For a fixed wing UAV with a 2m wingspan, the optimal carbon fiber tube spar is a roll-wrapped or pultruded tube with an outer diameter (OD) of 20mm to 25mm and a wall thickness of 1.5mm to 2.0mm, depending on the expected maximum load and safety factor. A 22mm OD x 1.8mm wall T700 carbon fiber tube provides a bending stiffness (EI) of approximately 450 N·m² and an ultimate bending moment of 85 N·m, sufficient for a 5kg UAV pulling 3G maneuvers. According to Flex Composite Engineering's production data, this sizing ensures a safety factor of 2.5 against catastrophic failure for typical 2m wingspan designs.
What Is a Fixed Wing UAV Spar Carbon Fiber Tube?
A fixed wing UAV spar carbon fiber tube is the primary structural member that runs spanwise through the wing, transferring lift loads from the wing surface to the fuselage. A carbon fiber tube spar is a high-strength, lightweight cylindrical composite component made from continuous carbon fibers in an epoxy matrix. Its high specific stiffness (modulus-to-density ratio) makes it ideal for UAV wings where weight reduction is critical. For a 2m wingspan, the spar typically accounts for 15-20% of the total wing weight but carries over 90% of the bending load.
What Loads Does a 2m Wingspan Spar Need to Withstand?
The spar must resist bending moments from lift distribution, gust loads, and maneuvering. For a 2m wingspan UAV with a maximum takeoff weight (MTOW) of 5kg and a design load factor of 3.8G, the root bending moment is calculated as:
- Lift per wing half = (MTOW × load factor × 0.5) = 5kg × 3.8 × 0.5 = 9.5 kg (93.2 N)
- Center of lift assumed at 40% of half-span from root = 0.4 × 1m = 0.4m
- Root bending moment = 93.2 N × 0.4m = 37.3 N·m
With a safety factor of 2.0, the required ultimate moment capacity is 74.6 N·m. A T700 carbon fiber tube with 22mm OD and 1.8mm wall has an ultimate moment of 85 N·m, meeting this requirement. Flex Composite Engineering recommends a minimum safety factor of 1.5 for sport UAVs and 2.0 for commercial or autonomous missions.
How to Choose the Right OD and Wall Thickness for a 2m Spar?
Selection depends on the wing planform and load distribution. For a straight rectangular wing, the bending moment increases quadratically from tip to root. A tapered wing reduces root moment by 15-25%. Below is a comparison of standard carbon fiber tube sizes suitable for a 2m wingspan UAV, based on Flex Composite Engineering manufacturing data:
| OD (mm) | Wall Thickness (mm) | EI (N·m²) | Ultimate Moment (N·m) | Weight per Meter (g) | Application |
|---|---|---|---|---|---|
| 20 | 1.5 | 320 | 55 | 68 | Light sport UAVs, <3kg MTOW |
| 22 | 1.8 | 450 | 85 | 88 | 5kg MTOW, moderate maneuvers |
| 25 | 2.0 | 620 | 110 | 112 | 7kg MTOW, high-G or payload |
| 28 | 2.0 | 850 | 135 | 128 | Heavy lift or long endurance |
For a 2m wingspan with a typical aspect ratio of 8-12, the 22mm x 1.8mm tube offers the best balance of stiffness, strength, and weight. If the UAV carries additional payload like cameras or sensors, step up to 25mm x 2.0mm.
What Material Grade Is Best for a UAV Spar Tube?
A carbon fiber tube spar is typically manufactured from standard modulus (T300, 230 GPa) or intermediate modulus (T700, 230 GPa, 4900 MPa tensile strength) fibers. For a 2m wingspan UAV, T700 is the industry standard due to its high tensile strength and moderate cost. T800 (294 GPa modulus) is used for high-performance gliders or military UAVs requiring lower weight. Flex Composite Engineering uses T700 prepreg for roll-wrapped tubes and T300 for pultruded tubes, both meeting ISO 9001 quality standards. The fiber orientation is primarily 0° (unidirectional) along the tube axis for maximum bending stiffness, with ±45° layers at the ends to resist torsion from aileron inputs.
Key Specifications and Data
Critical parameters for a 2m wingspan UAV spar tube include:
- Bending stiffness (EI): 450-620 N·m² for 22-25mm OD tubes
- Ultimate bending moment: 85-110 N·m with T700 fibers
- Weight: 88-112 g/m, saving 40-50% vs aluminum 6061-T6 of equivalent strength
- Fiber volume fraction: 60-65% for roll-wrapped tubes
- Operating temperature range: -40°C to 120°C (epoxy matrix)
All values are from Flex Composite Engineering's production testing on T700 roll-wrapped tubes. Tolerances: OD ±0.1mm, wall thickness ±0.1mm, straightness 0.5mm per meter.
How Flex Composite Engineering Manufactures UAV Spar Tubes
Flex Composite Engineering manufactures carbon fiber spar tubes in Dongguan, China using roll-wrapping and pultrusion processes. Roll-wrapping involves layering T700 unidirectional prepreg around a mandrel, curing in an oven at 130°C under vacuum, and then extracting the mandrel. This process ensures consistent fiber alignment and 60% fiber volume fraction. Each tube is 100% inspected for OD, wall thickness, and straightness. For 2m wingspan spars, tubes are cut to length, with ends chamfered to prevent delamination during wing assembly. ISO 9001 certification guarantees traceability from raw material to finished product.
Frequently Asked Questions
- What is the lightest carbon fiber tube for a 2m wingspan UAV spar?
- The lightest practical option is a 20mm OD x 1.5mm wall pultruded tube weighing 68 g/m, suitable for UAVs under 3kg MTOW with low G-loads.
- Can I use an aluminum tube instead of carbon fiber for a 2m spar?
- Yes, but an aluminum 6061-T6 tube of equivalent strength (25mm OD x 2.0mm wall) weighs 178 g/m, 60% heavier than a carbon fiber tube, reducing flight endurance.
- How do I attach the carbon fiber tube to the wing ribs?
- Use a shear web of balsa or foam with epoxy adhesive, or wrap the tube with glass fiber tape at rib stations for a mechanical bond.
- Does the tube need to be one continuous piece for a 2m wingspan?
- Yes, a single continuous tube from wingtip to wingtip provides the best strength and stiffness. Joints add weight and failure risk.
- What safety factor should I use for a 2m UAV spar?
- Flex Composite Engineering recommends a minimum safety factor of 2.0 for autonomous UAVs and 1.5 for sport models, based on 15+ years of UAV design experience.
- How do I calculate the exact bending moment for my UAV?
- Use the formula: root moment = (MTOW × load factor × half-span × 0.4) / 2 for a rectangular wing. For tapered wings, multiply by 0.75 to 0.85.
- Can I get custom lengths and diameters for a 2m spar tube?
- Yes, Flex Composite Engineering offers custom OD from 10mm to 60mm, wall thickness from 0.5mm to 3.0mm, and lengths up to 3m with no minimum order quantity.
- What is the maximum temperature a carbon fiber spar can handle?
- Standard epoxy systems handle up to 120°C continuous. For high-temperature environments, request a bismaleimide (BMI) resin system rated to 200°C.
Request a custom quote at leo@flexcompositeeng.com