Swan Necks
Rear Wing Structure
Swan Necks are the backbone of the rear wing mounting system. They are designed to carry the aerodynamic loads of the rear wing, which produces upwards of 360 lbs of downforce at a top speed of 90 mph. I worked to minimize weight, improving by over 50% from the previous design. The other elements of the mount are carbon tubes with spherical rod ends: a cross brace seen on top and vertical supports under the wing. This complicated 3D linkage presents challenges in calculating the expected loads and linkage behavior. The Swan Necks have three pin joints, with a truss structure optimized to carry the moment loads produced by the linkage.
Finite Element Analysis
By making a 2D simplification of the linkage geometry, I was able to model and simulate the loading conditions with SolidWorks FEA. The loads were applied at the pin joints, broken into lift and drag components, using Cl and Cd results from STAR CCM+ CFD simulations on the wing. Since downforce is twice the magnitude of drag, the Swan Neck is primarily loaded in 3-point bending. The middle joint is being pulled down since the wing’s aerodynamic center lies along the 1/4 chord of the airfoil, closest to the middle joint.
The exact loading is not as simple because of the other pin joints between the wing/endplate (modeled as a triangle), the vertical supports (left rod), and the fixed chassis (right rod). These members were assumed rigid to compute stresses in the Swan Neck. After 6 design iterations, running FEA on each to optimize parameters, the final design had a minimum safety factor of 2.5 for the von Mises yield criterion, weighing only 0.472 lbs each.
Buckling Analysis
I investigated stress normal to the cross section of the members to see that the entire structure was bending rather than just individual members, indicating a more stiff and weight-efficient structure. This is seen from the blue compressive stress along the top edge and yellow tensile stress along the bottom edge. I used this result at the members most likely to experience buckling, the longest members in compression, and calculated the thickness required to achieve a minimum safety factor of 2.5 for the buckling failure mode.