Title of the invention
A Novel Method to fabricate Carbon Fibre reinforced ARIS Pendulum for Advanced Light Choppers
Introduction
Pendulum absorber is a member of vibration isolation system, which helps in passive suppression of vibrations, mainly originated during rotation of main rotor blades in a chopper. Passive vibration isolation uses materials and linkages that absorb and damp the amplitude of vibration. Common passive isolation systems in use are Pneumatic isolators, Spring dampers. Pads or sheets of flexible materials, molded and bonded elastomeric mounts, wire rope isolators and tuned mass dampers. At present the Designers are striving to design an advanced, light weight, high strength vibration isolation absorbing system for light weight choppers. In this view, it's a challenging task to fabricate a composite absorber for ARIS (Anti-Resonance Isolation System) to dampen the ■ vibrations.
ARIS pendulum is a cylindrical-shaped variable cross sectional member, which houses the tuning mass and transfers the inertia forces to the structure through elastomeric bearings. It is a monolithic construction with uni-directional and bi-directional carbon epoxy fabric as the constituents. A perspective view of Composite ARIS Pendulum is demonstrated in Fig. 1.
Field of invention
The present invention relates to the field of Composites manufacturing, specifically for the light weight choppers. The method used for manufacturing composite Pendulum is unique, involving manual lay-up of carbon epoxy uni-directional & bi-directional prepregs with the aid of a specially fabricated closed mould.
Use of invention
The invention deals with a modern method for manufacturing of composite Pendulum (ARIS) for light weight choppers, with the use of advanced composite materials. The manufacturing of Pendulum using a closed mould technique, which is both robust and reliable, yields a defect free product.
Prior art
Patent no. US9090028B2 - Discusses method of forming a contoured composite part made of unidirectional pre-pregs having multiple legs, wherein the fibers are placed by automatic fiber placement machine.
Patent no. US9162380B2 - Discusses about method & apparatus to form highly contoured composite parts from multiple plies of pre-preg material. It employs mating re-

configurable dies to produce a variety of contoured parts, reducing or eliminating the wrinkles during processing on forming machines.
Patent no. US 9486951B2 - Discusses the closed mould method of manufacturing composite parts, wherein the injection & curing takes place in the injection tool, which is placed inside a pressure chamber. It aims to reduce the manufacturing cycle time and improve tool life, otherwise the moulds can sustain limited injection pressure, which might affect complete laminate saturation. It finds application in RTM processes.
Patent no. 4239455 - Invention relates to blade mounted centrifugal pendulum, which is connected to helicopter rotor grip and helps to isolate vibration in helicopters.
Patent no. 2892502A - Invention relates to vibration damping device for helicopter blades, wherein a pre-determined mass is pendulously mounted on a rotor blade surface. The pendulum actuates the control surface of aerofoil to dampen the vibrations.
Patent no. US89742I2B2 discusses about methods and systems for manufacturing composite parts, in particular it deals with the tooling system comprising a female mould surface configured to support a plurality of fibres covered by a sealing layer, which suits for U shaped channels and L shaped composite parts.
Patent no. 4619349 relates to a concept of vibration isolator with two degrees of freedom, to reduce the transmission of dynamic moments in addition to the prevention of dynamic forces using a pendulum rod connected with the leaf springs, wound in a semi-circle or arranged in a spring winding plane.
The available patent literature throws light on the methods of fabricating contoured components using female/male tools, usage of mating dies, processes dealing with closed moulds and on the concepts of vibration isolators mounted in different arrangements. No literature/patents are available on the art of manufacturing Composite Pendulums, as on date.
Draw backs of prior art
Drawbacks of the above referred patents are as follows:
Patent nos. US9090028B2 & US9I62380B2 deal with a method to form a contoured composite structure in general with the aid of Automated Fibre Placement machine, The later patent discloses a method to fabricate highly contoured parts with the use of mating dies instead of the hand layup to eliminate wrinkles during forming process. However, both the technique does not deal with fabrication of variable cross sectional cylindrical structure that best fits to manufacture the vibration isolation members like Pendulum.
Another US patent no. US 9486951B2 claims a closed mould technique that aims to reduce the process time and increases the tool life by placing the closed mould inside a pressure chamber. This technique best suits for RTM process in general and specifically does not deal with the present invention to fabricate light weight composite pendulums.

US Patent no. 4239455 claims the design of vibration reduction apparatus for a helicopter lift assembly, which includes a plurality of rotor blades. The scope of this invention is limited to only the concept a method to reduce the vibration at the hub of the helicopter rotor assembly. It does not deal with the process or the manufacturing of the vibration isolation parts, made of carbon epoxy composites.
US Patent no. 2892502A Invention claims a vibration damping device directly mounted on a helicopter rotor blade to minimize the magnitude of vibrations, in particular on aerofoil surface of rotor blades. However, this technique does not relate to the present invention of passive vibration isolation system proposed for light weight choppers.
Patent no. US8974212B2 claims a tooling system for manufacturing fibre reinforced resin parts, in particular for U shaped channels with angles ranging from 80-100 degrees. It does not deal with the present scope of invention.
Patent no. 4619349 claims a concept of vibration isolator comprising a pendulum rod and a leaf spring arrangement. However, this concept or method does not deal with the present invention of fabricating the carbon composite variable cross sectional pendulum, suitable to mount on circular shaped composite spring assembly.
It thus comes out that there is no patent or literature available, as on date, which deals with the art of fabricating composite pendulums, which is a cylindrical tubular member, with varying cross sectional area as well as varying thickness along the length. The present invention deals with one such unique manufacturing method for manufacturing of light weight composite pendulum absorber.
Aim of the invention
The aim of the invention is to provide a unique method for manufacturing of carbon fibre reinforced pendulum, suitable for highly efficient vibration isolation system for Light weight choppers.
Summary of the present invention
The usage of UD & BD carbon prepregs for the fabrication of variable cross sectional composite pendulum throws few challenges like occurrence of prepreg layer folds or wrinkles, poor compaction and achieving close dimensional tolerances along the length of component. It is an invention of developing and evolving a method to manufacture defect free composite pendulum for advanced choppers, overcoming the above challenges.
The process of manufacturing includes - Cutting and identification of the prepreg layers with the help of template, Casting of the intensifler using the same closed mould and simulator of the component, Positioning of the intensifier mandrel over the metal core, Manual lay-up of Carbon UD & BD layers on the intensifler mandrel, Intermediate debulking of the laid up plies, Final lay-up of the balance plies, Preheating the split

mould, positioning the laid up mandrel in the mould, Vacuum bagging, Loading & Curing of the bagged part in an autoclave, Final Demoulding and Trimming.
The usage of elastomeric intensifier coupled with closed mould, staggering of butt joints around the mandrel periphery and a proven autoclave cure cycle for curing vacuum bagged carbon epoxy composite pendulum are the special features of the invention.
Brief description of drawings
Fig. I demonstrates the construction of Pendulum (ARIS).
Fig. 2 illustrates the Intensifier made from elastomeric material.
Fig.3 shows the Inner core of the Split Closed Mould.
Fig.4 illustrates the split mould with inner core and intensifier.
Fig.5 shows the component simulator positioned over the inner core for casting
elastomeric intensifier.
Fig.6 shows the Intensifier positioned over the inner core, comprising the inner mandrel.
Fig.7 shows the Lay-up on the inner mandrel.
Fig.8 shows the Vacuum bagging for intermediate De-bulking.
Fig.9 illustrates the Inner mandrels with Completed lay-up
Fig. 10 demonstrates the Final De-bulking.
Fig. 11 illustrates the positioning of inner mandrel in the outside split mould .
Fig. 12 shows Close mould splits.
Fig. 13 illustrates a Finished Carbon fibre reinforced Pendulum.
Statement of invention
This invention specifically focuses on a unique method of manufacturing variable cross sectional cylindrical shaped, composite pendulum using high modulus high strength carbon fibre epoxy prepregs with the aid of specially in-house designed closed mould tooling.
Detailed description of invention
The proposed invention to manufacture variable cross sectional cylindrical shaped composite pendulum comprises a split tool with elastomeric intensifier and a clamping arrangement. Preferably the split tools are made of steel and the elastomeric intensifier is made of silicone. An embodiment of a silicone intensifier according to the invention may appear similar in cross section to that depicted in Fig. 2, wherein the intensifier is wrapped with the plies. The intensifier may be supported on the inside using an inner core, preferably made up of steel, to obtain dimensional stability and to avoid distortion

during the process. A perspective of the inner core is depicted in Fig. 3. The intensifier consisting of core, after layup, is encapsulated in a split external mould as demonstrated in Fig. 4.
The intensifier may be cast using the same set of tooling, using a simulation of the component, as'shown in Fig. 5. Depending on the shape of the pendulum, the intensifier may be fabricated in multiple sections, as illustrated in Fig. 6.
The plies to be wrapped around the intensifier may be divided suitably into sections and laid up one by one. with the sections forming butt joints around the periphery of the intensifier. During lay-up of the subsequent ply, the butt joint may be staggered suitably, so as to achieve the required strength and uniform thickness. Care should be taken before starting lay-up to clean the tool using a release agent and allowed to dry for 30 min. The lay-up of the component is demonstrated in Fig.7. Intermediate de-bulking is carried out during lay-up to remove the air entrapment between the layers and to achieve better compaction, which is illustrated in Fig.8. After de-bulking, remaining plies are laid up, as per the pre-defined lay-up sequence and a thin perforated release film is wrapped over the plies and subjected to final de-bulking for 8 hours, as shown in the Fig. 9 & 10. Later, the compacted lay-up is re-positioned into the pre-heated steel split mould and finally vacuum bagged.
Finally the vacuum bagged part is moved to autoclave for curing process, in particular the curing process is carried out under controlled temperature, pressure and vacuum; specifically the cure cycle was designed to cure these closed mould components in an autoclave under elevated pressure and temperature. Later the cured part is de-moulded and trimmed to the required dimensions.
Manufacturing of Composite Pendulum with the subject method provides the following advantages:
> Uniform pressure application on the component along with vacuum results in void free & excellent compaction properties
> Achieving variable thickness of the tube section along the part length with the use of intensifier
> Achievement of close dimensional tolerances.

Claims
Following Claims are made for this invention:
1. This invention claims a unique method of manufacturing variable cross sectional carbon fibre composite Pendulum for advanced light weight choppers.
2. This invention claims the method of incorporating staggered butt joints during manual lay-up of the component to avoid wrinkles over the varying circular tube section.
3. This invention claims the design and usage of split closed mould tooling in conjunction with the elastomeric intensifler mandrel.
4. This invention claims the method of casting silicone material mandrel using the same split closed mould and simulating the varying pendulum thickness across several cross sections along the length.
5. This invention claims the in-process room temperature debulking after placement of optimum number of pre-preg layers.
6. This invention claims that the present- method is an improvement over the filament winding process, thereby saving time and cost.
7. This invention claims that the curing process of this pendulum can be carried out along with other composite components in an Autoclave, rather than curing in separate mechanical press, thus saving process cost.