Aircraft Landing Gear

Landing Gear Lead for Cal Poly DBF

Overview

Cal Poly Design Build Fly (DBF) competes annually in the SAE Aero Design West competition, where teams from around the world design and build electric, radio-controlled aircraft to meet mission objectives defined by SAE each year. This year, I was responsible for redesigning the front landing gear strut. The previous trailing-arm configuration limited steering performance and required a small wheel, which led to reduced stability during takeoff and landing.

 Figure 1. Previous front landing gear design

Mechanical Design

Inspired by full-scale, manned aircraft, an oleo-strut landing gear system was selected utilizing a gas spring. Mechanical linkages constrain the spring, enabling steering while maintaining vertical compliance, and a custom bung interfaces with the spring to connect the steering column. Linkages, spacers, wheels, and bungs were 3D printed, with polycarbonate used for load-bearing components and PLA used for non-structural connection components, allowing for rapid prototyping while minimizing overall weight. Following testing, the main structural bung was redesigned as a lathed aluminum component for improved strength and O-rings were added for better surface traction.

 Figure 2. Exploded view of final assembly

Linkage Design

The linkages were designed with consideration for yield stress, in-plane and out-of-plane buckling, and bearing stress at the shoulder bolt interfaces. Hand calculations were performed for a 5g landing condition with 20 degree descent rate and was later implemented in Excel to iteratively solve for the minimum component volume while maintaining a design factor of safety of 1.5. Design variables included linkage length, width, thickness, bolt size, and the number of cross-brace supports. Buckling analysis assumed rectangular cross-sections with fixed–fixed end conditions

 Figure 3. Design factor calculations for rectangular cross-section linkages

An I-beam cross-section linkage was also evaluated using both Johnson and Euler buckling criteria in Excel, but was ultimately ruled out due to the added complexity of incorporating cross-braces and manufacturing limitations related to 3D print quality. Additionally, the material properties used for the 3D-printed components were intentionally conservative, as the buckling theories applied assume isotropic, linear-elastic behavior, which does not fully capture the anisotropic nature of additively manufactured materials

 Figure 4. Design factor calculations for I-beam cross-section linkages

 Figure 5. Upper linkage final design

Contact Stress

Contact stresses in the wheel were also evaluated using Hertzian contact theory. The landing gear wheel was modeled as polycarbonate (PC), while the ground was approximated as a flat asphalt surface with an effectively infinite radius of curvature. This model is subject to the same assumptions outlined above, including isotropic, linear-elastic material behavior

 Figure 6. Wheel contact stress plots

Finite Element Analysis

Finite Element Analysis (FEA) was conducted on both wheels and linkages to validate the theoretical calculations. Each component was modeled as an elastic–perfectly plastic material, exhibiting a linear elastic response up to yield. The analysis was performed to identify stress and plastic deformation concentrations arising from geometric features and to determine the locations where yielding would first occur. In the final linkage model, the shoulder bolts were treated as rigid bodies, and a prescribed displacement load was applied to simulate compression of the linkage assembly. All FEA simulations were performed using Abaqus 2025

 Figure 7. Linkage FEA result 1

 Figure 8. Linkage FEA result 2

Testing

The testing plan consisted of manual impact tests on the ground and integrated flight tests. The primary insights gained through testing were strength failures in the linkages and connection bung, as well as stiffness concerns due to excessive flexibility in the wheels. First, 3D-printed parts were switched from PLA to PC and printed in orientations where the layer lines were not aligned with expected stresses. As mentioned earlier, the main connection bung was changed to an aluminum bung after a flight test failure. Washers were added to allow the nut to better constrain a larger surface area of the wheels, helping reduce off-axis wheel flexibility. 

 Figure 9. Final design & manufacturing aluminum bung