Airbus Helicopters uses multiple preforms and a single infusion, replacing the aluminum main structure with CFRP. #airbus #focusondesign

Four-piece preforms for resin-infused ring frames Airbus Helicopters redesigned the aluminum H135 ring frame to carbon fiber reinforced epoxy resin, using a qualified adhesive dry fabric, and using four preforming tools to form it. The 1, 2, 3A, and 3B preforms are shown here and are assembled with unreinforced polymer gusset fillers (shown in Figure 4) to form a one-piece main structure, infused with cost-effective resin And oven curing is made. Photo source for all images: Airbus Helicopters

According to reports, the H135 light twin-engine helicopter is the number one emergency medical service (EMS)/air ambulance helicopter in Europe, known for its reliability, versatility and cost competitiveness. H135 is also used for search and rescue, military, police and wind farms and other offshore facilities. It is manufactured by Airbus Helicopters in Donauworth, Germany, and is equipped with the company's iconic Fenestron tail rotor. Touted by Airbus Helicopters as the helicopter with the lowest operating and maintenance cost of its kind, there are more than 1,350 H135 helicopters in service in more than 60 countries/regions, providing services to more than 300 operators.

Over the years, as the flight time of the H135 fleet has increased, regular routine maintenance has shown that the aluminum ring frame that connects the carbon fiber reinforced polymer (CFRP) tail boom to the CFRP tail rotor guard may experience fatigue and corrosion problems . This requires more detailed inspections and increases maintenance costs. In order to improve H135 inspection and raise safety standards, Airbus Helicopters seeks corrosion- and fatigue-resistant designs.

"Our first goal was to use titanium alloys," recalls Jan-Christoph Arent, senior tool design expert at Airbus Donauworth Helicopters. "But that must be machined, which is expensive, and the raw materials are also expensive." Then, the multidisciplinary development team used prepreg, Airbus's patented vacuum assisted infusion process (VAP) and resin transfer molding (RTM) to analyze the composite material . "Compared with aluminum and titanium, the CFRP ring frame not only provides the strongest solution, but also reduces weight," he explained. "And we have developed a lower cost design than titanium alloy." The design will be produced using multi-piece dry preforms and VAP.

The corrosion- and fatigue-resistant CFRP design improves robustness and reduces operator inspection and maintenance costs.

Compared to the original aluminum design, the weight is reduced by 25% and the cost is reduced by 50% compared to the possible titanium solution.

Develop low repetitive cost designs by using cost-effective tools and manufacturing infused with VAP resin.

The development of ring frame design raises several questions. The ring frame is the main structure connecting the tail boom tube and the tail boom guard. As the new ring frame replaces existing components that cannot change the surrounding structure, design options are limited. “We cannot change the tail beam design or shield in the interface area,” points out Thomas Kunkel from the Donauwoerth Structural Design Office. "We have to make parts with the same connection surface and size, but we can change the riveting pattern and the number of rivets."

After meeting with design, stress, and production engineers, the team had enough information to continue evaluating RTM, VAP, and prepreg. "Our early analysis shows that VAP will be the cheapest method," said Franz Stadler of Donauwoerth's Innovation Department. "So, we then started working with designers to develop parts that could be produced at a lower recurring cost. We also tried to reduce tool costs as much as possible. Therefore, we looked for something that we already have in production and we already have experience. Standard material."

The dry carbon fiber fabric selected by the team has been used in other Airbus helicopter projects. "Our stress department and testing laboratory are already familiar with the strength and size models," said Hans Otto from Donauwoerth's Industrialization Department. G0986 2x2 twill fabric from Hexcel (Stamford, Connecticut, USA) is stabilized with epoxy adhesive and is very suitable for resin infusion preforms. The resin chosen for infusion is Hexcel's one-component RTM6 epoxy resin, which has passed Airbus Helicopter's resin infusion and RTM certification.

However, due to the extreme inexpandability of the geometry (expandable surface is a surface that can be easily formed from 2D sheets), some cuts and notches must be designed in the flat preform to avoid compression in the flat layer When wrinkles are produced and molded into a circumferential layer (see Figure 1). During the preform design, the development and industrialization departments work closely to ensure that the production is cost-effective and meets the load requirements.

“We found that we had to make some cuts on the preforms to prevent wrinkles,” said Antonia Horstmann from Donauwoerth's pressure department. "For certification, we cut several cured ring frames into sections, and tested the ring frame parts under compression and tension. Compression tests were performed to test the compressive strength of the inner flange. Static and dynamic tensioning was performed. Tension test to test the ability to expand and the interface between the CFRP ring component and the unreinforced polymer gusset filler." These test results show that the expected loads can be handled safely and the new CFRP ring frame design is very robust.

Preform image of H135 CFRP ring frame

Otto said that the idea behind the preform design is not to make too many preform elements and not to make too many cuts. "We ended up with four prefabs, each of which is made up of multiple layers of pre-assembly," he explained. The picture on the right shows the four preforms numbered 1 and 2, where 3 includes preforms A and B. The shape number 4 is an unreinforced polymer gusset filler to ensure proper laying in the T-joint area. These preforms are manufactured on four special preform tools (Figure 1), and then assembled with annular gusset fillers to form the final configuration into a multi-piece curing tool (Figure 2).

Figure 1. Preform tool The preform tool is used to create preforms 1, 2 and 3 based on the opening image. Preforms 3 and 1 are shown at the bottom of the left (orange) and right (blue) panels. The preform assembly diagram is displayed in all three panels, the position of the preform is highlighted in color, and the unreinforced gusset filler 4 is shown as a triangle. The dashed line indicates the central axis of the ring-shaped preform assembly. The preform tools for preforms 2 and 1 are shown at the top of the center (green) and right (blue) panels. Please note that the C-profile preform 2 is inverted from its preform tool and assembled with the C side facing up. The Z-profile preform 1 forms the bottom of the preform assembly.

Figure 2. Curing tool Airbus Helicopters has developed a multi-piece aluminum curing tool, including a core tool connected to the substrate and an external tool divided into multi-piece upper and lower ring parts. The assembled preform is shown as turquoise.

Arent describes the curing tool (Figure 2): "The complete tool is made of aluminum alloy. We have the inner core tool [blue] fixed on the base plate [grey]. The outer ring tool [green] and the upper ring tool [orange, Yellow] is divided into multiple parts. This function prevents the thermal clamping force introduced by tool shrinkage during the cooling process after curing."

The preform shown in Figure 1 is assembled in a curing tool and shown as a single unit (turquoise) in Figure 2. The preform assembly in the curing tool starts with the placement of Z-shaped preform 1. Next is the gusset packing. After placing the C-profile preform 2, the two external preforms 3 (A and B) are stacked. "The positioning of the upper and external tools completes this process," Arent points out.

The thermocouple for curing cycle control is integrated into the core. After layup and tool assembly, vacuum bag material is applied and the preform is infused with RTM6 epoxy resin.

After layup and tool assembly, a typical VAP vacuum bag material sequence is applied, using semi-permeable membranes. The vacuum immersion is then completed at room temperature, and the tool and stack are heated in an oven. “You can use self-heating tools for this infusion, but we didn't do it because we didn't produce a lot of parts that needed to be done,” Arent explained. “The curing cycle quickly rises to the first platform for resin infusion. After the resin is filled, we will heat up to 180°C for the standard RTM6 curing cycle. There is no post-treatment. After curing, we cool down and then demold. Then the parts are ready for edge processing and then drilling. The drilling of the rivets must be done with high precision. Therefore, they are performed in the assembly fixture by drilling through the ring frame and the connecting structure [tail arm and tail cover] at once of."

Demoulding CFRP ring frame for H135.

The parts must maintain a tolerance of ±0.4 mm. To ensure that these geometric requirements are met, Arent explained that a distortion analysis was performed using finite element (FE) simulation. "The results of our selected parts and fiber design only showed some unimportant displacements," he pointed out. "With these results, we are confident that our mold design will meet the geometric requirements'first time success'. In fact, there are no tolerance issues and no special effort is required for hard shimming. We only use liquid gasket adhesives It can connect the tail boom, ring frame and shield."

The redesign of the ring frame was completed about a year ago, but it still takes some extra time before switching to production. "We have to make sure that many things-the design and all the process steps are effective, and there are no problems in mass production and the completion of the tail beam," Arent said. "You must ensure that you master the entire process, not just produce individual parts. We must also ensure that the factory's production instructions are changed and everyone is prepared for the new design, even surface treatment and painting. Before we started Before mass production, everyone must ensure that it is effective."

He added that training is important, “because when you switch from metal to composites, you have to understand and consider different things. Not only do you have to modify existing parts, but you also have to machine some reinforcements or some additional layers. This is a technological change." In the end, everything went well, and all new H135 helicopters adopted this composite ring frame.

What is the biggest challenge? "Match the interface without making changes," Arent said. "Tolerances are not a real challenge-these are well managed by tool development and selected designs. If you look at the tool diagram, you will see that internal and external tools build interfaces and they come out accurately . To make the production process robust and efficient, the real challenge is preforming."

Arendt said the redesign was considered a success for several reasons. "By replacing the aluminum design with a corrosion-resistant, fatigue-free CFRP ring frame, safety standards have been significantly improved, and inspection measures have been improved. We were able to reduce weight by approximately 0.5 kg. All of these provide our customers with huge advantages. "

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