys are the predominant
mate-rial used in the construction of both fixed- and adjustable-pitch propellers. In addition, some com-posite materials are now being utilized because of their light weight and flexibility.
WOOD
Wood was the most reliable material for fabrication of propellers for many years. Hardwoods such as birch, maple, and several others possess the flexi-bility and strength required for a propeller used on low horsepower engines of small aircraft. The mol-ecular structure of wood allows it to absorb engine vibration to a large degree and does not support res-onant vibrations. However, unless wood materials are coated with a tough protective layer of resin or other material, they are susceptible to damage from gravel and debris during ground operations.
ALUMINUM ALLOY
Today, the vast majority of propellers used are con-structed of an aluminum alloy. Aluminum is more desirable than wood because it allows thinner, more efficient airfoils to be constructed without sacrific-ing structural strength. In addition, the airfoil sec-tions on an aluminum propeller typically extend close to the hub providing better airflow for engine cooling. Furthermore, aluminum propellers require much less maintenance than wood propellers, thereby reducing the operating cost.
STEEL
Steel propellers and blades are found primarily on antique and older generation transport aircraft. Because steel is a heavy metal, steel blades are nor-mally hollow consisting of steel sheets attached to a rib structure. The hollow area is then filled wida a foam material to help absorb vibration and maintain a rigid structure.
COMPOSITE
Composite propeller blades are slowly gaining in popularity. Some advantages of composite pro-pellers include the fact that they are lightweight and extremely durable. In addition, composites absorb vibration and are resilient, making them resistant to damage and corrosion.
FIXED-PITCH PROPELLERS
The simplest type of propeller is a fixed-pitch pro-peller. As its name implies, the blade angle on a fixed-pitch propeller is fixed and cannot easily be changed. Because of this, fixed-pitch propellers achieve their optimum efficiency at a specific rota-tional and forward speed.
FIXED-PITCH CLASSIFICATIONS
A typical fixed-pitch propeller installed on a light aircraft has a diameter between 67 and 76 inches and a pitch between 53 and 68 inches. The exact diameter and pitch required for a specific airplane is specified by the aircraft manufacturer. In some cases, a manufacturer may authorize multiple pro-pellers, each with a different pitch. In this case, a propeller with the lower blade angle provides the best performance for takeoff and climb and, there-fore, is often called a climb propeller. The low blade angle allows the engine to develop its maximum rpm at the slower airspeeds associated with climbout. However, once the aircraft reaches its cruising altitude and begins to accelerate, the low blade angle becomes inefficient.A fixed-pitch propeller with a slightly higher blade angle is called a cruise propeller. A cruise propeller is designed to be efficient at cruising speed and high altitude flight. However, because of the higher pitch, cruise propellers are very inefficient during takeoff and climbout.
A standard propeller is often referred to as a com-promise between a climb propeller and cruise pro-peller. Each aircraft manufacturer usually desig-nates a standard propeller which is designed to pro-vide the best all-around performance under normal circumstances.
When an aircraft-engine combination is type certifi-cated with a specified standard, climb, and cruise propeller, the aircraft operator may choose the type of fixed-pitch propeller which provides the best performance for the flight operations most often conducted. For example, an aircraft which
fre-quently operates from short runways, or high field elevations generally perform better with a climb propeller. On the other hand, aircraft which are nor-mally operated at sea level from airports with long runways may be equipped with a cruise propeller.
PROPELLER CONSTRUCTION
Almost all fixed-pitch propellers produced are made of either wood or aluminum. Of the two, alu-minum is the most common, especially on produc-tion aircraft. However, there are still several classic and experimental aircraft that utilize wood pro-pellers.
WOODEN PROPELLERS
A majority of fixed-pitch propellers were made from wood until World War II and wooden propellers are still in limited use on small utility aircraft. Hardwoods such as ash and birch are typically used to build a wooden propeller. However, other hard-woods that have been used include mahogany, maple, cherry, oak, and black walnut. Whatever type of wood is used, it must be free of grain irregu-larities, knots, pitch pockets, and insect damage. A wooden propeller is constructed of a minimum of five layers of wood that are kiln-dried and lami-nated together with a waterproof resin glue. Each layer is normally the same thickness and type of wood; however, alternate layers of different wood types may be used. The reason laminated wood is used instead of a solid block of wood is that a lami-nated structure is less likely to warp. Once the lay-ers of wood are laminated together, they form what is called a propeller blank.
During fabrication, the blank is rough-cut to shape and then allowed to season for a period of time. The waiting period allows the moisture in the wood to disperse equally through all of the layers. The rough-shaped blank, referred to as a white, is then finished to the exact airfoil and pitch dimensions required. In addition, the center bore and bolt holes
Figure 12-15. (A) The first step in the manufacture of a wood propeller is to laminate planks together to form a propeller blank. (B) he 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.
are drilled and a metal hub assembly is inserted through the hub bore to accommodate the mounting bolts and face plate.
Once a propeller white is finished and sanded smooth, a cotton fabric is sometimes glued to the last 12 to 15 inches of the propeller blade. The fab-ric acts to reinforce the thin tip sections. Once applied, the fabric is doped to prevent deterioration caused by weather and the sun's rays. The entire propeller is then finished with clear varnish to tect the wood surface. In some cases, wood pro-pellers may be finished with a black or gray plastic coating that provides additional protection against chipping. In this case, the propeller is said to be armor coated. [Figure 12-15]
Monel, brass, or stainless steel tipping is applied to the leading edge and tip of most wooden propellers to prevent damage from small stones. In order 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 are used in the thick blade sections while copper rivets are used in the thin sections near the tip. Once in place, the screws and rivets are secured with solder. Using a number 60 drill, three small
holes are then drilled 3/16 inch deep into the tip of each blade. These holes allow moisture to drain from behind the metal tipping and allow the wood to breathe. [Figure 12-16]
Figure 12-16. Metal 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.
ALUMINUM ALLOY PROPELLERS
Wood has given way to aluminum as the most often used material for fixed-pitch propeller fabrication. As mentioned earlier, propeller blades can be made thinner and more efficient without sacrificing structural strength when using aluminum instead of wood. In addition, aluminum has the strength and flexibility to accommodate the high horsepower engines available in today's small aircraft.
One of the biggest advantages of using aluminum is that aluminum alloy blades are less susceptible to damage from gravel and debris normally incurred during ground operations. Another advantage is that infrequent damage such as small nicks and upsets are easily dressed out with special files, making aluminum blades eas-ier to repair than wooden blades. In addition, fixed-pitch aluminum propellers may be re-pitched to an approved blade angle by certified propeller repair stations when desired by the aircraft operator.
Although aluminum propellers offer several advan-tages over wood propellers, there are areas where aluminum propellers do not perform as well. For example, aluminum propellers are much more sus-ceptible to damage caused by resonant vibrations. Because of this, aluminum propellers must be vibrationally tested during the certification process. In addition, aluminum propellers gener-ally weigh more than a comparable wood propeller. On a light aircraft the difference can equate to sev-eral pounds.
F i g ur e 1 2 -1 7 . O n M c C a ul e y f ix e d - p i t c h pr o p e ll er s , t h e builders name, model designation, serial number, type cer -tifi ca te n um b er , a n d pr o du cti on c er tif ica te n um b er ar e stam ped ar ound the pr opeller hub.
Almost all propellers begin as a high strength alu-minum forging. Once forged, the propeller is ground to the desired airfoil shape by machine and manual grinding. The final pitch is then set by twisting the blades to their desired angle. Once the blade angle is set, the propeller is heat treated to relieve internal stresses.
tical balance is obtained by attaching balance weights to the side of the propeller hub. Once the propeller is balanced, the surfaces are finished by anodizing and painting.
To help prevent excessive vibration, all new pro-pellers are balanced both horizontally and verti-cally. Horizontal balance is typically achieved by removing metal from the blade tip while vertical balance is achieved by removing metal from a blade's leading and trailing edges. Some propeller models may be horizontally balanced by placing lead wool in balance holes near the boss while
ver-PROPELLER DESIGNATION
The Federal Aviation Regulations require that all propellers be identified with the builders name, model designation, serial number, type certificate number, and production certificate number if there is one. To comply with the FARs, most man-ufacturers of fixed-pitch propellers stamp all the required information on the propeller hub. [Figure 12-17]
Of the information presented on a propeller, the manufacturers model number provides the majority of the information you will need to be familiar with as a maintenance technician. For example, a McCauley propeller designated as 1A90/DM 7651 has a basic design designation of 1A90. The "DM" component of the designation indicates the type of crankshaft the propeller will fit, the blade tip con-tour, and other information pertaining to a specific aircraft installation. The "7651" indicates the pro-peller diameter is 76 inches, and the pitch of the propeller at the 75 percent station is 51 inches. As another example, a Sensenich propeller desig-nated as M74DM-61 has a desigdesig-nated diameter of 74 inches. The "D" component of the designation iden-tifies the blade design, while the "M" ideniden-tifies a specific hub design along with mounting informa-tion. The "61" designates the blade pitch in inches at the 75 percent station. [Figure 12-18
Figure 12-18. A Sensenich aluminum propeller has informa-tion stamped on the hub which identifies its hub design, blade design, blade length, and pitch. In addition, the num-ber 1 stamped on one of the blade roots identifies that blade as blade number one.
ADJUSTABLE-PITCH PROPELLERS
The design and construction of adjustable pitch pro-pellers permit the aircraft operator to change the propeller blade angle. This offers the advantage of being able to set the propeller blade angle to obtain the maximum possible efficiency from a particular propeller/engine combination. While a few of the older adjustable pitch propellers could only be adjusted on the ground by a maintenance techni-cian, most modern adjustable pitch propellers per-mit a pilot to change the propeller pitch in flight. The first adjustable pitch propeller systems devel-oped offered two pitch settings; a low pitch setting and a high pitch setting. Today, however, nearly all adjustable pitch propeller systems are capable of a range of pitch settings.
GROUND-ADJUSTABLE
PROPELLERS
As mentioned in Section A, ground-adjustable pro-pellers are constructed in a way that allows the blade angle to be changed when the aircraft is on the ground and the engine is shut down. This type of propeller is seldom used today and is usually found on older aircraft equipped with radial engines. The hub of a ground adjustable propeller consists of two aluminum or steel halves that are machined to form a matched pair. The interior of 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 blades are used, the shoulders are cast or machined into a metal sleeve that is fastened to the blade shank by lag screws. [Figure 12-19]
Once the blades are inserted between the two hub halves, bolts are normally used to secure the hub halves when steel blades are used. However, when wood or aluminum alloy blades are used, either bolts or clamp rings may be used to hold the hub halves together. [Figure 12-20]
Figure 12-19. To help ensure that the propeller blades are not pulled out of the hub on a ground-adjustable propeller, shoulders are machined into the base of each blade shank. These shoulders fit into grooves that are machined into each hub half.
Figure 12-20. Ground adjustable propellers utilize either clamp rings or bolts to secure the hub halves and hold the blades tightly.
CONTROLLABLE-PITCH
PROPELLERS
Controllable-pitch propellers have an advantage over ground adjustable propellers in that the blade angle may be changed while the propeller is rotating. This allows the propeller to assume a blade angle that provides the best performance for
Propellers
a particular flight condition. The number of pitch positions may be limited, as with a two-position controllable propeller; or the pitch may be adjusted to any angle between a minimum and maximum pitch setting.
TWO-POSITION PROPELLERS
One of the first controllable-pitch propellers that became popular was the Hamilton-Standard coun-terweight propeller. This propeller was developed in the 1930'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 so the engine would turn at its maximum rpm and develop its full rated horsepower. On the other hand, the high pitch setting was used during the cruise phase of flight to permit more efficient high-speed flight while increasing fuel economy.
The primary components of a two-position pro-peller include the propro-peller hub, propro-peller blades, and a piston assembly. At the center of the 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 made from an aluminum alloy and have hollow ends which fit over the arms of the spider. Once the blades are inserted on the hub, counterweight brackets are attached to the base of each blade. Like the ground-adjustable propeller, the Hamilton-Standard propeller consists of a two piece hub that encloses the spider and holds the propeller blades in place. To allow the propeller blades to rotate between the low and high pitch stops, each blade rides on a set of roller bearings. In addition, a counterweight bracket is installed at the base of each propeller blade.
The blade angle on the Hamilton-Standard pro-peller 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. The hydraulic force used to decrease blade angle is derived from engine oil that flows out of the crankshaft and acts on a piston assembly that is mounted on the front of the propeller hub. The flow of engine oil into the piston assembly is con-trolled 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 assembly is linked to each counterweight bracket so that, as the piston moves out, it pulls the
counter-Figure 12-21. When low pitch is selected, engine oil pres-sure forces the cylinder forward. This motion moves the counterweights and blades to the low pitch position.
weights in and decreases the blade angle. Once the blades reach their low pitch stop in the counter-weight assembly, oil pressure holds the blades in this position. [Figure 12-21]
To move the blades to a high pitch position, the pro-peller control lever is moved aft, rotating the selec-tor valve to release oil pressure in the propeller hub. With the oil pressure removed, the centrifugal force acting on the counterweights causes them to move outward, rotating the blades to their high pitch posi-tion. As the blades rotate, oil is forced out of the propeller cylinder and returned to the engine sump. The blades stop rotating when they contact their high pitch stops located in the counterweight assembly. [Figure 12-22]
In most cases, the pitch stops on a two-position pro-peller can be adjusted. To do this, a pitch stop adjusting nut is rotated until the desired blade angle is obtained.
Figure 12-22. When high pitch is selected, engine oil pres-sure is removed from the piston assembly allowing cen-trifugal force to move the counterweights outward. This rotates the blades to the high pitch position.
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Although the operation of a two-position propeller is fairly straight forward, there are some operational things you need to know. For example, prior to engine shutdown, the propeller should be placed in the high pitch position. If you recall, this retracts the piston assembly, which, in turn, helps protect it from corrosion and accumulations of dirt. Furthermore, most of the oil is forced from the pis-ton where it could congeal in cold weather.
MULTIPLE-POSITION PROPELLERS
As technology advanced, the two-position pro-peller was improved to allow the operator to select any blade angle between the high and low pitch stops. This way, optimum engine/propeller effi-ciency can be maintained over a wider range of power settings and airspeeds. For example, during takeoff, the propeller blade angle is set at its lowest blade angle so the engine can generate its maxi-mum power output. Then, once the aircraft is estab-lished 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 a variable-pitch or controllable-pitch propeller, is the most common type of adjustable-pitch propeller used on aircraft today. The main 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. The primary reason why a constant-speed propeller is more efficient than other propellers is because it allows the operator to select the most efficient engine rpm for the given conditions. Once a specific rpm is selected, a device called a governor automatically adjusts the propeller blade angle as necessary to maintain the selected rpm. For example, after selecting a desired rpm during cruising flight, an increase in airspeed or decrease in propeller load will cause the propeller blade angle to increase as necessary to maintain the selected rpm. On the other hand, a reduction in airspeed or increase in propeller load will cause the propeller blade angle to decrease. - . iThe 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. However, once the propeller blades contact a pitch stop, the engine rpm will increase or decrease as appropriate with changes in airspeed and propeller load. For example, once a specific rpm has been selected, and if aircraft speed decreases enough to rotate the propeller blades until they contact the low pitch stop, any further decrease in airspeed will cause engine rpm to decrease the same way as if a fixed pitch propeller were installed. The same holds true when an aircraft equipped with a constant-speed propeller acceler-ates to a faster airspeed. As the aircraft acceleracceler-ates, the propeller blade angle increases to maintain the selected rpm until the high pitch stop is reached. Once this occurs, the blade angle cannot increase any further and engine rpm to increases.
On aircraft that are equipped with a constant-speed propeller, engine power output is controlled by the throttle and indicated by a manifold pressure gauge. The propeller blade angle, on the other hand, is con-trolled by a propeller control lever and the resulting change in engine rpm caused by a change in blade angle is indicated on the tachometer. By providing the operator a means of controlling both engine power output and propeller angle, the most efficient combination of blade angle and engine power output can be maintained for a variety of flight conditions. For example, during takeoff you want the engine to develop its maximum power; therefore, the throttle and the propeller control are advanced full forward so the engine can turn at its maximum rpm on take-off. On the other hand, after the aircraft is estab-lished in cruise flight, the throttle can be retarded so the engine runs at a more economical speed and the propeller blade angle can be increased to increase propeller efficiency for higher speed flight.
One thing you must keep in mind when operating a constant-speed propeller is that, for a given rpm set-ting, there is a maximum allowable manifold pres-sure. Operating above this level may cause internal engine stress. Therefore, as a general rule, you should avoid high manifold pressures with low rpm settings.
OPERATING PRINCIPLES
Most constant-speed, non-feathering propellers rely on a combination of hydraulic and centrifugal forces to change the propeller blade angle. They use high-pressure oil to increase the propeller blade angle and the centrifugal twisting force inherent in all spinning propellers is utilized to decrease the blade angle. On the other hand, most feathering
pro-Propellers
pellers utilize counterweights and centrifugal force to pull the blades to high pitch and oil pressure to force the blades to low pitch. To help prevent con-fusion between the operation of feathering and non-feathering propellers, the following discussion will focus on a typical non-feathering, non-counter-weighted propeller assembly.
The device that is responsible for regulating the flow of high-pressure oil to the propeller is called the gov-ernor. 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 pro-peller, and it senses the rotational speed of the engine. A propeller governor is typically mounted either on the front of an engine near the propeller shaft or on the engine accessory case. In addition, all governors consist of three basic components; a gear-type boost pump, a pilot valve, and a speed sensitive flyweight assembly. [Figure 12-23]
A typical governor boost pump is installed in the base of a governor and boosts the oil pressure to between 180 and 300 psi depending on the system require-ments. The force needed to drive the boost pump is provided by a drive shaft that extends into the engine where it mates with an engine drive gear. In some cases, the drive shaft may be hollow to provide a pas-sage for return oil to flow back to the engine. Since
Figure 12-23. 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.
most boost pumps are constant displacement pumps, excessive oil pressure is produced at high engine speeds; therefore, a spring-loaded relief valve is pro-vided to prevent damage to seals and other compo-nents. This way, when the oil pressure increases enough to overcome the spring pressure acting on the relief valve, the valve opens and routes the excess oil back to the inlet side of the boost pump.
The valve that is responsible for routing oil into and out of the propeller hub is called a pilot valve. Although the design of a pilot valve varies between manufacturers, they all perform the same basic function; they direct oil into and out of the propeller hub. A typical pilot valve is a shuttle-type valve that alternately covers and uncovers oil passages allow-ing oil to flow into or out of the propeller hub. In order for a governor to adjust the propeller blade angle to maintain a constant rpm, it must be able to sense engine speed. The portion of a governor that senses engine speed is referred to as the flyweight assembly. A typical flyweight assembly consists of a set of flyweights mounted on a flyweight head that is driven by the same drive shaft that drives the boost pump. The pilot valve is located inside the drive shaft and extends into the weight assembly where it rests on the toe of each fly-weight. This way, as the flyweights tilt in and out, the pilot valve is moved up or down. To allow the operator to select, or set, a desired blade angle, a speeder spring is provided to adjust the amount of pressure acting on the flyweights and pilot valve. [Figure 12-24]
Figure 12-24. 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 act-ing on the flyweights and pilot valve can be adjusted.
When the engine is operating, the governor boost pump and flyweight assembly are driven by the gov-ernor drive shaft. When the propeller control in the cockpit is in the full forward, low pitch position, the speeder spring is fully compressed so that it holds the pilot valve down and allows no oil into the propeller hub. With no oil pressure acting on the pitch change mechanism in the propeller hub, centrifugal twisting force holds the blades in their low pitch position. This propeller setting is typically used during takeoff when the engine's maximum power output is needed. When the propeller control in the cockpit is moved aft, speeder spring pressure is decreased and centrifu-gal force begins to tilt the flyweights outward. As the flyweights move outward, the pilot valve is pulled up and governor oil is directed to the propeller hub to increase the propeller blade angle. As the blade angle increases, engine rpm decreases causing a reduction in the amount of centrifugal force acting on the fly-weights. This allows the flyweights to tilt inward and lower the pilot valve until the flow of oil to the pro-peller hub is cut off. At this point, speeder spring pres-sure and the centrifugal force acting on the flyweights are in balance and the governor is said to be on-speed. Once a given rpm is selected, the governor automat-ically adjusts the propeller pitch to maintain the selected rpm. Therefore, any change in airspeed or load on the propeller results in a change in blade pitch. For example, if a climb is initiated from level flight, aircraft speed decreases and load on the pro-peller blades increases. As this occurs, engine speed begins to decrease which, in turn, reduces the amount of centrifugal force acting on the flyweights. With less centrifugal force acting on the flyweights, speeder spring pressure forces the flyweight to tilt inward. When this happens, the governor is said to be in an under-speed condition. [Figure 12-25] As the flyweights move inward, the pilot valve moves downward and oil is ported out of the pro-peller hub and back to the engine. As oil is ported from the propeller hub, centrifugal twisting force moves the blades to a lower pitch. The lower pitch reduces the load on the propeller and allows the engine to accelerate. As the engine rpm increases, the centrifugal force acting on the flyweights also increases and causes the flyweights to tilt outward and return the governor to an on-speed condition. This same process can be applied when airspeed increases or the load on the propeller decreases. However, in either of these situations, engine rpm increases causing the centrifugal force acting on the flyweights to increase. When this occurs, the
fly-Figure 12-25. When a governor is in an under-speed condi-tion, speeder spring pressure is greater than the centrifugal force and the flyweights tilt inward.
weights tilt outward creating an over-speed condi-tion. [Figure 12-26]
As the flyweights move outward, the pilot valve moves up and boost pump oil is directed to the pro-peller hub to increase blade pitch. The increased pitch increases the load on the propeller and slows the engine. As the engine rpm decreases, the cen-trifugal force acting on the flyweights also decreases and causes them to tilt in ward and return the gov-ernor to an on-speed condition.
Figure 12-26. When a governor is in an over-speed condi-tion, the centrifugal force acting on the flyweights over-comes the force of the speeder spring to tilt the flyweights outward.
Changes in throttle settings have the same effect on the governor as changes in airspeed and/or changes in propeller load. For example, advancing the throt-tle increases power output which, in turn, increases engine rpm. Therefore, to maintain the selected rpm, the governor must increase the propeller blade. By the same token, pulling the throttle back decreases power output and causes engine rpm to decrease. Therefore, in an attempt to maintain the original rpm setting, the governor must decrease the blade angle to unload the engine and propeller. One thing to keep in mind though is that, once the pro-peller blades reach the low pitch stop, the engine rpm will decrease with further power decreases. As a safety feature to help protect the engine from over-speeding, all governors incorporate an adjustable stop screw that limits how low the blade pitch on a given constant-speed propeller can go. In addition, some governors incorporate a balance spring above the speeder spring that automatically sets the governor to produce a cruise rpm should the propeller control cable break.
McCAULEY CONSTANT-SPEED PROPELLERS
The McCauley constant-speed propeller system is one of the more popular systems used on light and medium size general aviation aircraft. For example, most Cessna aircraft that use a constant-speed pro-peller utilize a McCauley propro-peller system.
Currently, there are two types of constant-speed propellers that are installed on aircraft; the threaded series and the threadless series. Both use the same pitch change mechanism in the propeller hub; how-ever, the method used to attach the propeller blades to the hub does differ. For example, a threaded series propeller uses a retention nut which screws into the propeller hub and holds the blades in the hub. This differs from the threadless-type blades which employ a split retainer ring to hold each blade in the hub. [Figure 12-27]
Both series of McCauley propellers are non-feath-ering and non-counterweighted. Therefore, oil
Figure 12-27. (A) McCauley threaded blades use retention nuts and threaded ferrules 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 propeller blade in place.
pressure is used to increase the propeller's blade angle while centrifugal twisting force and an inter-nal spring are used to rotate the blades to low pitch. With this type of system, when a higher blade pitch is selected, high pressure oil is routed to the pro-peller hub where it pushes against a piston. Once the pressure builds enough to oppose the spring inside the hub and the centrifugal force exerted on the blades, the piston slides back toward the hub. This movement is transmitted to the propeller blades through blade actuating links to actuating pins located on each blade butt. [Figure 12-28] The propeller blades, hub, and piston are made from an aluminum alloy. On the other hand, the propeller cylinder, blade actuating pins, piston rod, and spring are manufactured from either chrome or cadmium plated steel. To help prevent metal parti-cles from wearing off the blade actuating links and becoming trapped inside the propeller hub, most actuating links are made of a phenolic material. To prevent pitch change oil from leaking into the cen-ter of the propeller hub and down the propeller blade, O-ring seals are installed between the piston and the cylinder, the piston and the piston rod, and the piston rod and the hub. This way, the components making up the propeller's pitch-change mechanism can be lubricated with a grease and not depend on engine oil for lubrication during operation.
Some models of McCauley propellers use dyed oil permanently sealed in the hub. If red appears on the hub or blades, it is an indication that the hub may have a crack. In such case the propeller should be removed for repair.
Like other constant-speed propellers, the model designation codes of McCauley constant-speed pro-pellers are longer to provide additional information. Important parts of the designation that you should be familiar with include the dowel pin location, the C-number, and the modification, or change letter, after the C-number. The modification or change des-ignation indicates that a propeller complies with a required or recommended alteration. [Figure 12-29]
Figure 12-29. The McCauley designation system provides important information that must be known when deter-mining which propeller will fit a specific aircraft.
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.
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.
In addition to the designation code found on the propeller blades, several McCauley propeller hubs have a designation code of their own. In most cases, the code consists of two groupings of numhers that indicate the year the huh was manufactured and the number of hubs that were made in any one year. For example, a designation code of 72 1126 indicates that the hub was manufactured in 1972 and there were a total of 1,126 hubs made in that year.
instead of the 180 to 200 psi Hamilton-Standard. [Figure 12-30]
Most McCauley governors use a control arm instead of a pulley to adjust the speeder spring pressure act-ing 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 flex-ible control cable, or a combination of the two to be connected to the arm.
McCAULEY GOVERNORS
McCauley governors use the same basic operating principles as the generic governors discussed ear-lier. That is, the governor directs high pressure oil to the propeller hub to increase the propeller blade angle. If you recall, this is directly opposite from the way the Hamilton-Standard system works. Another difference between the McCauley and Hamilton-Standard systems is that the McCauley governor produces an oil pressure of approximately 290 psi
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 maxi-mum 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. Both the
Figure 12-31. 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. high and low rpm stops can be adjusted by a set screw on the governor head. Depending on the engine and governor combination, one turn of the screw changes the rpm by 17, 20, or 25 rpm. [Figure 12-31]
Like other governors, McCauley governors utilize their own unique designation system. Some of the information provided in the designation code includes the specific features of a given model, any modifications that have been done, and ability to interchange the governor with another. For a full explanation of a given designation, it is best to con-sult a McCauley propeller maintenance manual. [Figure 12-32]
HAMILTON-STANDARD CONSTANT-SPEED PROPELLERS
As propeller technology advanced, the two-position Hamilton-Standard counterweight propeller system was transformed into a constant-speed propeller system. To do this, a flyweight governor was devel-oped and installed in place of a selector valve. The propeller used with the Hamilton-Standard constant-speed system is essentially the same coun-terweight propeller used as the two-position pro-peller discussed earlier. However, since a counter-weight propeller is used, oil pressure provides the force required to decrease blade angle while cen-trifugal force acting on the counterweights is used to increase the blade angle.
The governor used with a Hamilton-Standard con-stant-speed propeller is divided into three parts; the
Figure 12-32. The model, modifications, interchangeability, and specific features of a McCauley governor are identified in the McCauley designation code.
head, the body, and the base. The head contains the flyweights and flyweight assembly while the body and base house the pilot valve and boost pump. In most cases, a Hamilton-Standard governor has a designation code that is stamped on the governor body. The designation system indicates the design of the head, body, and base. For example, a governor with a designation code of 1A3-B2H identifies a "1" head design, an "A" body design, and a "3" base. In addition, the "B2H" indicates the modifications made to the head, body, and base respectively. HARTZELL CONSTANT-SPEED
PROPELLERS
Hartzell constant-speed propeller systems are widely used in modern general aviation airplanes and share the market with McCauley. Currently, Hartzell produces two types of constant-speed pro-pellers, a steel hub propeller and a Compact model. The Hartzell steel hub propeller is similar to the Hamilton-Standard constant-speed propeller in that the pitch change mechanism is exposed. On the
Figure 12-33. (A) 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.
other hand, the pitch change mechanism on a Hartzell compact propeller is housed inside the pro-peller head. [Figure 12-33]
Regardless of the type of hub used, Hartzell typi-cally stamps a model designation code on both the propeller hub and the propeller blades. A typical designation system identifies the specific hub or blade model, any modifications that have been made, mounting type, and any specific features. [Figure 12-34]
STEEL HUB PROPELLERS
Hartzell steel hub propellers may or may not be counterweighed. If the propeller has counter-weights, oil pressure is used to decrease blade angle while centrifugal force acting on the counterweights is used to increase blade angle. On the other hand, steel hub propellers that have no counterweights use oil pressure to increase blade angle and cen-trifugal twisting force to decrease the blade angle. The central component of a Hartzell steel hub pro-peller 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. Once the blades are placed on the spider arms they are secured by two-piece steel clamps, to provide a means of changing pitch, a steel cylin-der is threaded onto the front of the spicylin-der and an aluminum piston is placed over the cylinder. The piston is connected to the blade clamps on each
Figure 12-34. Hartzell constant-speed propellers carry two model designations numbers; one for the propeller hub and the other for the propeller blades.
12-24
Figure 12-35. 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.
hlade hy a sliding rod and fork system. This way, as oil is directed into and out of the propeller huh, the piston moves in and out and the propeller blades rotate as appropriate. [Figure 12-35]
COMPACT PROPELLERS
Hartzell compact propellers are more modern than steel hub propellers and incorporate several fea-tures that make the compact propeller hub smaller, lighter, and more dependable. A typical compact propeller hub is forged out of an aluminum alloy as two separate halves. Each half is machined out so that the shank of each propeller blade can be held between the two hub halves and so the entire pitch change mechanism can be contained. Once the blades and pitch changing mechanism are installed between the two hub halves, bolts are used to secure the halves together. [Figure 12-36]
Hartzell constant-speed propeller systems typically utilize either a Woodward governor or a modified Hamilton-Standard governor. Woodward governors are usually adjusted to produce approximately 275 psi of oil pressure when installed on an engine with a normal engine oil pressure of 60 psi. On the other hand, if a modified Hartzell governor is used, the output pressure does vary between different models. Therefore, to determine the pressure setting for a given Hartzell governor, you must refer to the appropriate Hartzell maintenance manual.
Figure 12-36. 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.
Like other governors, Hartzell propeller governors utilize their own unique designation systems. For example, a typical Hartzell designation code con-sists of a three-character code that designates the governors basic body style and major modifications, any major adjustments that were made to make it compatible with a specific system, and any minor adjustments that were made that do not affect eligi-bility. [Figure 12-37]
FEATHERING PROPELLERS
When an engine fails in flight, the propeller contin-ues to windmill, or turn, slowly as air flows over the blades. This creates a considerable amount of drag that can adversely effect an aircraft's flight charac-teristics. 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. This is known as feathering a propeller and eliminates the drag created by a windmilling pro-peller because it presents the smallest blade profile to the oncoming airstream. Today, all modern multi-engine, propeller-driven aircraft are equipped with feathering propellers. [Figure 12-38]
The operating principles discussed previously for constant-speed propellers also apply to feathering propeller systems. However, the propeller control
Figure 12-37. In the sample designation above, an F-6-3A propeller governor consists of a 4G8 governor with a major adjustment to obtain compatibility and a minor adjustment not effecting eligibility.
Figure 12-38. 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 the airstream and decreases aerodynamic drag.
lever in the cockpit typically incorporates an addi-tional position that, when selected, rotates the pro-peller blades to their feathered position. In most cases, the propeller control is pulled all the way aft to feather the blades. If a propeller is feathered from a low blade angle, the blades will move from a low angle through a high angle before they reach their feathered position. On the other hand, if a propeller is feathered from a high blade angle, the blades will move from their high pitch setting directly into the feathered position.
On most aircraft, feathering functions are indepen-dent of constant-speed operation. In other words, the operator can override a constant-speed system to feather the propeller at any time. In fact, in some systems the propeller can be feathered without engine rotation.
Most manufacturers of constant-speed propellers also build feathering propellers. However, to help simplify this discussion only the Hartzell
com-pact feathering propeller and the
Hamilton-Standard hydromatic propeller will be discussed in detail.
HARTZELL COMPACT FEATHERING PRO-PELLERS
The constant-speed operation of a Hartzell com-pact feathering propeller is the same as the con-stant-speed model with one difference; the feath-ering propeller uses both governor oil pressure and centrifugal twisting force to rotate the blades to low pitch while some combination of a high-pressure nitrogen charge, an internal spring, or counterweights are used to increase blade angle. For example, one type of Hartzell compact feathering propeller utilizes a high-pressure nitrogen charge and a mechanical spring to increase blade angle and to feather the blades. The nitrogen charge is stored in the cylinder head and works in conjunction with a spring that is also contained within the propeller hub.
Another model of Hartzell compact propeller uses a combination of air pressure and centrifugal force acting on counterweights to feather and increase the angle of the propeller blades. With this type of propeller, the high-pressure nitrogen charge is again stored in the propeller hub while a