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Please help me understand link between "motor size" and motor characteristics

#1
Please help me figure out how to translate "motor size" into actual motor parameters.
For example. on this page: https://www.amainhobbies.com/flite-test-sea-duck-electric-airplane-kit-1422mm-flt-1040/p673704
it says that it need "(2) Brushless Motors: 425 Size, 1200kV Minimum"
What is "425 size" motor?
FT's store lists motors using different set of parameters and "motor size" is nowhere to found there:
https://store.flitetest.com/emax-gt2215-09-1180kv-brushless-motor-emx-mt-0409/p694266

So, when I see "required motor of XXX-size" how do I convert it to something what I can find?

Another part of this question is motor names. In several videos I heard motors being referred as "Blue Wonder" or something else, but again - what it is? I cannot find "Blue Wonder" in FT store..
 

d8veh

Well-known member
#4
Motor sizes and designations are an absolute mystery. There's two things that I do to choose a motor: Tell people what model you have and ask for a recommendation; look at the information that they provide on the motor's listing. what you're looking for is thrust with whatever battery and propeller or watts with whatever battery and propeller. Unfortunately, they often give vague information, like Watts 250w, battery 2s - 4s, but that doesn't help, in which case i move on to another motor.

It's a bit weird because you can get those tiny little green 1805 racing motors that give over 1kg of thrust and other ones four times the size and weight that only give 800g.
 

Merv

Well-known member
#5
What is "425 size" motor?
425 goes back to the early days of electric, the size of brushed electric motor. Now days with brushless we have a different system. Take a 2212 1400 kv motor for example. The 22 is the diameter of the inside of the can, 22mm. The 12 is the length of the inside of the can, 12mm. The 1400 is the RPM/volt of the motor. So a 1400kv will spin about 16,800 rpm on 12 volts (12x1400) and 22,400 on 16 volts.

Instead of trying to convert xxx size to the new standard, I find it useful to look at the watts (watts = amps x volts) the motors are rated at. Two motors of similar watts and kv rating will produce about the similar power and use a similar prop. Looking at the recommended prop will also give you a hint, two motors of similar kv and that use the same prop will produce similar power.
 
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Merv

Well-known member
#6
For example. on this page:
Lets look at the EMAX GT2215/09 1180kV you mentioned from FT. The chart said it will draw 26A on 3S with a 10x4.7 prop and 24A on a 10x6 prop. I guessing those amp numbers have been switched. So both props are 10 inch diameter, the difference is the pitch of the blade. In theory the 4.7 will go forward 4.7 inches with each revolution. So with the 10x4.7, the plane go slower than the 10x6. The 10x4.7 will have more pulling power at lower air speeds, like in a hover, than the 10x6.

An invaluable tool is a watt meter. I test any new size prop with a watt meter. At a given rpm, bigger diameter pulls more amps, higher pitch pulls more amps. Likewise with a given prop size, the faster is spins, pulls more amps. So you try different diameters and pitch's to find the one that will use all the amps with out going over the max rating. You don't' want to let the magic smoke out of the wires. If you change voltages (RPM's), you will need to test all over with different size props.

If you wanted to go faster, you might try a 9x8 prop, to see if it pulled to many amps. If you want to 3D go with the recomeded 11x3.8 prop.
 
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Hai-Lee

Old and Bold RC PILOT
#7
I still find the motor designations a little strange. It almost appears like the old Mode 1 Vs Mode 2 debate. Each has their supporters and both work quite well for their group.

Rather than having a brain hemorrhage trying to come to grips with each type of motor classification and the equivalents I just default to the power the plane needs to fly, (Watts). Normally around 200 Watts per pound. (More for high performance and lower for trainer and gliders etc).

Then I determine what dia prop I wish to swing. My last thing to spec is the battery voltage I wish to use. With these things decided I then look for a motor or the desired power that will swing the prop I am going to use at the speed I want it to turn. As a result I normally select fairly low Kv motors that swing large dia props. SF for those Stol, 3D, and cruising machines and high pitch props for those that I only want to see as a blur!

Over time if you design your own aircraft you will end up with a small range of preferred motors similar to the FT offerings and you will design around those.

Just what works for me!

Have fun!
 

Chuppster

Well-known member
#9
It'll take you a bit to get the feel for things. Really, what you need is a good power to weight ratio. You need the thrust to get off the ground. Here's a link that can give you an idea of how much power you need given the weight of your model:

https://www.rc-airplane-world.com/watts-per-pound.html

Now, not all motors of a given wattage give you the same thrust. This is a complicated thing to understand but basically it comes down to propeller size, which is related to the motor's KV. A high KV motor will swing a smaller prop than a low KV motor of the same size. A smaller prop will give you less static thrust but it will give you more speed. Generally 1100kv is a good balance between the two for a 3s or 4s setup.

Keeping that in mind, you have to know your flying style and AUW before you can choose the ideal motor for your setup. If flying 3d, try and get something with a bigger prop diameter. If flying a racer get something that can turn a smaller prop really fast. And never, ever be afraid to ask questions here!
 
#11
Chuppster, Merv - Thank you! I am not afraid to ask questions, and got great support. Glad to have all you knowledgeable and smart people around and ready to help! This site/forum/community (amount of knowledge and atmosphere) got me started with hobby. Thank you all!