Boeing Thermoplastic Composite Wing

Boeing Thermoplastic Wing Project

Goal: Understanding the capabilities of thermoplastic composite in aerostructure applications and designing a prototype aimed at minimizing weight

Prototype design made to serve as the rare flap of a Formula SAE Vehicle

**The curing temperature and time for composites (Thermoplastic and Thermosets)**

Currently, Thermoset composites are used extensively in numerous fields. The manufacturing techniques for thermosets have become well established and the material characteristics are well understood. The challenge with Thermoset composites is that they can no longer be melted after curing, limiting their potential for recycling and joining. For that reason, Boeing aims to explore another type of composite called Thermoplastics. Although the curing temperature required for Thermoplastic composites is generally higher, Thermoplastic composites can still melt after curing and have shorter curing times. The ability to melt allows Thermoplastic composite to be joined through welding instead of traditional fasteners and adhesives. Ultimately, the welding possibility plus the shorter curing time could improve the production time for composite components

Flap and Tooling Design

The Flap is designed to be manufactured in two halves. Autoclave molding will be the primary method for curing the CF/PEEK (Polyether ether ketone) composite for this wing. To manufacture the two skin halves, plies of prepreg Boeing BMS8 PEEK fabric will be stacked in the desired orientation on the mold (determined later through composite laminate theory optimization). Vacuum bags are then utilized to seal the fabric inside the mold. An example of this mold is shown in the bottom right image.

Stiffeners (Stringers)

The spar shape was chosen to be L-shaped stringers because there is only one edge that needs to be welded to the skin, and it is simple and light as shown on the figure to left. Other stringers that were considered were U-shaped stringers and T-shaped stringers. For U-shaped stringers, two edges would need to be welded on the skin, adding additional process time. For T-shaped stringers, the centroid of the stringer is further from the skin compared to that of L-shaped, which reduces the stiffening effect of the stringers. To summarize, L-shape stringers were chosen for their faster processing time and their superior stiffening capabilities. In terms of material, like the ribs, PEEK chopped fibers are utilized.

Joining Technique (Welding)

A morphological chart was used to compare various joining techniques include mechanical fasteners, adhesive bonding, induction welding, ultrasonic welding, and resistance welding. Induction welding and resistance welding were two techniques that offered similar levels of advantages in strong weld strength and short production time. Ultimately, sequential resistance welding, a type of resistance welding technique that includes a series of voltage sources and heating elements, was chosen for its cost and simplicity. Having only a single resistance welding process over the 3 feet span of the flap would lead to temperature variation causing inconsistencies in the weld. For that reason, various spot welds would be made using sequential resistance welding to ensure that temperature variation is minimal [3].

Design Optimization using CFD and MATLAB

CFD was utilized to help determine the pressure distribution around the airfoil. This was used to help find the lift and drag on the flap. In Matlab, Boom area idealization and composite laminate theory were used to determine the prepreg sheet stacking orientation for the thermoplastic composite and the location of the stringers. To elaborate, the airfoil was discretized using the boom area idealization. Subsequently, stringers would be added to reinforce the divided boom areas. The goal of this optimization is to minimize the number of prepreg sheets and the size of stringers.

Optimization Results

The final optimized design has stringers with a cross-sectional area of 0.0276 in^2 with the skin composed of two stacked prepreg sheets of PEEK(Polyether ether ketone) thermoplastic stacked in a [0 90] orientation. This optimized flap weights 0.64 lbs and has a 2nd moment of inertia of 0.034 in^4, which allows it to withstand an aerodynamic load 22lbf (determined from CFD). For the 2nd moment of inertia, the results from the boom area idealization were within 5% from the results from the SOLIDWORKS cross-sectional analysis, adding validity to the method.

References

[1] Cheng, Roger, et al. Boeing Thermoplastics Composite Wing Design. The Boeing Company, 2018, pp. 1–83, Boeing Thermoplastics Composite Wing Design.

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[4] Zhao, Tian, et al. “Towards Robust Sequential Ultrasonic Spot Welding of Thermoplastic Composites: Welding Process Control Strategy for Consistent Weld Quality.” Composites Part A: Applied Science and Manufacturing , vol. 109, 2018, pp. 355–367., doi:10.1016/j.compositesa.2018.03.024.

[5] Yousefpour, Ali, et al. “Fusion Bonding/Welding of Thermoplastic Composites.” Journal of Thermoplastic Composite Materials , vol. 17, no. 4, 2004, pp. 303–341., doi:10.1177/0892705704045187.

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[7] Lence, Fernando Rodríguez, et al. “IN-SITU CONSOLIDATION OF PEEK COMPOSITES BY AUTOMATED PLACEMENT TECHNOLOGIES.” 20th International Conference on Composite Materials Copenhagen, 19-24th July 2015 , July 2015.

[8] “Incorrect Lift Theory.” NASA , NASA, 5 Apr. 2018, www.grc.nasa.gov/www/k-12/airplane/wrong1.html.

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[10] T. W. Clyne and D. Hull, An Introduction to Composite Materials , 3rd ed. Cambridge: Cambridge University Press, 2019.

[11] W. A. Rees, David, 2009. Appendix B: Plate Buckling Under Uniaxial Compression . [online] Wiley Online Library. Available at: <https://onlinelibrary.wiley.com/doi/pdf/10.1002/9780470749784.app2>

[12] Davies, P., et al. “Joining and Repair of a Carbon Fibre-Reinforced Thermoplastic.” Composites , Elsevier, 11 June 2003, www.sciencedirect.com/science/article/pii/001043619190199Q.