Abstract
Compared with traditional energy aircraft, electric/hybrid aircraft show the advantages of environmental protection and low carbon and have been considered one of the most promising directions for the future development of the aviation industry. However, the battery (battery box) structure employed in electric/hybrid aircraft generally faces several problems (e.g., insufficient rigidity/strength and low structural load-bearing efficiency). In this study, the composite cap-type-reinforced wall plate structure commonly applied in electric aircraft is developed to fill the battery structure using the internal cavity of the cap type, which leads to the formation of an integrated structure type of a composite cap-type-reinforced wall plate filled battery. The basic static strength analysis of the integrated structure was conducted to measure its basic load-bearing capacity. To investigate the impact resistance of the integrated structure, a series of bird collision simulations were conducted to analyze the impact position, impact angle, impact speed, and other factors on the integrated structure and the design direction of the composite cap-type-reinforced wall plate-filled battery-integrated structure against bird collision was summarized.
1. Introduction
Electric/hybrid aircraft refer to aircraft using electricity completely and partially as a power source [1]. Compared with traditional energy aircraft, they show the advantages of environmental protection, quietness, and excellent performance and they have an integrated design. Electric/hybrid aircraft have become one of the most promising directions for the future development of the aviation industry [2, 3]. The battery structure, the most critical structure of electric/hybrid aircraft, determines the range of electric aircraft. Electric/hybrid aircraft were subjected to the problems of excessive weight, insufficient strength, and low load-bearing efficiency of the battery structure [4, 5]. Moreover, the allowable deformation of lithium batteries is small and exceeding the deformation limit of lithium batteries will lead to short circuits, explosions, fires, and other dangerous accidents [6]. Thus, the design rationality of the lithium battery installation method has become one of the critical factors relating to the safety of electric aircraft.
There have been relatively few integrated installation solutions for electric aircraft battery structures in the domestic and international literature, which remain at the exploration stage. In general, the lithium batteries are installed in groups into the fuselage, wings, and other structures, thus taking up more space in the fuselage. These batteries are placed in fixed positions and generally do not play a role in the load bearing. To arrange the battery components more flexibly and save the space of the fuselage, a non-load-bearing battery-integrated structure filled and installed in the cap-type-reinforced composite wall plate is designed and the deformation and strain of the integrated structure are investigated under different static loads (e.g., bending, shear, and compression). Considering the risk of bird strike on the outer skin of the fuselage, this study focused on the dynamic response of the integrated structure under bird strike load. Furthermore, the reinforcement design of the composite cap-type-reinforced wall plate structure is conducted to effectively improve the impact resistance of the integrated battery structure [7, 8].
2. Composite-Reinforced Wall Panel-Filled Battery-Integrated Structure Design Scheme
2.1. Composite-Reinforced Wall Panels
An electric aircraft wing cap-type-reinforced T300 composite wall panel (cover) structure was selected for the study (Figure 1), consisting of a long truss of the cap-type section and skin, and the wall panels were connected to other structures of the wing by rivets all around.
To increase the finite element mesh quality and computational efficiency, the geometric features (e.g., the beveled edges of the two ends of the long truss and the smaller diameter mounting holes) were simplified. The simplified geometric model is illustrated in Figure 2.
2.2. Non-Load-Bearing Battery Box Design Solutions
Based on the requirements of service safety, disassembly convenience, and maintenance accessibility of the Li-ion battery structure, the battery box structure of the Li-ion battery assembly was designed not to participate in the structural load-bearing of the body, i.e., the battery box structure is not in contact with the cap truss, and sufficient deformation space was reserved between the outer wall of the battery box and the inner wall of the cap truss (Figure 3). Since the battery box structure was not subject to an excessive structural load, the battery box structure was designed with a T700 composite material.
Based on the internal space of the hat-shaped truss and the general shape of the lithium battery pack, the battery box structure was designed as a rectangular body. To fully use the internal space of the hat-shaped long truss, the thickness of the upper and lower wall plates of the battery box was set to 5 mm and the thickness of the left and right wall plates was set to 3 mm through repeated dimensional adaptation and preliminary static strength/stiffness analysis.
The design variable of the battery box was the width and height of the battery box, and the design constraint of the battery box was the size of the trapezoidal truss (the minimum gap between the battery box and the truss) and the volume of the battery box. The design goal of the battery box is to maximize the storage volume of the battery under the premise of ensuring safety.
Table 1 lists the parameters of the battery box. The design space of the battery box is quite limited due to the limitation of the girder size. The increase in battery box size results in a small increase in battery storage space; it is preferred to leave a certain safety space between the battery box and the long truss to ensure the safety of the battery box. Therefore, the length, width, and height of the battery box are set to 79 mm, 28 mm, and 14 mm, respectively.
3. Composite-Reinforced Wall Panel-Filled Cell-Integrated Structure Modeling
3.1. Finite Element Mesh
The simplified composite cap-type-reinforced wall plate-filled battery-integrated structure meshed is depicted in Figure 4. Since the battery box, the skin, long truss, and other components were thin-walled members, the quadrilateral shell cell (S4R) was used for dissection (triangular shell cell S3 was used at the local curved edge). To ensure computational accuracy, a small mesh scale (between 1 mm and 5 mm) was chosen. 149973 cells were divided into the whole structure, which comprised 149285 quadrilateral shell cells and 688 triangular shell cells.
(a) Battery box
(b) Long truss
(c) Skinning
(d) Wall plate structure
3.2. Material Properties
The cap-reinforced composite wall panel was T300/epoxy composite laminate with a single-layer thickness of 0.25 mm. The battery box structure was T700/epoxy composite laminate with a single-layer thickness of 0.25 mm. The choice of composite materials was based on cost factors. The strength of T300 and T700 already met the demand, while the higher strength T1000 and T1200 were too costly. Tables 2–5 list the basic properties of both composites. Table 6 lists each part’s composite layup properties.
3.3. Loads and Boundary Conditions
In order to investigate the static load-bearing performance of the composite cap-type reinforced wall plate integrated structure more comprehensively, in this chapter, three typical load conditions, including shear, compression, and bending, were selected to introduce the load and boundary condition application methods.
3.3.1. Shearable Working Condition
Two shear conditions were set up, including shear on the short side and shear on the long side. Short side shear is presented in Figure 5. The left side of the short side of the wall plate was fixed, and the right side of the short side of the sheer force was sized as 1000 N. The long side shear is illustrated in Figure 6, i.e., the upper side of the long side of the wall plate was fixed, and the lower side of the long side of the sheer force had a size of 1000 N. The abovementioned setup was used to measure the strength and deformation of the composite cap-type-reinforced wall panel-filled cell-integrated structure under the in-plane shear force.
3.3.2. Pressurized Working Conditions
The pressurized condition is presented in Figure 7, where the adjacent two sides of the wall panel were solidly supported and the other two sides were subjected to an in-plane uniform pressure of size 1 MPa. The abovementioned setup was employed to measure the strength and deformation of the composite cap-type-reinforced wall panel-filled cell-integrated structure under the in-plane pressure.
3.3.3. Bending Conditions
The bending condition is illustrated in Figure 8, where the adjacent two sides of the wall panel were solidly supported, and the other two sides were subjected to a 0.5 MPa normal uniform pressure. The abovementioned setup was adopted to investigate the strength and deformation of the composite cap-type-reinforced wall panel-filled cell-integrated structure under the bending moment.
4. Composite-Reinforced Wall Panel-Filled Cell-Integrated Structure Typical Static Load Analysis
The static analysis of the integrated structure under three typical load conditions, including shear, compression, and bending, was conducted to study the deformation and strain behavior of the composite-reinforced wall panel-filled battery-integrated structure under static load and the relative position change and possible mutual extrusion contact behavior of the long truss cavity and the battery box structure.
Tables 7 and 8 list the displacement and strain distribution of various typical static load analyses. As depicted in these tables, the maximum deformation of the long truss under the bending condition at the edge and the maximum structural strain under the shearing condition at the short edge were 0.00134, less than the allowable strain of T300 composite material 0.005. Under different typical static load conditions, the maximum displacement of the long truss was obtained as 1.143 mm and no mutual crushing contact was found between the outer wall of the battery box and the inner wall of the cap-type long truss. No mutual extrusion contact behavior occurred, i.e., the composite-reinforced wall panel-filled battery-integrated structure was safe under the abovementioned typical static load.
5. Composite-Reinforced Wall Panel-Filled Battery-Integrated Structure Bird Impact Response Analysis
5.1. Introduction to the Bird Collision Model
In this section, the smooth particle hydrodynamics algorithm (SPH method) was adopted. The bird impact response analysis of the composite-reinforced wall panel-filled cell-integrated structure was conducted [12–15]. The bird body modeling was selected as a hemisphere at both ends and a cylinder in the middle [16, 17]. The bird modeling was based on a hemispherical model with two ends and a cylindrical model in the middle. In accordance with the relevant requirements [18], the weight of the bird was 1.8 kg, the radius of the two hemispheres was 53 mm, and the length of the cylinder was set as 141.8 mm. The bird body model is illustrated in Figure 9. To ensure computational accuracy and solution efficiency, the bird body was dissected by tetrahedral cells (C3D4) with a mesh scale of 6 mm and a total of 61101 dissected cells [19]. Table 9 lists the main material parameters of the bird body. Since the bird strikes often occur during aircraft takeoff and landing, the bird body impact velocity here was 35763 mm/s (nearly 80 mph) based on the takeoff and landing speed of the electric aircraft. To fully simulate the whole bird strike process, the bird strike analysis step duration was 10 ms.
Considering the placement and support of the composite-reinforced wall panel-filled battery-integrated structure in the wing and the efficiency of the impact dynamics simulation calculation, the four-sided solid support of this wall panel served as the boundary constraint, as illustrated in Figure 10.
5.2. Bird Impact Response Analysis
In terms of the real bird collision process, the bird’s body shape is irregular and the aircraft’s takeoff and landing attitude is complicated. In general, it is difficult to determine how the bird’s body will impact the aircraft during the collision. Accordingly, the bird impact should be simulated several times in accordance with different impact positions and impact angles [20].
5.2.1. Impact Location Influence Analysis
To explore the effect arising from bird impact on the battery structure at different impact positions, the battery structure was installed at 3 equal locations A, B, and C as the impact points and the impact angles were all along the 15° direction. Three impact conditions were designed as illustrated in Figure 11.
The response of the skin after impact is illustrated in Figure 12. As depicted in Figure 12, the deformation response of the respective part was similar, i.e., the position near the center of the impact point was deformed more by the impact force, while the direction away from the impact of the bird body was affected by the residual splash SPH particles after the impact, thus making the deformation extend to one side. The deformation at the proximal end of the impact center was large, whereas the deformation at the distal end was found to be small. Besides, the largest deformation was at the front end of the impact center, which formed the force performance of the skin after impact.
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As revealed by relevant literature [9–11], the design of the battery box was primarily judged from two perspectives whether it meets the performance requirements of impact resistance. First, the maximum strain of the battery box should not be higher than the allowable strain of T700 material 0.006; second, according to the design principle, no mutual extrusion contact should occur between the outer wall of the battery box and the inner wall of the cap-type long truss during the impact.
The response of the battery box at different impact points is illustrated in Figure 13 and Table 10. Impacted by the distance from the solid support end of the spacer frame, the stiffness of the corresponding central part of the skin was smaller than the stiffness of the parts on both sides of the skin. Thus, the deformation of the battery box is the most significant when the impact location was B and the maximum strain was 0.0009682 (not exceeding the allowable strain of T700 material 0.006) and the maximum displacement of the battery box close to the impact center was 1.937 mm due to the excessive displacement of the skin. The outer wall of the battery box did not exhibit a mutual extrusion contact behavior with the inner wall of the cap-type truss, and the minimum gap was obtained as 2.228 mm. Furthermore, the battery boxes on both sides showed a large displacement of different degrees on the side near the impact point. The deformation of the battery box at A and C of the bird impact was found to be better than that at B. The maximum displacement and maximum strain were improved as compared with those at B.
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The failure coefficient of the composite-reinforced wall panels is illustrated in Figure 14. Impact damage analysis of composite laminates was carried out using the Tsai-Wu tensor strength theory. When the impact angle of the bird is 15 degrees and the speed of the bird was 60 mph, the structure suffers less normal impact force. Therefore, the overall failure coefficient of the structure is kept in a very small range and there is no risk of failure.
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5.2.2. Impact Angle Influence Analysis
Figure 15 shows the impact angle which refers to the angle between the direction normal to the bird body and the direction tangential to the skin. The total velocity of the bird body was kept constant during the impact. The impact angle was different, and the tangential and normal velocities of the bird along the skin are also different, where the normal velocity had a direct effect on the magnitude of the impact force, whereas the tangential velocity was correlated with the magnitude of the shear force on the skin.
Considering the source direction of the possible bird impact on the skin of the aircraft during takeoff and landing [21], six angles, including 15°, 25°, 30°, 45°, 60°, and 75°, were selected for numerical simulation several times and the calculation indicated similarity in the structural response under the same impact angle at different impact locations. Thus, only the calculated results under impact point B were selected for analysis (Figures 16–21).
The morphological changes of the bird body at different angles are presented in Figure 22. With the increase in the impact angle , the collision between the bird body and the skin was also more intense. When the angle was small, after the collision between the bird and the skin, a considerable number of SPH particles splashed away along the tangent direction of the skin; when the angle increased, after the bird hits the skin, the splashed SPH particles were scattered in an “explosion” along the skin with the impact point as the center, so the skin was subjected to a larger impact force [22].
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The response of the battery box at different angles is illustrated in Table 11, Figures 23 and 24. To be specific, with the increase in the impact angle, the maximum displacement of the battery box at the center of the impact point increased and the maximum displacement reached 8.093 mm at 75°; accordingly, the maximum strain of the battery box increased significantly. The minimum gap between the battery box and the long truss decreased with the increase in the angle and the normal velocity of the bird, so the size of the skin to withstand the collision force of the bird increased. However, even if the impact angle increased to 75°, the maximum strain of the battery box was obtained as 0.004693 (not exceeding the allowable strain of T700 composite material 0.006) and the battery box did not collide with the long truss and crush each other, thus revealing that the size design of the battery case is reasonable [23].
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The failure coefficient of the composite-reinforced wall panels is illustrated in Figure 25. When the angle is small, there is no risk of failure of all parts of the structural material; as the angle increases to 75 degrees, there is a small band of failure where the skin contacts the long truss.
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5.2.3. Impact Velocity Influence Analysis
However, the pilot could control the electric aircraft and choose to fly at a lower speed to avoid excessive bird impact damage when the electric aircraft encounters birds [24]. The most dangerous “75°bird impact point B” was selected as the calculation condition. Five sets of speeds, including 60 mph, 70 mph, 80 mph, 90 mph, and 100 mph, were set for calculation based on the takeoff and landing speed of the electric aircraft.
The response of the skin under different bird impact velocities is illustrated in Figure 26; the response of the battery box is illustrated in Figures 27 and 28 and Table 12. As depicted, when the flight speed was 90 mph, the maximum strain of the battery box was 0.0065, which exceeded the allowable strain of T700 composite material 0.006. Accordingly, it would be recommended that the speed of electric aircraft should not exceed 80 mph during takeoff and landing to avoid damage to the structure by bird impact.
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The failure coefficient of the composite-reinforced wall panels is illustrated in Figure 29. It can be seen that as the angle increases to 75 degrees, there is a small band of failure where the skin contacts the long truss. The speed of the bird increased to 100 mph, and the area of failure extended to the contact between the skin and the wall plate. The speed of the bird decreases from 80 mph, and the whole structure can be kept in a safe range without failure.
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6. Conclusion
(1)Preprocessing was performed for the initial wing model to build the finite element calculation model. For the optimized long truss, the skin measurement geometry was adopted to get the simplified battery structure installation space and the design was achieved for the non-load-bearing battery box design solutions(2)To investigate the specific deformation of the battery structure under different static forces and impact loads after installation in the long truss, the performance of the battery structure under static loads was verified, with emphasis on a series of calculations of bird body impact on the battery structure. Moreover, the structural response under different impact points and different impact angles was determined and investigated(3)Based on the takeoff and landing speed of the electric aircraft, the numerical calculation of the bird impact at different speeds was performed and the response of the battery box at different speeds was integrated to determine the recommended safe flight speed for the takeoff and landing of the electric aircraft
Data Availability
No data were used to support this study.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This research was funded by the National Natural Science Foundation of China (Grant nos. 11972301, 11201375, and 11972300), the Fundamental Research Funds for the Central Universities of China (Grant no. G2019KY05203), the Natural Science Foundation of Shaanxi Province (Grant no. 2018JQ1071), and the State Key Laboratory of Structural Analysis for Industrial Equipment (China) (Grant no. GZ18107).