Power Systems Talk about motors, ESC speed controllers, gear drives, propellers, power system simulators and all power system related topics

Watts per ounce of motor weight?

Old 12-01-2010, 03:00 PM
  #1  
av8djc
Old Fart
Thread Starter
 
av8djc's Avatar
 
Join Date: Jun 2009
Location: WA and AZ
Posts: 240
Default Watts per ounce of motor weight?

Somewhere in my browsing recently I read a rule of thumb for the max wattage one should take from a motor. I know I wrote it down because of my grey hair syndrome but I think it got buried among my massive stack of notes to self. Any help would be appreciated. I'm now two years into my electric experiences and am now working with motors up to 2000w as I convert all my glow to electric. Still learning something new every day
av8djc is offline  
Old 12-01-2010, 05:03 PM
  #2  
Turner
Super Contributor
 
Turner's Avatar
 
Join Date: Sep 2009
Location: Tallahassee, FL
Posts: 1,533
Default

Saw this recently:

http://www.wattflyer.com/forums/show...58&postcount=2
Turner is offline  
Old 12-01-2010, 05:10 PM
  #3  
flydiver
Super Contributor
 
flydiver's Avatar
 
Join Date: Jan 2007
Location: Seattle, WA
Posts: 2,668
Default

This what you are after?

Rough rule of thumb #1: 4-7 watts per ounce of aircraft weight will fly a trainer adequately. 10-15 watts per ounce will fly a 3D aerobatic plane, sport plane or scale warbird. 15-20 watts per ounce will fly a fast prop jet or flying wing. Speed/power freaks will naturally want more. Both low-speed thrust and high top speed require power. All else being equal, increasing speed requires more power than increasing thrust.
• Rough rule of thumb #2: a brushless motor can handle 80-100 watts per ounce of motor weight. High quality motors are at the high end and cheap motors are at the low end.
flydiver is offline  
Old 12-01-2010, 05:24 PM
  #4  
philipa_240sx
Super Contributor
 
Join Date: Jul 2009
Location: Ontario, Canada
Posts: 1,046
Default

What flydiver said...

Most motors can comfortably run at 3g/W or 0.01oz/W

Or to put it another way....

30g per 100W

or

1oz per 100W

This assumes you have adequate cooling. Results may vary.
philipa_240sx is offline  
Old 12-01-2010, 10:39 PM
  #5  
flyboyanderson
Member
 
Join Date: Oct 2005
Location: West Grove Pa
Posts: 51
Default

I believe it works this way:

> typical .45 size plane weighs about 6lbs
> If you want aerobatic performance figure 150 watts per lbs.
> 6lbs X 150 watts = 900 watts of power required
> Divide 900 watts by 3watts/gram of motor (900 watts /3 grams = 300 gram motor.

Therefore an average .45 ARF requires a 300 gram motor.

Checked against an Eflite Power 46 Brushless Outrunner Motor, 670Kv and sure enough it weights 290 grams. Pretty darn close. I think you will find this works well against all the Eflite motors.

Now the KV comes into play when figuring out what diameter prop you can swing. Which on a typical .45 size ARF might well be the prop clearance. Lower KV bigger prop; Higher KV smaller prop.

This, of course, is simplistic but it gets you into the ball park of what you need. You still need to make similar "Glow" decisions like quality, performance, reliability, etc across the entire power system (Motor, ESC, Battery).

However, I won't buy a plain bearing OS engine if I can buy a quality ball bearing engine for the same price. still have to do your homework.
flyboyanderson is offline  
Old 12-01-2010, 10:40 PM
  #6  
Dr Kiwi
Super Contributor
 
Join Date: Aug 2005
Posts: 1,726
Default

Originally Posted by philipa_240sx View Post
What flydiver said...

Most motors can comfortably run at 3g/W or 0.01oz/W

Or to put it another way....

30g per 100W

or

1oz per 100W

This assumes you have adequate cooling. Results may vary.
Some crazy math/units here! Input (W) for motor weight:

3W/g is the typical "rule of thumb" [or 85W/oz]
Dr Kiwi is offline  
Old 12-02-2010, 01:54 AM
  #7  
av8djc
Old Fart
Thread Starter
 
av8djc's Avatar
 
Join Date: Jun 2009
Location: WA and AZ
Posts: 240
Default

Yes to all for the kind answers. That was what I was looking for. 3w per Gram. Maybe I'll tattoo that to my..well, put it on a post it on the computer monitor for a while
av8djc is offline  
Old 12-02-2010, 12:41 PM
  #8  
philipa_240sx
Super Contributor
 
Join Date: Jul 2009
Location: Ontario, Canada
Posts: 1,046
Default

Originally Posted by Dr Kiwi View Post
Some crazy math/units here! Input (W) for motor weight:

3W/g is the typical "rule of thumb" [or 85W/oz]
Yeah, I got the math wrong. Thanks Kiwi
philipa_240sx is offline  
Old 12-06-2010, 06:04 AM
  #9  
z-8
Banned
 
Join Date: Oct 2010
Posts: 422
Default

Originally Posted by Dr Kiwi View Post
Some crazy math/units here! Input (W) for motor weight:

3W/g is the typical "rule of thumb" [or 85W/oz]
Motor weight is obviously meaningless, so disregard any rule of thumb posted above. Motors have all sorts of different designs, so the resulting total weight even for the same coil and magnet size can vary by a huge amount.

More importantly, motor efficiency is bad, not good for performance in aviation applications. Efficiency comes at the expense of performance. You want the least efficient motor that still flies, not the most efficient. The reason is obvious, the harder a motor has to work per unit weight, the higher the thrust to weight ratio and the better your performance. The less hard a motor has to work, the more of a relative pig it is when installed that particular airplane.

For example, the 2.2 oz Super Tigre .10 (steel bell without any rear casing, not traditional chunky aluminum) weighs exactly 50% much as the e-Flight Power 10, yet it generates exactly 89% of the Power 10's RPM using the same prop. It weighs 58% as much as the Turnigy 35-36c, and generates 94% of the RPM using the same prop.

The most efficient motor that still flies, by definition, would weigh so much that the plane could never climb out of ground effect. Efficiency is your enemy, not your friend.

You want effective, not efficient. Or, the absolute minimum amount of efficiency that still works for your particular application.

Same thrust output: 32,000 lbs
Inefficient: GE F110, 8:1 Thrust to Weight Ratio
Efficient: CFM-56, 4:1 Thrust to Weight Ratio
There is a reason the F-16 doesn't choose the way too big and heavy, efficient motor.

10x7 Prop
Power 10, 4.4 oz, 10:1 Thrust to Weight Ratio
Turnigy 35-36, 3.76 oz: 11:1 Thrust to Weight Ratio
Super Tigre .10, 2.2 oz: 18:1 Thrust to Weight Ratio

Rule of thumb
If you reverse the recommendations of toy engine testers without an aviation background, you'll generally get the correct solution. Good aviation design is neither simple nor intuitive.
Attached Thumbnails Click image for larger version

Name:	All.jpg
Views:	472
Size:	81.5 KB
ID:	140314  
z-8 is offline  
Old 12-06-2010, 04:07 PM
  #10  
rcers
Super Contributor
 
rcers's Avatar
 
Join Date: Jul 2005
Location: Trophy Club TX
Posts: 6,314
Default

Originally Posted by z-8 View Post
Motor weight is obviously meaningless, so disregard any rule of thumb posted above. Motors have all sorts of different designs, so the resulting total weight even for the same coil and magnet size can vary by a huge amount.
Rule of thumb does not mean 100%. As a general rule, that formula works VERY well. But you are right some lighter motors made with other materials may do better than that.

And to be honest I have not seen anyone as impressed as you about the ST .10 motor.

Mike
rcers is offline  
Old 12-07-2010, 12:22 AM
  #11  
Turner
Super Contributor
 
Turner's Avatar
 
Join Date: Sep 2009
Location: Tallahassee, FL
Posts: 1,533
Default

Z8, you are either yanking everyones chain or I don't think you understand the word efficiency or how it applies to electric motors. A motor with higher efficiency will have a higher usable power output for a given amount of electrical energy than one with less efficiency. Yes, more efficient motors are desirable unless that efficiency comes at too great a cost to some other important criteria.
Turner is offline  
Old 12-07-2010, 08:13 AM
  #12  
JetPlaneFlyer
Super Contributor
 
JetPlaneFlyer's Avatar
 
Join Date: Mar 2009
Location: Aberdeen, Scotland
Posts: 6,121
Default

Originally Posted by z-8 View Post
More importantly, motor efficiency is bad, not good for performance in aviation applications.
What complete non-sense.. Since when was having a more fuel efficient motor/engine bad for any application? Providing efficiency can be gained without impacting other motor specifications (like weight, size, reliability etc.) then efficiency is always a good thing.

The F16 example you quoted is meaningless and quite inaccurate.. Actually both engines have much closer thrust/weight ratios. If you look variants of both engines that have the highest specific thrust/weight ratio, the difference is minimal. In any case the two engines are totally different designed for totally different applications. One is a high bypass ratio turbofan intended for subsonic airliners, the other an afterburning low bypass type intended for supersonic flight. True mechanical efficiency of either engine is hard to calculate because thrust is force, not power. Calculating the output power of a turbojet is not straightforward.

As for the 1oz per 100W 'rule of thumb'.. This is a general guide to roughly what maximum power a motor is capable of safely tolerating. Your tests running the same size prop prove absolutely nothing as to what the motors are ultimately capable of. If you want to test this 'rule of thumb' then you would have to run the motors at higher and higher power until each failed. The point just before failure would indicate the maximum power the motor was capable of.

Steve

Last edited by JetPlaneFlyer; 12-07-2010 at 08:58 AM.
JetPlaneFlyer is offline  
Old 12-07-2010, 06:00 PM
  #13  
kyleservicetech
Super Contributor
 
kyleservicetech's Avatar
 
Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 8,952
Default

Originally Posted by JetPlaneFlyer View Post
As for the 1oz per 100W 'rule of thumb'.. This is a general guide to roughly what maximum power a motor is capable of safely tolerating. Your tests running the same size prop prove absolutely nothing as to what the motors are ultimately capable of. If you want to test this 'rule of thumb' then you would have to run the motors at higher and higher power until each failed. The point just before failure would indicate the maximum power the motor was capable of.

Steve
Problem is, these electric motors we use in our models are rather "Dumb". Any of our motors can easily put out several times their "rated" watts. At least for a very short time. But, in the process, the motors windings may overheat, cooking the insulation on the windings, leading to eventual failure. This could also de-magnetize the magnets on the cheaper motors.

Some of those "inexpensive" imported motors are running some 150 watts per ounce of motor or even more. I've got a Great Planes Edge 540T model. This is a direct quote from the manual on their suggested motor. "The motor gets hot. Do not touch it during or after operation". Motocalc suggests this motors windings will be running near 400 degrees during a flight.

Something else, as the motor is pushed further and further into severe overload, the motors efficiency usually takes a severe nose dive, resulting in even more heat loss in the motor and its windings.

This efficiency thing is easily checked by many computer programs such as www.motocalc.com. Taking one of my Hacker A50 motors, and running it at 1500 watts, the motor efficiency drops from near 90% to about 50- 60%. That results in a whopping 600 watts of power in direct heat loss inside the motor itself. Even a very good quality motor will have problems with that.

Last edited by kyleservicetech; 12-07-2010 at 06:17 PM.
kyleservicetech is offline  
Old 12-07-2010, 06:03 PM
  #14  
kyleservicetech
Super Contributor
 
kyleservicetech's Avatar
 
Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 8,952
Default

???Duplicated Response, deleted by kyleservicetech
kyleservicetech is offline  
Old 12-08-2010, 04:05 AM
  #15  
Prof100
Member
 
Join Date: Jun 2007
Posts: 847
Default

Originally Posted by Turner View Post
Z8, you are either yanking everyones chain or I don't think you understand the word efficiency or how it applies to electric motors. A motor with higher efficiency will have a higher usable power output for a given amount of electrical energy than one with less efficiency. Yes, more efficient motors are desirable unless that efficiency comes at too great a cost to some other important criteria.
Just ignore him. The posts are just to provoke. This guy did the same thing on Rcgroups and got banned.
Prof100 is offline  
Old 12-08-2010, 04:44 AM
  #16  
kyleservicetech
Super Contributor
 
kyleservicetech's Avatar
 
Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 8,952
Default

Originally Posted by Dr Kiwi View Post
Steve, Dennis and Turner,

You need to understand that the delusional Z-8 seems to imagine that he is the guy in the Dos Equis ads..... "The most interesting man in the world".

From his own pontificating (though none of it documented) he has been chief test pilot to most of the aircraft in the US arsenal, he has advanced degrees in Aeronautical Engineering..... and one assumes he has a Nobel Prize or two.... yet he spends his vacuous life dispensing dangerous/false information, and decrying the efforts of scientifically trained, experienced and knowledgeable RC guys who provide helpful, realistic and testable information to fellow RC enthusiasts.
Reminds me of an Electronics Engineer we had years ago that had 10 patents to his name, but NEVER designed anything that was manufacturable. And his designs that did get manufactured eventually got recalled. 100%. Management kept promoting him, to get him out of their division. (I had to repair the crap he designed, and explain to our customers how this stuff ever made it through production tests. He also designed the production tests.)

Same guy was overheard in the men's restroom, talking to himself as usual, "SOB, I got my underwear on backwards. Again" ???Again???.
kyleservicetech is offline  
Old 12-08-2010, 07:27 AM
  #17  
ministeve2003
Class Clown
 
ministeve2003's Avatar
 
Join Date: Sep 2007
Location: Montclair, California
Posts: 4,472
Default

yup, 100w is a good average... as someone stated, we tend to run some motors much higher like 150w, it depends on the application... such as 3d flying... where good airflow and short time at full throttle will be used...

other factors come into play with them too such as Thrust and Pitch speed,

such as a 3g/w high KV motor setup might have a 90mph pitch, and 1:1 thrust

and a 5g/w Low KV motor setup might be 30mph, and 3:1 thrust,

also... Good Cooling makes alot of difference, you can have the best motor aound, but propped wrong, with lots of heat, and no cooling, and it will still just be a garbage setup...

some other factors for runing them at 150w/oz that most people won't admit... might include "Cost effective Longevity"... or... "is it going to last long enough (in terms of amount of fun I get from it) to pay for the abuse I'm gonna do to it" (cause when I'm done it's going in the trash can...)

SK
ministeve2003 is offline  
Old 12-10-2010, 04:38 AM
  #18  
z-8
Banned
 
Join Date: Oct 2010
Posts: 422
Default

Originally Posted by Dr Kiwi View Post
Steve, Dennis and Turner,

You need to understand that the delusional Z-8 seems to imagine that he is the guy in the Dos Equis ads.....
Wow you sure know how to ruin a forum.

What a disgrace to WattFlyer.

Last edited by z-8; 12-10-2010 at 04:59 AM.
z-8 is offline  
Old 12-10-2010, 04:49 AM
  #19  
Bub Steve
Super Contributor
 
Bub Steve's Avatar
 
Join Date: Aug 2006
Location: Shadeville Fl,
Posts: 7,189
Default

Easy my Bub's,, we don't need to get the Mod's in,
Talk tec,, Not Trash,, we all get peved sometime, let's agree to Disagree, bubsteve
Bub Steve is offline  
Old 12-10-2010, 04:55 AM
  #20  
z-8
Banned
 
Join Date: Oct 2010
Posts: 422
Default

Originally Posted by ministeve2003 View Post
yup, 100w is a good average... as someone stated, we tend to run some motors much higher like 150w, it depends on the application... such as 3d flying... where good airflow and short time at full throttle will be used...

other factors come into play with them too such as Thrust and Pitch speed,

such as a 3g/w high KV motor setup might have a 90mph pitch, and 1:1 thrust

and a 5g/w Low KV motor setup might be 30mph, and 3:1 thrust,

also... Good Cooling makes alot of difference, you can have the best motor aound, but propped wrong, with lots of heat, and no cooling, and it will still just be a garbage setup...

some other factors for runing them at 150w/oz that most people won't admit... might include "Cost effective Longevity"... or... "is it going to last long enough (in terms of amount of fun I get from it) to pay for the abuse I'm gonna do to it" (cause when I'm done it's going in the trash can...)

SK
The problem with silly rules of thumb is that motors actually have different designs. A super heavy casing (ala Turnigy and Power XX series) obviously buys you nothing in terms of aircraft performance, even if it may make the motor physically more resistant to crashes.

If the weight is added to the actual working components of the motor (i.e not the case, mount, etc.), it might give you more thrust, or it might not. If it doesn't, then the motor could be more efficient, but of course that benefit pulls directly from potential performance. One can't get something for nothing. The most efficient electrical engine, that still flies in a particular aircraft will, by definition, barely be able to haul its own weight aloft.

Edited by Mod--is that for every additional unit of weight you pile on an airframe, 6 units of additional weight result (more weight means more drag means a bigger motor means more fuel means larger tanks means more lift means larger wings means more drag means ...and so on and so on, integrated over the trade space). So lighter motors with similar thrust (= less efficient) are almost always a vastly better choice for aviation applications.

Last edited by rcers; 12-10-2010 at 11:34 PM. Reason: Personal attack
z-8 is offline  
Old 12-10-2010, 05:16 AM
  #21  
z-8
Banned
 
Join Date: Oct 2010
Posts: 422
Default

Originally Posted by JetPlaneFlyer View Post
What complete non-sense.. Since when was having a more fuel efficient motor/engine bad for any application? Providing efficiency can be gained without impacting other motor specifications (like weight, size, reliability etc.) then efficiency is always a good thing.

The F16 example you quoted is meaningless and quite inaccurate.. Actually both engines have much closer thrust/weight ratios. If you look variants of both engines that have the highest specific thrust/weight ratio, the difference is minimal. In any case the two engines are totally different designed for totally different applications. One is a high bypass ratio turbofan intended for subsonic airliners, the other an afterburning low bypass type intended for supersonic flight. True mechanical efficiency of either engine is hard to calculate because thrust is force, not power. Calculating the output power of a turbojet is not straightforward.

As for the 1oz per 100W 'rule of thumb'.. This is a general guide to roughly what maximum power a motor is capable of safely tolerating. Your tests running the same size prop prove absolutely nothing as to what the motors are ultimately capable of. If you want to test this 'rule of thumb' then you would have to run the motors at higher and higher power until each failed. The point just before failure would indicate the maximum power the motor was capable of.

Steve
The T:W difference of the F110 and CFM-56 is 100% at the same thrust output, that is substantial. The manifest difference is one of purpose; one engine is designed for optimum efficiency, the other for optimum performance. You cannot have both.

If you test an electrical motor to the point just before failure it will be very inefficient in all cases. That is the point. You want an aviation engine to work reasonably hard, as opposed to being a heavy pig with little thrust, if, you want good aircraft performance.

Engine output, measured alone, is obviously irrelevant to performance--a Mack truck might sport 600 hp; a Lotus Elise only has a wimpy 200 hp. And aircraft add a whole other dimension to the total meaninglessness of engine-only testing.

Last edited by z-8; 12-10-2010 at 05:32 AM.
z-8 is offline  
Old 12-10-2010, 08:15 AM
  #22  
JetPlaneFlyer
Super Contributor
 
JetPlaneFlyer's Avatar
 
Join Date: Mar 2009
Location: Aberdeen, Scotland
Posts: 6,121
Default

Originally Posted by z-8 View Post
The T:W difference of the F110 and CFM-56 is 100% at the same thrust output, that is substantial. The manifest difference is one of purpose; one engine is designed for optimum efficiency, the other for optimum performance. You cannot have both.
I disagree. The real difference between the two engines is that one is designed to be optimised for subsonic flight (high bypass ratio) and the other optimised for supersonic flight (low bypass and afterburning).

Both engines are designed for maximum efficiency under their specific operating regime. Fuel efficiency is every bit as important for an F16 as it is for a Airbus 320. More fuel efficiency means ability to carry more weapons and/or increased operational radius.
JetPlaneFlyer is offline  
Old 12-10-2010, 08:30 AM
  #23  
JetPlaneFlyer
Super Contributor
 
JetPlaneFlyer's Avatar
 
Join Date: Mar 2009
Location: Aberdeen, Scotland
Posts: 6,121
Default

Originally Posted by z-8 View Post
.......So lighter motors with similar thrust (= less efficient)
Here's the nub of my disagreement with your 'reasoning'.

Efficiency is defined as the ratio: useful power output / total power input

So what does weight have to do with it Why does a light motor have to be less efficient than a heavy one?. Why would a motor that had a heavy case be more efficient than a motor with a light case (all other specifications being equal). If you have any form of engineering degree (or even a high school education) than I cant imagine how you could even have considered that weight would be directly linked to motor efficiency.

What if i take a light motor and glue some lead to it.. does it magically become more efficient

Steve
JetPlaneFlyer is offline  
Old 12-10-2010, 11:36 PM
  #24  
rcers
Super Contributor
 
rcers's Avatar
 
Join Date: Jul 2005
Location: Trophy Club TX
Posts: 6,314
Default

Guys, keep the posts on topic, or I will close the thread. Please all review the posting guidelines for WF do NOT harass or flame other members.

Thank you.

Mike
rcers is offline  
Old 12-11-2010, 05:10 AM
  #25  
Dr Kiwi
Super Contributor
 
Join Date: Aug 2005
Posts: 1,726
Default

Going right back to Post #1 the OP asked for a "rule of thumb". Indeed the one most of us use (3W/g) was promulgated initially by Louis Fourdan, a highly qualified (yes, "technical degrees") expert on electric vehicles and holder of several world records for solar powered vehicles. He also has a sophisticated lab for testing model aircraft motors, and conducted hundreds of scientifically valid/repeatable tests before deciding upon a "value" for his "rule of thumb".........I and others have simply followed Louis's lead since we assumed he had good reason for his suggested figures.

For Z-8 to suggest that "Motor weight is obviously meaningless, so disregard any rule of thumb posted above." .... followed by remarks indicating that everything I have ever posted is "100% wrong".... is disingenuous and less than useful!

If Z-8 cares to post documentation of his "technical degrees" or his professed "aeronautical engineering expertise" and "military test piloting experience"... then let him do so. Then we might listen..... or not!

Last edited by Dr Kiwi; 12-12-2010 at 01:57 PM.
Dr Kiwi is offline  

Quick Reply: Watts per ounce of motor weight?


Contact Us - Archive - Advertising - Cookie Policy - Privacy Statement - Terms of Service - Do Not Sell My Personal Information -

Copyright 2018 MH Sub I, LLC dba Internet Brands. All rights reserved. Use of this site indicates your consent to the Terms of Use.

Page generated in 0.11716 seconds with 14 queries