In recent years, unmanned aerial vehicles (drones) have been widely used in various fields, from agriculture, infrastructure inspection, disaster investigation and other industrial applications to leisure applications. There are various types of UAVs, but the mainstream is multi rotor aircraft with four or more rotors to control motion and height. A multi wing fuselage must be both lightweight to increase payload and rigid to support thrust. Although the structure is easy to manufacture, some problems must be considered, such as the weight increase due to the joint of multiple parts and the limitation of the body structure due to the material shape. In order to achieve portability and functionality, it is an ideal way to use 3D CFRP to manufacture fuselage. By optimizing the material arrangement, a certain amount of unnecessary material can be eliminated, and the fibers can be oriented to take advantage of the anisotropy relative to the load. Optimization of fuselage structure by topology The collaborative team designed the initial shape of the upper part for analysis, using the existing fuselage as the basis. Although existing fuselage designs have edges, they redesign smooth, continuous surfaces and adequate draft angles, taking into account the machinability of the mold. The center is flat and can accommodate the GPS receiver antenna.
Initial shape model for analysis.The center is flat and can accommodate the GPS receiver antenna.
The existing body is made of ABS resin with a thickness of about 1.5mm, and has a completely closed bag shaped monomer shell structure. For this reason, a shell with a thickness of 2 mm is used as the analysis model to calculate the design range. Since the carbon fiber will be placed on a flat area around the GPS antenna, this area is not included in the design scope. The fuselage is composed of the upper part and the lower part of the fuselage, which are connected by a plurality of joints and screws. The fastening state is simulated by linking screw position elements. Boundary conditions and topology optimization results
When an aircraft hovers in the air, the body is subjected to various forces that are difficult to measure or estimate. In this project, as a model case without using actual conditions, the team fixed the base on which the load would be attached and created the conditions for six different load / torque variations applied to the four corners of the rotor (load case). Then, the shape that produces the highest stiffness under six different load cases is determined. The analysis results of this paper are the optimization results for the specific model situation, and can not be widely applied to the actual machine.
Redesign the shape according to the optimization results Considering all six load cases, the optimization results lead to the shape completely covered by relatively uniform grid pattern. Setting multiple boundary conditions may lead to potentially high-throughput results. The results of the analysis are finite element mesh data, which can not be used as CAD data. Therefore, the improved fuselage shape is reconstructed according to the results. Manufacturing method: customized fiber placement technology (TFP) customized fiber placement is one of the methods used to make preforms, in which a bunch of continuous long carbon fibers are sewn onto the base cloth. Although the method has been applied in aircraft parts and other applications, there is almost no commercial case in Japan, and the future business development can be expected by establishing the technology as soon as possible. In this study, by using this method, carbon fibers are arranged according to the optimization results to enhance the performance of the aircraft without losing anisotropy. Because the preform is made in plane form, the preform must be designed in such a way that the shape after forming is flat and unfolded, so that the 3D shape can be reconstructed in the mold during the forming process.
VARTM CFRP molding VARTM is a resin transfer molding (RTM) technology in which the mold is used for molding and vacuum pressure suction is applied during liquid resin impregnation. A preform is placed in a single-sided aluminum mold (female mold on the outer surface of the fuselage) and sealed with bagging material. Vacuum suction is used to aid in impregnation of thermosetting resins, which are then cured in an autoclave. Due to the slightly larger size of the preform, the fibers of the first prototype are serrated. In order to correct this, in the design phase of the second prototype, the dimensions are adjusted by changing the offset value between the center plane of the preform and the fuselage surface. The first prototype has quality problems, including insufficient resin impregnation in the carbon fiber and residual space and voids in the carbon fiber bundle. In the process of vacuum bagging, impregnation and autoclave production, the following countermeasures were adopted: changing the glass cloth cutting process; Reduce the viscosity of resin; Reversing the mold side of the preform setting; Change the impregnation process. The results show that the impregnation effect of glass cloth is better, but there are more voids on the surface and inside of carbon fiber bundle. At present, further research is needed to improve the quality of molded parts. The team conducted flight tests on the aircraft and evaluated its practicability. The pilot tested the maneuverability. The results are satisfactory, because the response during steering is better than that of ABS resin aircraft. The combination of topology optimization and CFRP materials is expected to provide high-performance structural parts with both lightweight and high rigidity. This study verified that the three-dimensional CFRP structure can be formed by using TFP preform on multi wing body. In the future, by accumulating the knowledge of preform design and carbon fiber composite molding methods, it can be expected that it will be widely used in various products from the aerospace field.
