What FEA Is — and What It Can't Do

Finite element analysis (FEA) is a computational method that divides a 3D model into thousands of small elements and calculates how stress distributes through the part when a load is applied. The software shows you where material is under-stressed (wasteful mass you could remove) and where it is over-stressed (failure points you need to redesign). For drone motor mounts and arm cross-sections, this is genuinely useful information.

What FEA can tell you: whether your geometry has obvious stress concentrations, whether your material choice provides adequate safety margin under peak motor thrust, and where removing material creates a failure point.

What FEA cannot tell you: exactly how a printed part will perform in a real crash, because crash loading is dynamic and the anisotropic nature of FDM prints (layer adhesion is always the weak axis) is difficult to model accurately without material-specific data. Use FEA as a design aid, not as a pass/fail certification tool.

  • FEA is useful for comparing geometry variants — "does a 3mm wall vs 4mm wall change stress distribution significantly?"
  • FEA is useful for identifying stress concentrations at sharp internal corners — these are the most common geometric failure cause
  • FEA is not a substitute for physical test prints — always validate with a test part before production quantities

Setting Up SimScale — Free Account and Project

SimScale is a cloud-based FEA platform with a free community tier that allows unlimited public simulations with up to 3,000 core hours per year. For hobbyist use — running one or two geometry checks per design iteration — this is more than sufficient. No software installation is required; everything runs in the browser.

  1. Create a free account at simscale.com — no credit card required for the community tier
  2. Create a new project and name it (e.g., "Motor Mount FEA")
  3. Upload your motor mount STEP or STL file — STEP files from Fusion 360 produce cleaner meshes than STL
  4. Select "Structural Mechanics" as the simulation type, then "Static Analysis"

If you are exporting from Fusion 360, export as STEP (File → Export → STEP). STL files lose parametric geometry information and produce lower-quality mesh elements in FEA solvers. The quality difference is significant for thin-walled drone parts.

Assigning Material Properties

SimScale includes a material library with common engineering polymers. For printed drone parts, the closest available materials and their approximate real-world printed equivalents are:

  • PETG printed parts: use "PET" from the library — tensile modulus ~2.5 GPa, tensile strength ~50 MPa
  • ASA / ABS printed parts: use "ABS" — tensile modulus ~2.2 GPa, tensile strength ~40 MPa
  • PETG-CF printed parts: no direct match — use a custom material with tensile modulus ~6–8 GPa (conservative estimate for short-fibre CF composite)
  • Nylon-CF printed parts: use "PA 6" as a base and manually increase modulus to ~5 GPa

Important caveat: these values are for isotropic materials. Printed parts are anisotropic — they are significantly weaker perpendicular to layer lines than parallel to them. If your motor mount's load path is perpendicular to layer lines (common if the part is printed flat), apply a 30–40% reduction to tensile strength in your analysis to account for inter-layer bond strength.

Applying Loads and Constraints

This is where most beginners make mistakes. Incorrect boundary conditions produce meaningless results regardless of how accurate the rest of the simulation is.

Constraints (Fixed Supports)

Apply a fixed support to the faces that bolt to the frame — typically the bottom face of the motor mount boss and any bolt hole surfaces. This tells the solver "these faces cannot move." Do not over-constrain by fixing every surface, as this artificially stiffens the model.

Motor Thrust Load

Apply a force load to the motor mounting face in the vertical direction (away from the frame). For a typical 5-inch FPV motor producing 2.5kg thrust, apply 24.5N upward. For a safety factor of 2x (recommended), apply 49N. The direction matters — thrust acts axially away from the propeller plane.

Torque Load

Motor torque (reaction torque from spinning the prop) is often overlooked. For a 2306 motor at peak power, reaction torque is approximately 0.4–0.6 Nm. Apply this as a moment load around the motor shaft axis on the motor mounting face. In most motor mount designs, this produces significant stress at the mounting bolt boss corners.

◆ Quick Reference — Load Values

5-Inch FPV Build, Standard 2306 Motor

Thrust per motor: 24.5N (2.5kg) • Safety factor applied: 49N • Reaction torque: 0.5 Nm • Bolt preload: 8N per bolt (M3 at finger-tight). Apply these as your baseline — adjust upward for heavier motors or high-performance builds.

Reading the Results — What to Look For

After running the simulation (typically 5–15 minutes on SimScale's free tier), you will see a colour-coded stress map. The colour scale runs from blue (low stress) through green and yellow to red (highest stress). Red areas are your focus — they indicate where the material is working hardest and where failure is most likely to initiate.

Common Failure Patterns in Motor Mounts

Sharp internal corners: The most common problem. Stress concentrates at 90-degree internal corners — the tip of a notch acts as a stress amplifier. Add fillets with a minimum 1mm radius to all internal corners. For heavily loaded corners (bolt boss bases), 2–3mm fillets reduce peak stress by 40–60%.

Thin walls at bolt bosses: The cylindrical boss around a motor bolt is a common stress concentration point, particularly where it meets the base plate. Increase the boss-to-plate fillet radius and ensure wall thickness at this junction is at least 2.5mm for PETG or 2mm for CF composites.

Layer-direction loading: If your simulation shows peak stress on faces that correspond to inter-layer bond interfaces in your print orientation, consider re-orienting the part for printing or switching to a stronger material. FEA cannot model this directly, but understanding your print orientation relative to the stress map is critical.

◆ AeroInfill Verdict

A single FEA run before printing takes under an hour and can save multiple failed parts. The key insight is not the exact stress numbers — the material data is approximate — but the stress distribution pattern. Where stress concentrates in the simulation is where your part will crack in the field. Fix those geometry points before you print.