Reference Beta
NACA Submerged Duct
A flush intake that takes air from a surface with nothing protruding into the stream. The ramp walls diverge along its length, which rolls a pair of vortices that drag the boundary layer aside so the ramp can reach clean air — the feature that makes a submerged inlet work rather than just being a hole.
This part is in beta. It renders and exports, but the geometry is still moving: dimensions may change between versions, and it has not been through a printed fitment check. Worth knowing before nine hours and a kilogram of filament.
Printing it
- Material
- PETG / ASA
- Minimum wall
- 2 mm
- Bed splitting
- Cut into tiles across the part, so a board wider than any printer bed still prints. Joints carry dovetails, so tiles cannot pull apart sideways.
Print flange-face down on the bed: that face is the visible aero surface and comes out smoothest against glass. The collar then points up and wants support under it. PLA will sag on a sunlit bonnet.
What it works out for you
Recomputed from your numbers as you type, and shown beside the viewer.
- Ramp length (mm)
- Throat area (cm²)
- Throat aspect
- Overall length (mm)
- Depth below skin (mm)
- Pieces
Parameters
Every number this part is generated from. The geometry is built from these rather than scaled from a fixed model, so a value you can measure off your own car is a value you can type in.
Throat
| Parameter | Range | Default | What it does |
|---|---|---|---|
| Throat width | 60–240 mm | 100 mm | Across the car at the throat, which is the widest point of the ramp and where the air leaves it. |
| Throat depth | 12–60 mm | 26 mm | How far below the bodywork surface the ramp has descended by the time it reaches the throat. Together with the width this sets the throat area, which is what actually limits flow. |
Ramp
| Parameter | Range | Default | What it does |
|---|---|---|---|
| Ramp angle | 4–12 ° | 7 ° | NACA tested 5 to 9 degrees and 7 is the usual choice. Shallower recovers more pressure and makes a longer part; steeper is shorter and starts to lose the boundary-layer scrubbing that makes a submerged inlet work at all. |
| Entry width | 15–60 % | 30 % | How wide the ramp is where it breaks the surface, as a percentage of the throat. This is the start of the divergence: too close to 100 and the walls are parallel, which is the one thing a NACA duct must not be. |
| Wall divergence | 1–3 | 1.6 | How the walls open out along the ramp. 1 is a straight taper; higher holds the ramp narrow for longer and then flares. The published shape is a curve, not a straight taper. |
| Wall thickness | 2–6 mm | 3 mm | Shell thickness of the scoop and the collar. |
Hose collar
| Parameter | Range | Default | What it does |
|---|---|---|---|
| Hose bore | 38–102 mm | 51 mm | Nominal inner diameter of the ducting hose the collar plugs into. 51 is 2 inch, 76 is 3 inch, 102 is 4 inch. |
| Collar drop | 0–90 mm | 20 mm | How far below the throat centreline the round collar sits. A hose is almost always fatter than the throat is deep, so it has to drop far enough to stay under the bodywork. |
| Collar offset | -60–60 mm | 0 mm | Sideways offset of the collar, for steering the hose around something behind the panel. |
| Transition length | 20–120 mm | 45 mm | Length over which the rectangular throat becomes a round spigot. Longer is gentler on the flow and makes a longer part. |
| Collar barb | On or off | On | Retention ridge so a hose pushed on resists pulling off. Turn it off for a silicone coupler and a clamp, which want a plain parallel spigot. |
Flange
| Parameter | Range | Default | What it does |
|---|---|---|---|
| Flange margin | 10–40 mm | 18 mm | Flange material outboard of the duct walls. The fastener row lives in it, so it has to clear a countersink. |
| Nose margin | 0–80 mm | 25 mm | Flange ahead of where the ramp breaks the surface. Gives the leading edge something to bond or bolt through. |
| Flange thickness | 2–6 mm | 3 mm | The flange's visible face is the bodywork surface, so this is how far the part stands proud unless you recess it. |
| Corner radius | 0–40 mm | 12 mm | Plan-view rounding of the flange corners. |
| Fastener size | M4 countersunk · M5 countersunk · M6 countersunk | M5 countersunk | Countersunk so the heads sit flush with the aero surface. |
| Fastener pitch | 40–150 mm | 70 mm | The largest gap allowed between holes down each side. The count follows from it. |
Splitting for printing
| Parameter | Range | Default | What it does |
|---|---|---|---|
| Fit to my printer | On or off | Off | Work the number of pieces out from the bed size below instead of setting it by hand. The duct is cut across its length, never along it: a lengthwise seam would run down the ramp floor, in the airflow. |
| Printer bed | 100–600 mm | 220 mm | The largest square that fits on your bed. 220 for an Ender 3 or a Bambu A1 mini, 256 for a P1S or X1C, 350 for an X1E or a large Voron. |
| Pieces | 1–8 | 1 | How many pieces to cut the duct into along its length, when you are not fitting to a bed. |
| Piece to render | 1–8 | 1 | Which piece this is. Piece 1 is the leading end. Use “Download every piece” to get them all in one go. |
| Joint clearance | 0.05–0.5 mm | 0.15 mm | Slip fit between a spigot and the socket it slides into. Raise it if the pieces bind, lower it if they rattle. |