**2.4 Tooling**

One clear thread throughout the development of composite fuselage fabrication processes that was recognized and considered very early on, was tooling. The fabrication of large composite fuselage structures was also enabled by the tooling required to support it. The ability of industry to produce tools using specified materials and built to the size, scale and accuracy required by aerospace and defense applications were critical factors. Large scale machining, laser measuring systems, and innovative thinking supported the transition to today's composite fuselage manufacturing capability.

The ACT program demonstrated how the producibility of large, integrated, composite fuselage structures depend heavily on the tooling to ensure compatibility of the skin cure tool, the cocured or cobonded stringer tooling and the frame tooling. Controlling these elements is necessary to minimize gaps and interference fit between cured detail components. Understanding the effect of tolerance accumulations, warpage, liquid and hard shim allowances and fastener pull-up forces creates the ability to calculate the impact on fuselage structural arrangement and weight, part manufacturing cost and risk and fuselage assembly and integration time. These elements become even more critical as the size of the fuselage grows to 787 and A350 proportions.

One important note was the need for the stringer tooling to be extractable after cure and flexible enough to be able to accommodate skin thickness variations especially the "joggles" or transitions up-across-down at each of the frame stations. These requirements drove the team toward silicone or flexible laminate mandrels reusability was also a key consideration. The mandrels needed to be rigid enough for handling or to be used as drape or vacuum forming mandrels; durable and capable of withstanding a 350°F autoclave cure cycle and still be able to conform to skin ply sculpting and tailoring; and be able to be extracted after cure.

While the use of silicone mandrels and the flexible IML tooling proved adequate for controlling hat stiffener shape, quality and location for the demonstration panels, it was also recognized that silicone mandrels presented many challenges

**107**

**Figure 15.**

*C-17 horizontal stabilizer.*

*The Evolution of the Composite Fuselage: A Manufacturing Perspective*

bladder is released making it possible to remove the bladder.

cure cycles the bladder could be used for (**Figure 16**).

ues today in support of new programs and applications.

bag (**Figures 17**–**20**).

in both scale-up and production scenarios. Boeing started to develop hat shaped silicone bladders that fed autoclave pressure into the bladder throughout the cure to provide uniform pressure throughout the stringer. After cure, pressure in the

At this same time Rubbercraft was working with engineers on the C-17 program to develop and manufacture inflatable silicone bladders for use on the replacement composite tail (**Figure 15**). In 1991 on aircraft 51, a composite tail was integrated into the program. Rubbercraft produced silicone bladders with FEP film molded to the OML of the bladders that were used in IML tools to cocure hat stiffeners to the skin of the horizontal stabilizers. The tooling, bladders and hat stiffener design allowed for the bladders to be manufactured with substantial excess length that supported multiple cure cycles despite the dimensional shrinkage of the bladder in the longitudinal direction. The reusability over multiple cure cycles is key to the cost effectiveness of the inflatable bladder system. Rubbercraft product improvement was focused on bladder attributes that supported increasing the number of

While Boeing was developing flexible IML tooling for cocuring hat stringers and cobonding frames on the ACT program, they evolved away from one-piece overall cauls to separate, individual flexible cauls constructed from graphite/epoxy fabric with a layer of Viton® fluoroelastomer and an outer layer of FEP film. The fluoroelastomer was shown to be more resistant to the epoxy resin and thus more durable than silicones or other rubbers. An added benefit—but perhaps not as well understood at the time—is the added resistance to permeability offered by both the FEP film and the Viton rubber. This helps to minimize the amount of autoclave gas on the inside the bladder from being introduced into the laminate through the permeability of the bladder system. Fluoroelastomer bladder development contin-

A comparison of OML and IML cure tool approaches demonstrates some of the tradeoffs that must be considered. OML tooling is less complex, less expensive, can be initiated as soon as the OML of the aircraft is established and is more forgiving of change than an IML tool. The IML tool requires less labor and risk for locating and maintaining locations of stiffeners and other elements and is much more simple to

The ACT program also looked at separate male winding mandrels for AFP and then transferring the uncured skin to an OML cure tool. The male layup mandrel improved layup rates and proved to be a less expensive approach to meet production

*DOI: http://dx.doi.org/10.5772/intechopen.82353*

## *The Evolution of the Composite Fuselage: A Manufacturing Perspective DOI: http://dx.doi.org/10.5772/intechopen.82353*

*Aerospace Engineering*

**2.4 Tooling**

*AFP process and tooling.*

**Figure 14.**

One clear thread throughout the development of composite fuselage fabrication processes that was recognized and considered very early on, was tooling. The fabrication of large composite fuselage structures was also enabled by the tooling required to support it. The ability of industry to produce tools using specified materials and built to the size, scale and accuracy required by aerospace and defense applications were critical factors. Large scale machining, laser measuring systems, and innovative thinking supported the transition to today's composite fuselage manufacturing capability. The ACT program demonstrated how the producibility of large, integrated, composite fuselage structures depend heavily on the tooling to ensure compatibility of the skin cure tool, the cocured or cobonded stringer tooling and the frame tooling. Controlling these elements is necessary to minimize gaps and interference fit between cured detail components. Understanding the effect of tolerance accumulations, warpage, liquid and hard shim allowances and fastener pull-up forces creates the ability to calculate the impact on fuselage structural arrangement and weight, part manufacturing cost and risk and fuselage assembly and integration time. These elements become even more critical as the size of the fuselage grows to 787 and A350 proportions.

One important note was the need for the stringer tooling to be extractable after cure and flexible enough to be able to accommodate skin thickness variations especially the "joggles" or transitions up-across-down at each of the frame stations. These requirements drove the team toward silicone or flexible laminate mandrels reusability was also a key consideration. The mandrels needed to be rigid enough for handling or to be used as drape or vacuum forming mandrels; durable and capable of withstanding a 350°F autoclave cure cycle and still be able to conform to skin ply

While the use of silicone mandrels and the flexible IML tooling proved adequate

for controlling hat stiffener shape, quality and location for the demonstration panels, it was also recognized that silicone mandrels presented many challenges

sculpting and tailoring; and be able to be extracted after cure.

**106**

in both scale-up and production scenarios. Boeing started to develop hat shaped silicone bladders that fed autoclave pressure into the bladder throughout the cure to provide uniform pressure throughout the stringer. After cure, pressure in the bladder is released making it possible to remove the bladder.

At this same time Rubbercraft was working with engineers on the C-17 program to develop and manufacture inflatable silicone bladders for use on the replacement composite tail (**Figure 15**). In 1991 on aircraft 51, a composite tail was integrated into the program. Rubbercraft produced silicone bladders with FEP film molded to the OML of the bladders that were used in IML tools to cocure hat stiffeners to the skin of the horizontal stabilizers. The tooling, bladders and hat stiffener design allowed for the bladders to be manufactured with substantial excess length that supported multiple cure cycles despite the dimensional shrinkage of the bladder in the longitudinal direction. The reusability over multiple cure cycles is key to the cost effectiveness of the inflatable bladder system. Rubbercraft product improvement was focused on bladder attributes that supported increasing the number of cure cycles the bladder could be used for (**Figure 16**).

While Boeing was developing flexible IML tooling for cocuring hat stringers and cobonding frames on the ACT program, they evolved away from one-piece overall cauls to separate, individual flexible cauls constructed from graphite/epoxy fabric with a layer of Viton® fluoroelastomer and an outer layer of FEP film. The fluoroelastomer was shown to be more resistant to the epoxy resin and thus more durable than silicones or other rubbers. An added benefit—but perhaps not as well understood at the time—is the added resistance to permeability offered by both the FEP film and the Viton rubber. This helps to minimize the amount of autoclave gas on the inside the bladder from being introduced into the laminate through the permeability of the bladder system. Fluoroelastomer bladder development continues today in support of new programs and applications.

A comparison of OML and IML cure tool approaches demonstrates some of the tradeoffs that must be considered. OML tooling is less complex, less expensive, can be initiated as soon as the OML of the aircraft is established and is more forgiving of change than an IML tool. The IML tool requires less labor and risk for locating and maintaining locations of stiffeners and other elements and is much more simple to bag (**Figures 17**–**20**).

The ACT program also looked at separate male winding mandrels for AFP and then transferring the uncured skin to an OML cure tool. The male layup mandrel improved layup rates and proved to be a less expensive approach to meet production

**Figure 15.** *C-17 horizontal stabilizer.*

**Figure 16.** *Inflatable bladder.*

**Figure 17.** *OML sector panel tool. Source: Premium Aerotec.*

**Figure 18.** *IML tool. Source: Boeing.*

rate than two cure tools. This also plays to the argument for a combined IML controlled layup mandrel and cure tool—as Boeing selected for the 787 program.

One concern using IML controlled cure tooling is the ability to adequately control the aerodynamic shell of the fuselage. For the ACT program this meant meeting surface waviness criteria of ±0.025″ over a 2″ length using caul plates. The concern

**109**

*The Evolution of the Composite Fuselage: A Manufacturing Perspective*

over aerodynamic surface control seems to be greatly diminished when you look at what has evolved on the 787 program. The recognition that every airplane has a slightly different OML based on a number of factors such as exact resin content percentage in the prepreg (within the nominal tolerance range of ±5%), the amount of resin bleed experienced during cure and the amount of cured material removed during the sanding, smoothing and preparation for painting process. The skin of a composite fuselage allows for greater tailoring of the skin thickness than is usually incorporated into a metal fuselage. At the base, the fuselage is skin is thicker because it carries more load related to passengers, cargo and landing gear. The structural loads at the top of the fuselage are limited primarily to overhead bins, air ducting, and electrical wiring and this allows for lower weight, thinner skins that predominantly function as aerodynamic surfaces. Regardless of where in space it exists, and even though it varies from aircraft-to-aircraft, the surface is sanded smooth enough to satisfy the surface waviness allowance and negligible difference between aircraft. The ATCAS team envisioned scenarios that included full one piece barrel fabrication. Significant cost savings were estimated from the elimination of longitudinal splices and the need to compensate for tolerance accumulation in assembly. Material out-time, segmented full barrel cure tooling and barrel warpage were the primary

risks identified with full scale single piece barrel fabrication.

*DOI: http://dx.doi.org/10.5772/intechopen.82353*

**Figure 19.**

**Figure 20.**

*IML and OML tooling.*

*IML and OML cure tools [3].*

*The Evolution of the Composite Fuselage: A Manufacturing Perspective DOI: http://dx.doi.org/10.5772/intechopen.82353*

**Figure 19.** *IML and OML cure tools [3].*

*Aerospace Engineering*

**Figure 16.** *Inflatable bladder.*

**Figure 17.**

**Figure 18.**

*IML tool. Source: Boeing.*

*OML sector panel tool. Source: Premium Aerotec.*

**108**

rate than two cure tools. This also plays to the argument for a combined IML controlled layup mandrel and cure tool—as Boeing selected for the 787 program.

the aerodynamic shell of the fuselage. For the ACT program this meant meeting surface waviness criteria of ±0.025″ over a 2″ length using caul plates. The concern

One concern using IML controlled cure tooling is the ability to adequately control

**Figure 20.** *IML and OML tooling.*

over aerodynamic surface control seems to be greatly diminished when you look at what has evolved on the 787 program. The recognition that every airplane has a slightly different OML based on a number of factors such as exact resin content percentage in the prepreg (within the nominal tolerance range of ±5%), the amount of resin bleed experienced during cure and the amount of cured material removed during the sanding, smoothing and preparation for painting process. The skin of a composite fuselage allows for greater tailoring of the skin thickness than is usually incorporated into a metal fuselage. At the base, the fuselage is skin is thicker because it carries more load related to passengers, cargo and landing gear. The structural loads at the top of the fuselage are limited primarily to overhead bins, air ducting, and electrical wiring and this allows for lower weight, thinner skins that predominantly function as aerodynamic surfaces. Regardless of where in space it exists, and even though it varies from aircraft-to-aircraft, the surface is sanded smooth enough to satisfy the surface waviness allowance and negligible difference between aircraft.

The ATCAS team envisioned scenarios that included full one piece barrel fabrication. Significant cost savings were estimated from the elimination of longitudinal splices and the need to compensate for tolerance accumulation in assembly. Material out-time, segmented full barrel cure tooling and barrel warpage were the primary risks identified with full scale single piece barrel fabrication.

The sector panel construction used on the A350 allows for the use of invar for all the fuselage tooling. This includes the IML controlled sector panels fabricated by Spirit for Section 15. The approach Spirit applied is very similar to the one used on the 787 with the exception of the use of sector panels instead of a one piece barrel breakdown mandrel (**Figure 21**).
