My own custom designed Lowly Worm apple helicopter for DUPLO.

It's not exactly plug and play 😅

You will need the following to assemble:

• Super glue (cyanoacrylate)
• 3.2 or 3.3mm drill bit
• M4 thread tap
• 10mm M4 countersunk bolt
• Small round file

For glossy finish you will need:

• 120-5000 grit wet sanding paper (perhaps it can be polished after a lower grit than 5000, but that's what I used)
• Autosol Plastic Polish

I used organic supports for the Apple_top so the Dash Board is unaffected.
I used brim for the studs.

To avoid supports on, the bottom of the apple consists of four parts glued together: Apple_bottom, Bottom_lip and 2 x Bottom_studs.

I glued them with Super glue (cyanoacrylate)

The same goes for the rotor, where the hub with supports for the blades are glued on.

I have uploaded prints for templates/jigs for glueing on the Bottom_lip and Rotor_hub, to make sure they are glued on precisely. (see pictures)

Need to glue on the Lip_bottom on the Apple_bottom before the studs in order to use the template.

The chassis with wheels is an original Lego Duplo part and so far I haven't made a printable model for that.
The little Lowly Worm figure is bought and I glued on a Duplo stud under his sole so it would fit Duplo.

I sanded the apple with the bottom and top assembled. The first sanding with 120 is the most time consuming. For the best result, it's important to keep sanding until there is no visible shiny fillament left. Take your time before going from 180 to 240 and 240 to 320 to make sure all the scratches from the lower grit is gone - if you move on too quickly, they will show up in the end result.
I suppose this could be done with vapor smoothing using another kind of filament and save you a lot of time.

I sanded the rotor before gluing the hub on. But I don't think it makes much difference.

The hole in the Rotor_axle needs to be drilled up using a 3.2 or 3.3mm drill but. Then it needs threading using a M4 thread tap. Make sure you don't drill or tap all the way through.
If you don't have a thread tap I suppose you could glue a smaller bolt in instead. (I haven't tried that)

The rotor can spin. In order to press it on the axle you will need to carefully file the small protusions inside the hub with a small round file. File some and try to press it on. Keep doing that until it can be pressed on. I pressed it on while there was a considerable resistance.

Before gluing on the studs on the bottom I sanded the conical part with grit 180. I used small spring clamps afterwards and let it set for 15 minutes.

I have included both .stl and .stp files for all the parts. PrusaSlicer can handle STEP-files aswell and you get a smoother result.

Let me know in the comments if anything is unclear or you have questions.

Prusa

Prusa XL

No

Yes

0.1 - 0.2

30%

PETG