Need help with gearbox info/setup for Fun Cub - RC Groups
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Aug 08, 2012, 02:35 PM
Registered User

Need help with gearbox info/setup for Fun Cub

Hey guys,
excuse any noobness. This is my 'unofficial' first post.

I'm going to be building a Multiplex Fun Cub and I decided to go with a geared outrunner so I can swing a 13x10 or maybe 14x10 prop.

I'm looking into using this motor or something similar in the kV range from HK

I've done as much searching as I can on the topic and most of the links or gearbox suggestions I followed up on turned out to be discontinued, 404s, or in the hundred+ price range.

I think someone somewhere had referred this one.. it seems to be around still..kinda pricey though

So my questions are

1- how do I know if the gearbox will fit onto the motor?
2- what gearbox model suggestions do you guys have (and where to buy them)?
3- how to I adjust and calculate the gear ratio?
4- do you think the motor and gearbox will fit into a Fun Cub and how to mount?

my goal is 50oz of thrust @ 28amps

edit: forgot to ask...can I use a Heli motor? What's the difference?
Last edited by Var.; Aug 08, 2012 at 02:44 PM.
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Aug 09, 2012, 05:45 PM
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Dr Kiwi's Avatar
A Cobri Rotating case mount from would probably fit and you can get a huge range of ratios to play with. Figure out what is the biggest diameter prop which will fit the aircraft... then look at gear ratios which will get that prop spinning at ~75% of Kv x V.

The HK suggestions for maximum power are well over-the-top... that 87g motor is probably good for 3W/g = 260W... so if you want to draw 28A on 3s you are probably just okay.

Badcock doesn't have thrust data for 13x10 but here's 13x8:

That seems to be plenty of thrust (60oz!)... and by computing Kv x V x .75 to get your prop speed of 6500rpm from 2700Kv and 10.5v (3s).. you would need about 3.25:1 gearbox ratio.

Certainly you could use a heli motor... they are essentially the same, but typically higher Kv.
Last edited by Dr Kiwi; Aug 09, 2012 at 05:57 PM.
Aug 09, 2012, 06:26 PM
Registered User


I'm aiming for 50oz of thrust on a 40oz AUW

If I hit 60 i won't be complaining

(btw i did find 13x10 thrust data)
Last edited by Var.; Aug 09, 2012 at 06:35 PM.
Aug 09, 2012, 06:31 PM
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Dr Kiwi's Avatar
Obviously this theoretical computation may bear only a slight resemblance to reality, but hopefully it may be somewhere in the ballpark... good luck with the experimentation!
Aug 09, 2012, 10:54 PM
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scirocco's Avatar
Scorpion 3020_780 on 3s will deliver well over 50oz on a 13x6 for about 28A. No need for complexity/cost of a gearbox.
Aug 10, 2012, 10:12 AM
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Dr Kiwi's Avatar
Perhaps weight doesn't matter, but 166g v. (87g + 15g gearbox) is a big jump.
Aug 10, 2012, 06:08 PM
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scirocco's Avatar
The Multiplex recommended Himax 3516-840 motor is 134g. Similar result to the Scorpion, perhaps a little less efficient.

The FunCub to me just doesn't sit out on the extremes eg extra low Kv relative to weight such as for a slow flyer, very narrow nose such as a glider, or requiring ultimate efficiency to justify pursuing a geared setup, especually when there is any number of outrunners sitting right on the required weight and Kv
Aug 12, 2012, 09:20 PM
Registered User
It seems more and more likely that ill go with direct drive simply because of space limitations.

I did email about the cobri setup to get the dimensions with a motor attached to it. I don't feel crazy about modification to the foam to make it fit
Aug 13, 2012, 01:30 AM
Former Futaba employee
I'm working on a couple Multiplex easy cubs for training others and thinking of mounting the motor (28-30 60 gram power 400) on the outside for better cooling. Most 35-36 100 gram motors should be enough or 35-42 135 gram if more power is needed for the fun cub. The motor below should be enough but will need to purchase collet adapter.
Last edited by keyman923; Aug 13, 2012 at 01:39 AM.
Aug 13, 2012, 07:26 PM
Registered User
Problem with Fun Cubs is I keep reading about them being nose heavy. People are putting batteries as far back as physically possible to get decent CG.

What do you think about this motor? 73 grams

I'm thinking it'll only turn an 115.5 prop but I might settle. I really want a geared setup still, but only if it'll fit.

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