Axial fan (rotor)

A printable axial fan rotor — twisted blades at the pitch you set, on a bored hub.

An axial fan is a hub and a ring of twisted blades, and it blows along its own axis — the propeller-shaped kind, as opposed to the impeller, which throws air out sideways. The twist is the whole design, and it comes from one number.

Pitch is the twist. Pitch is how far the fan would advance in a single turn if it screwed through a solid, and it is the figure every propeller has ever been sold on. From it, the blade angle at each radius follows exactly — `arctan(pitch / 2 pi r)` — which means the blade is steep near the hub, where it is moving slowly, and flat out at the tip, where it is moving fast. That is the free-vortex distribution, and it is not something you have to author: there are no per-station angles to invent, and the panel reports the two ends it produced. A coarser pitch is a more aggressive blade that moves more air per turn and asks more of the motor.

Get the rotation right — but not for the reason you think. Both senses blow the same way; the blade is mirrored, not flipped. What the setting has to match is the motor, and picking the wrong one gives you a fan that pulls when you wanted it to push.

The section, and why its edges are blunt. Choose a flat plate — the simplest thing to print and the hardest to get wrong — or one of the published NACA four-digit aerofoils. On paper an aerofoil closes to a knife edge, and no nozzle can lay that down: at 12% thickness on a 10 mm chord it tapers from 1.2 mm to nothing. So both edges are blunted to Edge thickness, which is the thinnest the blade is allowed to get anywhere.

The tips are cut on the circle they sweep. A blade section lies across the airflow, so its corners sit further from the axis than the station they belong to — a blade built out to the stated radius would sweep a couple of millimetres wider than that. The rotor is therefore trimmed by the swept circle, which also gives the tip the arc a real fan has. The consequence is the useful one: Tip diameter is the number that has to clear your frame.

Watch the roots. Blades crowd where they leave the hub — that circle is the shortest one they cross, and the blade lies most nearly along it there. Add enough blades, or enough chord, and the roots run into each other and fuse into a solid disc that still looks like a fan. The Gap at the blade roots row is the thing that will tell you.

It will probably want supports. A twisted blade printed flat has a shallow underside — at ordinary pitches the tip is only a few degrees off horizontal, well past what a slicer will bridge. The Blade underside at the tip row reports that angle and warns below the usual 45 degree rule of thumb. Turn supports on, or raise the pitch. This is a fact about twisted blades rather than about this generator, and it is better said than discovered on the plate.

What it does not tell you. No airflow, no pressure, no efficiency, no noise figure. Those depend on the shroud around the rotor, the tip clearance and the motor driving it, none of which this part knows about, and we have no test rig. What it does claim is geometric and checkable: the blade sits at the pitch you asked for at every radius, the section is the aerofoil you named, the blades clear each other, the rotor sweeps the diameter on the field, and the bore fits the shaft you measured. Run Print check over the result before you commit filament to it.

1. Open the Generate panel from the left sidebar and choose Axial fan (under Fans & airflow).
2. The rotor drops in as one printable body. The parameters open in the Properties panel.
3. Set the Tip diameter to the circle it has to sweep, and the Hub to what fits around your shaft.
4. Set the Pitch — that alone is the whole twist, and the panel reports the hub and tip angles it gives.
5. Set Turns to match your motor, then the blade count and the chord at each end, watching the root gap row.
6. Enter your Shaft diameter (the shaft, not the hole), and a Shaft flat if it is a D-shaft.
7. Check the blade underside row — a twisted blade usually needs supports — then print it hub-down.