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Prop Clearance Question?

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Richard. W.21/12/2018 23:50:31
63 forum posts

Hi, All.

This is just a general question rather than about any specific model so I can't quote any actual sizes, BUT.....

If I build a scale or sport type model with a large radial cowling, do I hit the same problem with electric power as I do with I/C, in getting a useful amount of the prop past the edge of the cowl?

With I/C I'd use a motor with plenty of torque and a bigger prop. If I over prop it, it doesn't really matter as It won't harm anything but with electric (if I have this right) the motor is going to try to reach it's stated rpm whatever it's swinging around.

Am I right in thinking that I'd need a motor with a lower rpm but a battery and esc with higher ampage values so the motor can reach it's speed with the bigger prop but nothing is cooked by the current it needs to do it?

Don Fry22/12/2018 06:37:25
3728 forum posts
42 photos

No, you need a motor with a lower KV, wot will swing a bigger prop, at lower revs for the same amps. And you get better efficiency from the bigger prop.

Andrew76722/12/2018 06:49:50
809 forum posts
4 photos

Same as Don but do use a wattmeter.


J Moyler22/12/2018 07:43:01
143 forum posts
56 photos

4-Max have a lot of information on choice of electric motors.

To answer your question regarding prop size with large cowling, I have found that electric motors tend less problems as they tend to use larger props than an equivalent IC engine.

If there is a problem you can just use a lower KV motor as suggested by Don, or by lowering the voltage you can use a larger prop.

eg Using a PO-3547-700 Purple Power Motor

3 Cell 11.1 Volts - 15 x 10 Prop

4 Cell 14.8 Volts - 13 x 6.5 Prop

5 Cell 18.5 Volts - 12 x 6 Prop

Hope this helps you.


Richard. W.22/12/2018 13:18:17
63 forum posts

It's certainly a help, guys...Any positive input is a help to me at the moment, lol.

I'm reading all the information I can get from these beginners threads to try and get to grips with the whole electric thing.. I know the easy way is to simply ask what I'd need to fly a specific model by giving it's size, weight, etc every time but I'd also like to learn the equations myself.. "Monkey see, Monkey do" isn't how I like to work. thumbs up

Simon Chaddock22/12/2018 19:41:33
5405 forum posts
2824 photos


Whilst learning all the technical "equations" is to be lauded there is some advantage. at least initially. in going down a fully recommended solution simply because you know it will all work. If it then don't fly too well its probably down to you.wink 2

Although I have a good grasp of the technical issues of magnetism and electricity I had little idea of what combinations of battery, motor and prop would work well enough to fly an RC plane.

Having achieved controlled flight with an out of the box ready to fly plane this formed a starting point for me to change things a bit a a time to alter its performance so it better suited my flying conditions and abilities!

Richard. W.23/12/2018 14:01:51
63 forum posts

Hi, Simon.

Thanks for your response and you're correct......My apologies if I wasn't being clear..

Initially it's certainly wise to go with a fully recommended set up but I'd still like to understand the calculation behind the answer so I can work it out for myself in future.

"If it then don't fly too well".....etc.... Erm. Yep.. That definitely sounds like me! crying 2

Geoff Sleath23/12/2018 15:06:55
3272 forum posts
251 photos

I am in broad agreement with the advice regarding motor kv and prop size but I wonder if a large diameter cowling really does demand a prop which is big enough to extend well outside it. I'm sure I read somewhere (I'm no expert in this) that a prop will work perfectly well regardless (well almost regardless) of the size of the cowl because it's drawing air from the front, which is unrestricted.

Is that the case?


Richard. W.23/12/2018 15:39:26
63 forum posts

That's a good point, Geoff and I'm sure I've read a similar thing myself in the past.. Much of it may well be aesthetic and I know that going back to my free flight days with scale, we'd have two props. One just for static judging (to scale) and then the flying prop. The main thing there was that you could get enough clearance to flick it but thinking back, they still flew OK with minimal clearance.

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