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What effect does a large cowl have on the efficiency of propeller?

Or: How important is an inch?

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David Pearce 402/07/2019 10:32:05
319 forum posts
37 photos

I have a Great Planes Mr Mulligan ARTF. The recommended motor is Rimfire 32 with 800Kv and a 12” x 8” prop. That’s a rather expensive, and rare, motor and I’m hoping to use a motor I already own. This is a 900Kv motor which delivers the required 800+ watts using an 11” x 5.5” prop.

My question is; will the large fat cowl have an effect on the power and would I be better off buying a new motor, of lower Kv, to swing a large diameter prop? Is that 1” important? (Fitting a larger prop to my existing motor would raise the current too much.)

Your advice would be appreciated.


Edited By David Pearce 4 on 02/07/2019 10:33:14

J D 802/07/2019 11:39:20
1588 forum posts
87 photos

It is often said that 1" more is important.winkdevil

There is often a concern that with radial cowls much of the inner part of the blades is in front of the motor but most of the thrust comes from the outer half of the blade so I think in your case you need to stick with the recommended diameter.

  Larger prop with less pitch would likely be alright.

Edited By J D 8 on 02/07/2019 11:41:09

Dwain Dibley.02/07/2019 12:02:08
1523 forum posts
1506 photos

Interesting much so, I have just measured all of my radially cowled models and their props. All three were pretty much 2:1. That is to say the prop was twice as wide as the cowl. The biggest, a 50 cc Gilmore has a 22 X 10 prop and the cowl is 12" across.

This would seem to confirm what JD8 has said above.


Simon Chaddock02/07/2019 15:09:31
5777 forum posts
3055 photos


As a rule of thumb the outer 1/3 of any prop does 2/3 of the work so you need to check the cowl diameter to your 11" prop.

As quite a proportion of the prop will be 'blanked' by the cowl you might find a 12 x 5.5 when in place does not actually exceed the motor's current limit.

Experimentation and a Watt meter are the order of the day!.

David Pearce 402/07/2019 16:43:59
319 forum posts
37 photos

Thank you all for your input.

I agree more testing and measuring is required. I'd rather get it right the first time so, if there's any doubts, I may well go for a lower KV motor.

Thanks again.

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