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Old Aug 20, 2015, 07:04 PM
rcbuilder65 is offline
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Question
What ropeller do I use?

Hi,
I have bought a Suppo Brushless Motor A2208/14 1450kv for m scratch build trainer airplane because by other motor broke. I put a 9x4.7 prop on it and it started to heat up and to smell. I use a 3c lipo. Whats the best prop for this motor? And also for the plane the listed prop size is a 10x6 with a 1400kv motor but in the specs for those motors the suggested prop sizes are all between 8x6 and 9x6? Which should i use? sorry I'm just getting into the hobby
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Old Aug 20, 2015, 07:15 PM
Doug Bartley is offline
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First, you should use std electric props, not slow fly types on these higher kv motors. The SF typically pull more amps than std E props.The slowfly prop is designed for slower speeds, far below the rpms this motor is capable.
If the motor suggest 8/6 and 9/6 they likely intend 9/6 on 2S and 8/6 on 3S. The plane mfg suggestion of 10/6 means nothing to the motor, except you need to look for a motor capable of handling a 10/6 on 3S . If you need or want to run a 10/6, you need a larger motor, something like a 2836-1100. fwiw Doug B
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Old Aug 20, 2015, 07:38 PM
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but does it matter if i use a 8x6 prop with my 1400kv motor on a plane with a 40inch wing span? like for the plane it says that it needs a 10x6 prop but can i still use a smaller one (8x6) since thats what the motor requires?
This is the prop i have. is this one good? https://www.hobbyking.com/hobbyking/...propeller.html
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Old Aug 21, 2015, 07:38 AM
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An itch?. Scratch build.
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Have a look at this chart --

http://www.flybrushless.com/motor/view/27
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Old Aug 21, 2015, 09:47 AM
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Thank you so much guys!!!!
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Old Aug 21, 2015, 11:01 AM
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Get a way to measure your power consumption. ... wattmeter. 3 watts per gram of motor is a standard rule of tbumb.

If your motor is 30 grams, it should be happy at 90 watts or so.
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Old Aug 22, 2015, 09:37 AM
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It is almost impossible to know if a system will have enough "power" to fly any given plane unless you can test the output of the motor.
I choose a motor by first selecting a prop size I want to use, suitable to the plane size. The I estimate the all up weight of the plane including battery. For normal flight I use 100w/lb of plane. If your plane weighed 2 lbs, you'd require 200watts. Fot acrobatic flight, you need 125-150w/lb, for 3D you need 200w/lb or more. For a speed plane, 200w/lb or more.
Once you have a "WAtt" number you want to achieve, you can look for a motor that can deliver the wattage wanted, spinning the desired prop, on the battery cell count you wish to use. Once all the pieces are selected, you need to confirm everything with a Wattmeter and stay within the limits prescribed by Mfg's for Esc and Motors. Hope that helps Doug B
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Old Aug 23, 2015, 08:04 AM
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Quote:
Originally Posted by rcbuilder65 View Post
Hi,
I have bought a Suppo Brushless Motor A2208/14 1450kv for m scratch build trainer airplane because by other motor broke. I put a 9x4.7 prop on it and it started to heat up and to smell. I use a 3c lipo. Whats the best prop for this motor? And also for the plane the listed prop size is a 10x6 with a 1400kv motor but in the specs for those motors the suggested prop sizes are all between 8x6 and 9x6? Which should i use? sorry I'm just getting into the hobby
That is a 36g motor. Applying the 3W per gram rule of thumb to it says it should be able to handle a prop load that makes the input power about 108W (3 x 36 = 108) when running at it's continuous rating.

You don't say what brand prop you used but if you go to flybrushless.com and look at the data for the APC SF 9 x 4.7 you can see how much power that prop needs:

APC SF 9x4.7 - www.flybrushless.com/prop/view/76

That shows that the APC SF 9 x 4.7 has an input of around 100W at 7,500 RPM and it get's up to 165W at 9,000 RPM. So if you were running it at full throttle or close to full throttle that would explain why the motor over heated.

The GWS RS or Reduction Series slow fly props are thinner and lighter than the APC props (and also not as strong) and they draw less power and have a lower RPM limit. If you look at that prop:

GWS RS 9x4.7 - www.flybrushless.com/prop/view/90

The GWS RS 9x4.7 prop it has an input power of about 90W at 6,500 RPM and gets up around 100W at 8,000 RPM. So it would put a lighter load on the motor and may work for you.

There will be an obvious difference in the performance between those two props, the APC will do much better than the GWS and need more power too.

You can also look at the thrust that you are getting and get a feel for whether it is enough to fly your plane. A trainer will typically fly nicely with a thrust to weight ratio of 0.33:1 so if you plane's is around three time the thrust readings seen there you should be OK.

For better performance and as you acquire more flying skills, if you get the thrust to weight ratio up to 0.5:1 that will satisfy most sports flying basic aerobatics needs.

Jack
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