Almost every FPV pilot has been there — a hard crash and a cracked or delaminated arm. Before you order a $40 replacement, consider a faster, cheaper alternative: 3D printed arm splints. When designed properly, a printed splint can get you back in the air the same day for pennies in filament.
When Can You Splint Instead of Replace?
Not all arm damage is splintable. Here is a quick triage guide:
- Splintable: Hairline cracks, partial delamination (layers separating but arm still in one piece), chips at the motor mount edge, cracks that do not run through the full thickness.
- Replace: Complete breaks, cracks through the full arm width, damage near the center plate (high stress concentration), arms on over 5-inch quads that will see aggressive flying.
For micro quads (3-inch and below), a well-splinted arm can last indefinitely. For 5-inch freestyle quads, consider a splint as a temporary fix.
Designing the Splint
1. Take Measurements
Use calipers to measure your arm exact dimensions. You will need: arm width, arm thickness, crack location and length, and distance from motor mount to the crack.
2. Design in CAD
The splint should be a U-shaped channel that wraps around the arm. Key design rules:
- Length: Extend the splint at least 15mm beyond each end of the crack.
- Thickness: 2-3mm wall thickness for the splint itself.
- Clearance: Model the splint 0.2mm wider than the arm for a snug press-fit.
- Zip-tie channels: Include 2-3 slots (3mm wide) for zip ties to secure the splint.
Printing the Splint
For splint material, TPU is the best choice for the same reasons it excels at camera mounts — it absorbs vibration and flexes instead of cracking. PETG is an acceptable alternative.
- Material: TPU 95A (ideal) or PETG (acceptable)
- Perimeters: 4-5 walls. Set infill to 0% if walls fill the entire cross-section.
- Orientation: Print flat on its side so layer lines run along the length of the arm.
Installation
- Clean the arm with isopropyl alcohol.
- Apply a thin layer of CA glue or epoxy inside the splint (optional).
- Press the splint onto the arm — it should fit snugly.
- Zip-tie it down with 2-3 small zip ties through the channels.
- Check motor clearance by spinning the prop by hand.
3D printed splints are not just about saving money — they are about maximizing air time. Keep a few splints in your field bag and you will never end a flying session early because of a cracked arm.