This extremely large and powerful ducted fan was designed and built primarily for powering a jet go-kart, but it has an array of other applications including scooters, boats or R/C aircraft. Using two independent rotors, this absolute unit of a fan has 8 blades on the first stage and 9 on the second stage. This difference in blade number dampens blade-on-blade interference noise making it slightly quieter during operation than a traditional contra-rotating propeller. The motor mounts ideally should be manufactured out of metal or carbon fiber, but the printed versions should still hold up to the heating if enough air is forced through the nacelle. This project uses A LOT of filament (>3700g) so beware that multiple 1kg rolls may be used up.

Performance:
I use two 4Kw 170kv brushless motors on a 12s lipo battery to power this thruster, but more powerful motors may yield better thrust. The theoretical thrust of this unit (https://rcplanes.online/calc_thrust.htm) at 4Kw is in the range of 140N -180N with a large margin of error from simplifying the blade geometry. I have yet to measure the full scale performance but it should be close to the theoretical using my best performing blade geometry. This was designed around using 6374 electric skateboard motors which are available in a variety of speeds and power levels. (170kv @ 3500w is ideal) Motor may become warm, so a cooling fan in the tailcone is strongly recommended. When stress testing the 3D-printed blades out of PETG, they each could handle about 70N of force on their centroid of loading before significant deformation and withstood more than I could measure radially. This gives a safety factor of about 6 with my highest thrust scenario.

3D printing:
I used PETG for the entire thing and printed parts on a Creality CR-10 and a Raiscube A8r. Some parts are very large and need 310mm x 310mm bed space The CR-10 works well for this but those with smaller printers and modeling skill may need to separate it out into smaller sections.
Here are the parameters for the different pieces:
300mm rotors - 1.6mm wall - 1mm top/bottom - 40% cubic infill - use support and brim
cowling sections - 1.2mm wall - 1mm top/bottom - 20% cubic infill - brim optional
bearing adapter - 1.2mm wall - 100% infill - brim optional
Nose and tailcone - 0.8mm wall - 1mm top/bottom - 20% cubic infill - brim optional
nozzle and intake duct - 0.8mm wall - 1mm top/bottom - 20% cubic infill - brim optional

materials: ≈ $250
4x - 1kg filament rolls, Ideally something high temp like PETG, PC, Nylon CF, etc.
1x - plastic glue (Cyanoacrylate, Epoxy, etc.)
2x - 170kv 6374 brushless motors rated to at least 2800w
1x - 6x60x38 box fan (server fans are more powerful and can keep things cooler)
1x - 52x25x15 ball bearing
6x - small zip ties (optional, for cable management)
16x - M4x15mm machine screws (motor mounting)
18x - #8 x 1/2" thread forming screws
20x - #8 x 1" thread forming screws
18x - #8 x 1-1/2" thread forming screws
4x - #8 x 2" thread forming screws
Paint, filler, or other finishers are optional, but a smooth surface may increase performance slightly.

assembly:
assemble the duct in top and bottom halves separately with the glue of your choice, using 20mm sections of 1.75mm filament as alignment pins between glued sections. with the two duct halves done, attach the motors to the motor mounts and run the wires through the internal wire passage. Run cooling fan wires through the rear motor mount to power the fan in the tailcone. Attach the rotors to the motors with 4 machine screws and insert a 52x25x15 ball bearing into the front rotor. Use the bearing adapter to connect the front and rear rotors about their central axis. Place the rotor assembly into the bottom half of the duct and align the wire passage to the one in the duct. Use thread forming screws to attach the rotor assembly to the bottom of the duct and then attach the top half of the duct to the bottom assembly using the same screws. Use screws to attach the nosecone and attach the 60mm box fan to the inside of the tailcone making sure the wires are connected through the rear motor mount passage. Lastly, screw on the tailcone and base mount to finish the thruster assembly.

final notes:
assembly can be complicated with so many parts. be patient and use sandpaper to adjust tolerances between interlocking parts. hasty assembly leads to damaged or cracked parts.
do not spin the rotor over 15,000 RPM or else it might suffer a sudden kinetic disassembly. Do not let any loose objects or body parts to come in contact with spinning blades or serious injury can occur.
Improper printing or use of damaged parts can also lead to catastrophic failure - inspect all parts before assembly and use.

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