In this study, we focus on the structural design of a vertical takeoff and landing (VTOL) fixed-wing drone with a maximum takeoff weight of 30 kg. The objective is to integrate lithium batteries into the composite wing structure so that they not only store energy but also participate in load bearing. This approach enhances the strength, stiffness, and operational stability of the wing, which is particularly important for high-altitude environments with complex wind fields and low air density. We adopt the NACA 4412 airfoil for its favorable lift-to-drag ratio and wide low-drag range. The wing parameters are summarized in Table 1.
| Parameter | Value |
|---|---|
| Wingspan | 4 m |
| Chord length | 0.4 m |
| Aspect ratio | 10 |
| Wing loading (design) | 188 N/m² |
| Safety factor | 1.6 |
| Ultimate wing loading | 300 N/m² |
| Single wing ultimate load | 240 N |
The wing structure consists of composite spars, ribs, and skin made from T700 unidirectional prepreg. The material properties and layup sequences are critical for achieving the required mechanical performance. The ribs and spars have an average thickness of 2 mm with a layup of [0/45/90/-45/0/0/45/90/-45/0]. The skin thickness is 1.6 mm with a stacking sequence of [0/90/0/90/90/0/90/0]. The T700 prepreg properties are provided in Table 2.
| Property | Value |
|---|---|
| Longitudinal modulus E₁ (GPa) | 119 |
| Transverse modulus E₂ (GPa) | 9 |
| Through-thickness modulus E₃ (GPa) | 9 |
| Poisson’s ratio ν₁₂ | 0.309 |
| Poisson’s ratio ν₁₃ | 0.309 |
| Poisson’s ratio ν₂₃ | 0.35 |
| Shear modulus G₁₂ (GPa) | 4.21 |
| Shear modulus G₁₃ (GPa) | 4.21 |
| Shear modulus G₂₃ (GPa) | 3.33 |
| Laminated equivalent strength (MPa) | 480 |
Initial Wing-Battery Integrated Structure Design
We embed a lithium battery pack inside the wing cavity, positioned between the front and rear spars near the wing root. The battery pack dimensions are 222 mm × 96 mm × 30 mm, and it is bonded to the spars and ribs using structural adhesive. This configuration forms an integrated wing-battery structure where the battery acts as a load-bearing component. The initial design does not establish direct contact between the battery and the wing skin; the battery is only connected to the internal spars and ribs. We perform finite element analysis using ABAQUS to evaluate the mechanical response.
Finite Element Modeling and Load Application
We generate a finite element model with appropriate mesh convergence. The wing skin is meshed using S4R shell elements with a global size of 10 mm, resulting in 16,800 elements. The remaining parts (spars, ribs, battery) are meshed with C3D4 tetrahedral elements; the majority are 4 mm, while local corners use 2 mm, totaling 225,301 elements for the initial model. The load is applied as a uniformly distributed pressure on the lower wing surface, simulating aerodynamic lift. The root of the wing is fixed, and the battery weight (1.5 kg) is applied as a downward gravity force opposite to the lift direction. The total equivalent lift load is 240 N.
Initial Analysis Results
We first examine the stress distribution. The maximum von Mises stress in the initial design reaches 182.2 MPa at the trailing edge of the wing root, which is well below the allowable strength of 480 MPa. The overall stress level is mostly below 30.4 MPa. Strain analysis reveals a maximum true strain of 263.0 με at the root, and a minimum of 26.9 με near the tip. The tip deflection is 9.5 mm. The modal analysis yields 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 significantly higher than typical high-altitude wind excitation frequencies (usually below 10 Hz), indicating good dynamic characteristics.
The load transfer distribution within the initial design shows that the battery experiences loads between 11.6 and 34.8 MPa, but the distribution is uneven. More importantly, the battery does not receive direct load from the skin, leading to inefficient load sharing. This observation motivates the optimization.
Optimization Strategy: Adding Force-Transfer Medium
To improve load distribution and enhance the stiffness of the fixed-wing drone wing, we introduce a force-transfer medium between the battery and the wing skin. This medium is made of the same T700 composite material used for spars and ribs. The medium connects the top and bottom surfaces of the battery to the inner surface of the skin, allowing direct load transmission from the skin to the battery. The optimized configuration is shown schematically in the following figure.
We remesh the model with the added medium, resulting in a total of 226,194 elements. The boundary conditions and loading remain identical to the initial case.
Optimized Analysis Results
The stress analysis of the optimized fixed-wing drone wing indicates a more uniform stress distribution. The maximum stress decreases to 173.7 MPa, occurring at the front spar near the root. The overall stress level is reduced, confirming that the additional load path relieves stress concentrations. The strain distribution shows a maximum true strain of 186.5 με at the front spar, a reduction of 76.5 με compared to the initial design. The minimum strain is 33.8 με near the inboard rib, indicating that the battery region experiences moderate deformation. The tip deflection is now 8.9 mm, a 0.6 mm reduction. The first six natural frequencies increase to 14.3 Hz, 57.8 Hz, 62.3 Hz, 68.3 Hz, 82.9 Hz, and 86.0 Hz. These improvements demonstrate enhanced global stiffness and better resistance to external excitation.
We evaluate the load transfer through the battery in the optimized structure. The force distribution across the battery becomes more uniform, ranging from 7.6 to 22.7 MPa with an average of 15.2 MPa. The battery now effectively participates in load bearing, with the highest loads observed at the interfaces with spars and ribs near the root. This confirms that the added medium successfully integrates the battery into the primary load path.
Quantitative Comparison and Optimization Rate
To quantify the improvement, we define the optimization rate as
$$ \text{Optimization rate} = \frac{X_O – X_N}{X_O} \times 100\% $$
where \(X_O\) is the initial value and \(X_N\) is the optimized value. Table 3 summarizes the comparison of key performance indicators.
| Parameter | Initial Design | Optimized Design | Optimization Rate |
|---|---|---|---|
| Maximum stress (MPa) | 182.2 | 173.7 | 4.7% |
| Maximum strain (με) | 263.0 | 186.5 | 29.1% |
| Tip deflection (mm) | 9.5 | 8.9 | 6.3% |
| 1st natural frequency (Hz) | 13.9 | 14.3 | 2.9% |
| 2nd natural frequency (Hz) | 54.2 | 57.8 | 6.6% |
| 3rd natural frequency (Hz) | 60.4 | 62.3 | 3.1% |
| 4th natural frequency (Hz) | 67.3 | 68.3 | 1.5% |
| 5th natural frequency (Hz) | 69.2 | 82.9 | 19.8% |
| 6th natural frequency (Hz) | 82.4 | 86.0 | 4.4% |
The strain optimization rate is the most remarkable (29.1%), indicating a significant reduction in deformation. The stress optimization rate is modest (4.7%), but the redistribution leads to more uniform loading. The modal frequencies all increase, with the fifth mode showing a substantial 19.8% increase due to the added stiffness from the force-transfer medium.
Discussion on Stiffness and Dynamic Behavior
The improvement in stiffness can be explained by the relationship between natural frequency \(f\), stiffness \(k\), and mass \(m\):
$$ f = \frac{1}{2\pi} \sqrt{\frac{k}{m}} $$
Since the mass of the optimized fixed-wing drone wing increases only negligibly (due to the added medium), the increase in frequency directly reflects a higher stiffness. The optimized structure achieves a first-mode frequency of 14.3 Hz, compared to 13.9 Hz initially, further distancing from the typical low-frequency wind excitation (below 10 Hz). This reduces the risk of resonance and enhances the wing’s ability to withstand gust loads common in high-altitude environments. Moreover, the increased spacing between consecutive modes lowers the probability of modal coupling, improving the overall dynamic response of the fixed-wing drone.
Load Distribution in the Battery
We further analyze the load distribution within the battery itself. In the initial design, the battery experienced a non-uniform load range of 11.6–34.8 MPa. After optimization, the range narrows to 7.6–22.7 MPa. The average load is 15.2 MPa, and the distribution is smoother, with peaks concentrated at the structural connections. This indicates that the battery now acts as a true structural element, effectively transmitting forces from the skin through the medium to the internal spars and ribs. The dual functionality—energy storage and load bearing—is achieved without compromising safety.
Conclusion
We have successfully designed and optimized a composite wing-battery integrated structure for a VTOL fixed-wing drone. The key innovation is the addition of a composite force-transfer medium that connects the battery to the wing skin, allowing the battery to participate in load bearing. The finite element analysis demonstrates that the optimized fixed-wing drone wing exhibits a more uniform stress distribution, a 29.1% reduction in maximum strain, a 6.3% reduction in tip deflection, and increased natural frequencies across all six modes. The battery load becomes more evenly distributed, confirming effective structural integration. This design approach enhances the structural stiffness and dynamic stability of the fixed-wing drone, making it more suitable for challenging high-altitude operations. The methodology can be extended to other types of fixed-wing drones and provides a promising path toward multifunctional lightweight structures.