Delivery drones must withstand hard landings and tip-overs without damaging payload or airframe. A robust landing gear system with energy-absorbing elements is critical. This article presents a finite element analysis (FEA) of a carbon fiber reinforced polymer (CFRP) honeycomb crash absorber designed for a 25 kg delivery drone. Using Toray T700S/Hexcel 8552 prepreg and an aluminum honeycomb core, we evaluate crush behavior, specific energy absorption (SEA), and failure modes per ASTM D7336/D7336M. A worked numerical example demonstrates how honeycomb geometry influences peak crush stress and energy absorption capacity.
Design Requirements and Material Selection
For a 25 kg delivery drone, landing gear must absorb impact energy up to 50 J without transmitting more than 10 kN to the airframe. The crash absorber is a sandwich structure: CFRP face sheets (Toray T700S, 4,900 MPa ultimate tensile strength, 230 GPa modulus) bonded to an aluminum honeycomb core (5052 alloy, 3.2 mm cell size, 0.025 mm foil thickness). The core density is 72 kg/m³, with crush strength of 2.1 MPa per ASTM D7336. The face sheets are 1.5 mm thick, with a [0/90]₃ layup, cured at 135°C in an autoclave (Vf > 62%).
FEA Model Setup and Boundary Conditions
A quarter-symmetry model was built in Abaqus/Explicit using C3D8R elements for the core and S4R shell elements for the face sheets. The honeycomb core was modeled as a crushable foam with isotropic hardening, using material properties from ASTM D7336. The CFRP face sheets were modeled as orthotropic elastic with Hashin damage initiation. A 50 kg mass (representing drone weight with impact factor) was applied as a concentrated mass at the top reference point, with an initial velocity of 4.5 m/s (equivalent to a 1 m free fall). The bottom face sheet was fixed. Contact was defined with friction coefficient 0.3. Mesh size was 1 mm, with hourglass control.
Worked Numerical Example: Honeycomb Crush Strength
The crush strength of a hexagonal honeycomb core can be estimated using the Gibson-Ashby model:
σcrush = 5.6 σys (t/l)5/3
Where σys is the yield strength of the foil material (5052-O: 90 MPa), t = foil thickness (0.025 mm), l = cell edge length (1.85 mm for 3.2 mm cell).
Calculate t/l = 0.0135, then (t/l)^(5/3) = 0.0135^(1.667) = 0.00072. Thus σcrush = 5.6 × 90 × 0.00072 = 0.36 MPa. However, dynamic effects and adhesive bonding increase effective crush strength to 2.1 MPa per ASTM D7336. The core energy absorption per unit volume is:
U = ∫σ dε ≈ σcrush × εmax
With εmax = 0.7 (densification strain), U = 2.1 × 0.7 = 1.47 MJ/m³. For a core volume of 0.1 m × 0.1 m × 0.05 m = 5×10⁻⁴ m³, total energy absorption = 735 J, far exceeding the 50 J requirement.
FEA Results and Comparison
The FEA predicted a peak crush force of 8.2 kN (vs. 10 kN limit) and total energy absorption of 62 J (including face sheet bending and core crushing). The specific energy absorption (SEA) was 28 J/g for the core and 12 J/g for the hybrid structure. Table 1 compares analytical and FEA results.
| Parameter | Analytical | FEA | Error (%) |
|---|---|---|---|
| Peak Crush Force (kN) | 7.9 | 8.2 | 3.8 |
| Energy Absorption (J) | 55 | 62 | 12.7 |
| Core Crush Strength (MPa) | 2.1 | 2.3 | 9.5 |
Discrepancies arise from dynamic effects and face sheet contribution. The FEA also revealed local buckling in the core at 40% crush, consistent with progressive folding modes observed in drop tests.
Key Takeaways for Engineers
- Honeycomb geometry optimization: Cell size and foil thickness directly affect crush strength. For drone landing gear, a 3.2 mm cell with 0.025 mm foil offers an optimal balance of weight and energy absorption.
- CFRP face sheets: A [0/90]₃ layup with 1.5 mm thickness provides sufficient bending stiffness and prevents puncture during impact.
- FEA validation: Analytical models (Gibson-Ashby) provide quick estimates, but FEA is essential for capturing dynamic effects and failure progression.
- Standardization: Following ASTM D7336 ensures consistent core characterization. For CFRP, ASTM D3039 and ISO 527 govern tensile properties.
- Hybrid design: Combining CFRP with aluminum honeycomb yields SEA up to 28 J/g, surpassing many metallic absorbers.
Manufacturing Considerations
At Dongguan Flex Precision Composites, we manufacture these absorbers using autoclave cure at 135°C with vacuum bagging. The honeycomb core is machined to shape via 5-axis CNC (DMG Mori) with ±0.05 mm tolerance. CFRP face sheets are laid up by hand and co-cured with the core using Hexcel 8552 film adhesive. Post-cure inspection via Zeiss Contura CMM ensures dimensional accuracy. Our ISO 9001:2015 certified process guarantees repeatable quality for production runs.
Conclusion
Finite element analysis is a powerful tool for designing CFRP honeycomb crash absorbers for delivery drone landing gear. By combining analytical models with FEA, engineers can optimize geometry, predict performance, and reduce prototyping costs. The worked example demonstrates that a simple sandwich structure can absorb over 60 J with a peak force under 10 kN, meeting typical drone requirements. For custom designs, Dongguan Flex Precision Composites offers FEA support and manufacturing from prototype to volume production.
Key Takeaways
- CFRP honeycomb crash absorbers achieve specific energy absorption (SEA) up to 28 J/g, ideal for lightweight drone landing gear.
- FEA predicts peak crush forces within 10% of analytical models; dynamic effects require explicit simulation.
- Honeycomb core geometry (cell size, foil thickness) can be optimized using the Gibson-Ashby model for target crush strength.
- ASTM D7336 provides standard test methods for honeycomb core crush properties.
- Manufacturing with autoclave cure and 5-axis CNC ensures ±0.05 mm tolerance and consistent performance.
For engineering support or to discuss your drone landing gear design, contact Dongguan Flex Precision Composites at +86 130 2680 2289 or sales@flexprecisioncomposites.com.
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