Illini Electric Motorsports FSAE Team

Design and Analysis

The core structural challenge was resisting spanwise bending, torsional twist, and localized loads at mounting interfaces within a tight mass budget. Previous ribs and spars were balsa wood, which was lightweight but too weak to carry large bending and shear loads on their own. To compensate, the skins were built as a Nomex-cored sandwich layup for strength, which restricted the achievable airfoil shape and added significant weight. The redesign moved the primary load path into the internal structure itself rather than relying on core. We decided on ribs and spars machined from flat carbon fiber plate and waterjet-cut to final geometry, which would be strong enough to carry spanwise bending and shear directly. This let the skins shrink to a thin laminate, no longer needing a structural core, and freed the airfoil shape from the constraints the old sandwich skin imposed. Assembly tolerance was addressed through an interlocking cross-lap joint system, where opposing slots in the ribs and spars let each component self-align onto the main spar rather than accumulating positional error through jigs and sequential placement. 

The structure was validated in FEA against four load cases: 

The final design reduced mass from 7.5 lb in previous seasons to 5.86 lb while meeting every case above, achieved by removing material in low-stress regions and reinforcing areas of concentration.

As part of a broader team effort, I contributed to developing an FSI workflow using ANSYS Mechanical and STAR-CCM+ to generate the aero load case. The workflow moves beyond static deflection targets by capturing how 3D wing deformation affects aerodynamic performance. Loading can mean spanwise bowing closing slot gaps, torsional twist shifting angle of attack, and surface deformation altering the airfoil shape. Pressure field data is exported from CFD as a point cloud and mapped directly onto the structural mesh, providing significantly higher fidelity than averaged surface loading, which is what the team previously used.

Manufacturing

Manufacturing decisions were driven by FEA results and the need for repeatable fabrication within a tight production timeline. Composite layups were optimized for each structural member based on stress distribution and stiffness requirements. The ribs use a [1/C/1] symmetric layup with carbon plies on either side of a Nomex core, while the spars use a five-ply carbon [0₅] layup. The five-ply spar maintains adequate geometric stiffness in the flat-stock configuration, while the cored ribs place material only where structurally necessary. All parts are laid up flat and waterjet-cut to final geometry, eliminating mold tooling and ensuring edges are dimensionally accurate.

The mainplane and flap skins were manufactured using Vacuum Assisted Resin Transfer Molding (VARTM), infusing dry carbon fiber over a shaped mold. The process produces skins with good fiber consolidation and a consistent surface finish, and avoids the cost and lead time of prepreg materials. While the layup schedule was designed by other team members, I was involved in the manufacturing process and contributed to validation efforts. 

The front wing mainplane leading edge presented an unexpected challenge. The original design called for a high-density foam core to distribute aerodynamic loads and absorb impact energy during cone strikes, but budget constraints made the specified foam unavailable. An interim solution used a 3D printed core designed to replicate the deformation and energy absorption behavior of the original foam, modeled as an anisotropic material in ANSYS to account for the directional properties introduced by the print geometry and infill structure. Following structural review during final assembly, the team transitioned to a carbon fiber pad-up at the leading edge, which better addressed both the mass penalty and long-term structural concerns of the printed core while maintaining the load distribution goals of the original concept.

With all composite skins and structural parts complete, assembly required careful alignment. All bond surfaces were sanded prior to assembly, with EA9120 epoxy used throughout. Bonding followed a deliberate sequence: ribs and spars were bonded together first as a subassembly, then that structure and the aluminum strut inserts were bonded to the bottom skin before the top skin was closed out. This order kept components from shifting during cure and gave access for clamping at each stage. Alignment proved more challenging than anticipated, particularly for the aluminum strut inserts, which interface with the car at an odd angle. 3D printed bonding jigs were developed to address this, referencing the struts and car mounting points directly. Clamps were used to apply pressure during cure, though in hindsight a dedicated pressure jig would have given better control over bond thickness and pressure distribution.