Scaling libdrone to a new frame size

Summary¶

libdrone is designed to scale from 5-inch to 10-inch and beyond. The same architecture — sandwich X body, platform, backplane, dual GX12-7 payload interface — works at any wheelbase because most geometry is parametrically derived from the wheelbase variable. However, not everything scales automatically. Prop clearance, battery rail dimensions, EASA category, and motor/ESC selection all require explicit human judgement at every new scale. The payload interface is deliberately scale-independent — a payload built for libdrone 6 plugs into libdrone 10 without modification.

Concept¶

Frame-driven vs electronics-driven geometry¶

Scaling intuition says "bigger frame = bigger everything." This is wrong for libdrone. The frame geometry scales. The electronics geometry does not.

Frame-driven variables are derived from wheelbase. When wheelbase grows, the arms get longer, the X body core gets larger, the rod length changes. These variables cascade automatically through the parametric model.

Electronics-driven variables are set by the components, not the frame. A 10-inch libdrone uses the same Matek H7A3-SLIM FC, the same Pilotix 75A ESC, the same GX12-7 connectors, the same HDZero VTX. The platform that carries them does not grow with the frame. The platform is long enough to fit the electronics — and that length is independent of how big the arms are.

The critical insight: when scaling from 6-inch to 10-inch, the arms grow by roughly 70 mm each. The platform stays approximately the same length.

Payload interface scale-independence¶

The dual GX12-7 connector standard is deliberately fixed at all scales: - Connector geometry: identical - Boss pad M3 spacing (20 mm): identical - Backplane post spacing: follows electronics zone, not frame size

A payload built for libdrone 6 plugs into libdrone 10 without modification. This is the core commercial and community argument for the platform: builders invest in payloads once and run them on any frame size. Breaking this compatibility requires a version bump of the payload interface standard, not a routine scale change.

What requires human judgement¶

Parametric models can propagate known relationships. They cannot make decisions that require new calculations:

Prop clearance — the platform width at the arm root zones must give sufficient clearance between the prop disc and the platform edge. This depends on prop diameter, arm angle, and platform width. At every new scale, this geometry must be calculated from scratch. The V2.4.6 values (40 mm narrow / 50 mm electronics) are correct for 330 mm wheelbase and 6-inch props only.

Battery selection — a larger frame carries a larger battery. Battery rail dimensions are set by the physical battery dimensions. Update BattRailInnerWidth, BattLength, BattWidth, BattHeight after selecting the battery for the new scale.

EASA category — all-up weight determines regulatory category. Recalculate AUW at every scale. A 10-inch libdrone will almost certainly exceed the 900 g Open A2 boundary and operate as A3.

Reference¶

Variables that change at new scale¶

Variables that do not change at new scale¶

PlatformLength, StackPattern, GX12PositionY, MIPIChannelWidth, FanSlotDim, GX12BossDia, GX12ChimneyOD, GX12ChimneyBoreFlatFlat, MastBossSpacing, MastBossSpacingY

Procedure¶

Scaling sequence¶

1. Duplicate the .FCStd file. Rename: libdrone_[size]inch_V10.FCStd
2. Open the Variables spreadsheet. Change Wheelbase only.
3. Recalculate prop clearance manually from new wheelbase and prop diameter. Adjust PlatformWidthNarrow if needed. Adjust PlatformWidthElec only if the electronics zone prop clearance is affected.
4. Recalculate ArmShaftLength from new wheelbase geometry.
5. Update RodLength (verify exact value in Assembly — Box-in-Box check).
6. Choose battery for new frame size. Update all battery rail variables.
7. Recalculate mass budget. Verify EASA category at estimated AUW.
8. Select motor and ESC for new frame size. Update mass budget.
9. Run full Assembly verification:
10. Rod clearance check (Box-in-Box)
11. Corner post check
12. Chimney clearance check
13. Print coupons before any production parts. Fit tests are more critical at a new scale than at an iterated known scale.

Rationale¶

Why payload interface is fixed at all scales¶

A scalable hardware platform that requires payload redesign at every scale has no commercial advantage over a single-scale platform. The GX12-7 interface was chosen and dimensioned to be compatible with the physical scale range of libdrone (5–12 inch). Fixing it creates the ecosystem compounding effect: each new payload adds value to all past and future frame scales simultaneously.

Why platform length is electronics-driven¶

The instinct is to scale everything with the frame. But the platform exists to carry specific electronics — a FC with a known bolt pattern, a GPS module of known size, a VTX of known dimensions. These do not change with frame size. A 10-inch frame with a stretched platform wastes weight and moves the battery CG position without serving any engineering purpose. The platform length is derived from the electronics layout, not the frame.

Connections¶

requires: - parametric-modelling-philosophy - variable-table-structure related: - freecad-document-setup - frame-structure-overview - payload-architecture leads_to: - freecad-workbenches