Monocoque structure

Summary¶

A monocoque structure carries loads through its outer skin rather than through an internal skeleton. The skin is the structure — remove it and the load path disappears. A sandwich structure is a specific type of monocoque: two stiff face sheets separated by a core, working together to resist bending loads far more efficiently than either face sheet alone. libdrone's body uses a PETG-PCCF sandwich that behaves as a monocoque panel — the body shell is the primary load-carrying element, not internal ribs or posts. Understanding this principle explains why wall thickness and layer orientation matter more than infill percentage in the body panels.

Concept¶

Skin-bearing vs skeleton-bearing¶

A conventional framed structure — a truss, a space frame — carries loads through discrete members: beams, columns, and diagonal braces. The skin between members is non-structural; it can be removed without affecting the load path. Aircraft fuselages of this type used external fabric over a wood or metal frame. The frame is the structure; the skin is the cover.

A monocoque reverses this. The skin is curved or sandwich-stiffened to resist buckling, and it carries the axial, bending, and shear loads directly. Remove the skin and there is nothing left. Early aircraft monocoques used plywood shells; modern aircraft use aluminium or carbon fibre shells with stringers to prevent buckling. The weight saving over a framed structure is significant — the skin that was dead weight in a framed structure now does structural work.

Why sandwich is a monocoque¶

A flat panel fails in bending at very low load because its second moment of area is small. A sandwich panel with the same total thickness but with two face sheets separated by a light core has a second moment of area many times larger — the core separates the face sheets, putting more material far from the neutral axis where it resists bending most effectively.

For libdrone's PETG-PCCF sandwich, the PCCF layers are the face sheets (stiff, high modulus) and the PETG layers are the core (lower modulus but continuous). The sandwich resists bending loads that would crack a single-material panel of the same mass. See → sandwich-structure for the specific layup and geometry.

Monocoque and printed geometry¶

FDM printing produces quasi-monocoque parts by default: the perimeter walls are solid and stiff; the infill is sparse. The perimeter walls are the face sheets of a thin sandwich; the sparse infill is the core. This is why increasing perimeter count (wall thickness) improves part stiffness much more than increasing infill percentage — the perimeters are the structural elements, not the infill. A 3-perimeter PETG part with 15% infill is significantly stiffer in bending than a 2-perimeter part with 40% infill of the same mass.

For body panels that carry the full frame load, libdrone specifies 4 perimeters minimum. The infill provides compressive resistance (buckling prevention) but is secondary to the perimeter walls.

Reference¶

Key print parameters for monocoque behaviour:

Procedure¶

Rationale¶

The decision to use a sandwich monocoque for the libdrone body rather than an internal-rib design (which is common in hobby-grade printed frames) is driven by the specific load case: distributed bending from the CF rod pre-tension and the battery weight. An internal-rib design resists point loads well but distributes bending loads poorly across a panel — the panel skins delaminate from the ribs under cyclic bending. A sandwich monocoque distributes bending loads across the full panel area and is better suited to the rod pre-tension load case.

Connections¶

yaml requires: - [frame-structure-overview](<./frame-structure-overview.md>) - [sandwich-structure](<./sandwich-structure.md>) related: - [zonal-stiffness](<./zonal-stiffness.md>) - [pre-tensioning](<./pre-tensioning.md>) - [pccf](<./pccf.md>) - [petg](<./petg.md>) - [print-profiles](<./print-profiles.md>) leads_to: - [sandwich-structure](<./sandwich-structure.md>) - [zonal-stiffness](<./zonal-stiffness.md>)