Airbus successfully replaced the original aluminum alloy ring frame of H135 helicopter with carbon fiber reinforced polymer (CFRP) as raw material by using multi piece combined preform and one-time pouring process.
As an emergency medical service / air ambulance helicopter, Airbus H135 light twin engine helicopter has become the helicopter with the lowest operation and maintenance cost among Airbus's similar products because of its high reliability, multi-function and cost advantages. At present, more than 1350 H135 are in service in more than 60 countries or regions, providing services to more than 300 operators. However, with the increase of flight time of H135, relevant regular operation and maintenance data show that the aluminum alloy annular frame connecting CFRP tail beam and CFRP tail rotor shield is prone to fatigue and corrosion, which directly leads to the increase of operation and maintenance detection cost. In order to reduce the cost and ensure the safety of the body, Airbus began to study a new process design scheme for the corrosion and fatigue resistance of the annular frame.
Airbus first considered the scheme based on titanium alloy, but the machining process and raw materials of titanium alloy scheme are more expensive than aluminum scheme. Therefore, the multidisciplinary R & D team developed a new design scheme based on CFRP by using prepreg, vacuum assisted perfusion (VAP) and resin transfer molding (RTM) processes. After calculation, the weight of the new scheme is reduced by 25% compared with the aluminum scheme, and the cost is reduced by 50% compared with the titanium alloy scheme, which significantly reduces the detection and maintenance cost.
Engineers cannot change the existing design of the tail beam and tail rotor shield, and the annular frame is the main structure connecting the tail beam and tail rotor shield. Therefore, the CFRP annular frame must have the same connection surface and geometric dimensions as the original aluminum frame, which greatly limits the freedom of the design scheme. Through communication and evaluation, the engineering team and the design, stress and production engineers finally determine to use the standard specification materials that have been widely used in actual production, so as to minimize the R & D cost and subsequent batch manufacturing cost. Among them, the g0986 twill carbon fiber of American Hexcel company used in the new scheme has been widely used in the pouring molding of other helicopter projects of Airbus; The resin for pouring is one component rtm6 epoxy resin of hatch company, which has already passed the resin pouring and RTM certification of Airbus helicopter company.
Because of the extreme non deployability of the ring frame geometry, engineers designed additional cuts and notches in the preform to avoid wrinkles during molding. Subsequently, engineers tested the compressive strength of the inner flange, verified the deployment capacity of the CFRP annular frame and the interface between the CFRP annular frame and the unreinforced polymer gusset plate through static and dynamic tensile tests. The test results show that the new CFRP ring frame design is very robust and can safely handle the expected load.
For resin pouring, the engineering team developed four preforms as shown in the figure below. Among them, No. 3 preform includes two parts A and B; Preform No. 4 is a non reinforced polymer gusset plate to ensure the connection of the t-area.
For the four preforms, the engineering team developed their corresponding special tooling, and then assembled all preforms and annular gusset plates together into the final curing tool. The final complete curing tools are made of aluminum alloy, including blue tooling fixed on the gray base plate, green ring tooling spliced by multiple parts, and orange and yellow top ring tooling. The design purpose of multi-part splicing tooling is to prevent the shrinkage force caused by tooling cooling during curing and cooling.
After completing the assembly of laminates and tooling, the engineering team uses the most typical and cost-effective VAP process scheme for resin pouring. After curing, cooling and demoulding steps, the team carries out finishing machining for the edges of parts, and then carries out high-precision drilling for subsequent riveting work. In order to ensure the connection accuracy between the annular frame and the tail beam and tail rotor shield, the design tolerance of the connection shall be controlled within ± 0.4mm. The finite element analysis results show that the die design results of the new scheme are "one-time success" and can meet the tolerance requirements.
Airbus's new design for CFRP ring frame was completed about one year ago, but it took a long time to change from aluminum alloy to composite material. The company must consider many problems, such as the rationality of design, the verification and confirmation of process steps, the stability of mass production and so on. In addition, the staff training for the new process is also very important. It must be ensured that the engineering personnel in each link are ready to adapt to the new design scheme. Airbus believes that the conversion of ring frame manufacturing from aluminum alloy process to CFRP process is not only a simple modification of existing parts, but a technological change in Airbus helicopter department; Among them, the most challenging work is not to meet the geometric tolerance requirements, but the successful development of preforms.
At present, all newly produced H135 helicopters are equipped with new CFRP ring frames. Due to the excellent corrosion resistance and fatigue resistance of the new CFRP ring frame, Airbus has reduced the product weight of about 0.5kg, significantly improved the safety performance and reduced the detection times and cost.
