FreeCAD parametric scaling

Summary¶

Parametric scaling in libdrone means changing the wheelbase variable in the spreadsheet and having the frame geometry update automatically — arms grow longer, the body expands, rod channels reposition — without touching individual sketches. This works because every geometric dimension is an expression referencing a spreadsheet cell rather than a hardcoded number. However, not all variables scale with the wheelbase: electronics stack mounting patterns, GX12 connector geometry, and battery connector clearances are fixed by hardware and must be kept constant regardless of airframe scale. Understanding which variables are frame-driven and which are electronics-driven is a prerequisite for scaling the model without breaking it.

Concept¶

Two classes of variable¶

Frame-driven variables are geometric parameters that should change when the wheelbase changes. Arm length, body width, rod channel spacing, mast height — all of these are proportional to the airframe scale. If you double the wheelbase from 220 mm to 440 mm, arm length roughly doubles, the body widens, the rod spacing increases.

Electronics-driven variables are fixed by hardware constraints that do not change with airframe scale. The FC stack mounting pattern is 30×30 mm on all libdrone variants — a larger airframe does not get a larger FC. The GX12 chimney bore diameter is 14.5 mm regardless of airframe size. Battery connector clearance is fixed by the connector geometry, not the frame size. Scaling these variables with the wheelbase would produce a model where the FC holes are too far apart for any real FC, or where the GX12 bore is so large the connector falls through.

The scaling sequence¶

Changing the wheelbase variable is not the first step — it is step four of a validated sequence:

1. Read the scaling notes in the Variables spreadsheet before changing anything. The notes column on each variable identifies whether it is frame-driven, electronics-driven, or requires human judgement.
2. Change only frame-driven variables in proportion to the target scale. The wheelbase is the primary driver; derived variables (arm length, body width) will update automatically if their expressions are correct.
3. Verify electronics-driven variables are unchanged. Open the spreadsheet after the change and confirm that FC stack pattern, GX12 geometry, and connector clearances show their expected fixed values.
4. Check items that require human judgement. Some variables — GPS mast height, wall thickness, battery rail spacing — are not simple functions of wheelbase and require the designer to assess whether the scaled value is physically sensible. The Variables document flags these explicitly.
5. Run the FreeCAD model and inspect for sketch over-constraints or red geometry — indications that expressions evaluated to impossible values.

What requires human judgement¶

Parametric scaling does not replace engineering judgement. Specifically:

• Wall thickness does not scale linearly with wheelbase. A body designed for 220 mm may use 2.0 mm walls adequately; at 440 mm wheelbase, the same wall thickness may be insufficient for the increased bending moment. Thickness should be reviewed, not blindly scaled.
• Arm cross-section — a wider arm at larger scale may be needed even though the spreadsheet variable scales the arm length but not the cross-section.
• Battery rail spacing — determined by the specific battery form factor, not the airframe size.
• GPS mast height — must clear the prop arc at the new prop size, which grows non-linearly with wheelbase.

Reference¶

Variable classification (selected)¶

Full variable table: → variable-table-structure and → variable-table-values.

Procedure¶

Scale the model to a new wheelbase¶

1. Open LD_V245_Variables.FCMacro in FreeCAD. Run the macro to populate the spreadsheet if starting from scratch. See → prep-and-parametrics.
2. In the spreadsheet, change the wb cell to the target wheelbase.
3. Check all frame-driven variables: confirm they updated to proportional values.
4. Check all electronics-driven variables: confirm they remain at their fixed values (not scaled).
5. Identify all human-judgement variables. Evaluate each one for the new scale: is the wall thickness sufficient? Does the mast height clear the new prop size?
6. Resolve any red (failed) geometry in the Part Design bodies.
7. Run the assembly check. See → freecad-assembly-workbench.
8. Print coupons at the new scale before committing to full production. See → coupon-validation.

Rationale¶

The frame-driven / electronics-driven classification was made explicit in the Variables document after a scaling attempt that changed the GX12 chimney bore in proportion to the wheelbase, producing a bore too large for the connector. Labelling each variable with its class in the spreadsheet notes column prevents this category error. The human-judgement category was added because some builders interpreted "parametric" as meaning the model required no judgement — which is incorrect and leads to geometrically consistent but physically wrong designs.

Connections¶

requires: - parametric-modelling-philosophy - prep-and-parametrics - variable-table-structure related: - variable-table-values - freecad-document-setup - freecad-assembly-workbench - topological-naming-problem leads_to: - freecad-assembly-workbench - coupon-validation - stl-export-and-slicer-setup