As the low-altitude economy rapidly emerges as a strategic industry, unmanned aerial vehicles (UAVs) have become fundamental platforms for a wide range of aerial activities. Among them, the vertical takeoff and landing fixed-wing UAV combines the advantages of high endurance, high lift-to-drag ratio, and convenient takeoff and landing, making it particularly suitable for complex terrains such as plateaus. To achieve green flight, hybrid power systems integrating hydrogen fuel cells and lithium batteries are gaining traction. Typically, the hydrogen fuel cell is placed in the fuselage as the cruise power, while the lithium battery acts as the power source for the vertical lift rotors and is often located inside the wing. If the lithium battery can be designed as a load-bearing structural component of the wing, it can improve the structural strength, stiffness, and stability of the wing, which is crucial for high-aspect-ratio wings operating in complex wind environments. This paper presents the design of a composite wing-battery integrated structure for a 30 kg maximum takeoff mass lightweight fixed-wing UAV. The battery is embedded as a structural element, and the entire wing is optimized to achieve better load distribution. Finite element simulations using ABAQUS are conducted to compare the initial and optimized designs, demonstrating significant improvements in stress, strain, deflection, and modal frequencies. The proposed integrated structure not only provides energy storage but also enhances the mechanical performance of the fixed-wing UAV, offering a promising approach for future UAV design.
1. Wing Overall Design
To meet the requirements of high plateau operations, the NACA 4412 airfoil is selected due to its high lift-to-drag ratio, large lift coefficient, and wide low-drag range. The airfoil profile is standardized and provides favorable aerodynamic characteristics. The wing structure is designed as a conventional beam-rib-skin configuration using T700 unidirectional prepreg composite material. The material properties of T700 are listed in Table 1.
| Parameter | E1 (GPa) | E2 (GPa) | E3 (GPa) | ν12 | ν13 | ν23 | G12 (GPa) | G13 (GPa) | G23 (GPa) | Equivalent Strength (MPa) |
|---|---|---|---|---|---|---|---|---|---|---|
| Value | 119 | 9 | 9 | 0.309 | 0.309 | 0.35 | 4.21 | 4.21 | 3.33 | 480 |
The wing ribs are laminated plates with an average thickness of 2 mm, using a stacking sequence [0/45/90/-45/0/0/45/90/-45/0]. The wing spar has a rectangular cross-section and is made of the same material and layup as the ribs. The skin thickness is 1.6 mm with a stacking sequence [0/90/0/90/90/0/90/0]. The key design parameters of the wing for the fixed-wing UAV are given in Table 2.
| Parameter | Wingspan (m) | Chord (m) | Aspect Ratio | Wing Loading (N/m²) | Safety Factor | Wing Load per Side (N) |
|---|---|---|---|---|---|---|
| Value | 4 | 0.4 | 10 | 188 | 1.6 | 240 |
2. Initial Wing-Battery Integrated Structure Design
In the initial design, a lithium battery module (dimensions: 222 mm × 96 mm × 30 mm) is placed between the front and rear spars near the wing root. The battery module does not contact the skin; instead, it is bonded to the spars and ribs using structural adhesive. The battery acts as a structural element, sharing part of the wing load. This configuration aims to integrate energy storage without compromising the load-bearing capacity of the fixed-wing UAV.
3. Finite Element Simulation of the Initial Structure
3.1 Mesh Convergence Study
To ensure computational accuracy and efficiency, a mesh convergence study is performed on the skin component. Three mesh sizes (5 mm, 10 mm, 15 mm) are evaluated, and the tip deflection is compared. The results are shown in Table 3. The 10 mm mesh achieves a deflection error of only 1.0% relative to the 5 mm mesh, while reducing calculation time by 14.6%. Therefore, a 10 mm mesh is selected for the skin.
| Mesh Size (mm) | Deflection (mm) | Computation Time (s) | Error (%) |
|---|---|---|---|
| 5 (baseline) | 8.81 | 528 | – |
| 10 | 8.90 | 451 | 1.0 |
| 15 | 9.47 | 392 | 7.5 |
The final mesh consists of 16,800 S4R shell elements for the skin (10 mm), and 225,301 C3D4 tetrahedral elements for the ribs, spars, and battery module (with local refinement at corners to 2 mm).
3.2 Loading and Boundary Conditions
A distributed aerodynamic lift load of 240 N is applied to the lower wing surface, representing the equivalent lift under a 1.6 safety factor. The wing root is fixed. Additionally, the weight of the lithium battery (1.5 kg) acts in the opposite direction (gravity). The simulation is performed using ABAQUS Standard with geometric nonlinearity enabled.
3.3 Simulation Results for Initial Design
The stress distribution shows a maximum stress of 182.2 MPa at the wing root trailing edge, well below the material’s allowable strength of 480 MPa. The maximum strain is 263.0 με at the root, and the maximum deflection is 9.5 mm at the wing tip. Modal analysis reveals the first six natural frequencies: 13.9 Hz, 54.2 Hz, 60.4 Hz, 67.3 Hz, 69.2 Hz, and 82.4 Hz. These frequencies are all significantly higher than typical plateau wind excitation frequencies (below 10 Hz), indicating no resonance risk. However, the load transfer through the battery module is uneven, with loads concentrated at the connection points (11.6–34.8 MPa), while the skin does not directly transfer force to the battery.
4. Optimization of the Wing-Battery Integrated Structure
To improve load distribution and fully utilize the battery’s structural capacity, an optimization is implemented. A thin composite load-transfer medium (same material as the spars) is added between the battery module and the skin, allowing direct force transmission. The geometry is modified accordingly, and the mesh is updated to include the new component (total elements: 226,194). The loading conditions remain identical.
4.1 Optimized Structure Simulation Results
The optimized structure exhibits a maximum stress of 173.7 MPa (reduction of 8.5 MPa), located at the front spar near the root. The maximum strain decreases to 186.5 με (reduction of 76.5 με), and the deflection reduces to 8.9 mm (reduction of 0.6 mm). The modal frequencies increase across all six modes: 14.3 Hz, 57.8 Hz, 62.3 Hz, 68.3 Hz, 82.9 Hz, and 86.0 Hz. The load transfer through the battery module becomes more uniform, with values ranging from 7.6 to 22.7 MPa (average 15.2 MPa), and the battery now participates effectively in carrying the wing load.
5. Comparative Analysis and Discussion
To quantify the improvement, an optimization rate is defined as:
$$ \text{Optimization Rate} = \frac{X_O – X_N}{X_O} \times 100\% $$
where \(X_O\) and \(X_N\) are the initial and optimized values, respectively. The comparative results are summarized in Table 4.
| Indicator | Initial Design | Optimized Design | Optimization Rate (%) |
|---|---|---|---|
| Maximum Stress (MPa) | 182.2 | 173.7 | 4.7 |
| Maximum Strain (με) | 263.0 | 186.5 | 29.1 |
| Wing Tip Deflection (mm) | 9.5 | 8.9 | 6.3 |
The optimized wing-battery integrated structure for the fixed-wing UAV shows a 4.7% reduction in maximum stress, a 29.1% reduction in maximum strain, and a 6.3% reduction in deflection. These improvements indicate a more uniform load distribution and enhanced stiffness. The modal frequencies increased, confirming better vibrational resistance. The battery module now serves a dual function: energy storage and structural load-bearing, which is critical for the endurance and safety of the fixed-wing UAV in complex high-altitude wind conditions.
6. Conclusion
This paper presents a comprehensive design and optimization of a composite wing-battery integrated structure for a lightweight fixed-wing UAV. By introducing a load-transfer medium between the battery and the skin, the load distribution becomes more uniform, and the mechanical performance is significantly enhanced. The optimized design yields a 4.7% lower peak stress, a 29.1% lower peak strain, and a 6.3% reduction in tip deflection. The first six natural frequencies increase, improving the structural dynamic behavior and reducing resonance risk. The integrated battery not only powers the vertical lift system but also contributes to the overall strength and stiffness of the wing.
This design philosophy demonstrates a promising approach for future fixed-wing UAVs, especially those operating in challenging environments such as plateaus. The combination of structural composites and energy storage elements opens new avenues for lightweight, multifunctional aircraft structures. Future work will focus on experimental validation of the integrated design and its performance under dynamic loading conditions.