CF plate arms

Summary¶

CF plate arms are the structural arm architecture used on Ghost. Where Pro, Bandit, and Core use 3D-printed polymer arms that clamp onto CF rods, Ghost uses arms fabricated from 2 mm 3K carbon fibre flat sheet — two plates per arm, sandwiching and clamping onto the existing 3 mm CF rod system. This architecture provides the structural stiffness required to carry 12-inch propellers at the 540 mm wheelbase without the arm flex that would cause vibration at low RPM. Arms are not 3D-printable by the community but are field-repairable without tools: unclamp, slide new plates on, reclamp. Fabrication uses the same DXF workflow as the rest of the libdrone system.

Concept¶

Why not print at this scale¶

The 3D-printed arm architecture used on smaller variants works because the arm length (from rod clamp to motor mount) is short enough that PETG or PC-CF provides adequate stiffness. Ghost's 12-inch prop class requires an arm length of approximately 190–210 mm from body centre to motor centre. At this span, printed polymer arms flex enough under the gyroscopic precession loads of a 12-inch propeller to produce measurable vibration — the same vibration that couples into the IMU and degrades flight controller stability.

Flat carbon fibre plate resists bending in its plane. A 2 mm 3K CF plate at 37 mm width and 210 mm length is approximately 15× stiffer in bending than an equivalent PETG arm cross-section. The stiffness allows Ghost's low-RPM propulsion to produce a clean vibration profile.

The two-plate sandwich¶

Each Ghost arm is two identical CF plates, one above and one below the CF rod. M3 bolts through both plates and the rod clamp recess provide clamping force that locks the arm geometry. The 3 mm CF rod runs through a semi-circular channel cut into the inner face of each plate; the clamping force around the rod replaces the printed rod channel used on polymer arms.

Using two identical plates reduces the DXF complexity to a single profile cut eight times (four arms × two plates). The plates are interchangeable — no left/right distinction. See → cf-rod-architecture for the rod system that Ghost shares with the rest of the family.

Field repair¶

Arm replacement on Ghost requires no tools beyond an M3 hex driver. Procedure: loosen the M3 clamp bolts on the damaged arm, slide the plates off the rod, slide replacement plates on, retighten. Replacement takes under 15 minutes in the field. Plate stock is flat sheet — it can be carried as pre-cut spares or fabricated from a DXF file at any laser cutting service.

Reference¶

Mass estimate: 8 plates × ~15 g = ~120 g total arm structure.

Fabrication note: Ghost arm DXF is planned but not yet produced as of v0.2. Plates can be hand-cut from 2 mm CF sheet using the geometry in Ghost Spec §6 as a reference until the DXF is available.

Procedure¶

Install a CF plate arm on Ghost¶

1. Cut 2 plates per arm from 2 mm CF sheet to the ghost_arm.dxf profile. Deburr all edges with 220-grit sandpaper.
2. Slide one plate below and one above the CF rod at the arm station.
3. Align the semi-circular rod channels around the rod.
4. Insert M3 × 10 mm bolts through the plate stack and rod clamp holes.
5. Apply Loctite 243 to bolt threads. Torque to 0.5 N·m.
6. Mount motor to the motor-end plate holes using M3 × 8 mm bolts with nylon washers to protect the CF surface.
7. Verify arm is perpendicular to the body axis (measure motor-to-motor diagonal to confirm geometry).

Replace a damaged arm in the field¶

1. Remove motor from the damaged arm (4× M3 bolts).
2. Loosen arm clamp bolts (3× M3 per arm position).
3. Slide damaged plates off the CF rod.
4. Slide replacement plates onto rod. Retighten clamp bolts.
5. Reinstall motor. Verify geometry before flight.

Rationale¶

The two-plate identical profile was chosen over a single thicker plate or a machined arm because it eliminates the top/bottom distinction and reduces the number of unique parts to one. A single 2 mm plate would need to be 3–4 mm thick to achieve equivalent bending stiffness, increasing mass and cost. A machined arm offers no stiffness advantage over flat plate for the loads on Ghost, while costing significantly more and requiring a machine shop rather than a laser cutter.

Connections¶

yaml requires: - [cf-rod-architecture](<./cf-rod-architecture.md>) - [frame-structure-overview](<./frame-structure-overview.md>) related: - [ghost-variant](<./ghost-variant.md>) - [sandwich-structure](<./sandwich-structure.md>) - [floating-motor-mounts](<./floating-motor-mounts.md>) - [stl-export-and-slicer-setup](<./stl-export-and-slicer-setup.md>) leads_to: - [ghost-variant](<./ghost-variant.md>) - [acoustic-signature-design](<./acoustic-signature-design.md>)