CASA B1-12c Helicopter Systems Airframes 2018 PDF

Student Resource Subject B1-12c: Helicopter Systems Airframes Copyright © 2018 Aviation Australia All rights reserved. No part of this document may be reproduced, transferred, sold, or otherwise disposed of, without the written permission of Aviation Australia CONTROLLED DOCUMENT 2018-05-14 B1-12c Helicopter Systems Airframes Page 2 of 9 CONTENTS CONTENTS........................................................................................................................................ 3 DEFINITIONS.................................................................................................................................... 4 STUDY RESOURCES...................................................................................................................... 5 INTRODUCTION............................................................................................................................... 6 Topic 12.4.1 Transmissions (MRGB).......................................................................................... 6 Topic 12.4.2 Clutches, Freewheel Units and Rotor Brakes..................................................... 6 Topic 12.4.3 Tail Rotor Drive Trains and Gearboxes............................................................... 6 Topic 12.9.1 Emergency Equipment and Systems (ATA Chapter 25)................................... 7 Topic 12.9.2 Cabin Furnishings (ATA Chapter 25)................................................................... 7 Topic 12.11 Fuel Systems............................................................................................................ 7 Topic 12.12 Hydraulic Power....................................................................................................... 8 Topic 12.14 Landing Gear............................................................................................................ 8 2018-05-14 B1-12c Helicopter Systems Airframes Page 3 of 9 DEFINITIONS Define To describe the nature or basic qualities of. To state the precise meaning of (a word or sense of a word). State Specify in words or writing. To set forth in words; declare. Identify To establish the identity of. List Itemise. Describe Represent in words enabling hearer or reader to form an idea of an object or process. To tell the facts, details, or particulars of something verbally or in writing. Explain Make known in detail. Offer reason for cause and effect. 2018-05-14 B1-12c Helicopter Systems Airframes Page 4 of 9 STUDY RESOURCES B1-12c Student Handout 2018-05-14 B1-12c Helicopter Systems Airframes Page 5 of 9 INTRODUCTION The purpose of this subject is to familiarise you with the basic construction, requirements and maintenance of helicopter airframe structures. On completion of the following topics you will be able to: Topic 12.4.1 Transmissions (MRGB) State their purpose and explain the construction, mounting, and principles of operation of the following: M/R Gearboxes and Masts. Identify types of inspection and explain the maintenance requirements. Topic 12.4.2 Clutches, Freewheel Units and Rotor Brakes State their purpose and explain the constructional features and operation of the following: Clutches; Freewheel Units; Rotor Brake Units; Couplings and Drive Train Systems. Topic 12.4.3 Tail Rotor Drive Trains and Gearboxes State their purpose and explain the construction, mounting, and principles of operation of the following: Intermediate and T/R Gearboxes. Explain the construction and operation of typical tail rotor drive systems comprising of the following: Couplings; Pitch Change Mechanism; Shafts and Universal Joints. Identify types of inspection and explain the maintenance requirements. 2018-05-14 B1-12c Helicopter Systems Airframes Page 6 of 9 Topic 12.9.1 Emergency Equipment and Systems (ATA Chapter 25) Identify and state the purpose of aircraft emergency equipment and describe their operation. Describe the general construction and installation of: Passenger and crew seats and Harnesses and seatbelts. Describe the typical layout and operation of the following Helicopter lifting systems: Rescue Hoist System and Cargo Hook System. Topic 12.9.2 Cabin Furnishings (ATA Chapter 25) State the operation of Helicopter Emergency Flotation Systems. Identify and state the requirements of the following: Cabin Lay-Out; Equipment Lay-Out; Cabin Furnishing Installation and Cargo Handling and Retention Equipment. Topic 12.11 Fuel Systems Describe the typical layout of fuel systems. Identify components of and explain the operation of the following fuel systems / components: Tanks; o Integral, o Auxiliary and o Self-sealing types. Supply; Dumping, Venting and Draining; Cross-Feed and Transfer and Refuelling and Defuelling. 2018-05-14 B1-12c Helicopter Systems Airframes Page 7 of 9 Explain the operation of fuel system indication and warning systems. Identify hazards that cause unsafe conditions and interpret the following concepts associated for fuel tank safety and maintenance activities: Fuel Tank Entry and Exit Procedures; Clean working environment; Configuration control; Wire separation and Bonding of system components. Topic 12.12 Hydraulic Power Describe the system layout of typical aircraft hydraulic systems. Identify and state the properties of various aviation hydraulic fluid types and explain the associated handling precautions. Identify components of and explain the operation of the following hydraulic systems / components: Reservoirs and Accumulators; Electrical, Mechanical and Pneumatic Pressure Generation; Emergency Pressure Generation; Pressure Control; Power Distribution; Indication and Warning and Interface with Other Systems. Topic 12.14 Landing Gear Explain the construction of the following landing gear systems: Skid type application; Extension and retraction systems, and Float attachment Systems. Explain the operation of the following landing gear systems: Shock absorbing; Normal and emergency extension and retraction and Steering applications. Explain the construction of various aircraft wheels types and state their application. Identify various aircraft tyre classifications, explain their construction and state their application. 2018-05-14 B1-12c Helicopter Systems Airframes Page 8 of 9 Interpret the precautions to be observed during inflation of aircraft tyres. Explain inspection procedures for tyres and identify normal and abnormal wear patterns. Explain the construction of and explain operation of Helicopter brake systems Explain the operation of landing gear and brake indication and warning systems. 2018-05-14 B1-12c Helicopter Systems Airframes Page 9 of 9 TOPIC 12.4.1: TRANSMISSIONS - MRGB Table of Contents List of Figures....................................................................................................................................... 2 TOPIC 12.4.1: TRANSMISSIONS - MRGB...................................................................................... 3 Helicopter Transmission Systems........................................................................................................ 3 Purpose......................................................................................................................................................... 3 Transmission System Components............................................................................................. 4 Main Rotor Transmission..................................................................................................................... 5 Bell 47 Transmission Mounting............................................................................................................ 7 Bell 206 Transmission Mounting.......................................................................................................... 9 Casings........................................................................................................................................................ 13 Input Drives................................................................................................................................................ 13 Output Drives..................................................................................................................................... 14 Mast / Shaft........................................................................................................................................ 15 Mast Maintenance............................................................................................................................. 16 TOPIC 12.4.1.2: Maintenance – MRGB...................................................................................... 17 Servicing Points.................................................................................................................................. 17 Chip Detectors.................................................................................................................................... 18 Oil Cooler............................................................................................................................................ 20 Temperature Sensing Bulbs............................................................................................................... 21 Bell 206 Transmission Maintenance.................................................................................................. 23 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 1 of 24 Training Material Only List of Figures Figure-1. Relationships of Engines and Transmissions............................................................................ 3 Figure-2. Transmission System................................................................................................................ 4 Figure-3. Dual-needle Tachometers used on Helicopters....................................................................... 5 Figure-4. Inside a Main Rotor Transmission............................................................................................ 6 Figure-5. Bell 47 Transmission Mounting................................................................................................ 7 Figure-6. Sprag Mount System used on the Bell 47................................................................................ 8 Figure-7. Safety Cable System used on Bell 47....................................................................................... 8 Figure-8. Bell 206 Transmission Mounting.............................................................................................. 9 Figure-9. A’ Frame Link or coat hanger Mounting.................................................................................. 9 Figure-10. Spike and Isolation Mount on Bell 206.................................................................................. 10 Figure-11. Bell 206L Isolation Mount...................................................................................................... 11 Figure-12. Nodal Beam System used on several Bell Models................................................................. 12 Figure-13. Dual engine MRG inputs........................................................................................................ 13 Figure-14. Dual Engine MRG System....................................................................................................... 14 Figure-15. Bell 47 Mast........................................................................................................................... 16 Figure-16. Oil filter.................................................................................................................................. 17 Figure-17. Oil level.................................................................................................................................. 17 Figure-18. Chip Detectors....................................................................................................................... 18 Figure-19. Bell 206 Transmission Oil System........................................................................................... 19 Figure-20. Lubricating Oil Cooler............................................................................................................ 20 Figure-21. Temperature Sensors............................................................................................................. 21 Figure-22. Gauge Pin Placement............................................................................................................. 22 Figure-23. Bell 206 Transmission Maintenance...................................................................................... 23 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 2 of 24 Training Material Only TOPIC 12.4.1: TRANSMISSIONS - MRGB Helicopter Transmission Systems Purpose The function of the main rotor gearbox is to: transmit engine power to the main and tail rotors provide the necessary speed reduction between the engine and rotors transmit forces from the main rotor head to the fuselage provide a mounting for the main rotor head drive the tail rotor provide drive for accessories change the angle of drive As with all other components, each manufacturer has a different way of designing, mounting, and powering the transmission. These have also changed considerably as the technology of the helicopter has developed from reciprocating power to turbine power. The powerplants and their relationship to transmissions are quite important to the basic design requirements. Typical relationships of engines and transmissions used in helicopters are shown. Figure-1. Relationships of Engines and Transmissions 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 3 of 24 Training Material Only Transmission System Components The transmission system transfers power from the engine to the main rotor, tail rotor, and other accessories. The main components of the transmission system are the main rotor transmission, tail rotor drive system, clutch, and freewheeling unit. Helicopter transmissions are normally lubricated and cooled with their own oil supply. Figure-2. Transmission System 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 4 of 24 Training Material Only Main Rotor Transmission The primary purpose of the main rotor transmission is to reduce engine output RPM to optimum rotor RPM This reduction is different for the various helicopters, but as an example, suppose the engine RPM of a specific helicopter is 2,700. To achieve a rotor speed of 450 RPM would require a 6 to 1 reduction. A 9 to 1 reduction would mean the rotor would turn at 300 RPM Most helicopters use a dual-needle tachometer to show both engine and rotor RPM or a percentage of engine and rotor RPM. The rotor RPM needle normally is used only during clutch engagement to monitor rotor acceleration, and in autorotation to maintain RPM within prescribed limits. Figure-3. Dual-needle Tachometers used on Helicopters In helicopters with horizontally mounted engines, another purpose of the main rotor transmission is to change the axis of rotation from the horizontal axis of the engine to the vertical axis of the rotor shaft. 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 5 of 24 Training Material Only The inside of a main rotor transmission is shown. Figure-4. Inside a Main Rotor Transmission 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 6 of 24 Training Material Only Bell 47 Transmission Mounting Some of the early light helicopters had engines mounted directly to the transmission, forming one unit. This system required only one mount, which served as the engine and transmission mount. Figure-5. Bell 47 Transmission Mounting The Bell 47 mount system consists of an adapter plate, a basket assembly, rubber Lord mounts, and a mount system on the bottom of the engine. This mount is usually referred to as an engine mount, but it essentially carries both the engine and transmission as a unit. The adapter plate is bolted to the engine and suspended by two trunnion pins. This gives the engine and transmission some flexibility from side to side. The trunnions pass through the tubular steel mount, often referred to as the basket, which is attached to the center section of the helicopter through two rubber Lord mounts. The mounts allow some flexibility in the fore and aft direction and a slight amount of movement for torque. The mount system consists of an adapter plate, a basket assembly, rubber Lord mounts, and a mount system on the bottom of the engine. This mount is usually referred to as an engine mount, but it essentially carries both the engine and transmission as a unit. The adapter plate is bolted to the engine and suspended by two trunnions pins. This gives the engine and transmission some flexibility from side to side. The trunnions pass through the tubular steel mount, often referred to as the basket, which is attached to the center section of the helicopter through two rubber Lord Mounts. The mounts allow some flexibility in the fore and aft direction and a slight amount of movement for torque. In addition, a sprag mount system is placed on the bottom of the engine. This allows the bottom of the engine to move with the trunnions movement and at the same time offers some rigidity. This rigidity is quite necessary because of the arm length of the rotor assembly, which includes the mast, transmission, and the engine length. 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 7 of 24 Training Material Only Figure-6. Sprag Mount System used on the Bell 47 A safety cable system is installed to hold the engine in approximate position in case of failure of the sprag system. Figure-7. Safety Cable System used on Bell 47 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 8 of 24 Training Material Only Bell 206 Transmission Mounting The Bell 206 has the engine located aft of the transmission with the power passing through a short shaft to the transmission. The transmission is mounted to the fuselage by two pylon mount links attached at two points to the cabin roof. Figure-8. Bell 206 Transmission Mounting The apex of the “A” frame link (coat hanger) contains a spherical bearing attached to a spindle on the transmission case. Figure-9. A’ Frame Link or coat hanger Mounting 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 9 of 24 Training Material Only Fore and aft movement must be restricted. To restrict this movement, an isolation mount is attached, through a plate with a spherical bearing, to the bottom of the transmission. This mount is made up of layers of elastomer and metal. Any fore and aft movement will place the elastomer in tension and compression. Because of this elasticity, the transmission and mast will return to its original position when the load is relieved. In addition to the isolation mount, a spike is attached to the same plate as the isolation fitting. This spike passes through a hole surrounded by a stop plate, in case of failure of the isolation mount. It also prevents unlimited movement of the transmission and mast assembly. Figure-10 shows the attachment of the transmission at the bottom with the isolation mount and spike. Figure-10. Spike and Isolation Mount on Bell 206 The pylon mount links carry the load of the helicopter and should be examined for nicks and scratches. Because of the loads imposed, the pylon mount links may be reworked only in specified areas. The pylon mounts are shimmed by the manufacturer to assure the alignment of the transmission. The shims are to remain with the helicopter. Interchanging of shims will require realignment at a major overhaul facility. The isolation mount is visually inspected like other elastomeric items and must be replaced as required. The mounts are subject to deterioration due to solvents. A cover is placed over the mount and should be in place at all times. 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 10 of 24 Training Material Only Figure-11. Bell 206L Isolation Mount 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 11 of 24 Training Material Only The nodal beam system is used on the Bell 206L, 222, and 214 models. The nodal beam virtually eliminates the two-per-revolution vibrations normally felt in semi-rigid rotor systems. This is accomplished by the flexing of the nodal beam, tuned for the two-per- revolution vertical vibrations. Figure-12. Nodal Beam System used on several Bell Models The transmission, mast and rotor are isolated from the fuselage by the nodal beam and the transmission restraint. The nodal beam uses four link attachments and two stop mounts bolted to the transmission. The four link assemblies are secured to the four support assemblies and the flexture assemblies. The support assemblies are attached to the cabin roof and contain elastomeric bearings to isolate and balance the vibration inputs into the flexture assemblies. The assemblies are the primary vibration absorbing unit and are provided with tuning weights for the fine tuning of the nodal beam system. 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 12 of 24 Training Material Only Casings The casing of the main rotor gearbox houses the internal gearing of the gearbox. Transmission casings vary in construction depending on drive set up, loads to be taken and transmission size. Most casings are of a cylindrical shape consisting of upper and lower halves. They are usually constructed of cast aluminium or magnesium to help reduce weight. Input Drives Input drives transmit the drive from either the front or rear of the engine by couplings and shafts to the main crown wheel by an input pinion. The upper section of the main crown wheel becomes the sun gear in the epicyclic gear. Figure-13 shows the location of the input drives in a twin engine helicopter's gearbox. Figure-13. Dual engine MRG inputs 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 13 of 24 Training Material Only Output Drives The output drive transmits the drive from the main rotor gearbox to the tail rotor. Additional output drives are provided to drive accessories fitted to the gear box such as hydraulic pumps alternators/generators and tacho-generators Figure-14 shows output drives for accessories and tail rotor drive. Figure-14. Dual Engine MRG System 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 14 of 24 Training Material Only Mast / Shaft The mast or shaft provides a mounting point for the main rotor. The mast is a tube attached to the transmission. The mast absorbs torsional and tension loads received from the engine torque and the weight of the helicopter in flight. Construction of the mast varies between manufacturers. Some masts provide drive via sets of splines for the rotor head and swash plate assembly; other masts are stationary and only support the rotor head. The mast is a critical item and in some instances has a finite life. The construction of the mast assembly varies considerably from one manufacturer to another. Some masts only support the head assembly, while others also may support the stabilizer bar assembly. The mast also drives the swashplate (azimuth star assembly) through which the flight controls operate. First, let’s consider the mast of the Bell 47 in Figure-15. This mast, mounted in and driven by the transmission, drives the flight control units and main rotor. For this purpose it is equipped with five sets of splines used as attaching points for the following items: 1. Main rotor 2. Stabiliser bar 3. Dampener bracket 4. Swashplate 5. Transmission Thread portions are provided for the mast nut at the top, and the mast bearing on the lower end. The mast bearing is a split inner race thrust bearing. The outer race is supported and held in place by a cap on top of the transmission. The inner race, locked to the flange of the mast by a nut assembly, rotates with the mast. This is the main support for the mast and provides the primary point of rotation and thrust. It is one of the more critical bearings in the helicopter. Other features of the mast include grooves for snap rings used to hold flight components. Some masts have a cork placed in the bottom, preventing transmission oil from entering the mast. In addition, an aluminium plug is placed in the top of the hollow mast, preventing distortion of the mast due to torque applied to the rotor head retaining nut. The rotor head is supported on a set of split cones at the rotor head trunnion. The trunnion is splined to accept the first set of splines. On top of the rotor are placed the stops for the rotor. The nut is threaded down on the mast, and secured with a locking retainer. 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 15 of 24 Training Material Only Figure-15. Bell 47 Mast Mast Maintenance All masts are made of machined steel forgings, making them susceptible to corrosion, stresses, scratches, and other damage associated with steel parts. As with all critical parts, rotor masts are to be inspected in accordance with manufacturers’ recommendations contained in their manuals. Masts are cadmium plated to prevent corrosion. As with most highly stressed parts, they cannot be replated in the field. Electroplating sets up internal stresses that cannot be relieved without special equipment. All scratches, nicks, and gouges in the mast must be carefully examined. In some instances they may be reworked to relieve any possible stress concentration. 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 16 of 24 Training Material Only TOPIC 12.4.1.2: Maintenance – MRGB Servicing Points Helicopter gearboxes with an internal lubrication system have servicing points located at accessible areas of the gearbox to enable servicing of the gearbox to be carried out. Figure-16 and Figure-17 show locations for the following servicing points that are normally found on a main rotor gearbox: a sight glass or dipstick to check the lubricating oil level a drain plug to allow the gearbox casing to be drained of oil a lubrication filter to remove foreign particles from the lubricating oil a filler point for the replenishment of lubricating oil Figure-16. Oil filter Figure-17. Oil level 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 17 of 24 Training Material Only Chip Detectors Magnetic chip detectors are situated at strategic points in the gearbox and are used to detect the presence of loose metal chips or particles in the oil system. They can be electrically connected to a central warning panel in the cockpit. If large chips of metal are present on the detector, the cockpit indicator will illuminate. Magnetic chip detectors can be fitted with circuits which, when activated, discharge a bank of capacitors to remove any minute metal particles which may have accumulated. (Sometimes called “Fuzz”) Figure-18 shows a typical chip detector. Figure-18. Chip Detectors 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 18 of 24 Training Material Only Figure-19. Bell 206 Transmission Oil System The filter head provides an attachment point for the filter and incorporates a thermoswitch, temperature bulb and an oil bypass valve. As the oil enters the head, temperature is sensed by the temperature switch and GQ temperature bulb, which operates an instrument in the cockpit. In normal operation the oil then enters the filter. If however, the filter should become clogged to the point that a differential of pressure exists, the oil will be bypassed rather than entering the filter unit. The oil then flows to the oil cooler. This unit is used to regulate the temperature of the oil entering the transmission. The cooler is located outside the transmission and is cooled by a fan driven from the tail rotor driveshaft. The cooler itself is a radiator equipped with a bypass system. During starting and cold weather, the oil bypasses the cooler. As the oil warms up, however, the bypass valve starts to open and oil flows through the core of the cooler before it is directed through the lubrication points and pressure regulator. The pressure regulating valve limits the system pressure. This is a common relief type valve which bleeds off pressure when oil pressure overcomes the spring setting. This valve is located in back of the filter and is field adjustable. Bypass oil is returned to the sump. Two oil jets are located on the upper and lower case. Oil flows from the oil cooler to the case through an external fitting, where it is distributed to both jets and the freewheeling unit. The oil passing through the jets flows through the transmission and returns to the lower case. The oil going to the freewheeling unit, however, goes out of the case through an external fitting and a restrictor fitting. The restrictor fitting also contains a low pressure warning switch and an oil pressure tap for a continuous reading indicator. Oil leaving the freewheeling unit is returned to the sump. 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 19 of 24 Training Material Only Oil Cooler The oil cooler is used to regulate the temperature of the oil entering the filter unit. The oil cooler is similar to a radiator, and is generally located outside the transmission. It is cooled by a fan which may be driven from the tail rotor drive shaft. The cooler is equipped with a by-pass system which allows the oil to by-pass the cooler in cold weather. Figure-20 and Figure-21 show examples of gearbox lubricating oil cooler installations. Figure-20. Lubricating Oil Cooler 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 20 of 24 Training Material Only Temperature Sensing Bulbs Sensing bulbs are fitted to the system to monitor the temperature of the lubricating oil. The indicator will provide a warning if the temperature increases above a predetermined figure. Figure-21. Temperature Sensors During operation, the transmission oil temperature is monitored by an indicator. One indicator will read the oil temperature of either the engine or transmission, depending upon the position of the selector. Temperature is always a good indicator of the condition of the transmission. Like any other component that uses oil, it is subject to oil leakage. The leakage will appear on the outside of the transmission. 0-rings are used to seal the parting surfaces of the cases. The 0-rings may be changed by separating the transmission case section. Conventional lip seals are used on all the accessory drive quills. These will often require the removal of the quill for replacement and, in some instances, the disassembly of the quill package. The quill assemblies have shims located between the sleeve flange and the transmission case. These shims determine the lash and pattern of the gears. If a quill is removed for inspection or seal replacement, the shims must be returned as originally placed, or the lash and gear pattern will be destroyed. If this should occur, the lash and gear pattern must be redetermined, and the proper shims installed. To check the condition of the shoes and drum, the clutch assembly may be removed by lifting the transmission from the adapter plate, without complete disassembly of the transmission. This is normally done at 600-hour inspections or whenever the clutch operation is questionable. Major inspections of the transmission require complete disassembly. All steel parts are subject to magnetic particle inspection and all nonferrous components will receive fluorescent penetrant inspection. Besides visual inspection for wear, parts are dimensionally checked and gears are measured for wear using gauge pins. See Figure-22 for the typical use of gauge pins. 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 21 of 24 Training Material Only Figure-22. Gauge Pin Placement During the major inspection it is not uncommon to replace certain parts, such as bearings, rollers, and springs, without inspection. These items are replaced at the discretion of the shop performing the work. When various parts are replaced, the lash and pattern of the accessory gears and quills may be disturbed. Lash and gear patterns are always checked on reassembly. 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 22 of 24 Training Material Only Bell 206 Transmission Maintenance From a maintenance standpoint, this transmission is much simpler than that of the Bell 47, even though it has its self-contained oil system. The probability of oil leaks is greatly reduced with only one parting surface and two quills. The upper case is sealed with an O-ring and lip seals are used on the quills. The jets which are mounted externally are sealed with O-rings. The transmission is equipped with a sight gauge in the lower case for checking the oil level. The oil and filter are changed at recommended intervals. At this time the chip detector is checked and the filter is cut open for examination. Any excessive metal will require the replacement of the transmission and cooler. The 206 transmission, like that of the 47, may be disassembled in the field for major inspection and parts replacement. Because of fewer parts, this inspection is somewhat simplified, but the same basic checks are made on the ferrous and nonferrous parts. The lash and pattern requirements have been somewhat simplified with a lash only check being made. The reason for the lash only check is that shim rings are placed in the case by the manufacturer to determine the lash and pattern requirements of the gears. The lash check is really only an indication of the gears and the assembly procedures. If the rings are removed and lost from the case, new ones must be ground by the manufacturer for that particular case. These rings are not interchangeable. Figure-23. Bell 206 Transmission Maintenance 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 23 of 24 Training Material Only This page intentionally left blank 2018-05-14 B1-12.4.1 Transmissions - MRGB Page 24 of 24 Training Material Only TOPIC 12.4.2: CLUTCHES, FREEWHEEL UNITS AND ROTOR BRAKES Table of Contents List of Figures....................................................................................................................................... 2 TOPIC 12.4.2: CLUTCHES, FREEWHEEL UNITS AND ROTOR BRAKES................................................ 3 Introduction......................................................................................................................................... 3 Clutches..................................................................................................................................... 5 Bell 47 Piston Engine.................................................................................................................................... 5 Hiller 12 Piston Engine.................................................................................................................................. 6 Clutch Maintenance............................................................................................................................. 7 Belt Tightener Manual Clutch.............................................................................................................. 8 Freewheel Units......................................................................................................................... 8 Bell 47 and Hillier........................................................................................................................................ 10 Bell 206....................................................................................................................................................... 10 Hughes 500................................................................................................................................................. 11 Aerospatiale Gazelle................................................................................................................................... 12 Maintenance – Hughes 500........................................................................................................................ 12 Rotor Brakes............................................................................................................................ 13 Rotor Brakes (Mechanical) – Aerospatiale Astar 350................................................................................. 14 Rotor Brakes (Hydraulic) – Bell 206............................................................................................................ 15 Drive Train Systems............................................................................................................................ 16 Bell 47 Piston Engine (Information Only)................................................................................................... 17 Hiller 12e Piston Engine (Information Only)............................................................................................... 18 Hughes 300 Piston Engine (Information Only)........................................................................................... 19 Drive Train Systems.................................................................................................................................... 20 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 1 of 20 Training Material Only List of Figures Figure-1. Westland Wessex (top), Bell 47 (centre), Enstrom (left) Hughes 500 (right).......................... 3 Figure-2. Helicopter Drive Train.............................................................................................................. 4 Figure-3. Bell 47 Clutch........................................................................................................................... 5 Figure-4. Hiller 12 Clutch......................................................................................................................... 6 Figure-5. Freewheel Units....................................................................................................................... 9 Figure-6. Bell 47 Freewheel Unit........................................................................................................... 10 Figure-7. Bell 206 Freewheel Unit......................................................................................................... 10 Figure-8. Hughes 500 Freewheel Unit................................................................................................... 11 Figure-9. Aerospatiale Gazelle Freewheel Unit..................................................................................... 12 Figure-10. Hughes 500 Freewheel Unit................................................................................................... 12 Figure-11. Rotor Brake lever................................................................................................................... 13 Figure-12. Rotor Brakes (Mechanical) – Aerospatiale Astar 350............................................................ 14 Figure-13. Rotor Brakes (Hydraulic) – Bell 206....................................................................................... 15 Figure-14. Rotor Brake Pressure Gauge.................................................................................................. 15 Figure-15. Helicopter Drive Train Systems.............................................................................................. 16 Figure-16. Drive Train Systems – Bell 47................................................................................................. 17 Figure-17. Drive Train Systems – Hiller 12e............................................................................................ 18 Figure-18. Hughes 300............................................................................................................................ 19 Figure-19. Main Drive Shafts................................................................................................................... 20 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 2 of 20 Training Material Only TOPIC 12.4.2: CLUTCHES, FREEWHEEL UNITS AND ROTOR BRAKES Introduction All helicopters require some way of transmitting engine power to the main rotor and tail rotor. A direct drive is not possible due to starting and autorotation requirements. The use of clutches on piston engine helicopters and freewheeling units on gas turbine helicopters is the norm. Rotor brakes are also fitted to allow a faster shut down and a smoother start in high winds as well as the ability to do some maintenance checks without turning the rotors. Figure-1. Westland Wessex (top), Bell 47 (centre), Enstrom (left) Hughes 500 (right) 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 3 of 20 Training Material Only Helicopters have changed considerably as the technology has developed from reciprocating power to turbine power. The powerplants and their relationship to transmissions are quite important to the basic design requirements. Purpose of the power train system is to convey the power developed by the engine/s to the main and tail rotor systems also providing drive for the accessories such as cooling fans, hydraulic pumps and generators. Figure-2. Helicopter Drive Train 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 4 of 20 Training Material Only Clutches The purpose of the clutch is to transmit power from the engine to the main and tail rotors and to allow the rotors to overrun the engine drive in the event of engine failure or during autorotation. The clutch is used in reciprocating engine and turbine powered helicopters that do not use a free turbine (i.e. with a direct–coupled turboshaft engine). The clutch is necessary to unload the engine during starting operation because the inertia required to move the rotor system would be too great. The free turbines do not require this because the engine does not have a direct drive between the compressor and the power turbine. Clutches are always located between the powerplant and the gear reduction of the transmission so the powerplant maybe started without immediate engagement of the rotor system. The clutches are quite varied in design and may engage automatically or manually. Bell 47 Piston Engine Automatic clutch systems vary somewhat in design. The Bell 47 clutch is a centrifugal unit using a set of shoes and a drum. The shoes are splined to the drive gear from the powerplant. As the engine increases in speed, the shoes move outward by centrifugal force making contact with the drum which drives the transmission. When the centrifugal force becomes great enough, the drum, through the transmission, turns the rotor. This engagement should be quite smooth with the rotor RPM lagging slightly behind the engine until the two attain the same speed. At that time the engine and rotor speed remain constant with the shoes riding with the drum. This process should take place in a few seconds. If a longer period of time is required, the clutch is slipping. Figure-3. Bell 47 Clutch 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 5 of 20 Training Material Only Hiller 12 Piston Engine Another centrifugal type clutch is used on the Hiller 12. This type of clutch is often referred to as a mercury clutch because mercury is used for the weight of the centrifugal force. The drive clutch assembly is the clutch housing or drum that is attached to the transmission drive train. The clutch assembly is surrounded by segments of shoe that will make contact with the drum when the engine is running at 500 RPM, with positive engagement at approximately 700 to 900 RPM. Figure-4. Hiller 12 Clutch The segments are moved outward by mercury held under the shoes by a rubber bladder. The shoes are held in place on each segment by springs that assist in returning the shoes to their static position. Because of the slippage that does occur prior to positive engagement, the drum is cooled by air passing through holes in the base of the transmission. The same holes are used to inspect the drive coupling mentioned previously in this section. It is important that no oil leaks into this area of the transmission because clutch slippage will occur. The Bell 47 clutch, however, runs in oil and depends upon it for cooling. 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 6 of 20 Training Material Only Clutch Maintenance The maintenance practices applied to these components, located between the engine and transmission, are as varied as the components themselves. Maintenance procedures must be performed in accordance with the manufacturer’s recommendations. The operation of the clutch and freewheeling unit must be checked in each preflight run-up, as both items are critical to the safety of flight. The clutch engagement should be smooth and positive. To check the clutch engagement, the tachometer is used. Most helicopters make use of a dual type instrument. One needle indicates rotor RPM and the other indicates engine RPM. In normal operation the two needles stay super-imposed, or married. During starting, or until the clutch is engaged, the engine RPM will be ahead of the rotor. After actual engagement takes place the rotor RPM should increase rapidly until the two needles are superimposed. The amount of time required for this to hap-pen is the engagement time. Since the clutch is actually slipping during this period, it is a critical period. Some helicopters are placarded to warn the pilot of the maximum engagement time. If this time is longer or the two needles do not marry, the clutch is slipping. No flight should be attempted under these conditions. The freewheeling unit is as critical to flight as the clutch. In the situation of engine failure or seizure, it is the freewheeling unit that will allow the helicopter to auto rotate. It, too, is checked with the use of the tachometer during preflight run-up. This is done by suddenly reducing the RPM of the engine while the two needles are superimposed. At that time the two needles should split, with the rotor RPM remaining at its present position and engine RPM rapidly decreasing. If this does not happen, the freewheeling unit is inoperative and the procedure should be discontinued. This would mean that the unit is frozen and auto-rotation is impossible. Slippage could occur between the engine and the transmission, giving the same indication as a slipping clutch. If the clutch units are used properly and overhaul procedures are followed, they will normally last from major inspection to major inspection or recommended overhaul. The centrifugal clutch used on the Bell 47 may require new shoe linings. The drum may have to be turned to return its surface to a serviceable condition be-cause it is subject to wear from the shoe contact and warpage from frictional heat. The shoe linings and drums are often replaced by specialty shops. If shoes or drums are to be replaced, the contact area of the two is critical and must be checked to insure that the contact area of the shoe covers the area of the drum. Quite often they must be lapped in place with sandpaper and checked with “Prussian Blue”. The Hiller 12 makes use of a mercury centrifugal clutch. This type of clutch has replaceable linings and a drum, both of which are susceptible to the same wear factors as Bell 47's. In addition, the mercury level is quite important because it is the weight of this mercury, coupled with centrifugal force, which engages the clutch. Mercury is very corrosive to metal. Loss of mercury will be noticed by clutch slippage. 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 7 of 20 Training Material Only Belt Tightener Manual Clutch Helicopters incorporating belt drive transmission use a belt tightener as a clutch. This simply increases the tension of the belt to the point that the pulley on the transmission begins to rotate. With this type of system, the clutch must be manually engaged by the pilot. Operation is not automatic. In one system, the clutch is manually engaged by a lever. Another system uses a solenoid to move the belt tightener into engagement. These systems are very simple mechanically, but they require skill from the pilot to engage and disengage at the proper time for a smooth engagement and to avoid engine overspeed upon disengagement. Since the load is removed by disengagement, the engine must be at low power before it is attempted. The operator’s manual must be closely followed. Automatic clutches have an integral freewheeling function that prevents the rotor from driving a non-rotating engine. This permits autorotation in the event of engine failure. A manual belt drive clutch does not possess this property. Consequently, belt driven helicopters incorporate a sprag clutch freewheel unit in the upper (gearbox) pulley of the belt drive. Freewheel units are explained next. Freewheel Units A freewheel unit is sometimes referred to as an overrunning clutch. A freewheel unit transmits torque in one direction only, and disconnects in the opposite direction. A freewheeling function must be incorporated into the design of all helicopters to allow the engine to drive the transmission, but prevent the rotor from driving a failed engine. Without this function, the engine would be driven by the rotor any time an autorotation is attempted. In addition, any seizure of the engine would make autorotation impossible. For this reason, a twin engine helicopter incorporates a freewheel unit between the power take-off of each engine and the reduction gearing in the transmission. Free-wheeling is an integral (and essential) function of the centrifugal clutch used on some piston engine helicopters and direct-coupled turboshaft powered helicopters. A helicopter powered by a free turbine turboshaft engine does not need a clutch, but does incorporate a freewheel unit to make autorotation possible. As mentioned in the previous paragraph, a manual (belt tightening) clutch on some piston engine installations also requires the addition of a freewheel unit. The location and size of freewheel units varies from one helicopter to another. The operation of a freewheel unit is always automatic. 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 8 of 20 Training Material Only Figure-5. Freewheel Units The most commonly used freewheeling unit on helicopters made in the U.S. is the sprag clutch. This clutch allows movement in only one direction by having an inner and outer race which are often at the end of the driveshaft. The sprag assembly is made up of a number of sprags resembling the rollers in a roller bearing. The sprags, unlike the circular bearings, have a figure-eight shape. The vertical height of each of these sprags is slightly greater than the gap between the ID of the outer race and the OD of the inner race. They are held in position by a double cage assembly spring loaded in the engaged position. This engaged position places the sprags against both races at a slight angle. Rotation from the engine on the outer race jams the sprags between the outer and inner races and this interference fit drives the inner race which is attached to the driveshaft. If the driveshaft attempts to drive the engine, the sprags will be relieved and the driveshaft will rotate without the engine. The same would happen if the engine stopped. 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 9 of 20 Training Material Only Bell 47 and Hillier Sprags may be designed to drive in either direction or on the inner or outer race. The actual application varies considerably from one installation to another, but the operation is the same. Both the Bell 47 and the Hiller have the freewheeling unit as an integral part of the transmission. The belt driven helicopters, however, use the sprag system in the upper pulley of the belt drive. Figure-6. Bell 47 Freewheel Unit Bell 206 The Bell 206 has a sprag system installed at the engine output end. This system operates as follows: a shaft from the power turbine drives the power takeoff gear shaft through the engine reduction gearbox. The freewheeling unit is mounted on the engine gearbox and its shaft is splined directly to the power takeoff of the gear shaft. The engine power is transmitted to the outer race of the free- wheeling unit, then through the sprags to the inner race, which is attached to the transmission driveshaft. Figure-7. Bell 206 Freewheel Unit 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 10 of 20 Training Material Only Hughes 500 The Hughes 500 sprag unit operates in a similar manner, but is different in installation. The sprag clutch is mounted on the front of the engine between the forward power takeoff and the main driveshaft. The clutch assembly is attached to the engine output pad by bolts. No gasket is placed between the two surfaces, but a drain hole is provided in the housing to allow any seal leakage to drain overboard. The unit has shafts projecting from each end of the housing. One of these is the inner race of the sprag unit and the other is the outer race of the sprag. The outer race is the engine side and the inner race is the trans-mission side. The inner and outer races are separated by two ball bearings and the sprag unit. This bearing arrangement is locked by a large nut and lock washer. The housing bearing on the output side of the sprag unit is in constant rotation. This bearing is hand packed with grease. The two bearings and sprag unit, however, operate in oil which is kept within the unit. Since no supply of outside oil is available, the level must be checked regularly and the unit must be inspected for leakage. Sprag units cannot operate dry. Figure-8. Hughes 500 Freewheel Unit 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 11 of 20 Training Material Only Aerospatiale Gazelle Another type of freewheeling unit is shown in the figure below. This particular unit is used on the Aerospatiale Gazelle. The operation of this unit is quite similar to that of the sprag unit, but rollers are used rather than sprags. The rollers are trapped between a lobed shaft and the freewheeling head. When rotation of the lobed shaft occurs from the engine, the rollers make contact with the lobed side and wedge the rollers to the driver and the driven portions, forming a solid unit. When the rotor goes to autorotation, the rollers change position and make contact with the lobe heel. In this position the lobed shaft may remain still with the outer freewheeling head in rotation. This unit is lubricated and sealed with oil in manufacture. Figure-9. Aerospatiale Gazelle Freewheel Unit Maintenance – Hughes 500 The Sprag unit requires servicing if it contains its own oil supply. These units must be checked for oil quantity periodically. The Hughes 500 unit is checked with a ruler. This can be done only after the removal of the driveshaft as shown in the figure below. The units are subject to overhaul unless the manufacturer provides exchange units. Figure-10. Hughes 500 Freewheel Unit 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 12 of 20 Training Material Only Some units may be disassembled for inspection, such as the unit on the Bell 206. Any components in this unit not meeting inspection criteria will be replaced. A wear factor is always created between the sprags, the driver, and driven races of these units. Replacement of parts in these areas is not uncommon. Rotor Brakes Provides a means for holding and stopping of the main rotor during: Engine starting Engine shutdown Emergency rotor shutdown The rotor brake may either be hydraulically or mechanically operated. Another component that may be located between the engine and transmission is the rotor brake. This component is used to stop the rotor on shut-down after the engine has ceased to power the rotor. Because of the inertia of the main rotor, it takes a few minutes for the rotor to come to a complete stop. When loading passengers or for fuelling operations a safety hazard exists during rotor coast down. Wind gusts will add to the hazard because the blades can suddenly dip to within four feet of the ground. For this reason, rotor brakes are often in-stalled, either as standard or optional equipment. Their use, however, is usually limited. Most operators confine their use to necessity rather than convenience because of the wear factors to the unit. The rotor has a very high inertia, requiring a great amount of braking force. For this reason the brake is never applied until the rotor has slowed down considerably on its own. Usually these brakes are a disc type, attached to the input to the transmission. They may be either hydraulically or mechanically operated. Figure-11. Rotor Brake lever 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 13 of 20 Training Material Only Rotor Brakes (Mechanical) – Aerospatiale Astar 350 A mechanically operated rotor brake is used on the Aerospatiale Astar 350 shown in Figure-12 below. This system consists of a fixed housing secured to the transmission (2) and a movable housing (4) that slides in to the fixed housing and supports the brake linings. The movement of this housing is in the fore and aft direction only. Between the fixed housing and the moveable housing is a spring (3) to keep housing off the disc when the brake is not applied. The brake itself is actuated by a control fork (7).The fork slides over the fixed housing and through a sleeve (8), and a diaphragm (5) moves the movable housing against the disc attached to the input of the transmission. Figure-12. Rotor Brakes (Mechanical) – Aerospatiale Astar 350 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 14 of 20 Training Material Only Rotor Brakes (Hydraulic) – Bell 206 The figure below has a view of the rotor brake system used on some Bell 206 models. This system is hydraulically operated with a master cylinder installed on the cabin roof. The master cylinder is equipped with a handle which the pilot pulls to apply brake pressure. The brake unit itself is a single disc unit with a dual brake pad system, similar to what might be found on many general aviation aircraft for wheel brakes. The disc attaches to the short shaft between the transmission and the engine so the freewheeling unit does not affect the braking action. This system, with its master cylinder, will require servicing with fluid because it does not use the hydraulic system of the aircraft. Figure-13. Rotor Brakes (Hydraulic) – Bell 206 Figure-14. Rotor Brake Pressure Gauge 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 15 of 20 Training Material Only Drive Train Systems Purpose of the power train system is to convey the power developed by the engine/s to the main and tail rotor systems also providing drive for the accessories such as cooling fans, hydraulic pumps and generators. Most of the early light helicopters had a reciprocating engine hung vertically and coupled to the transmission which drove the rotor. Some of the older heavier helicopters located the reciprocating engine in the nose of the helicopter and drove the transmission by shafting. At least one military helicopter had the engine in the rear and utilised shafting. Two different reciprocating powered helicopters manufactured today have engines mounted horizontally below a belt driven transmission. Figure-15 below shows some typical engine- transmission relationships in reciprocating powered helicopters. The turbine powered helicopters often have the engines and transmissions in different locations. In either case the power from the engine must be transferred to the transmission. This is done in a number of different ways depending upon the location of the engine and the transmission. Figure-15. Helicopter Drive Train Systems 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 16 of 20 Training Material Only Bell 47 Piston Engine (Information Only) To all outside appearances the Bell 47 engine and transmission appear to be joined into one unit. Actually, the Lycoming TVO-435 engine is bolted to the transmission by an adapter plate, and a gear that meshes with the transmission is placed on the engine. This is a very simple system and requires a minimum of maintenance. Figure-16. Drive Train Systems – Bell 47 In the system used on the Bell 47, with the gear installed on the engine crankshaft flange, the gear is lubricated from the oil system of the engine, eliminating the need of servicing with lubricant. Inspections are limited to major ones because the gear is not accessible except by removal of the transmission. The inspection of the gear will be quite similar to those for other gears and include the use of magnetic particle inspection and visual inspection of the teeth for wear. Since there are no repair criteria for the gear or it’s mating surfaces, it can only be removed and replaced. 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 17 of 20 Training Material Only Hiller 12e Piston Engine (Information Only) The Hiller 12e looks quite similar because of the mounting of the transmission and Lycoming VO-540 engine. However, the internal portion is quite different. In this system, a flexible rubber and steel coupling is bolted to the engine drive flange. The purpose of this coupling is to absorb torsional vibrations imposed on the clutch and drive train. Mounted on top of this unit is the clutch system which in turn has a small splined shaft that drives the transmission. Such a device is not uncommon on transmissions because torsional loads are usually taken into consideration by the manufacturer. Figure-17. Drive Train Systems – Hiller 12e The Hiller 12e, however, with its steel and rubber coupling, will require additional inspections. Lubrication is not used in this area because a dry clutch is located there. Holes are provided, however, for viewing the rubber coupling and its condition. It must be examined for deterioration of the rubber, separation of the rubber steel bonding, and deformation of the rubber. Any of these will require replacement of the coupling. 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 18 of 20 Training Material Only Hughes 300 Piston Engine (Information Only) On two other light helicopters, a belt system is used to transmit the power from the Lycoming HIO 360 engine to the transmission. One system uses eight V-belts placed on a pulley that adapts to the flange of the engine crankshaft as shown in the figure below. Another pulley is attached directly to the transmission. The other system is quite similar in design but uses one single belt of a multiple V-type. Belts loose for start Belts taut for normal operation Figure-18. Hughes 300 Helicopters with a belt drive will require inspection of the belts and pulleys. The pulleys may require greasing for lubrication at specified periods. As any other component requiring lubrication, the periods may be shortened due to adverse conditions. The belts must be inspected for cracks and deterioration. Belts are usually assigned an infinite life. However, as with other materials of this type, deterioration is also dependent upon age as well as use. Lack of use can affect the life as greatly as high usage. The environment will be a large determining factor in the belt life Exposure to weather and cleaning solvents will also contribute to deterioration. Still another system on some reciprocating powered helicopters, the driveshaft, is equipped with a rubber coupling to absorb torsional shock loads and is placed between the engine and the transmission. Items such as a clutch assembly are usually placed in line with this shaft. 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 19 of 20 Training Material Only Drive Train Systems Most turbine helicopters make use of a short shaft system to deliver power to the transmission. These short shafts vary in design, but all have some way to correct for misalignment and for movement of the transmission. Some of these shafts operate with no lubrication, while others require it. This lubrication is usually in the form of grease and is often hand-packed. Figure-19. Main Drive Shafts 2018-01-11 B1-12.4.2 Clutches, Freewheel Units and Rotor Brakes Page 20 of 20 Training Material Only TOPIC 12.4: TAIL ROTOR DRIVE TRAINS AND GEARBOXES Table of Contents List of Figures....................................................................................................................................... 2 TOPIC 12.4.3: TAIL ROTOR DRIVE TRAINS AND GEARBOXES........................................................ 3 Purpose................................................................................................................................................ 3 Tail Rotor Drive Train........................................................................................................................... 3 Bell 47........................................................................................................................................................... 6 Hughes 500................................................................................................................................................... 9 ASTAR 350.................................................................................................................................................. 10 Bell 212....................................................................................................................................................... 11 Sikorsky S-76............................................................................................................................................... 15 Flexible Couplings............................................................................................................................... 16 Caution:...................................................................................................................................................... 17 Tail Rotor System Maintenance......................................................................................................... 18 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 1 of 24 Training Material Only List of Figures Figure-1. Tail Rotor Drive Train............................................................................................................... 3 Figure-2. Tail Rotor Drive Train............................................................................................................... 4 Figure-3. 90 Degree Gear Box................................................................................................................. 4 Figure-4. Tail Rotor.................................................................................................................................. 5 Figure-5. Tail Rotor Pitch Change............................................................................................................ 5 Figure-6. Tail Rotor Drive Train – Bell 47................................................................................................ 6 Figure-7. 90-Degree Gearbox Assembly – Bell 47................................................................................... 7 Figure-8. Tail Rotor Drive Shaft – Hughes 500........................................................................................ 9 Figure-9. Tail Rotor Drive Train – Astar 350.......................................................................................... 10 Figure-10. Tail Rotor Drive – Astar 350................................................................................................... 10 Figure-11. Bell 212.................................................................................................................................. 11 Figure-12. Drive Coupling – Bell 212....................................................................................................... 12 Figure-13. Intermediate Gearbox – Bell 212........................................................................................... 13 Figure-14. 90-Degree Gearbox – Bell 212............................................................................................... 14 Figure-15. Sikorsky S76 Tail Rotor Drive Train........................................................................................ 15 Figure-16. Flexible Couplings – Sikorsky S-76......................................................................................... 16 Figure-17. Bearing Support Assembly- Sikorsky S-76.............................................................................. 17 Figure-18. Grease Point – Hanger Bearing.............................................................................................. 18 Figure-19. Hanger Bearing – Bell 206...................................................................................................... 18 Figure-20. Pin Used on Check Bonded Joint............................................................................................ 19 Figure-21. Non-Lubricated Coupling....................................................................................................... 20 Figure-22. Drive Shaft Alignment............................................................................................................ 21 Figure-23. Intermediate Gearbox............................................................................................................ 22 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 2 of 24 Training Material Only TOPIC 12.4.3: TAIL ROTOR DRIVE TRAINS AND GEARBOXES Purpose The purpose of the Tail Rotor is to provide directional control of the single main rotor helicopters. The tail rotor, like the main rotor, must be able to perform in much the same manner, with the blades being able to change in pitch and flap either independently or as a unit. The tail rotor blades must have a negative and a positive pitch capability to supply directional control under powered conditions and autorotation. Directional control is accomplished by foot pedals similar to those used in fixed wing aircraft to control the rudder. In fact, they are often referred to as rudder pedals, even though they aren’t. Tail Rotor Drive Train The drive for the tail rotor is supplied from the transmission of the helicopter, or at least by a connection with the transmission. It is necessary for the tail rotor to rotate at all times during flight, even if the engine is not operational. The tail rotor is driven by the transmission during autorotation. Figure-1. Tail Rotor Drive Train The tail rotor is driven through shafting from the transmission down the length of the tail boom. The booms generally have some limited flexibility which means the shafting must also have some ability to move in a fore-and-aft direction. This is normally accomplished by the use of hanger bearings, which not only support the shaft, but also provide shaft alignment. Alignment is very important from a vibration standpoint. 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 3 of 24 Training Material Only At times it may be necessary to divert the shaft if the tail rotor is mounted at the top of a tail boom pylon. In this type of situation an intermediate gearbox is used; or, in some instances, a universal joint. The majority of manufacturers use the intermediate gearbox, which is used to change direction and does not increase or decrease the speed. Figure-2. Tail Rotor Drive Train The tail rotor gearbox changes direction and increases or decreases the speed. Some manufacturers prefer to increase the speed while others decrease the output shaft. Figure-3. 90 Degree Gear Box In either situation the tail rotor turns faster than the main rotor. On some helicopters the speed is in excess of 3000 RPM. With others, operating over 2000 RPM is not unusual. 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 4 of 24 Training Material Only Tail rotors are made of a number of different materials and with different designs. Many of the newer blades are composites with metal blades still in use. Some helicopters will use a two-bladed system while others use a multi-blade system. Figure-4. Tail Rotor For the pitch-change mechanism, a number of different systems are used. Most newer helicopters use push-pull tubes to the tail rotor, while a few use cable. The pitch- change system may also have a hydraulic boost on the control system, which is operated from the same system used for the cyclic and collective. Figure-5. Tail Rotor Pitch Change 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 5 of 24 Training Material Only Bell 47 The tail rotor driveshaft assembly provides the mechanical connection between the main transmission, tail rotor output quill, and the tail rotor gearbox. The driveshaft assembly consists of three tubular sections, connected by splined couplings, and one universal joint. Figure-6. Tail Rotor Drive Train – Bell 47 In addition to the three drive-shafts and one universal joint, an extension drive is placed at the end of the shaft assembly. The extension drive assembly consists of an extension shaft, housing, and a gearbox. The extension drive is attached through a yoke to the upper end of the tail boom. The gearbox provides for a 90-degree directional change, and a gear reduction to the tail rotor. 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 6 of 24 Training Material Only Major components of the tail rotor gearbox: Gearbox housing Input sleeve Output shaft and front cap Pitch-change mechanism The tail rotor gearbox housing attaches to the extension tube flange by bolts. The housing is made of cast aluminium and is equipped with a filler plug, a drain plug, a sight gauge, and a mounting point for the brush guard. The input sleeve, made of an aluminium casting, fits into the gearbox housing. The sleeve houses the input shaft, thrust bearings, and alignment bearing. Figure-7. 90-Degree Gearbox Assembly – Bell 47 The output shaft is master splined to index the tail rotor controls on the pitch-change mechanism. The shaft itself is hollow, providing an area for the pitch-change rod. The thrust bearings are retained on the shaft by a flange and gear. 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 7 of 24 Training Material Only The pitch-change rod is a steel tube with solid ends. The outboard end holds the pitch- change bearing inner race which supports the shaft. The inboard end is splined to the spindle and acme threaded to the nut assembly. This arrangement changes the rotary drum motion to a linear movement to change the pitch of the rotor blades. The pitch-change drum is an aluminium casting with grooves and slots for the pitch change cable. The cable wraps around the outside of the drum. The cap and nut assembly are bolted to the drum to provide a dust cover. In the centre of the cap is an inspection hole and plug. Attached to the output shaft of the gearbox is a two-bladed rotor system. The major components of this tail rotor assembly are: Blades Yoke Trunnion Bearing housing Thrust plug Pitch-change horn The tail rotor blades are of bonded metal construction and are balanced against a master blade in the factory. Weights are added to these blades at the tip and at the inboard trailing edge. Neither of these weights is to be disturbed after manufacture. The tail rotor is controlled through pedal cables, a pitch-change mechanism, and the tail rotor. The pedals located on the floor are adjustable to the pilot’s leg length. The pedals act through push-pull tubes to move the jackshaft on the box beam. The jackshaft is connected to a cable drum. From the drum the cables go to the pulley on the tail rotor gearbox. The movement of the drum moves the pitch-change mechanism of the blades. 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 8 of 24 Training Material Only Hughes 500 The Hughes 500 has a very simple system, but is quite different from the Bell 47. The tail rotor driveshaft is a one-piece unit installed between the transmission and the tail rotor gearbox. The driveshaft is a dynamically balanced tube of bonded and riveted construction with flange coupling on each end. About halfway down the tube, a steel sleeve is bonded to the tube to act as a bearing surface for the driveshaft dampener. Figure-8. Tail Rotor Drive Shaft – Hughes 500 Splined couplings are provided at the output of the transmission and input to the tail rotor gearbox. The couplings are steel and are of the Bendix flexible type as used on the main drive shaft. These couplings allow for any misalignment that may occur. The balancing of the shaft is accomplished by brass weights bonded to the shaft in three locations. The driveshaft dampener, mounted in the aft fuselage boom, surrounds the steel sleeve which is bonded to the shaft. The dampener consists of a graphite centred TEFLON® block held in place by bolts, springs, and washers. This block requires a set friction which is set by spring tension. 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 9 of 24 Training Material Only ASTAR 350 The Astar 350 uses a two-section shaft assembly from the engine to the tail rotor gearbox. At first appearance, one would think that the tail rotor would be inoperative if the engine were to fail. It must be realised that the same shaft that drives the transmission with the freewheeling unit, will transmit power from the transmission to the tail rotor, in the event of an engine failure. Figure-9. Tail Rotor Drive Train – Astar 350 The drive shafts are connected to each other, to the engine, and the tail rotor gearbox by flexible couplings which allow for any misalignment that may take place. The short shaft next to the engine is made of steel because of the heat it is exposed to in that area. The flexible discs on the forward shaft are of larger diameter because more flexing occurs near the engine and transmission. The rear shaft is made of aluminium because of its length and not being exposed to the engine heat. It is supported by five or eight bearings, depending upon the engine installation. The bearing packages operate on rubber sleeves to absorb vibrations. Figure-10. Tail Rotor Drive – Astar 350 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 10 of 24 Training Material Only Bell 212 Figure-11. Bell 212 The Bell 212 has a drive system from the transmission rather than the engine. In this system a total of six shafts are used, five of which are the same length and one that is shorter. In addition to the use of the six shafts, four hanger assemblies and two gearboxes are used to deliver power to the tail rotor. The forward shaft extends through a tunnel beneath the powerplant to a hanger assembly on the engine deck. This shaft is connected to the tail rotor output quill of the transmission. The second, third and fourth driveshafts connect between hanger assemblies, with the second shaft being the short one. The fifth driveshaft connects between the hanger assembly and the 42° gearbox, with no hangers in between. Each shaft is made of aluminium alloy and has a curvic coupling, riveted to each end that mates with the hangers and gearboxes. Each of these is statically balanced with weights bonded to the shaft near the centre. Each end is attached to the couplings by V-band type clamps to secure the curvic couplings. The clamps must be replaced as a set because the two halves are manufactured together. The clamps are bolted with the heads in the direction of rotation. The clamps are installed 90° to the bolts on the preceding clamp for balance purposes. This is one of the few areas in which friction torque is used. This means the assigned torque value must have the torque of the locknut added to it for the correct value. Each of the hanger assemblies is used to support the shafts and allow flexing of the tail boom. The hanger assembly consists of a short splined shaft with a single row of ball bearings installed in a ring shaped hanger with mounting lugs. Couplings are splined to each end of the shaft. The front coupling is a flex coupling and the rear coupling is rigid. The flexible coupling consists of an inner and an outer coupling. The outer coupling is retained to the inner coupling by a seal. Both couplings are lubricated by hand-packed grease. The ball bearings assembly is permanently lubricated. The mounting lugs are bolted to the airframe. The bases of the lugs are master shimmed at the factory with the shims bonded to the airframe. The shims should not be removed because they determine alignment. 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 11 of 24 Training Material Only Figure-12. Drive Coupling – Bell 212 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 12 of 24 Training Material Only Located at the base of the tail fin is the 42° gearbox. This is used to change the direction of the shafting as it goes up the tail fin. The gearbox consists of a case with a quill attached at each end. There is no change of speed with this gearbox. The case serves as a reservoir for the oil used to lubricate the gears. It is equipped with a sight gauge filler cap, chip detector, and drain plug. The case, flange mounted to the tail boom, is master shimmed to the boom, as are the hangers. The two quills are removable and may be changed without disturbing the lash and pattern, because the case is permanently shimmed. Both quills are equipped with flexible couplings similar to those used on the hangers. A single shaft connects the 42 degree gearbox to the 90-degree gearbox. Figure-13. Intermediate Gearbox – Bell 212 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 13 of 24 Training Material Only The 90° gearbox, mounted at the top of the tail fin, provides a gear reduction and a 90° change in direction. The gearbox case houses the meshing input and output quill assemblies. This case, like that of the 42° box, acts as a reservoir for the lubrication and is equipped with a cap, sight gauge, and drain plug. This case uses ground shim rings for the placement of the quills. The components may be replaced without disturbing the lash and pattern. Figure-14. 90-Degree Gearbox – Bell 212 Attached to the output of the 90-degree gearbox is a two-bladed tail rotor. The hub is pre-coned and underslung, as is the main rotor. 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 14 of 24 Training Material Only Sikorsky S-76 The SIKORSKY S-76 has a drive system from the main gear box. The system is mainly consisted of intermediate gear box, tail gear box and tail drive shaft. The intermediate gear box, at the base of the vertical stabilizer, transmits torque, changes the drive angle of the tail drive shaft about 57°, and reduces tail drive shaft RPM from 3491 RPM to 3370 RPM. The tail gear box, mounted at the upper end of the vertical stabilizer, transmits torque and changes the angle of drive from the intermediate gear box to the tail rotor. The tail gear box reduces rpm from 3370 RPM at the input side to 1723 RPM at the tail rotor. It also provides a mount for the tail rotor servo and a pitch control mechanism for the tail rotor. The tail drive shaft transmits torque from the main gear box to the tail rotor. It consists of five sections, and extends from the tail takeoff flange of the main gear box, to the input side of the intermediate gear box, and from the output side of the intermediate gear box, to the input side of the tail gear box. Figure-15. Sikorsky S76 Tail Rotor Drive Train 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 15 of 24 Training Material Only Sections I and V consist of two flanges, a shaft, and two flexible couplings each. Sections II, III, and IV each consist of two flanges, a shaft, a flexible coupling, and a bearing support, and are interchangeable. Flexible Couplings The flexible couplings are between adjoining flanges of the tail drive shaft sections and gear boxes and serve to compensate for minor misalignments. Each flexible coupling is 1,173 to 0.185 inch thick and consists of a stack of 15 ± 1 stainless steel discs. Figure-16. Flexible Couplings – Sikorsky S-76 The discs are marked A and B. Each is indexed by flats to make sure they line up when stacked. In A discs, the grain direction is perpendicular to the indexing flats, and in B discs the grain direction is parallel to the indexing flats. When the discs are stacked to make a coupling, the grain direction of the A discs is laid 90° opposite that of the B discs. This is assured by alternating the A and B discs and making certain the indexing flats are in the correct position according to the Aircraft Maintenance Manual. 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 16 of 24 Training Material Only Caution: If any one of the flexible steel discs is damaged, replace entire stack. Do not separate discs during handling, as incorrect reassembly may occur. Information is provided for better understanding of operation only. Sections II, III, and IV each consists of a bearing support assembly, which supports the tail drive shaft. The main components of bearing support assembly consist of a grease- packed sealed ball bearing that is pressed into a viscous damper bladder and supported by a housing that mounts to an airframe interface. The bearing is expected to be lightly loaded since it doesn’t support any significant radial or axial loads, though those imposed from imbalance and misalignment occur in- service. Figure-17. Bearing Support Assembly- Sikorsky S-76 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 17 of 24 Training Material Only Tail Rotor System Maintenance The servicing has been reduced considerably on the newer helicopters because of the use of hanger bearings that are permanently lubricated. However, many of the older helicopters require grease in these bearings. The lubrication periods and the type of grease are specified by the manufacturer. Figure-18. Grease Point – Hanger Bearing Impending failure of any bearing will be indicated by a rise in the temperature of the bearing package and may result in high frequency vibration as the failure progresses. For this reason, on shafting that is not enclosed, the bearing packages are touched by hand during the post flight inspection to determine the temperature. On some of the enclosed shafting, a heat sensitive sticker is placed on the bearing package to indicate increased temperatures. At least one helicopter is using a hanger which allows the bearing to rotate in the housing in case the bearing should freeze, rather than allow the possibility of shaft failure. Figure-19. Hanger Bearing – Bell 206 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 18 of 24 Training Material Only The shafts require very little maintenance and inspection but are very sensitive to corrosion, scratches, and bends. The driveshafts are hollow and usually made of aluminium alloy. All corrosion, scratches and bends must be removed in accordance with the manufacturer’s recommendations. Particular attention should be given to the ends of the shafts during inspection. Some type of coupling attachment is accomplished at the shaft ends. This may be done with pins, rivets, bonding, or a combination of these. In any situation this area is heavily stressed. One method used to attach the coupling to the shaft uses a pin passing through a bonded joint. During inspection, the pin is checked for rotation, indicating whether the bonded joint is still holding. Figure-20. Pin Used on Check Bonded Joint Removal and replacement of the drive shafting requires special attention and some special maintenance practices. When removing shafting, care must be taken that the shafts are not nicked, scratched, or bent. Damage to the shafts can result in its rejection. If there are several shafts and they are of different lengths, it may be advisable to mark their locations. The couplings are of several varieties and may be all interchangeable on one aircraft, while another aircraft may have different types at different locations. Some of the couplings will require lubrication. This is accomplished with grease and may require either hand packing or the use of a grease gun. 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 19 of 24 Training Material Only On certain helicopters, a coupling such as seen in may be used. This coupling consists of a stack of indexed stainless steel discs and it is important that the stack be properly indexed. Once the stack is used, it should never be restacked. It will require removal as a stack and must be replaced as a stack. Figure-21. Non-Lubricated Coupling 2018-05-14 B1-12.4.3 Tail Rotor Drive Trains and Gearboxes Page 20 of 24

CASA B1-12c Helicopter Systems Airframes 2018 PDF

Document Details

Tags

Related

Summary

Full Transcript

Upgrade to continue