Illini Electric Motorsports FSAE Team

Design and Analysis

The core structural challenge was resisting spanwise bending, torsional twist, and localized loads at mounting interfaces, all within a tight mass budget. Previous internal components were complex curved 3D geometries that required separate molds, were difficult to layup consistently, and relied on sequential bonding that introduced tolerance stack-up. Compensating with adhesive and filler to close gaps added weight and undermined the design intent. To address this, the internal structure was redesigned around a flat-stock strategy: ribs and spars manufactured as flat carbon fiber plate and waterjet-cut to final geometry. This removed the need for mold tooling, reduced layup complexity and scrap rate, and ensured dimensional accuracy at cut edges. Assembly tolerance was addressed through an interlocking cross-lap joint system, where opposing slots in the ribs and spars allow each component to self-align onto the main spar rather than accumulating positional error through sequential placement using things like jigs.

Structural concepts were developed in CAD and evaluated using FEA to identify stress concentrations and excessive deflection. Early designs showed localized stress near mounting features, which were resolved through changes in rib geometry and laminate thickness. The final design reduced mass by from 7.5lb in previous seasons to 5.86lb while also improving strength, 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+. The workflow moves beyond static deflection targets by capturing how 3D wing deformation affects aerodynamic performance. Spanwise bowing closes slot gaps, torsional twist shifts angle of attack, and surface deformation alters 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.

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 woven 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. With the internal rib and spar structure carrying the primary bending and shear loads, the skins themselves are a relatively thin laminate, eliminating the need for a sandwich core and keeping weight low. 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 sequencing and alignment control. 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.