Title of Invention

MANUFACTURING PROCESS OF COMPOSITE TAIL ROTOR BLADE FOR HELICOPTER

Abstract

The present invention comprises manufacturing process of a composite tail rotor blade with novel feature of flex beam. The Flex Beam is an integrated part of the paired Tail Rotor Blade. These blade pairs are manufactured by laying up prepreg plies and foam cores at both the end of flex beam. The rotor blade of the present invention incorporates a number of novel features in order to increase its reliability and strength while reducing its weight. In certain embodiments, the tail rotor blade of the present invention is manufactured using unique methods of developing wet tapes, flex beam, polyurethane foam cores and tail rotor blade; machining of the blade and bonding of erosion protection strip, shoe-snubber bearing and bushes using specially designed fixtures.

Full Text

2- PURPOSE OF INVENTION Rotary wing aircraft i.e. helicopters usually have two rotors viz. Main rotor and tail rotor. The large main rotor provides thrust in the vertical direction and tail rotor counteracts the torque from the main rotor system. Pitch of the rotor blade develops the thrust. The Tail Rotor generates side thrust required to counter balance the reaction torque of the main rotor during forward movement /hover and provides Yaw control to the helicopter. The conventional rotor system adopted for earlier helicopters is called articulated rotor system wherein the blades are connected to a hub plate with physical hinges. They have more number of parts and number of areas to be lubricated. These aspects add to maintenance & life cycle costs. Most of the tail rotor design incorporate variable -pitch rotors and use bearings or bushes to change the pitch. Failure of these parts may cause catastrophic failure without any prior indication. Also life of these parts is another reason to worry. The technological advancement in material science especially composites, has led to the elimination of physical hinges in the rotor system. The present day system without mechanical hinges is called bearing less rotor. The multiple load path provided by the composite layers increases the reliability and eliminates catastrophic failure. There are many ways of manufacturing based on the design of the blade. Many relate to metal sandwich structure; some are related to metal blades. Some of these need preparation of top and bottom skins separately and bonding. Turbine blades use metal matrix and these are different from rotor blades of a rotary wing aircraft. The present invention relates to the process of manufacturing of Composite Tail Rotor Blade for helicopters. This tail rotor blade has been designed with a flex beam with continuous fibre along the length where a pair of blades one on each hand are made which is unique feature. The three articulations namely flap; lead lag and torsion are obtained through flexing and twisting of flex beam. This is pure sandwich in nature with polyurethane foam as core. No separate skin manufacturing or honeycomb cores are used. An unique technology of wet tape fabrication is used in this invention. The patents in reference cited above, do not have this type of structural configuration covered in this invention, 2. PRIOR ART AND DRAWBACKS A study is made on the prior patents done on the manufacturing techniques used for the fabrication of Tail Rotor Blades used in Helicopters as referred in para.1. The following draw backs are noted against these patents. Patents US4095322, US4252503, US3782856, US316701. US5041182, US5336367 and US5939007 use top and bottom skins and spars with honeycomb cores to manufacture aerofoil blade shape. These blades do not use flex beam concept and are directly attached to the tail rotor hub through mechanical means. The number of parts are more leading to maintenance problems. The aerodynamic smoothness is affected due to the use of honeycomb core. Use of honeycomb core causes dimpling of the face sheet while curing under pressure and putty is used to achieve aerodynamic smoothness that results in increase of weight of the blade. Fatigue life is less for such type of blades. Patents US4898515, US5431538, US5738494, US5820344 and US6659722 have flex beams but these are not having blades at either end. Each flex beam is mechanically attached to aerofoil section of one blade. The number of parts is more due to mechanical linkage and cause maintenance problem. Pitch change mechanism is less efficient in these blades as the aerofoil section of the blade is at a longer distance from center of rotation. Patent US6056838 is related to injection moulding of blade; patent US6263936 deals with fiber composite object forming; and the other patents are related to small turbine engine blades. These are not relevant to the present invention. The present invention overcomes the drawbacks of prior art/patents discussed above. In present invention, the flex beam and the aerofoil section of the blade along with pitch case are embedded together during manufacturing and blades are made in pair, one each on both the side of a flex beam so that two pairs of blades are required for a helicopter unlike other design where four separate blades are required for a helicopter. The number of parts is drastically reduced, as the pitch case is the integral part of aerofoil section. The control input given to the pitch case for pitch change is efficiently transferred to the aerofoil section due to near proximity of the blade region and the pitch case. The fatigue life for the system is very high in comparison to the prior art, due to no mechanical linkage and use of full fibre composite prepregs. The polyurethane foam cores used in aerofoil section provides a smooth surface and eliminates use of putty for smoothness unlike the prior art where honeycomb cores are used in aerofoil section. Due to superior design of this tail rotor blade, the manufacturing process is unique in certain aspects. In this blade, the material elements and the type of structure are put in proper sequence and in right location to achieve best product ratio. This tail rotor blade is so designed and manufactured that it has got excellent performance, reliability and maintainability. The innovative manufacturing process has kept the price, delivery time and life cycle cost to the lowest. Moreover this type of tail rotor blade can be used for pusher type of helicopters where the aerodynamic performance of the tail rotor is increased due to the reason that vertical stabilizer does not hinder the frontal airflow. 3. BRIEF DESCRIPTION OF THE DRAWINGS The accompanying figures illustrate the present invention and , together with the detailed description of the invention, serve to explain the principle of the present invention. Figure 1 is a side view of the helicopter suitable for use with the present invention -composite tail rotor blade 1. Figure 2 is a isometric view of tail rotor blade pair whose manufacturing process is covered in the present invention. Figure 3 is a drawing of Tail Rotor Blade - raw with lay up details for use with certain embodiments of the present invention. Figure 4 is a drawing of Tail Rotor Blade- post bonded for use with certain embodiments of the present invention. Figure 5 is a drawing of Tail Rotor Blade- machined for use with certain embodiments of the present invention. Figure 6 is a drawing of Tail Rotor Blade- painted for use with certain embodiments of the present invention. Figure 7 is a photograph of Flex beam which manufacturing process is a part of this invention and used in manufacturing of tail rotor blade. Figure 8 is a drawing of Flex Beam -raw with lay up details for use with certain embodiments of the present invention. Figure 9 is a drawing of Flex Beam -machined with cross section details for use with certain embodiments of the present invention. Figure 10 is a cut section of tail rotor blade profile area according to certain embodiments of the present invention. Figure 11 is a cut section of flex beam according to certain embodiments of the present invention. Figure 12 is a photograph of the roving machine used to fabricate Glass fabric wet tapes with certain embodiments of the present invention. Figure 13 is a photograph of moulding tool used for lay up of flex beam according to certain embodiments of the present invention. Figure 14 is a photograph of the lay-up of flex beam according to certain embodiments of the present invention. Figure 15 is a photograph of foam flex beam according to certain embodiments of the present invention Figure 16 is a photograph of tuning mass for flex beam according to certain embodiments of the present invention. Figure 17 is a photograph of the platen press with Flex beam mould under curing according to certain embodiments of the present invention. Figure 18 is photograph of the drilling holes in flex beam in the mould according to certain embodiments of the present invention. Figure 19 is a photograph of the machining of flex beam on CNC machine according to certain embodiments of the present invention. Figure 20 is a photograph of drilling holes in flex beam using drill jig according to certain embodiments of the present invention. Figure 21 is a photograph of moulding tool used for the lay up of Tail rotor blade according to certain embodiments of the present invention. Figure 22 is a photograph of silicon mandrel used to develop hollow section at pitch case area during lay up according to certain embodiments of the present invention. Figure 23 is a photograph of the lay-up of Tail Rotor Blade according to certain embodiments of the present invention. Figure 24 is a photograph of Tail rotor blade cores ( core leading edge, core trailing edge and tip core) according to certain embodiments of the present invention Figure 25 is a photograph of filler top and filler bottom used with lay-up of Tail Rotor Blade according to certain embodiments of the present invention. Figure 26 is a photograph of the platen press with Tail Rotor Blade mould under curing according to certain embodiment of the present Invention. Figure 27 is a photograph of drilling of holes at pitch case for assembly of Snubber Bearing in Tail Rotor Blade using drill jig according to certain embodiments of the present invention. Figure 28 is a photograph of drilling of holes at pitch case for assembly of Pitch Horn in Tail Rotor Blade using drill jig according to certain embodiments of the present invention. Figure 29 is a photograph of erosion Protection strip according to certain embodiments of the present invention. Figure 30 is bonding of erosion strip using bonding fixture according to certain embodiments of the present invention. Figure 31 is bonding of shoe- snubber bearing using fixture according to certain embodiments of the present invention. Figure 32 is chopped strand filling at pitch case area according to certain embodiments of the present invention. Figure 33 is bonding of bushes and shim according to certain embodiment of the present invention. Figure 34 is cure cycle for curing of flex beam for use with certain embodiments of the present invention. Figure 35 is cure cycle for curing of Tail Rotor Blade for use with certain embodiments of the present invention. Figure 36 is a flow chart depicting the process of manufacturing the fiex beam according to certain embodiments of the present invention Figure 37 is a fiow chart depicting the process of manufacturing the Raw Tail Rotor Blade according to certain embodiments of the present invention Figure 38 is a fiow chart depicting the post bonding operations, machining and painting according to certain embodiments of the present invention. 4. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION The embodiments and details given herein are presented to best explain the present invention and its practical application and thereby enable those working in the field to make and utilize the invention. The present invention makes use of a variety of novel features to overcome the inherent limitations of the prior art. The present invention achieves an increase in the life of the blade by reducing the number of moving parts. It also improves reliability and maintainability by eliminating catastrophic failure by using wet tapes, which provides multiple load paths to sustain the load in case of failure. The invention also facilitates easy repair and reconditioning A helicopter incorporating one embodiment of the present invention i.e. composite tail rotor blade 1 is shown in figure 1. The tail rotor blade 1 with flex beam 2 shown in fig 2 described here is of bearing less type. It uses the flexibility of composites in order to eliminate the need of hinges. The flex beam shown in fig. 7 is made up of unidirectional glass tapes of rectangular cross section. Carbon/epoxy prepregs are embedded in the rotor hub attachment area to increase stiffness. The flex beam connects the two blades to the rotor hub. It extends un-interrupted throughout the length of the blade. It carries centrifugal force, flap end lead lag moments so the hub is relieved of this load. It also allows pitch articulations by virtue of its low torsion stiffness. It is fabricated from UD glass/epoxy wet tapes. Carbon/epoxy prepreg layers at ±45° are interleaved in the hub attachment area. The hollow, enlarged and root end extension of the blade is called pitch case as shown in fig. 3. It is made out of ± 45° glass/epoxy layers. The aerofoil shaped blade is moulded around the flex beam. These section consist of UD glass/epoxy prepreg layers. Structural foam is used as core. The glass/epoxy skin with fibres oriented at ± 45° to longitudinal axis provides torsion stiffness. Rigid structural foam stabilizes the skin in the lateral direction. This keeps the shape of the aerofoil. The glass/epoxy UD trailing edge increases the lead-lag stiffness. The Tail Rotor Blade shown in fig 2 and fig. 3 for which manufacturing process is developed is fully composite in nature. The opposite rotor blades are fabricated as one unit. The main part of the tail rotor blade is the flex beam which connects the two blades of each blade pair. The flex beam shown in fig.7 and Fig 8 is designed to facilitate the required pitch control motion by its structural deformation. The flex beam is designed with uni-directional E-glass epoxy and interleaved carbon prepreg layers at hub attachment area. The pitch case and airfoil portion of the blade is contracted around the flex beam. The pitch case has hollow structure made of glass prepregs and further gets attached to airfoil section. The hollow structure of pitch case gives the flex beam sufficient clearance during pitch at articulation. The airfoil section tapered down at tip and made up of foam cores shown in fig. 15 and fig. 24, and glass prepreg layers where extension of flex beam acts as spar. The structural foam (Rohacell foam) is used to form the core for extended portion of flex beam and also for the remaining airfoil sections of the blade. E-glass epoxy prepregs are used as skin layers of the blade. Erosion protection is provided by bonding electro formed nickel strip shown in fig. 29. An helicopter incorporating one embodiment of the present invention is shown in figure 1. The flex beam gets assembled in between the machined metallic top and bottom hub plates through bolts. The bottom hub plate is integrated with tail rotor shaft. The pitch horn is attached to the blade on the pitch case area through the bolt. The other side of the pitch horn is connected to the spider mechanism which is operated by the control rod accommodated inside the tail rotor shaft United to the pedal through yaw control actuator. The process of manufacturing of Raw Tail Rotor Blade has been shown in flow chart in fig 37. It starts with manufacturing of flex beam machining of cores, and cutting of layers. The process of manufacturing of Flex Beam has been shown in flow chart at fig 36. Fabrication of flex beam starts with fabrication of Foam Flex beam shown in fig-15 from PMI (polymethacrylimide) foam by CNC machining. The next step in manufacturing of flex beam is fabrication of the wet tapes. These are manufactured by using roving impregnated machine shown in fig 12. In this machine, 16-20 spools of glass roving are loaded on rollers and filaments from spools are passed over the roller wetted with epoxy resin and pass over a set of roller to remove excess resin then pass through an aperture to maintain the width of the tape. The epoxy resin is kept at 50-60° C in a tray and roller touches the resin. When filaments pass over the roller it gets wet Sample from wet tapes are taken after every 10 metres length and weighed. The aperture is adjusted to maintain the weight. The wet tapes are kept in long tray with protection film and stored at (-)18° to (-) 22° C in cold store. The wet glass roving tapes are shown in fig.12(b). A pair of tuning mass shown in fig. 16 is used in fiex beam which is laid up during lay-up of wet tapes and prepreg layers. This tuning mass is manufactured by casting the lead in a mould. The Carbon bi-directional and unidirectional prepreg plies are used at center portion of the fiex beam. These unidirectional carbon plies 12,13,15.16,17,18.20,21,23,24,25.26,27 and glass unidirectional plies 6. 31,32,34,35,36,37, carbon bidirectional ply 27 shown in fig 8 are cut and punched using punching tool. Punching is one done by using a platen press. Lay up is done on top and bottom mould, developed by CNC programming and machining using CAD data for the air foil section The raw material used for the mould is Al-alloy billet. Before start of the lay up, the mould is cleaned and release agent (a water based wax compound) is applied. The foam fiex beam 2, wet tapes 7,8,9,10,14,19,22,29,30 layers, above punched layers and tuning mass 1 with chopped glass 3, epoxy resin and hardener 4 & 5 are laid up in a mould shown in fig. 8 and fig 14 in line with the fiow chart in fig 30. Top and bottom moulds are shown in fig. 13. After the lay up, the mould is closed. The closed mould is positioned on a platen press shown in figure 17 and curing is carried out as per cure cycle shown in figure 34. As depicted in flow chart in fig. 36, the drilling of 2 holes 1 and 2 in fig.9 is carried out in the flex beam when the part is still in mould as shown in fig. 18. Then the part is demoulded. After demoulding, the flashes are removed and part is loaded on CNC machine on a milling fixture and machined to maintain the thickness at center portion area 8 and 9 in fig.9 as shown in fig. 19. After milling, the holes for hub plate assembly 3 &4 and shoe snubber bearing assembly 5,6 & 7 shown in fig.9 are drilled using a drill plate on a vertical drilling machine as shown in fig. 20. The flex beam is statically balanced by removing material by drilling the holes in tuning masses as needed using steel balls on a surface table. Foam cores for Tail Rotor blade -raw are fabricated by CNC programming and machining. The figure 24 shows the cores. Next stage of raw tail rotor blade starts with the cutting of glass prepreg plies item nos.7,8,11 to 16,18 to 24 and 26 to 40, polyurethane strip item no. 10, Cu mesh ply no. 9, peel ply no. 17 as shown in flow chart given in fig. 37. Filler upper and lower (shown in fig 25) item nos. 5 and 6 are made out of 6 glass prepreg plies with 45 deg orientation item no. 46 to 51 shown in fig.3. These are laid up in pre-compaction tool and pre-compacted at 90 deg. C for 1 hour. Lay up for raw tail rotor blade is done on Top and bottom moulds shown in fig. 21. Two sides of the blade are made in two stages. First one side of the blade pair is laid up and then the other side of the blade pair is laid up using the same set of mould. The moulds are developed by CNC programming and machining using CAD data for the air foil section and pitch case area. The raw material used for the mould is Al. Alloy billet. The release agent is applied on the tool before starting of the lay up for ease of demoulding, once the part is cured. Lay-up of prepreg plies 7,8,11 to 20 on the top and bottom mould is carried out along with as per the lay-up shown in fig 3. Flex beam is positioned after removing peel ply from extended portion and laying up adhesive film item 41. Silicon mandrel developed for hollow section at pitch case is positioned and lay-up of glass prepreg layers for pitch case 21 to 38 is carried out. Filler upper and filler lower 5 and 6 in fig 3 also shown in fig. 25, foam trailing edge 3, foam leading edge 2 and foam tip 1 in fig3 and shown in fig. 24 are positioned on the moulds and gaps are filled with foaming adhesive. Mould is closed by positioning top mould on the bottom mould. The mould is positioned on the platten press as shown in fig. 26. Thermo couples are attached and lay-up is cured in platten press at 130° to 135° C. After curing, the part is demoulded and fiashes are removed. The above process is repeated for the other side to make the complete blade pair. Next stage is post bonding operations, machining and painting shown fig.4, fig. 5 and fig. 6 and stages are given in fiow chart at fig. 38. The erosion strip bonding is first stage of post bonding as shown in fiow chart at fig. 38. Erosion strips 3 in fig 4 made by electro-formed nickel shown in fig. 29 are sand blasted at inside bonding surface and then primed suitably. Araldite is applied on the bonding surfaces of the strip and the blade. Nylon fabric 10 in fig 4 is laid up on bonding surface of the blade to maintain glue line thickness. Strip is positioned on the blade 1 in fig 4. The blade with erosion strip is located in bonding fixture as shown In fig. 30 and cured at room temperature for 24 hrs. Flashes are removed after curing. Lap shear specimen is provided to check the quality of the bonding. In next stage, shoe -snubber bearing 2 in fig 4 is bonded. Corrosion resistant steel shoe- snubber bearing is fabricated by milling, grinding and jig boring. The surface of bonding of the part is sand blasted to prepare for bonding. First a rubber sheet is bonded on the shoe snubber bearing using araldite. Araldite is applied at bonding area at the location of shoe snubber bearing and on shoe-snubber bearing 2 in fig. 4 and same is bonded using a locating and bonding fixture shown in fig. 31. Lap shear specimen is provided to qualify the bonding. At pitch case area, chopped glass built up 11 in fig 4 is done so that pitch horn sits properly at pitch case area of the blade. Bonding fixture is provided for build up. Release agent is applied on the fixture Chopped glass and epoxy resin are mixed and applied on fixture and blade surface. Fixture is located with pins as shown in fig. 32. Curing is done at room temperature for 24 hrs. Fixture is removed after 24 hrs and flashes are removed. Bushes 4,5 and 6 as shown in fig.4 are bonded after vapour blasting using araldite. In next stage, Blade 1 in fig. 5 is loaded on drilling jig for drilling the holes in pitch case for bushes 2 to assemble pitch horn are drilled and reamed on radial drilling machine as shown in fig.27. Holes in pitch case area for bushes 3 to assemble snubber bearing are drilled and reamed on radial drilling machine as shown in fig. 28. Metallic bushes 2 and 3 required to be assembled in holes in pitch case area are machined at centre lathe and ground at cylindrical grinding machine. Aluminium shim 4 in fig.5 is milled to shape in vertical milling machine. Bushes 2 & 3 and shim 4 are bonded using araldite as shown in fig 33. Finally, the blades are painted by using epoxy primer and black and yellow paints as shown in fig 6. Claims We claim 1. A method of manufacturing variable cross section paired tail rotor blade having near elliptical hollow section at inboard gradually merged to aerofoil section at outboard with a flex beam running through and through, and embedded inside the aerofoil section. This comprises the steps of: making a composite flex beam and PMI (ploymethacrylimide) foam cores; laying up the prepreg plies on metallic moulds in two halves having the outer aerofoil contour and assembling flex beam and foam cores creating the hollow section by using silicon mandrel assembling on flex beam and laying up glass prepeg plies on the silicon mandrel; and then curing in a platen press after closing the mould. The pressure and heat are applied so that polymerization of epoxy resin in prepreg plies occurs and the flex beam, PMI foams and prepreg plies are integrated to give a final shaped blade. The holes are drilled thereafter and bushes & shims are bonded for assembling pitch horn and snubber bearing to provide twist during flight. An erosion strip of nickel material is bonded on leading edge to avoid erosion during flight. 2. The method according to claim 1, further comprising the step of manufacturing of flex beam having variable cross section which are made by laying up the wet tape with epoxy glass and carbon prepreg plies, machined PMI foam core and lead tuning mass on metallic moulds in two halves having the outer aerofoil contour; and then curing in a platen press after closing the mould. The pressure and heat are applied so that polymerization of epoxy resin occurs and the wet tape, PMI foam core, lead tuning mass and prepreg plies are integrated to give a final shaped flex beam. 3. The method according to claim 2, further comprising the step of manufacturing of wet tape using glass rovings passing through a tray having epoxy resin . 4. The method according to claim 2, further comprising the step of manufacturing Foam -Flex Beam from PMI foam using CNC machining. 5. The method according to claim 2, further comprising the step for drilling the holes before opening the mould and after that drilling the holes using jigs specifically made for the holes. 6. The method according to claim 1, further comprising the step of manufacturing foam core-leading edge, foam core-trailing edge and foam core-tip from PMI foam using CNC machining. 7. The method according to claim 1, wherein erosion protection strip of nickel is bonded with epoxy araldite using a metallic fixture. 8. The method according to claim 1, further comprising the steps bonding of shoe using bonding fixture, chopped strand filling at pitch case area, drilling hoes using drill jig, bonding bushes and shims in the holes.

Documents:

057-che-2004-abstract.pdf

057-che-2004-claims filed.pdf

057-che-2004-claims granted.pdf

057-che-2004-correspondnece-others.pdf

057-che-2004-correspondnece-po.pdf

057-che-2004-description(complete) filed.pdf

057-che-2004-description(complete) granted.pdf

057-che-2004-drawings.pdf

057-che-2004-form 1.pdf

057-che-2004-form 19.pdf

057-che-2004-form 3.pdf

057-che-2004-form 5.pdf