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Old Apr 30, 2011, 06:06 AM
Rhys - Forever Learning
United Kingdom, Wales, Nelson
Joined Feb 2007
360 Posts
Help!
How to choose a motor?

Hello all,

Right, I'ev been reading a few things about the thousands of different motors out there and their naming conventions. I just can't get my head around it.

How do you choose the most suitable, or a range of suitable motors, for any given model?

I'm starting to build an Ultron 3D Depron model that I have plans for, and just can't figure out what motor I need. Living in the UK I'm somewhat limited compared to the States.

Can someone please help?
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Old Apr 30, 2011, 07:16 AM
Registered User
Staffs, UK
Joined Nov 2003
10,258 Posts
Basically you need an idea of the finished weight then start with the power loading - watts per pound. For 3D you'll need at least 150W / lb model weight. You probably also want to run a large diameter low pitch prop so a fairly low Kv will be better than a high Kv. That's probably the two most important factors sorted out.

After that it's mainly down to deciding how much you want to spend. For "as little as possible" try Giantcod. For somewhat higher quality (at a cost) look for the well known names e.g. AXI, Hacker etc.

Steve
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Old Apr 30, 2011, 07:25 AM
Registered User
Joined Jan 2009
7,643 Posts
Check out WebOCalc. Here's the tutorial (click on 'Software'.) It's a free power-system calculator for electric RC planes.

The most important attributes of motors are: current/power limits, speed constant (Kv), and physical size. After a while you develop a feel for appropriate motor/prop combinations. And generally, when you buy or shop for a plane, the seller will recommend a power system -- so start there to find the appropriate numbers.

Couple of rules of thumb lead to an interesting result...
* 100 watts per pound of all-up flying weight
* a good modern brushless motor can produce about 100 watts per ounce of motor mass

Given these two (admittedly fungible and arguable) "rules" you end up with 1 ounce of motor per pound of plane. Quite typically, the battery will weigh considerably more than that, so it's not uncommon to have the power system account for 30% of the flying weight.
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Old Apr 30, 2011, 11:35 AM
Falling with style!
Rick Ludtke's Avatar
USA, WA, Freeland
Joined Nov 2010
223 Posts
I cheat, but so far it has worked well for me:

I ingore all of the watts, kv's, etc, and focus upon thrust to weight ratio. To do this you need a motor system provider that can provide thrust measurments for the power system, and you need to know your airplanes weight. Add to your measured weight, the weights for the power system components you are interested in, to arrive at the all-up flying weight estimate for your airplane. Compare this total weight to the thrust value provided for the power system. I target around a 1:1 thrust to weight ratio in a motor, prop, battery combination that will not hurt itself. 1:1 helps ensure that you are not underpowered, and allows you more throttle room to compensate for the models lack of scale inertia. Most of the time I fly at power settings well below full, for scale-ness.

For 3-D flying, something I haven't done, you probably need more than 1:1 power to weight.

I find that this simplifies the equation. If you know how much your model weighs, you can target an appropriate thrust value to suit your needs, and yet be conservative enough to protect from coming up short on power. Nobody wants to fly an underpowered airplane.

A great resource for power systems and other components with very affordable prices is HeadsUp RC http://www.headsuprc.com/servlet/StoreFront.. HeadsUp offers real test performance data for their motors, that allow you to select system component combinations based upon actual thrust output. They have great prices, and can ship affordably across the country in as few as a couple of days.

Rick
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Old Apr 30, 2011, 08:54 PM
Culper Junior
eastern pa
Joined Feb 2007
2,175 Posts
Ditto on the HeadsUp r/c site. Check out all his pages on motor information, Everything is written in laymans easy to understand language.

Also, here's a tutorial on choosing a power system. Note it is 4 years old, so some info might be getting stale, but the basic ideas are still usable. Don't get too involved with it, just use what you need and go back later for more info when you want it.
http://www.rcgroups.com/forums/showthread.php?t=739069

But definitely check out the HeadsUp r/c site.
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Last edited by aeronca52; Apr 30, 2011 at 09:01 PM.
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Old May 01, 2011, 02:49 AM
Registered User
Staffs, UK
Joined Nov 2003
10,258 Posts
Unfortunately Headsup might be nice people but as they don't ship overseas they aren't much use to those of us who live in the UK....like the OP .

Steve
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Old May 01, 2011, 09:31 AM
Lost but making good time
Joined Nov 2008
1,345 Posts
rhysduk,

Try the following:

www.brchobbies.co.uk
www.westlondonmodels.com
www.electricwingman.com
www.allelectricrc.co.uk

Also, go to the SEARCH area and look up noflyzone's excellent tutorial on choosing motors and props for an airplane. Here it is.

http://www.rcgroups.com/forums/showt...ight=noflyzone

Regards,

Hankg
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Old May 03, 2011, 03:18 PM
Culper Junior
eastern pa
Joined Feb 2007
2,175 Posts
Quote:
Originally Posted by slipstick View Post
Unfortunately Headsup might be nice people but as they don't ship overseas they aren't much use to those of us who live in the UK....like the OP .

Steve

Sorry Steve,
Not needing that information it never occurred to me to seek it out.
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Old May 03, 2011, 08:18 PM
Reduce the drama...
rick.benjamin's Avatar
USA, OR, Damascus
Joined Apr 2004
4,043 Posts
How do you convert a speed 380 to brushless?
Here's the thread.
http://www.rcgroups.com/forums/showt...0#post18137456

I looked at motor weight, prop size, shaft size, volts in, watts in.
A while ago, I made a database of motor parameters.
Listed shaft size, weight, KV, amps, watts, volts in, vendor.
I can sort for whichever parameter looks important.
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Old May 04, 2011, 02:49 AM
I'm flying ! (run for cover)
dtrip's Avatar
Greece, South Aegean, Thira
Joined Nov 2004
727 Posts
The best way I have found is to use the Watt power of the motor.

As this is often not mentioned in the specifications, you can calculate Watts as a product of Volts x Amps.

For example if a motor mentions 15 Amps with a specific propeller at 3S LiPo (3S means 11.1V), this yields 15 x 11.1 = 166.5 Watts power. This can easily fly a 1.5 pount airplane. You should have at least 100 Watts per pound of airplane for good performance. Less will fly it, but it will need a lot of distance to taxi on the ground or a very good toss, which makes take off more tricky.

The prop used changes the Watts, but not greatly (if it reduces the Watts too much, then it is not the good size and you should get a bigger one, if it increases the Watts too much it will burn the motor). So stick with the props that the specifications mention.
If the mention only slow fly, you can decrease-increase to get roughly same power, e.g. you can replace a 10x5 prop with a 9x7.

For example for a 1 pound slow fly you can get a motor that works at 12 Amps with a 9x4 slow fly propeller, since this yields 12 x 11.1 = 133.2 Watts, more than enough, it will fly Ok.
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