Feb 22, 2013, 09:45 PM Registered User Canada, BC, Vancouver Joined Oct 2011 119 Posts Discussion Hobby king outrunner sizing convention? Hi there, Just wondering what the sizing convention is for outrunner motors. What does 0.15 - 0.4 mean? Is that in like 1.5 cm to 4 cms? Thanks
 Feb 22, 2013, 09:55 PM Registered User Chattanooga, Tennessee, United States Joined May 2003 27,210 Posts In this case the .15 refers to glow equivalency.. the electric motor is supposedly capable of generating the same power as a .15 cu. in. glow motor (~2.5cc)... .4 is probably more correctly written as .40.... so .40 cu. in (~6.5cc). One "guesstimate of glow equivalency with electric" is to use watts = cubic inches x 2000... so a .15 glow motor might be replaced by a 300W-capable electric.... and a .40 by 800W. Sometimes aircraft are sized according the glow motor they might use.. a "40-size aircraft" seems to commonly used. Last edited by Dr Kiwi; Feb 22, 2013 at 10:01 PM.
 Feb 22, 2013, 11:26 PM Suspended Account United States, FL, Pompano Beach Joined Oct 2011 1,425 Posts I think when they refer to motors as an engine displacement equivalent they have wound it to spin as if it were an engine. Directly NOT taking advantage of why we use electric motors. The ability for an engine to make power is dependent on the burn time of the fuel. So it must turn "X"RPM to Make "X" amount of power. I think the convention is to make an airplane fly the same whether it uses an engine or an electric motor. Although they never will because fuel produces the same amount of power with the first drop of fuel as the last drop of fuel. The electric motor and battery power curve is down hill from the moment the throttle is opened.
 Mar 03, 2013, 04:53 PM Registered User Joined Mar 2013 97 Posts This is a bit confusing for me as well. HobbyKing typically will say something like this: Requires: 28-XX or 35-XX brushless outrunner motors But looking through their motors this naming convention is not used. Trying to figure it out myself am I guessing right in thinking that the first number is the diameter of the motors mounting plate and the second is the kv power? The more I look at the specs the more it confuses me? I hate being a newb!
 Mar 03, 2013, 05:51 PM Jack USA, ME, Ellsworth Joined May 2008 19,584 Posts It is fairly common to refer to motors by the diameter of the magnet housing and the length of the magnet housing and backplate, like 2836 or 3642 and the like. The length includes the back plate but not the shaft, and generally includes the small raised boss where the shaft emerges from the magnet housing. Those are generally the Length B (mm) and Diameter C (mm) dimensions shown in a figure on the pages for most motors at HK. Sometimes the length B will vary a couple of mm or so from a model name. But there are no rules for model naming conventions, for some motors the model name will refer to the diameter and length of the stator as that is the part of the motor that is most connotative the potential power of the motor. So then you learn that one company's 2211 motor is the same size and power as someone else's 2836. Jack
 Mar 03, 2013, 07:32 PM Registered User Chattanooga, Tennessee, United States Joined May 2003 27,210 Posts Requires: 28-XX or 35-XX brushless outrunner motors In this case the convention used will be, as Jack says, bell housing diameter and external length. Hence 2826 or 3530... But, equally, other companies may use stator diameter and depth.. so a 2826 is the same size as their 2212, and a 3530 is the same as their 2814. Some may add # of winding turns and/or Kv.. hence SunnySky X2212-9, or Turnigy SK3 3530-1340...... or just odd names: Grayson Supersonic v2... which is actually a 54g, 2212 stator [2826 external dimensions], 6-turn motor of ~2180Kv, rated for 14A-22A (28A burst).
 Mar 03, 2013, 07:46 PM 2012 NZ Speedcup - 231 MPH New Zealand, Tasman, Richmond Joined Mar 2006 2,396 Posts One good way I've found is to look at the motor weight: this is a good indication of the expected power potential. Of course not all motors are created equally, so you have to be cautious. Also look at the weight of the plane, and work out how much power you need based on what performance you are after. Forget the glow nomenclature - the sooner manufacturers drop that the better with regards to electrics, IMHO.
 Mar 03, 2013, 08:12 PM Jack USA, ME, Ellsworth Joined May 2008 19,584 Posts As DownUnderPilot says, the weight can eliminate a lot of mystery. There is a rule of thumb for average quality outrunners that they will handle an input power of 3 Watts per gram of the motor weight when running at full throttle and their continuous power rating (the load that does not eventually cause motor to overheat at full throttle). That rule works well for the motors from HK, better quality motors can raise the bar to 5 or 6 Watts per gram. Jack
Mar 04, 2013, 09:46 AM
Registered User
Joined Mar 2013
97 Posts
Quote:
 Originally Posted by jackerbes As DownUnderPilot says, the weight can eliminate a lot of mystery. There is a rule of thumb for average quality outrunners that they will handle an input power of 3 Watts per gram of the motor weight when running at full throttle and their continuous power rating (the load that does not eventually cause motor to overheat at full throttle). That rule works well for the motors from HK, better quality motors can raise the bar to 5 or 6 Watts per gram. Jack

So the following motor seems to fit in that equation (87g X 3 Watts = 261 Watts) Right at the max using a 4S LiPo:

Model: NTM Prop Drive Series 2836 750kv
Kv: 750rpm/v
Poles: 3
Motor Wind: 20T
Max current: 18A
Max Power: 165W @ 12v (3S) / 265W @ 15v (4S)
Shaft: 4mm
Weight: 87g
ESC: 20A
Cell count: 3s~4s Lipoly
Bolt holes: 16mm & 19mm
Connection: 3.5mm Bullet-connector

It's still difficult to choose the right motor looking at these specs especially since there is no prop data.

My TriCopter build is estimated at 1300g and the rule of thumb that I found for choosing a motor is: required thrust per motor = ( Aircraft weight x 2 ) / # motors (for Multirotor)

This formula for a tricopter puts the required thrust at 866g, How would I know this looking at motor specs?

Some of their motors actually show the thrust per prop as shown below:

Model: NTM Prop Drive Series 28-30A 800kv (short shaft version)
Kv: 800rpm/v
Max current: 20A
Max Power: 300W
Shaft: 3mm
Weight: 65g
ESC: 20~30A
Cell count: 3s~6s Lipoly
Bolt holes: 16mm & 19mm
Connection: 3.5mm Bullet-connector

Prop Tests:
8x4E - 22.2V / 310W / 13.9A / 1.11kg thrust
10x5E - 18.5V / 315W / 17.3A / 1,27kg thrust
11x7E - 14.8V / 260W / 17.8A / 1.05kg thrust
12x6E - 14.8V / 276W / 18.7A / 1.20kg thrust

Since both of these motors are similar, I'm assuming my first choice should produce similar results?
 Mar 06, 2013, 01:48 AM Registered User Canada, BC, Vancouver Joined Oct 2011 119 Posts Thanks for the replies all! From what I gathered from you guys, the naming convention seems to only be related to glow engine equivalents. My application isn't so much related to planes or helis or quads. My application is to basically put the electronics from a remote control car into a longboard which I would use as transportation. I'm trying to determine what kind of motor I should use. From what I've found online, it appears that many people use 2000 W of power to move their boards. Unfortunately, I want to keep costs low, and use what I have already therefore my aim is to have a 1000W system. The only constraint I have here are the batteries which are 3S 2200mAh 25c LIPO batteries. This must mean that I have about a safe maximum of 55 amp continuous current draw, and it must also mean that I have only 11.1V to work with. From what I gathered from these constraints, this is the motor I should be using: motor: http://www.hobbyking.com/hobbyking/s...dproduct=16092 esc: http://www.hobbyking.com/hobbyking/s...dproduct=10015 I could be using two of those motors with two ESCs and a couple LIPO batteries in parallel to power this system. Would this be a proper assessment, assuming 1000w is enough to carry me (80kg)?
 Mar 06, 2013, 07:03 AM Jack USA, ME, Ellsworth Joined May 2008 19,584 Posts The 25C rating on the battery is best considered to be a peak rating, not a continuous rating. I would consider it OK for maybe 15 to 30 seconds or so but not more. So if you can use a pair of those in parallel (making a 4400 mAH 3S2P pack), that will drop the load from 25C to 12.5C and give the batteries a better chance at being able to handle the load. When the load is applied the voltage will drop some and you want that to not get lower than 3.7-3.8V or so under load. And the pack temperature will be the real signal as to how the batteries are doing. The outside of the pack should not get more than warm, maybe 105-110F/43C or so at the most. When batteries get too hot they suffer unrecoverable damage. Beyond the battery, I'm not sure if 1000W will move you or not. It will all come down to how the power is transmitted from the motor to driving the board. I would think that some kind of planetary gearing would be needed or maybe a reduction drive with belt and pulleys. Do you have the details on that worked out? Jack
Mar 06, 2013, 01:48 PM
Registered User
Joined Oct 2011
119 Posts
Quote:
 Originally Posted by jackerbes The 25C rating on the battery is best considered to be a peak rating, not a continuous rating. I would consider it OK for maybe 15 to 30 seconds or so but not more. So if you can use a pair of those in parallel (making a 4400 mAH 3S2P pack), that will drop the load from 25C to 12.5C and give the batteries a better chance at being able to handle the load. When the load is applied the voltage will drop some and you want that to not get lower than 3.7-3.8V or so under load. And the pack temperature will be the real signal as to how the batteries are doing. The outside of the pack should not get more than warm, maybe 105-110F/43C or so at the most. When batteries get too hot they suffer unrecoverable damage. Beyond the battery, I'm not sure if 1000W will move you or not. It will all come down to how the power is transmitted from the motor to driving the board. I would think that some kind of planetary gearing would be needed or maybe a reduction drive with belt and pulleys. Do you have the details on that worked out? Jack
I have not worked out the details, but looking at the simplicity of a belt drive... this might be a better choice than compared to planetary gears. I'll have to work out the math to figure out if 1000W is acceptable for flat roads and small hills. For me, this system is just suppose to assist me; it is not suppose to replace the motions.

Here is a link to a place that has already done this: http://endless-sphere.com/forums/vie...p?f=35&t=47176