VTOL UAV CFRP Airframe Structural Redesign

A structural redesign of a VTOL UAV airframe, replacing a tubular truss architecture originally conceived for welded metallic construction with a frame designed natively for carbon fibre. Carbon fibre cannot be welded, and the original tube-to-tube joints relied on mechanical fasteners and multi-axis assembly that left them weak in shear and difficult to build accurately.

The proposed architecture replaces that tubular frame with interlocking 2D CFRP plates bonded with structural adhesive, engineered so that every primary joint carries load in the mode adhesive performs best in: pure shear.

The original four-longeron tubular frame carried over assumptions from welded metallic aircraft structures: tube-to-tube joints with six or more degrees of freedom to control during assembly, and a reliance on mechanical fasteners at every junction. Carbon fibre tubes can't be welded the way metal tubes can, so those joints instead depended on fasteners that introduce stress concentrations, and on tube walls that are inherently poor at carrying shear across a joint — a mismatch between the architecture and the material it was built from.

The redesign replaces the tube truss with a unified frame built from market-standard 20×20 mm CFRP square tubes and custom 2D CNC-cut CFRP plates that interlock directly with one another. Motor pylons and landing gear hardpoints are integrated directly into this plate structure rather than bolted on as separate sub-assemblies, and the frame is sized to preserve 200 mm of propeller clearance for folding props, with NACA 0018 aerodynamic fairings built around the resulting structural depth.

The core design decision behind the plate architecture is how adhesive joints carry load. Structural adhesive performs at essentially full strength in shear, but drops to 40–60% of that strength in tension (opening) and as low as 15–25% in peel. By making every primary joint a flat-surface plate interlock, all major loads transfer as pure in-plane shear across a bonded flat interface, rather than as opening or peeling forces at a joint with a moment arm. The mechanical interlock geometry itself then acts as fail-safe redundancy, carrying load even if a bond line were to be locally compromised.

This shift also simplifies manufacturing: the entire structure is built from CNC waterjet- or laser-cut flat plates and cut-to-length tubes, dry-fit for tolerance, then bonded with a CFRP-specific structural epoxy — no welding, moulding, or autoclave cure required. Combined, the composite-native architecture and all-shear joint philosophy point to an estimated 15–25% airframe weight saving over the original tube-truss reference design, while targeting the 50g impact resistance established by heritage designs of similar architecture.