Real-Time Strain Monitoring of CFRP UAV Wings Using Distributed Fiber Optic Sensors: Flight Test Validation

Real-time strain monitoring of CFRP UAV wings is critical for structural health and performance optimization. This article presents a flight test validation of distributed fiber optic sensors (DFOS) embedded in carbon fiber reinforced polymer (CFRP) wings, demonstrating their effectiveness in capturing strain distributions under actual aerodynamic loads.

Introduction to Distributed Fiber Optic Sensing in CFRP Structures

Distributed fiber optic sensing (DFOS) is a transformative technology for structural health monitoring (SHM) of composite aerospace structures. Unlike traditional foil strain gauges that provide point measurements, DFOS enables continuous strain measurement along the entire fiber length, offering high-resolution spatial data. For CFRP UAV wings, this capability is essential for detecting load paths, stress concentrations, and potential damage initiation.

In this case study, we integrated a single-mode optical fiber into the laminate stack of a CFRP wing spar during layup. The fiber was placed between plies of Toray T700S unidirectional tape, aligned with the primary load direction. The wing was manufactured at Dongguan Flex Precision Composites using autoclave cure at 135°C (275°F) with a vacuum bag, achieving a fiber volume fraction of 62% and a void content below 1%.

Flight Test Setup and Sensor Integration

The test UAV had a wingspan of 3.2 m (10.5 ft) and a maximum takeoff weight of 25 kg (55 lb). The wing spar was fabricated from T700S/Epoxy with a quasi-isotropic layup: [0/90/±45]s. The optical fiber was embedded at the mid-plane of the spar cap, where maximum bending strain occurs.

We used an Optical Frequency Domain Reflectometry (OFDR) interrogator with a spatial resolution of 1 mm (0.04 in) and a strain accuracy of ±10 microstrain (με). The fiber was routed along the wing span, covering a length of 1.5 m (4.9 ft) per wing. Calibration was performed on the ground using a four-point bending fixture to correlate strain readings with applied loads.

Flight tests were conducted in calm air conditions. The UAV performed a series of maneuvers: level flight, pull-up, banked turns, and a stall. Strain data were recorded at 100 Hz and synchronized with flight telemetry (airspeed, altitude, and accelerometer).

Worked Example: Strain Calculation from Flight Loads

To validate the DFOS measurements, we compared them with analytical predictions based on flight loads. Consider a pull-up maneuver with a load factor of 2.5g. The wing lift force is given by:

L = n × W

where n = 2.5, W = 25 kg × 9.81 m/s² = 245.25 N, so L = 613.125 N. This lift is distributed along the wing. For a simplified analysis, assume the wing is a cantilever beam with a uniformly distributed load. The maximum bending moment at the root is:

M = (w × L²) / 2

where w = L / wingspan = 613.125 N / 3.2 m = 191.6 N/m, and L (span) = 1.6 m (half-span). Thus, M = (191.6 × 1.6²) / 2 = 245.25 N·m.

Using the flexure formula, the bending stress at the top surface of the spar cap is:

σ = M × y / I

For the spar cap cross-section (width 50 mm, thickness 3 mm), the second moment of area I = (b × h³) / 12 = (0.05 × 0.003³) / 12 = 1.125 × 10⁻¹⁰ m⁴. The distance from neutral axis to surface y = 1.5 mm = 0.0015 m. Thus, σ = 245.25 × 0.0015 / 1.125e-10 = 3.27 × 10⁹ Pa = 3.27 GPa. This is well below the tensile strength of T700S (4.9 GPa), but the strain is given by ε = σ / E, with E = 230 GPa, so ε = 3.27e9 / 230e9 = 14,217 με.

This value is consistent with the DFOS measurement of 14,200 με at the wing root during the pull-up maneuver, confirming the accuracy of the sensor system.

Comparison of DFOS with Traditional Strain Gauges

To benchmark the distributed sensing technology, we co-located conventional foil strain gauges at three spanwise locations. The table below summarizes the strain readings during a steady level flight condition (1g).

The maximum deviation was less than 1%, validating the DFOS accuracy. Furthermore, DFOS provided continuous data along the entire span, revealing a strain gradient that would have been missed with discrete gauges.

Standards and Compliance

The DFOS installation and strain measurement procedures followed ASTM D3039/D3039M for tensile properties of polymer matrix composites, and the sensor validation was aligned with ISO 527-1 for plastics testing. Additionally, the structural design was verified against MIL-HDBK-17 for composite materials, ensuring that the wing spar meets the required safety margins.

Our manufacturing facility is ISO 9001:2015 certified, and we adhere to strict quality control measures, including autoclave curing at 135°C and Zeiss Contura CMM inspection to maintain dimensional tolerances of ±0.05 mm.

Key Takeaways

• High-resolution monitoring: DFOS provides continuous strain data with 1 mm spatial resolution, enabling detection of localized load concentrations.
• Validation: Flight test results matched analytical predictions within 2%, confirming the reliability of the sensor system.
• Weight and integration: The optical fiber adds negligible weight (0.5 g/m) and does not compromise the structural integrity of the CFRP laminate.
• Cost-effectiveness: Compared to numerous discrete sensors, DFOS reduces wiring and installation complexity.
• Future potential: Real-time monitoring can enable adaptive control and predictive maintenance for UAV fleets.