Rotating props in small scales

[Leif Ohlsson]

[This tutorial was originally published at Kartonbau.de:
Kartonbau.de - Alles rund um's Kartonmodell... | abgeschlossene Bauberichte | Learning to build in 1/87 scale
The text here is more or less the same. Reason to republish: I kind of like it myself, and it is nice to have it available at the correct place, here under "Tips & tricks".]

I have always disliked the idea of just sticking a pin through the prop and then into the engine or other part of the model. There's always the risk of the propeller being so thin that it will wobble, and then the pinhead itself is kind of large and noticeable in small scales.

Much better, I have thought, to manufacture a unit where the rotating prop is already attached to an axis and a casing, and then stick that entire unit into a hole in the model.

The idea, thus, is to stick a pin through a small plastic tube, glue the prop to that axis, check that the glued up unit rotates freely and well aligned, and then shove the whole thing into the finished model.

Here's my first attempt:



1. For both casing and prop hub I chose RC control rod inner tubing of 2mm outer diameter. The axis will be an ordinary pin, cut off at suitable length. Here I am drilling a 0.5mm hole in a length of tubing which will be the prop hub. The idea is to insert a piece of pin at right angles, to form a rigid framework for the prop blades.



2. The drilled end of the tube is filled with ordinary white glue (which is another thing I like; stick to white glue and water-soluble paints as long as possible).



3. A pin is stuck through the hole with white glue inside, and cut off at suitable lengths. The prop will be built up around these pin ends.



4. Prop stiffeners and base mounted. The prop governing mechanism is replicated by a rolled strip of 45g sketching paper ("vellum"). The whole thing is covered with white glue for stiffening. (Note actual parts sheet in the background, where the prop covers, front and back, still remain to be cut out.)



5. The whole thing is painted with grey watercolour pencil (and yes, it sticks also to the plastic part; at least well enough for the moment). Actually you only need to paint the centre part around the hub.

Outer skins of prop (front and back) are now added, and the plastic tubing cut off at suitable length to form the finished prop. Some kind of rounded front is created, for example by repeated blobs of white glue. I used a cut-off pinhead as a base, which isn't perfect, and I won't do it again.

The whole prop is varnished with gouache varnish matt, which is spirit-soluble, and the one exception from the water-soluble principle I allow myself. It is excellent for varnishing parts sheet before cutting out parts, since it doesn't wrinkle the paper, and equally excellent for varnishing finished sections and the whole model.



6. Here are the finished parts of the prop assembly, ready to be glued together. The axis is a cut-off pin, and the casing to be glued into the model is another piece of 2mm plastic tubing. The prop will rotate glued to the axis within this casing, which is to be glued into model.

Note that the pinhead is smaller than the 2mm plastic tube casing, which will allow inserting into a hole in model without problems. Note also that the construction sketch of the model in the background is to the same 1:87 scale as the prop.

The outer casing is a piece of RC control rod outer tubing. It is very much optional, and I don't think I will use it. If you go for it, you can glue it into the model from the start, and the prop assembly is then glued into that casing instead of into the card parts of the model.



7. The finished assembly is tested according to the familiar hair-dryer principle. It works just fine, and there is no wobbling (which can be seen from this long exposure; the prop shaft would be blurry if there were any wobbling).

Evaluation: I will use this method, but I won't make such a complicated prop hub. A rolled paper part is just as good, and can easily be stuck, and glued, to the prop shaft. In any case, the drilled plastic prop hub can only be used for two-bladed props, not three- or four-bladed ones.

But it was fun to make, just this once.

Leif

[THE DC]

Thanks for the tips!


The DC

[Don Boose]

This is an outstanding tutorial, Leif. Thank you very much for posting it here.

I have grabbed it and have added it to my notebook of paper modeling techniques.

Don

[legion]

Might be a small tip, but if you drill another .5 mm hole in front of the first hole, but at oblique angles to it, insert glue and a rod and then bend the rod to be at the same place as the other rod? That way you can get a 4 or even 6 bladed prop, though the whole hob would get a bit thicker then the 2 prop version.