Complete Specification
Of
Method of manufacturing a composite bottom shell for an advanced rotary wing
aircraft.

Title of the invention
Introduction
Field of invention
Use of invention
Prior art
Draw backs of prior art
Comparison between prior art and present invention
Aim of the invention
Summary of the present invention
Brief description of drawings
Statement of invention
Detailed description of invention
Drawings & diagrams

Title of the invention
The invention relates to a novel approach to fabricate an advanced high stiffened light weight composite bottom shell structure for an advanced light weight rotary wing aircraft such as a rotorcraft.
Introduction
The composite bottom shell assembly is to provide rigid and stiffened structure to mount & support hybrid rubberized fuel tanks and to bear the localized stresses of the connected undercarriage and fuselage structures like side shell and sliding doors during static and flying conditions. The shell is mounted under the floor board of the rotor craft.
Field of invention
The invention is a novel and unique method of manufacturing a hybrid composite bottom shell. More precisely, this invention concerns the domain of fabricating light weight composite rotary wing aircraft structures. The invention relates to technical field of fabricating a high strength single piece sandwich structure comprising low density foam, nomex honey comb core and metallic cores embedded in pre-preg layer along with metallic foil bonded on the surface. An important aspect of this invention is to integrate different core materials, rohacell foam, and composite tube with prepreg plies to obtain a flawless hybrid composite sandwich structure.
Use of invention
The invention is a contemporary method of fabricating a hybrid composite bottom shell sandwich structure with the use of advanced composite materials. This is a flawless method, which can be consistently applied for fabrication of composite sandwich structures to avoid the defects like core / foam crush, foam break, core debonding and core shifting, layer wrinkles & waviness.
Prior art
Patent US 7503368B2 deals with a method of manufacturing composite sections for aircraft fuselages and other structure.
Patent US 7413140 B2 deals in fabrication of under carriage or a force-transmission structure for a rotary Wing aircraft fuselage.
Patent US 66664078B2 deals with fabrication of retractable undercarriages normally comprise a housing structure for helicopters.
Patent US5183618 explains about process for manufacturing a Ski that is used in a helicopter.

Patent US 4167430 relates to a method of fabricating a bonded structure without vacuum bagging and autoclave curing.
Patent US3879245 discloses a method of manufacturing honeycomb cored structures using a matched mould die and cured in a press.
Patent US4593870 describes the invention of crushable energy absorbing composite panels to sustain overall bending failure.
Patent US 5037041 discloses the invention of fabricating cockpit at minimum weight and cost for a helicopter especially when size limitation is considered.
Patent US 6427945 relates to the invention of a subfloor structure of the fuselage cabin cell of a helicopter.
Patent US 6649096 B2 relates to a method of manufacturing of a foam filled honeycomb core parts.
European Patent EP2876042A1 relates to a helicopter airframe with a specific architecture of a subfloor structure that connects to the floor and the bottom shell.
Draw backs of prior art
The patent US 7503368B2 relates to an invention to manufacture a composite structure specifically for a fixed wing aircraft fuselage and is not related to the present method of fabricating the bottom shell of a rotary wing aircraft.
The patents US 7413140 B2, US 66664078B2, US 6427945 and EP2876042A1 describes the method for manufacturing undercarriage components and have not described about the bottom shell and its manufacturing process.
Patents like US 4167430, US3879245 and US4593870 deals with the invention of a new methods to manufacture bonded structures with honey comb cores for an helicopter, but these patents have not explained about manufacturing a composite structure that comprises of light weight foam combined with both honey comb core and metallic core that are embedded in prepreg layers to form a single component and to maintain its structural integrity, which a challenging task to fabricate and process it in an autoclave.
Few patents like US5183618 and US 5037041 describes a method to fabricate composite ski and cockpit for a helicopter at minimum weight and cost. This invention is in particular to a specific part of a helicopter and never deals with the fabrication of light weight high stiffened sandwich hybrid composite structures at optimum cost.

Comparison between prior art and present invention

SLNo. PRIOR PATENT PRESENT INVENTION
01 US 7503368B2 deals with a method of manufacturing composite sections for aircraft fuselages and other structure. A method of manufacturing a light weight high stiffened composite bottom shell of an advanced light helicopter.
02 US 7413140 B2 deals in fabrication of under carriage or a force-transmission structure for a rotary Wing aircraft fuselage This invention relates to fabrication of a shell that carries the fuel tanks and is a part of an undercarriage of an advanced chopper.
03 US 66664078B2 deals with fabrication of retractable undercarriages normally comprise a housing structure for helicopters This invention deals with a high strength sandwich structure used for housing fuel tank.
04 US5183618 explains about process for manufacturing a Ski that is used in a helicopter. The present invention describes a novel method to fabricate a bottom shell that is used in advanced light helicopter.
05 US 4167430 relates to a method of fabricating a bonded structure without vacuum bagging and autoclave curing. The present invention describes a method to bond a thin metallic sheet onto a cured composite structure.
06 US3879245 discloses a method of manufacturing honeycomb cored structures using a matched mould die and cured in a press. This invention discloses a method of manufacturing a typical sandwich structure that comprises of metallic core, honeycomb core and a foam.
07 US4593870 describes the invention of crushable energy absorbing composite panels to sustain overall bending failure. This invention describes a method of manufacturing a high stiffened bottom shell that absorbs the vibrations from the fuselage.
08 US 5037041 disclose the invention of fabricating cockpit at minimum weight and cost for a helicopter especially when size limitation is considered. The present invention discloses the fabricating of a light weight high strength bottom shell especially for an advanced light weight helicopters at optimum cost.
09 US 6427945 relates to the invention of a subfloor structure of the fuselage cabin cell of a helicopter The present invention relates to a structure which lies below the floor board of the fuselage.
10 US 6649096 B2 relates to a method of manufacturing of a foam filled honeycomb core parts. This invention deals with a unique method of integrating foam and different cores in a single sandwich structure.
11 EP2876042A1 relates to a helicopter airframe with a specific architecture of a subfloor structure that connects to the floor and the bottom shell. This invention relates to a specific process of fabricating a structure that connects the floor board and other [structural components of the fuselage.
I

Aim of the invention
The aim of this invention is to provide a unique method of manufacturing a hybrid composite bottom shell for advanced light weight helicopters wherein the foam , honeycomb core and metallic core is embedded in a pre-impregnated Carbon BD fabric and are processed in stages to obtain high strength and stiffness to absorb vibrations and to mount high capacity fuel tanks.
Summary of the present invention
The present invention is directed specifically to fabricate and process high stiffened composite bottom shell for advanced light helicopter. The major stages of this invention
are:
a) Development of the prepreg layer as per the design requirement.
b) Machining of high density foam to the requirement and foam curing.
c) Chamfering of 6 mm thick nomex honey comb core and 25 mm thick metallic aluminum core.
d) Core positioning and room temperature precompaction.
e) Wet Lay up to avoid the foam / core shift.
f) Final bagging and curing in autoclave.
g) Preparation of FTT blocks and samples for FTT test.
h) Ultrasonic testing of the cured part.
i) Preparation of bonding surface and aluminum alloy foils pickling.
j) Bonding of aluminum alloy foil in autoclave.
k) Final inspection and applying epoxy primer.
Brief description of drawings
Fig.l Layup sequence details
Fig.2 Half Sectional View - details of core and foam

Fig. 3 Nickel Coated tool for bottom shell layup.
Fig. 4 Layup of carbon layers and cured foam
Fig. 5 Machined metallic alloy core
Fig. 6 Positioning of metallic core
Fig. 7 Stacking of carbon epoxy prepreg layers over the core
Fig. 8 Vacuum bagging of the component
Fig. 9 Two step cure cycle for final curing of bottom shell
Fig. 10 Cured bottom shell
Fig. 11 Epoxy primer applied to bottom shell
Fig. 12 Sheet bonded to bottom shell
Statement of invention
This invention specifically focuses on a unique manufacturing process of Composite bottom shell wherein advantages of high modulus carbon prepregs with foam, nomex core and metallic core as filler materials are embedded and the cured part is bonded to an aluminium alloy foil to enhance its strength and stiffness.
Detailed description of invention
Composite bottom shell is manufactured using following raw materials:
i) 913/40/G801 carbon BD fabric prepreg material ii) 913/37/120 glass BD fabric prepreg material iii) Low density rohacell foam iv) Nomex honey comb Ox core v) A1. Alloy honey comb core vi) FM 73 adhesive layer
Composite bottom shell is manufactured in three stages and to provide rigid and stiffened structure to mount & support hybrid rubberized fuel tanks and to bear the localized stresses of the connected undercarriage and fuselage structures of advanced light rotary wing aircraft.
The carbon epoxy prepreg layers are cut using automated prepregs cutting machine and kitted as per the layup sequence. The SLE ( shelf life expiry) was ensured before using it for cutting, the tool used for part layup was cleaned using acetone and release-all was applied on the tool surface. The rohacell foam was machined to the required size and

foaming adhesive layer is laid over the foam and is cured in oven before using it for the layup to avoid moisture contamination. The nomex honey comb ox core of 6 mm thick and the aluminium alloy core of 25 mm thick was cut as per the drawing dimensions and the ends are chamfered using core chamfering machine as shown in the fig.5 The cut and kitted carbon prepreg layers, cured foam and cores are laid up as per the layup sequence as shown the fig. 1 & 2. Initially peel ply was applied over the nickel coated tool as shown in the fig. 3 and carbon layers were laid over peel ply to provide better surface for bonding of thin aluminium foil. After carbon layers are stacked the cured foam was positioned as shown in the fig.4.
Eventually the part was bagged and vacuum was applied and the vacuum bagged layers along with foam were left for precompaction at room temperature for one hour under 0.8 bar vacuum. This process will avoid the foam misplacement and improves the compaction between the layers, later the aluminium core was positioned as shown in the fig.6 in between the foam where in between the foam and the aluminum core foaming adhesive was used to bond between them during pecompaction process.
Nomex cores were placed in their positions as indicated in the drawing and the part was vacuum bagged as shown in the fig. 8. The vacuum bag was checked for vacuum leak and if the leak rate is less than 50 millibar for 5 min the bag was allowed for loading in to an autoclave for precompaction process. If not the component was rebagged or the leak was arrested.
The precompaction was carried out in an autoclave at 75±5 deg.C for 45 *15 min duration at 0.8 bar vacuum to ensure the foam and the core was bonded and the layers are compacted together. After precompaction the vacuum bag was removed and carbon layers were stacked as shown in the fig 7. over the core and the wet lay- up was done over the core chamfered portion to avoid core crush and part was vacuum bagged and retained for 6 to 7 hrs until the wet lay- up resin gets cured under room temperature.
Finally after resin gets cured the vacuum bag was inspected for the vacuum leak and was loaded in to the autoclave for curing as per the two step cure cycle as shown in the fig. 9 after curing the FTT( Flat Wise Tensile Test ) specimens were cut as per the standard dimensions from the test laminate which was fabricated and cured along with the actual component as shown in the fig. 10. Later the FTT specimens are bonded to an aluminium blocks using an adhesive and tested in an universal testing machine as per the standard testing procedure to check the flatwise tensile strength.
Meanwhile the actual component was inspected for any defects are the debonding using ultrasonic A scan or the ultrasonic fluoroscopy. If the component is defect free the peelply which was applied initially during layup was removed to obtain rough surface to bond the aluminium foil which has undergone pickling of 4 hrs. Adhesive film layer FM73 was applied over the surface where the foil is bonded and again the component was bagged and checked for vacuum leak.
The vacuum bagged component was loaded in to the autoclave for sheet bonding process which was carried out at 120 deg.C for 60 min duration at minimum 0.2 bar vacuum. Later the sheet bonded component as shown in the fig. 12 was inspected and will be cleared for applying epoxy primer as shown in the fig. 11.

The two step cure cycle as claimed in the claim 7, said the cure cycle comprises three parameters like temperature, pressure and vacuum which are maintained with time. The temperature was maintained above the set temperature suggested as per the prepreg datasheets and the pressure was maintained with the type of foam or the core materials embedded in the prepreg layers. As the component claimed in the claim 1, comprised of different core and foam materials, it is a challenging task to cure this component without core crush or the core shift. Hence a unique technique called as the wet layup of resin was used over the core material before final curing in autoclave.
The wet layup process as claimed in the claim 8, said the wet lay was carried to avoid the core shift or the core end shrink due to high pressure applied during curing process and due to the internal stresses generated between the core material and the prepreg layers.
). The sheet bonding process as claimed in 5, said a thin aluminum foil that has undergone picking process for four hours was used to bond over the surface of the cured bottom shell component to provide better strength to the component.