Making Your Propellers Spin Freely

[sequoiaranger]

Most of my stuff has a "fan" on it. Not that I hate jets or anything, but I just like prop-jobs. To me, "in flight" is the ideal arena for an aircraft, and of course the prop would be spinning to accomplish that. Holding a model at arm's distance, who hasn't blown hard on the propeller to make it spin and create the illusion that the propeller is holding up the model, not a hairy arm. All too often, though, despite best intentions when constructing, the propeller is reluctant to spin, or comes to a screeching halt once the blower stops blowing. Doesn't have to be that way!

Another one of the difficulties of fan-job models is that if the propeller needs to be attached before the model is painted, then there are multiple problems of painting around the prop blades, whether for the cowling or the engine inside. The easiest way is just to cement the prop in a fixed position at the end and forget the spinning part. Again, doesn't have to be that way.

So what is my solution?

The "HUB", bub!

SOME models have "my" solution, i.e., having a minimal, spinning "Hub" to which the painted prop is attached after the model is finished [see "A" in my illustration]. Most don't. The solution is to make your own "HUB", bub.

As on the left of my admittedly crude illustration below, most prop models have the prop, hub, and shaft as one piece, with some sort of retainer that goes on the inside of the engine or cowling to hold the prop in. Ya gotta put the prop-and-shaft through the engine cowling and "retain" it during the construction phase, and tape/mask off the prop and blades while painting. Too much handling of the propeller, at least on the 1/72 scale and smaller, risks that accidental snapping-off of a propeller blade and the resultant tricky re-glue to get the prop at all the right angles, etc.

Ideally, all you whiffers out there have a "Moto-Tool" or some other motorized rotary tool that you can attach a thin rotary saw blade to. If you don't have it, you NEED one next "gift" time that rolls around. Anyway, if you saw a thin piece of propeller hub off the end of your propeller hub that will butt up against the engine/cowling, you can cement and retain THAT piece, and glue the rest of the prop and hub on later.

Easier said than done, and is yet another reason to buy a second main model to use as a practice dummy or as a spare.  In the case of my recent single-engined "Jinpu-Kai", I had used a four-engined bomber as a parts source, so I had FOUR props I could experiment with (but I only needed two as I had hoped). I deliberately broke off the prop blades on one prop hub and cut a generous piece for the HUB, then cut off another prop and vigorously filed down the good prop-and-hub so that the combo would have the same "projection" from the cowl when assembled.

The above makes the prop spinnable, but not necessarily "freely". The "freely" part has two elements:

One is to limit friction between moving parts. Obviously, a lot of friction will make the prop hard to get started, and it will come to a halt rather quickly. The "secrets" are: a perfectly round, smooth hole through the engine/cowling, a round shaft that is free of mold marks. raised edges, or other imperfections, and very smooth mating surfaces of the end-plates. Some eyeball scrutiny before assembling is the best guarantee--get a 10X hand lens and look hard at the surfaces. Use polish (like toothpaste) if you like. If the plastic doesn't shine at the mating surfaces, it is too rough and will create unwanted friction.

For additional lessening of friction, use a lubricant that will stick around. Some graphite or teflon powders work well and I prefer them, but even a VERY LIGHT oil will work. I will sometimes lightly press the hub up against my moto-tool and turn it on low to keep it spinning and wear down any high spots before I attach the prop.

Next comes a somewhat overlooked adjunct to a well-spinning prop---CLOSE TOLERANCES. That is, if the hole and shaft are nice and round, but the hole is too much bigger than the shaft, the shaft will wobble and cut down on the spinnability AND wear out the bearing surface faster for MORE wobble. The weight of the prop is obviously more than the retaining ring, so the retaining ring has to be "tight" up against the cowling opening or the prop will tend to sag downward with leverage. A longer shaft and longer hole-through-the-cowling helps that. Also, a slight counterweight on the inside will compensate for the added weight (however slight you may think it) of the prop. THEN...if you add a metal washer on the inside (hopefully just a tad less than the diameter of the cowling), you will create a "flywheel effect" with some extra angular momentum that will keep the prop spinning longer!! With a washer, you don't want a long shaft because now the weight will have the opposite leverage effect (too much weight on the inside).

As always, "trial-and-error" is needed to get it right.

Ready to try it (some of it?)?

[sequoiaranger]

OK, below is a pic of my "Jinpu-Kai" cowl (Renazn "Rita" cowl, actually, with the flared cooling flaps ground down) with the "hub" ("A" assembly in illustration) in place as I explained. It revolves freely, and I can attach the propeller post-painting.

The item on the upper right is the "flywheel" setup. The round green thing is an extra retaining ring to glue onto the actual retaining ring to allow the flywheel to clear the immediate cowling around the retaining ring (slightly sunken into the cowl piece). The white is just a piece of sheet plastic to be able to glue onto the retaining ring with styrene cement and also to cover the "hole" in the washer. I used RC white glue to glue the white plastic onto the steel washer. Now I will cement green to green (INSIDE the cowl), and the flywheel will be inside the cowl and ready to keep the prop whirrin'.

Better than the "flywheel", but more difficult to arrange, is a "ring" of metal the same diameter as the washer. You physics folk will note that a ring of the same mass and the same diameter has greater angular momentum than a mere disk. The drawback is that it might take an extra bit of huffing and puffing (us old farts need all the breath we can muster) to get the prop up to speed, but it will STAY at speed longer.

[sequoiaranger]

Eating a little humble pie here.

My "flywheel setup" I touted in the last post BROKE APART on me AFTER I had glued the cowling to the fuselage and tack-glued a propeller on, too. So...luckily the whole model wasn't assembled, and I could carefully pry the cowling off. Luckily, too, I have two more spare cowlings from my Renzan four-motor bomber which I was parting out. So STARTING ALL OVER AGAIN  the thought suddenly hit me that perhaps the flywheel was out of balance, and that when I was gleefully spinning the prop with forced air that the resulting imbalance had shaken loose the tenuous bond of the prop shaft to the retaining pin. Slapping my head in a "DOH!" moment, I realized how important balance is to a flywheel, remembering my experiences with an slightly-out-of-balance flywheel on my own Jeep engine (way back in 1973) that shook like crazy.

To strengthen the whole assembly and to make sure the flywheel was balanced, I carefully drilled a hole through the center of the entire propshaft assembly and inserted a pin with a bent-over end. Next, using the "rails" from a large tweezer, I placed the assembly on the rails and rolled it along the rails to test balance. If it was in balance, there wouldn't be a place along the rails that the assembly would "settle" regularly.

Sure enough, the assembly would settle in one place all the time, and slight tilting of the rails would not budge it (a balanced arrangement would immediately start to roll). So I Super-glued on tiny counterwieghts, barely visible in the pictures, and used the bent-over part of the pin to fine-tune the balance until the assembly did not settle when rolled along the rails. Whew!!

Now I will dis-assemble the assembly and re-assemble the whole cowling/propshaft setup. The green part in the pictures is the part on the outside of the cowling and will hold the final propeller and spinner. I'll snip off the excess protruding pin when I'm done.

This was WAAAY more work than I had expected, but I did learn something from it. Hope you did, too (if nothing more than to adhere to the K.I.S.S. principle and stop futzing around with complicated engineering--this is supposed to be FUN, not JMN stuff!)