Motor / ESC / prop combo for 5.5lbs plane - RC Groups
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Feb 11, 2012, 04:05 PM
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metalfred's Avatar

Motor / ESC / prop combo for 5.5lbs plane

Hello, I am scratchbuilding my first plane and I need help with its power system. AUW estimation is about 5.5lbs, I would like to use 2 x 4S 3300mah bats in parallel, 11x7 3-blade prop, NTM 35-42 1000kv motor and a 80 Plush ESC.

I am trying different combinations on this site.

Is 2816g of static thrust enough for 5.5lbs AUW ?

Any help is appreciated.
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Feb 11, 2012, 06:07 PM
Frequent Flyer
whitecrest's Avatar
Given an average sport airframe, 6.2 pounds is plenty of thrust for a 5.5 pound plane. You may not need such a large battery for sport flying.

Here's a great calculator for problems like this:

Just input your plane's vital statistics. The "Help" button leads to an excellent tutorial. Good luck with your project!
Feb 11, 2012, 06:12 PM
Registered User
Doug Bartley's Avatar
2816 gr = 99oz thrust = 6.2 lbs. My concern here is the motors ability to handle the 11/7/3 on 4S. Check loads with a wattmeter!
A 3542/1000 Turnigy (not NTB) is good for 400watts on 3S with a 12/6/2, and no more than 550 ( I think thats pushing it). FWIW Doug B
Feb 11, 2012, 07:22 PM
Registered User
Dr Kiwi's Avatar
Doug is right... 11x7x3 on 4s will fry that motor very fast... at 141g, the 3W/g "rule of thumb" says 420W max... one of the HK reviewers measured 440W with a 12x6 APC E on just 3s!!!!!!
Feb 11, 2012, 11:07 PM
Registered User
Doug Bartley's Avatar
I tested the Turnigy 3542/1000 with 12/6 Apc using a 2200/3S/25C Flightmax.
My wattmeter showed 430 watts in, a 30 second run made the motor luke warm on the bench. While it likely could handle a little more in the air, I left it alone at that reading. FWIW Doug B
Feb 12, 2012, 09:00 AM
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metalfred's Avatar
This is the motor.

It is supposed to be a 700 watts motor?!
Feb 12, 2012, 09:21 AM
Registered User
Dr Kiwi's Avatar
It is supposed to be a 700 watts motor?!

Over-rating of cheap motors from HK is a way of 146g the 3W/g "rule of thumb" says about 440W is closer to what the motor should be asked to deal with. If you push 700W into it you may well be at <60% efficiency, so where does that lost 300W go... into waste heat, cooking your motor.

Just for fun I fudged numbers for an AXI of similar size and Kv and loaded it with ~700W input (way beyond its rating)... even then, it would still run at 75% efficiency, so it would have to dissipate only half the amount of waste heat your NTM would have to.

This AXI is more conservatively rated at only 37A max.(~400W-500W)!
Last edited by Dr Kiwi; Feb 12, 2012 at 09:38 AM.
Feb 12, 2012, 12:01 PM
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metalfred's Avatar
Ok thanks for your help, I really appreciate. I still have lots to learn in this field!

The only other motor I have here is a Hacker A30-10L v2 not sure how it would behave with that setup.

Basically I should buy a motor of a reputable brand for this bird.

What do you think of this 3S setup.

82% efficienty, 424watts, 10x6 2-blades, 2285g of static thrust.
Last edited by metalfred; Feb 12, 2012 at 12:20 PM.
Feb 12, 2012, 12:21 PM
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metalfred's Avatar
5lbs of thrust for a 5.5 lbs plane is a bit low, no?
Feb 12, 2012, 12:32 PM
Registered User
Dr Kiwi's Avatar
5lb should be plenty unless you intend to go vertical or hover! For gentle flying you may get away with as little as 1lb thrust per 2-3lb of plane.

Here is a passage from Jeff at headsuprc:

How do I choose an outrunner motor for my plane?

The 'watts per pound' method of choosing a motor is often used, but it's a little complex and confusing to some people. I prefer a simpler method - 'thrust to weight' ratio. For this method, you need to know two things: the actual amount of thrust a motor will produce, and the estimated weight of the plane. I provide thrust data for all of the motors that I sell, and the all up weight of a model is usually fairly easy to determine. Most models will fly with about a 50% thrust to weight ratio, so a 20 oz model would need about 10 oz of thrust to fly at a very modest level of performance. I prefer at least an 80% thrust to weight ratio, so a 20 oz plane would require 16 oz of thrust, and would fly rather nicely. Even better is a 100% thrust to weight ratio, and this is what I recommend for trainer type planes. You want plenty of thrust to get you out of trouble, and a low pitch prop to keep the speed down. For unlimited vertical performance and 3D flying, I suggest at least a 120% thrust to weight ratio, so you would want at least 24 oz of thrust with a 20 oz plane. This high level of performance is easy to achieve with todays brushless motors and Lipo batteries. When choosing your motor, keep in mind that outrunners that spin larger propellers are generally more efficient.

Jeff is a pretty experienced RC guy.
Last edited by Dr Kiwi; Feb 12, 2012 at 12:39 PM.
Feb 12, 2012, 04:20 PM
Falling with style
metalfred's Avatar
Awesome that setup will work well then Thanks a lot for the help !!!!!!!

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