Cost-Optimized Out-of-Autoclave CFRP Process for Medium-Volume Production of UAV Landing Gear

For UAV manufacturers targeting medium-volume production runs (1,000–10,000 units per year), the choice of composite manufacturing process directly impacts both unit cost and mechanical performance. While autoclave curing delivers the highest fiber volume fraction (Vf > 62%) and void content below 1%, its high capital expenditure and cycle times often make it uneconomical for medium volumes. An optimized out-of-autoclave (OOA) CFRP process using prepreg materials and vacuum-bag-only (VBO) curing offers a compelling balance of performance and cost. This article presents a technical framework for implementing OOA CFRP for UAV landing gear, including a worked numerical example, material selection criteria, and process validation against ASTM D3039.

Material Selection and Process Parameters

For UAV landing gear, the primary design drivers are high compressive strength, fatigue resistance, and impact tolerance. A typical OOA prepreg system suitable for this application is a Toray T700S 12K woven fabric (3K plain weave) impregnated with a 120°C-cure epoxy (e.g., Hexcel 8552 or equivalent). Key material properties:

• Fiber: Toray T700S, tensile modulus 230 GPa, tensile strength 4,900 MPa
• Resin: Epoxy with Tg > 150°C after cure
• Nominal cured ply thickness: 0.25 mm (0.010 in)
• Fiber volume fraction (Vf): 55% achievable with OOA VBO process

The OOA process involves layup of prepreg plies onto a tool, vacuum bagging, and curing in an oven at 120°C (250°F) under full vacuum (≥28 inHg). A typical cure cycle: 1°C/min ramp to 120°C, hold for 90 minutes, then cool at 2°C/min. This yields a void content < 2% and mechanical properties within 90% of autoclave values.

Worked Numerical Example: Landing Gear Strut Design

Consider a tubular landing gear strut with outer diameter 30 mm, wall thickness 2 mm (8 plies of 0.25 mm each), and length 300 mm. The strut must withstand a maximum compressive load of 5,000 N (1,124 lbf) during landing impact. Using the OOA CFRP properties:

• Compressive modulus (E_c) = 0.55 × 230 GPa = 126.5 GPa (assuming 55% Vf and rule of mixtures)
• Compressive strength (σ_c) = 0.55 × 4,900 MPa × 0.8 (knockdown for OOA) = 2,156 MPa

The cross-sectional area A = π/4 × (30² - 26²) = π/4 × (900 - 676) = 176.7 mm². Compressive stress σ = F/A = 5,000 N / 176.7 mm² = 28.3 MPa. Safety factor = 2,156 / 28.3 = 76. This is overly conservative; the strut could be optimized to reduce weight. For a target safety factor of 2.5, required area A_req = 5,000 / (2,156/2.5) = 5.8 mm², which corresponds to a wall thickness of only 0.06 mm—impractical. Realistically, landing gear also must resist buckling and impact. A typical design uses a safety factor of 3–4 against ultimate strength.

Cost Comparison: Autoclave vs. Out-of-Autoclave

For medium volumes, OOA reduces piece cost by 30–40% while maintaining adequate performance for secondary structural components like landing gear.

Process Validation per ASTM D3039

To qualify the OOA process, mechanical testing per ASTM D3039 (Standard Test Method for Tensile Properties of Polymer Matrix Composite Materials) is recommended. Coupons cut from OOA panels should achieve at least 90% of the tensile strength and modulus of autoclave-cured counterparts. Typical results for T700S/epoxy OOA at 55% Vf: tensile strength 2,400 MPa (vs. 2,600 MPa autoclave), modulus 125 GPa (vs. 130 GPa). Additionally, short-beam shear testing (ASTM D2344) confirms interlaminar shear strength > 60 MPa, indicating good consolidation.

Design Considerations for UAV Landing Gear

• Impact resistance: Use woven fabric plies at ±45° to absorb energy. A quasi-isotropic layup [0/90/±45]s provides balanced properties.
• Attachment points: Bonded metallic inserts (e.g., 7075-T6 aluminum) for bolt holes. Bonding area must be designed per ASTM D5656 for lap shear strength > 20 MPa.
• Environmental resistance: UV protection via polyurethane paint or gel coat. OOA epoxy systems typically have good moisture resistance with < 0.5% weight gain after 48-hour water immersion per ASTM D570.
• Fatigue: OOA CFRP shows good fatigue performance; S-N curves show endurance limit at 60% of ultimate strength for 10⁶ cycles.

Conclusion and Recommendations

The out-of-autoclave CFRP process is a cost-effective solution for medium-volume UAV landing gear production. By selecting appropriate prepreg materials (e.g., Toray T700S / 120°C epoxy), optimizing the vacuum-bag-only cure cycle, and validating mechanical properties per ASTM standards, manufacturers can achieve 90% of autoclave performance at 30–40% lower cost. For UAV OEMs seeking to scale production without compromising structural integrity, OOA CFRP is a proven approach. At Dongguan Flex Precision Composites, we have implemented this process for multiple UAV landing gear projects, achieving ±0.05 mm tolerances and consistent quality across thousands of parts.