PTC B1.1 Notes - Sub Module 17.2 (Propeller Construction)

PIATrainingCentre(PTC) Module17PROPELLERCategory A/B1 Sub Module 17.2 Propeller Construction

ISO 9001 - 2008 Certified For Training Purpose Only PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.2 Mar 2014

MODULE17

SubModule17.2

PROPELLERCONSTRUCTION


ISO 9001 - 2008 Certified For Training Purpose Only PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.2 - i Mar 2014

ContentsCONSTRUCTIONMETHODSANDMATERIALSUSEDINWOODEN,COMPOSITEANDMETALPROPELLERS1

BLADESTATION,BLADEFACE,BLADESHANK,BLADEBACKANDHUBASSEMBLY10

FIXEDPITCH,CONTROLLABLEPITCHANDCONSTANTSPEEDPROPELLERS13

PROPELLERINSTALLATION17

SAFETYINGAPROPELLER19

SPINNERINSTALLATION21


ISO 9001 - 2008 Certified For Training Purpose Only PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.2 - ii Mar 2014

Page Intentionally Left Blank


ISO 9001 - 2008 Certified For Training Purpose Only PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.2 - 1 Mar 2014

CONSTRUCTION METHODS AND MATERIALS USED IN WOODEN, COMPOSITE AND METAL PROPELLERS Propellers have been constructed using wood, metal and composite materials. We will take a look at each of these materials starting with simple wooden and metal propeller construction. Wooden Propellers Wooden propellers are generally fixed pitch or ground adjustable and have two blades. They are normally to be found installed on single engine, light aircraft. Variable pitch wooden propellers have a metal hub and pitch change mechanism. The individual wooden blades are fitted with metal root fittings designed to mate with the hub. We will concentrate on the all wood, two blade fixed pitch propeller at this stage. The wood most frequently used these days is yellow birch but other woods such as mahogany, cherry, maple or walnut have been used either singly or in combination. Mahogany was extensively used in earlier times. Prior to its use the wood must be inspected to ensure that it is free from knots, shakes, cracks, signs of decay, mildew, wormholes or staining. The grain should be straight and the wood correctly seasoned. The more common defects that can be found in wood are worth noting. Dote disease is the worst condition and can be recognized by the presence of oily brownish yellow spots scattered over the wood. The affected areas appear soggy and the wood will break out in pieces when picked with a knife.

The condition is infectious so the only remedy is to burn the wood. Decay normally reveals itself as dry brown patches that either flake or come out in chunks. It is often referred to as dry rot because the fungus that causes it feeds on moisture in the wood. Shakes are splits that run along sap lines or around annular rings. They may occur during the growth of the tree or during seasoning. They can also be caused by frost if the wood is wet. Shakes are hard to spot and often are not seen until the wood is varnished. Knots appear in several forms but ail will cause problems. A thorough inspection is essential to save a lot of wasted effort. A propeller is not carved from a block of solid wood, it is laminated. The construction process commences by gluing and clamping together seasoned planks that have been roughly profiled to form a blank shape containing a minimum of five layers of the selected wood. The use of a laminated construction is designed to reduce the risk of the wood warping during the service life of the propeller. Gluing is carried out using plastic resin glue under carefully controlled pressure and temperature conditions. The glued blank is then left for a further period to allow the layers to season together. Seasoning is a drying process carried out to reduce the possibility of shrinkage and warping of the propeller after manufacture. It also improves the strength of the timber and its resistance to decay. The blank should be kept horizontal during this time to ensure even moisture distribution or the shrinkage will be uneven.



Figure 17.2.01



The blank is then roughly shaped using an automatic profiling machine. This rough shape is called a white. The roughly shaped block is then shaved into its correct form using templates and blade protractors to check the shape and pitch. This part of the process requires a highly skilled craftsman. The thin tip region of each blade is then bound with cotton fabric to prevent splitting and doped to avoid moisture ingress. A sheet brass anti-erosion sheath covering the leading edge and the tip of each blade is screwed into position. The leading edge will have been rebated to accept the sheath. The countersunk screws securing the leading edge sheath are normally locked using solder to till the countersink depressions over the screw heads. Copper rivets are sometimes used to secure the metal cap onto the thin sections near the blade tip. These are also capped with solder to prevent loosening in use. The hub centre bore and the holes for the propeller mounting bolts are drilled through the centre boss. The most frequently used surface finish for wooden propellers is waterproof clear varnish and this is applied over the whole of the propeller using several coats. Alternatively, cellulose paint may be used. The appropriate metal hub is then inserted and secured to the propeller with through bolts and a faceplate. The hub assembly adapts the propeller for installation onto the engine drive shaft. There are a number of different propeller drive methods. The hub may be adapted to accept shaft splines, a flange plate or a keyed taper shaft. A typical example only is shown in the figure 17.2.02.

Three small holes are drilled into the blade tip to act as a vent for moisture allowing the wood in that region to breathe. The propeller will finally be checked for correct static and dynamic balance. On simple wooden propellers, applying and smoothing solder onto the light blade tip metal cap may trim horizontal static balance. Vertical static balance may be adjusted by screwing a brass or lead compensating balance plate onto the light side of the propeller boss face. Very small balance adjustments may be carried out by the application of additional coats of clear varnish to the light blade. During the post manufacture testing of wooden propellers it is important to check for vibration and that they do not suffer resonance in any part of the operating range. With piston engines, this test should take into account the effects of misfiring or inoperative cylinders.



Figure 17.2.02



Metal Propellers Improvements in metallurgy and manufacturing techniques have enabled metal propellers to replace wood propellers for modern commercially manufactured aircraft. Figure 17.2.03 shows a metal construction propeller blade. Metal propellers are forged from high-strength aluminium alloy, and after being ground to their finished dimensions and pitch, are anodized to protect them from corrosion. Metal propellers cost more than wood for the same engine and aero plane, but their increased durability, resistance to weathering, and ability to be straightened after minor damage have made them more cost effective in the long term.

Some propellers have blades made of steel with the blade halves stamped of thin sheet steel and brazed together along the leading and trailing edges. The blade shell is then installed over a tubular steel shank. A few propellers with hollow steel blades are still flying, but these are usually found only on special purpose aero planes.



Figure 17.2.03



Composite Propellers Laminated wood, forged aluminium alloy, and brazed sheet steel propellers have been standard for decades. But the powerful turboprop engines and the demands for higher-speed flight and quieter operation have caused propeller manufacturers to exploit the advantages of modern advanced composite materials in modern turbo-prop engines. Composite materials used in the propeller manufacturing consist of two constituents: the fibres and the matrix. The fibres most generally used are glass, graphite and aramid (Kevlar), and the matrix is a thermosetting resin such as epoxy. The strength and stiffness of the blades are determined by the material, diameter and orientation of the fibres. The matrix material supports the fibers, holds them in place and completely encapsulates them for environmental protection. Because the fibres have strength only parallel to their length, they are arranged in such a way that they can sustain tensile loads. Hartzell Blade Construction The typical Hartzell composite propeller, like that in figure 17.2.04 has a machined aluminium alloy shank, and molded into this shank is a low density foam core. Slots are cut into the foam core and unidirectional Kevlar shear webs are inserted. The leading and trailing edges are solid sections made of unidirectional Kevlar and laminations of pre-impregnated material are cut and laid up over the core foundation to provide the correct blade thickness, aerofoil shape, pitch distribution, planform and ply orientation.

The outer shell is held in place on the aluminium alloy shank by Kevlar filaments impregnated with epoxy resin wound around the portion of the shell that grips the shank. Some Hartzell blades have a stainless steel mesh under the final layer of Kevlar to protect against abrasion, and a nickel leading edge erosion shield is bonded in place. The entire blade is put into a blade press and cured under computer-controlled heat and pressure. Hamilton-Standard Blade Construction The Hamilton-Standard blade has tremendous strength and fatigue resistance because of its solid aluminium alloy spar enclosed in a glass fibre shell (Figure 17.2.05). The spar is machined to its correct configuration and placed in a mould cavity, and the core foam is injected around it. The foam is cured and removed from the mould. Glass fibre cloth, with the correct number of plies and the proper ply orientation, is then laid over the cured core. The complete item is then placed in a second mould that has the shape of the finished blade. The resin matrix is injected to impregnate all the fibres, and is cured with heat and pressure.



Figure 17.2.04

Figure 17.2.05



Dowty Rotol Blade Construction The Dowty Rotol composite propeller blade has two carbon fibre spars that run the length of the blade on both the face and back and come smoothly together at the blade root. The carbon fibres and pre-impregnated glass fibre cloth are laid with the correct number of plies and the correct ply orientation and are placed in a mould. Polyurethane foam is injected into the inside of the blade, and the entire unit is cured under heat and pressure. The Dowty Rotol blade is secured in the hub by expanding the carbon fibre spars with tapered glass fibre wedges and locking them between the inner and outer sleeves (Figure 17.2.07).

Figure 17.2.06

Figure 17.2.07



BLADE STATION, BLADE FACE, BLADE SHANK, BLADE BACK AND HUB ASSEMBLY The typical propeller blade can be described as a twisted aerofoil of irregular planform, as shown in the figure 17.2.08. For the purposes of analysis, a blade can be divided into segments, which are located by station numbers in inches from the centre of the blade hub. The blade hub assembly is the supporting unit for the blades. The blade shank and the blade butt are also identified the figure 17.2.08.

Figure 17.2.08



The aircraft propeller consists of two or more blades and a central hub to which they are attached. Propeller blades have a shank, butt, hub, tip, trailing edge, leading edge, face and a back. The blade shank is the thick, rounded portion of the propeller blade near the hub, which is designed to give strength to the blade. The blade butt, also called the blade base or root, is that end of the blade, which fits in the propeller hub. The blade tip is that part of the propeller blade farthest from the hub, generally defined as the last 6 inches of the blade. The cambered side of a blade is called the blade back. The flat side of the blade is called the blade face.



Figure 17.2.09



FIXED PITCH, CONTROLLABLE PITCH AND CONSTANT SPEED PROPELLERS There are various types or classes of propellers, the simplest of which are the fixed-pitch and ground-adjustable propellers. The complexity of propeller systems increases from these simpler forms to controllable pitch and complex automatic systems. Fixed-Pitch Propeller As the name implies, a fixed-pitch propeller has the blade pitch, or blade angle, built into the propeller. The blade angle cannot be changed after the propeller is built. Generally, this type of propeller is one piece and is constructed of laminated wood or aluminium alloy. Fixed-pitch propellers are designed for best efficiency at one rotational and forward speed. They are designed to fit a set of conditions of both aircraft and engine speeds, and any change in these conditions reduces the efficiency of both the propeller and the engine. The fixed-pitch propeller is used on aircraft of low power, speed, range, or altitude.

Ground-Adjustable Propeller The ground-adjustable propeller operates as a fixed-pitch propeller. The pitch or blade angle can only be changed when the propeller is not running. It is done by loosening the clamping mechanism, which holds the blades in place. After the clamping mechanism has been tightened, the pitch of the blades cannot be changed in flight to meet variable flight requirements. Like the fixed-pitch propeller, the ground-adjustable propeller is used on aircraft of low power, speed, range, or altitude. A ground adjustable propeller may have blades made of wood or metal. The hub is usually of two piece steel construction with clamps or large nuts used to hold the blade securely in place. When the angle of the blade is to be changed, the clamp or blade nuts are loosened and the blades rotated to the desired angle as indicated by a propeller protractor. The angle markings on the hub are not considered accurate enough to provide a good reference for blade adjustment, so they are only used for reference.



Figure 17.2.10



Controllable Pitch Propeller Ground-adjustable propellers were a step in the right direction, but with only minor added weight and complexity, the propeller could be made far more efficient by allowing the pilot to change the pitch of the blades in flight. The first popular controllable-pitch propellers were hydraulically actuated by engine lubricating oil supplied through a hollow crankshaft. A counterweight on an arm is attached to each blade root so that centrifugal force rotates the blade into a high pitch angle. A fixed piston in the end of the propeller shaft is covered by a movable cylinder attached through bearings to the counterweight arms. For takeoff, the two-position propeller control is placed in the low pitch position that directs engine oil into the cylinder and moves it forward over the piston. This pulls the counterweights in and rotates the blades into their low pitch position. When the airplane is set up for cruise flight, the pitch control is moved to the high pitch position. This opens an oil passage, allowing the oil in the propeller cylinder to drain back into the engine sump. Centrifugal force on the counterweights moves them outward into the plane of rotation, and rotates the blades into their high pitch position. This same configuration of propeller, when equipped with a flyweight governor to control the oil into and out of the cylinder, is a popular constant speed propeller.

Figure 17.2.11



Constant-Speed Propeller A constant speed propeller system is a system in which the propeller blade angle is varied by the action of a governor to maintain a constant speed. The pitch changing devices for constant speed propellers include electric motors, hydraulic cylinders, and devices in which centrifugal forces act on flyweights and combinations of these methods. The tremendous advantage of being able to change pitch in flight opened new possibilities for increased efficiency. Replacing the two-position valve with a flyweight-controlled valve in a governor allows the blade pitch angle to be continuously and automatically adjusted in flight to maintain a constant and efficient engine speed. Throughout and immediately after World War II, an electrically controlled constant speed propeller was used with some degree of success. A small reversible DC motor mounted in the centre of the propeller hub drove a speed reducer with an attached bevel gear. This gear meshed with bevel gears on the root of each blade to change the pitch so the propeller could maintain an air load on the engine that produced the RPM called for by the governor. The large amount of maintenance required for electric propellers caused their demise. There are only two types of propellers installed on current production airplanes: fixed-pitch propellers for the small and simple airplanes, and hydraulically actuated constant-speed propellers for complex airplanes. The blades of constant-speed propellers are held in high-strength steel hubs, with roller or ball type thrust bearings that enable their pitch angle to be changed with a minimum of force.

Figure 17.2.12



PROPELLER INSTALLATION Flanged Shaft Propeller Installation Flanged propeller shafts are used on most flat/horizontally opposed and some turboprop engines. The front of the crankshaft is formed into a flange four-to-eight inches across and perpendicular to the crankshaft centre line. Mounting bolt holes and dowel pin holes are machined into the flange and, on some flanges, threaded inserts are pressed into the bolt holes.

Figure 17.2.13

Tapered Shaft Propeller Installation Tapered shaft crankshafts are found on older model engines of low horsepower. This type of crankshaft requires a hub insert to adapt the propeller to the shaft. The method used to attach the propeller to the engine crankshaft will vary with the design of the crankshaft.

Figure 17.2.14

Module 17 Category A/B1 Sub Module 17.2 Propeller Construction


Splined Shaft Propeller Installation

Splined crankshafts are found on most radial engines and some flat/ horizontally opposed, inline, and even popular for large turboprop engines. The splined shaft has grooves and splines of equal dimensions, and a master, or double-width, spline so that a hub will fit on the shaft in only one position.

Figure 17.2.15



SPINNER INSTALLATION All modern propeller-driven airplanes have spinners over their propeller hubs. These spinners have the dual aerodynamic function of streamlining the engine installation and directing cool air into the openings in the Cowling. Figure 17.2.18 shows a typical spinner installation over a constant-speed propeller. The spinner bulkhead is installed on the propeller shaft flange and held in place by six spinner attaching bolts. The propeller is then installed so that the dowel pins in the propeller hub align with the holes in the flange. The propeller attaching nuts are installed and tightened to the torque value specified in the airframe maintenance manual. If a spinner support is required, it is installed and the spinner is secured to the bulkhead with the proper machine screws. The propeller spinner and bulkhead are critical components, and cracks in either one can be repaired only if they do not exceed the allowable limits. Repair them using the procedures specified in the airframe maintenance manual and take special care not to add weight where it could cause vibration.

Module 17 Category A/B1 Sub Module 17.2 Propeller Construction


Figure 17.2.18

PTC B1.1 Notes - Sub Module 17.2 (Propeller Construction)

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