Most foam wings need a spar — a carbon tube (or hardwood strip) buried in the core that carries the bending load. Cutting that channel by hand with a soldering iron or router guide is the least fun part of any wing build. The wing generator's Spar tunnel option cuts it for you, in the same pass as the profile.
How the wire cuts a hole in the middle of foam
A hot wire can't teleport into the middle of a block — but it can cut its way in and back out along the same thin line. The spar tunnel is a keyhole path spliced into the aerofoil outline:
- From the bottom skin, the wire cuts a thin entry slit (≈0.8 mm wide) straight up into the core at your chosen chord position.
- It then traces a full round tunnel of your chosen diameter, centred mid-thickness between the upper and lower skins.
- It exits back down the same slit and carries on around the profile.
The result is a round tunnel connected to the surface by a slit thinner than the wire kerf. Push the tube through the slit — foam flexes, the tube clicks into the round channel and the slit closes behind it. A drop of foam-safe glue (or a strip of tape over the slit) finishes the job.
Settings & fit rules
- Ø (mm) — the tube diameter. Use the real tube size; the tunnel is cut to match and foam compresses enough for a snug push-fit.
- Position (% chord) — distance from the leading edge. 25–35% is the sweet spot: near the section's maximum thickness (most room) and close to the centre of lift.
- Fit rule: the tunnel needs the section to be at least Ø + 2 mm thick at that station (1 mm foam wall above and below). Too thin — e.g. a 6 mm tube at 70% chord of a small tip — and the generator skips the tunnel and tells you rather than cutting through the skin.
Taper, sweep & washout
The tunnel is cut into both the root and tip profiles at the same % of chord, and the 4-wire morph rules a straight line between them — so a straight tube fits through a tapered, swept panel. With washout the tip's tunnel twists a degree or two with the profile; at normal washout values (≤3°) the deviation is far smaller than the foam's give, so a straight spar still slides home.
Tips
- One spar at ~30% chord suits most sport wings; heavy or large wings often add a second smaller tube at ~60–70% (generate twice and cut two cores, or run the second channel by hand).
- Keep the slit on the bottom surface (the generator does this automatically) — the top skin carries compression and stays unbroken.
- Glassing or taping over the slit restores essentially all the skin stiffness.
Related
See the wing generator for the full panel workflow, kerf & cut quality for why the slit ends up wire-kerf wide, and the ready-made wing presets to start from a proven planform.