Wattflyer RC Network: RC Universe :: RCU Magazine :: RCU Forums :: RCU Classifieds :: RCU User Reviews :: RCU YouTube
Home Who's Online Calendar Today's Posts RealTime Post Spy Mark Forums Read
Go Back   WattFlyer RC Electric Flight Forums - Discuss radio control eflight > R/C Electric Power - Batteries, Chargers, ESCs and More > Power Systems
Register Members List Wattflyer Extras Articles Search Today's Posts Mark Forums Read Social Groups

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

Thank you for your support (hide ads)
Reply
 
Thread Tools Display Modes
Old 02-10-2008, 04:44 AM   #1
sgf323
Member
 
Join Date: Sep 2007
Location: Omaha, NE
Posts: 317
View sgf323's Gallery12
Thanked 16 Times in 15 Posts
iTrader: (1)
Friends: (1)
Default Making sense of Watts, RPM, Amps and how to calculate thrust?

I've got an eLogger and I've been doing some playing around with it. With a nearly charged battery here are some readings I just got with my Park 450.

11x7E: 190 watts, 17 amps, 7800 rpm
10x7SF: 220 watts, 20 amps, 7300 rpm

The 20 amps is just above the max burst for the motor but, how would these two setups differ in the real world? Which would provide more thrust?

Since the 11x7E is pulling less amps the watts are lower but that doesn't mean it's a less powerful setup, right? Being that both are x7 props with the SF just having more prop area I'm assuming the 11x7E would be faster and provide more thrust since it is turning at a higher RPM. Is that right?

I though I read somewhere that you could calculate thrust based on several parameters, including RPM, but I can't find it anywhere. Anyone know how to do this?
sgf323 is offline  
  Reply With Quote
Old 02-10-2008, 05:17 AM   #2
Dr Kiwi
Super Contributor
 
Join Date: Aug 2005
Posts: 1,726
Thanked 180 Times in 174 Posts
Awards Showcase

Outstanding Contributor Award 
iTrader: (0)
Friends: (3)
Default

Here is a good way to get a ballpark estimate of thrust if you know your rpm: http://www.badcock.net/cgi-bin/powertrain/propconst.cgi

The other way is for me to check my spreadsheet with 8500 entries for motor/prop/rpm/thrust etc and match your rpm for each of those props.

Back in a minute!

11x7 APC E: My measurement from my thrust stand: 7770rpm = 1374g/48.52oz

From badcock - his figures tend to be just a fraction low: 7800rpm = 1334g/47.00oz

10x7 APC SF: I've never dared take one that high - safe rpm limit for that prop (as listed by the manufacturer) is only 6500rpm; from badcock: 7300rpm = 1268g/44.65oz
Dr Kiwi is offline  
  Reply With Quote
Old 02-10-2008, 05:21 AM   #3
Murocflyer
WAA-08 Pilot #1
 
Murocflyer's Avatar
 
Join Date: Mar 2007
Location: Edwards AFB, CA
Posts: 7,018
Thanked 313 Times in 296 Posts
Awards Showcase

WAA-08 Pilot  Globetrotter Pilot  AV Contest  AP Contest Winner 
iTrader: (8)
Friends: (0)
Default

Originally Posted by sgf323 View Post
I've got an eLogger and I've been doing some playing around with it. With a nearly charged battery here are some readings I just got with my Park 450.

11x7E: 190 watts, 17 amps, 7800 rpm
10x7SF: 220 watts, 20 amps, 7300 rpm

The 20 amps is just above the max burst for the motor but, how would these two setups differ in the real world? Which would provide more thrust?

Since the 11x7E is pulling less amps the watts are lower but that doesn't mean it's a less powerful setup, right? Being that both are x7 props with the SF just having more prop area I'm assuming the 11x7E would be faster and provide more thrust since it is turning at a higher RPM. Is that right?

I though I read somewhere that you could calculate thrust based on several parameters, including RPM, but I can't find it anywhere. Anyone know how to do this?
I gave up on trying to figure that out long ago.

Here is what I use when I need to know that: http://www.gobrushless.com/testing/t...calculator.php

I'm sure someone knows how to do that. Looking forward to a better response.

Frank

Let's Help Newcomers! << Click Here
Murocflyer is offline  
  Reply With Quote
Old 02-10-2008, 05:23 AM   #4
Murocflyer
WAA-08 Pilot #1
 
Murocflyer's Avatar
 
Join Date: Mar 2007
Location: Edwards AFB, CA
Posts: 7,018
Thanked 313 Times in 296 Posts
Awards Showcase

WAA-08 Pilot  Globetrotter Pilot  AV Contest  AP Contest Winner 
iTrader: (8)
Friends: (0)
Default

Originally Posted by Dr Kiwi View Post
Here is a good way to get a ballpark estimate of thrust if you know your rpm: http://www.badcock.net/cgi-bin/powertrain/propconst.cgi

The other way is for me to check my spreadsheet with 8500 entries for motor/prop/rpm/thrust etc and match your rpm for each of those props.

Back in a minute!

Cool! Another one for the reference book.

Thanks!

Frank

Let's Help Newcomers! << Click Here
Murocflyer is offline  
  Reply With Quote
Old 02-10-2008, 05:43 AM   #5
sgf323
Member
 
Join Date: Sep 2007
Location: Omaha, NE
Posts: 317
View sgf323's Gallery12
Thanked 16 Times in 15 Posts
iTrader: (1)
Friends: (1)
Default

Heh, I guess I never even thought about seeing if I was taking the SF prop too high!

I guess I'll try the 11x7E out for a while!

Thanks
sgf323 is offline  
  Reply With Quote
Old 02-10-2008, 08:48 AM   #6
Larry3215
Look out for that tree!!!
 
Larry3215's Avatar
 
Join Date: Mar 2006
Location: Gig Harbor, Wa USA
Posts: 6,836
View Larry3215's Gallery63
Thanked 692 Times in 671 Posts
Club: Kitsap ARCS & E-FLAPS
Awards Showcase

90 Minute Thermal Duration  60 Minute Thermal Duration  30 Minute Thermal Duration  4kW 
iTrader: (6)
Friends: (21)
Default

This illustrates one other thing - diameter is king when it comes to thrust.

Even tho the 10x4.7 prop had more power and was turning at a significantly higher rpm, it developed less thrust than the 11" prop.

Thrust is a function of two things - prop diameter and prop rpm.

Thrust goes up as the 3rd power of RPM but it goes up as the 4th power of diameter. In other words - if you double the rpm you get 8 times the thrust 2x2x2=8. But if you double the diameter you get 16 times the thrust 2x2x2x2=16.

So if all your concerned about is thrust - larger props are your friend

Larry
Larry3215 is offline  
  Reply With Quote
Old 06-28-2013, 10:54 AM   #7
hs25
New Member
 
Join Date: Jun 2013
Posts: 1
Thanked 0 Times in 0 Posts
iTrader: (0)
Friends: (0)
Default Quadcopter

Hello frnds!
I am making a Quadcopter. I need to knw hw to calculate the thrust produced by the motor..
Motor : Park 250 Brushless Outrunner Motor 2200Kv
ESC : 10-Amp Pro Brushless ESC
Propeler : Slow Flyer Propeller, 8*6, SF
Battery : 3200mAh 2s 7.4v 20c
hs25 is offline  
  Reply With Quote
Old 06-28-2013, 01:23 PM   #8
JetPlaneFlyer
Super Contributor
 
JetPlaneFlyer's Avatar
 
Join Date: Mar 2009
Location: Aberdeen, Scotland
Posts: 4,492
Thanked 492 Times in 458 Posts
Awards Showcase

5kW  Outstanding Contributor Award  1kW 
iTrader: (0)
Friends: (8)
Default

As noted in posts above, there is no simple way to calculate thrust. best way is to build a threust test rig and measure thrust, next best option is to use one of the online calculator tools to estimate thrust. for instance e-calc

I'm not sure that a 8x6 prop would be good for a quadcopter, you would normally be looking to have as large a diameter and as low a pitch as possible for a helicopter or quad.
JetPlaneFlyer is offline  
  Reply With Quote
Old 06-28-2013, 04:47 PM   #9
pizzano
Behold The Renaissance
 
pizzano's Avatar
 
Join Date: May 2012
Location: So. Calif
Posts: 1,238
Thanked 83 Times in 81 Posts
Club: AMA, Marks, Pomona Valley, Prado Dam
iTrader: (0)
Friends: (3)
Default

As stated above, proper thrust is not easily calculated with only motor, esc, battery and prop info...particularly for copters!
The prop size you've given appears to have more pitch than what is generally used on any type of vertical lift craft......the size of the frame (dimensions), balance, weight, and vertical height from ground floor to blade are very important as well (info not provided).....

Thumb through these web sites (forum) and see what examples fit your application:

http://www.multirotorforums.com/forum.php

http://www.rcheliresource.com/category/helicopters-uav/

http://www.rcgroups.com/multirotor-talk-659/

http://www.wattflyer.com/forums/showthread.php?p=880713

http://www.rcuniverse.com/forum/forumid_487/tt.htm

AMA 928214
pizzano is offline  
  Reply With Quote
Old 06-28-2013, 06:43 PM   #10
CNY_Dave
Member
 
Join Date: Aug 2011
Location: near Cortland NY
Posts: 774
Thanked 36 Times in 35 Posts
iTrader: (0)
Friends: (1)
Default

Might be spinning that slow-flyer prop too quick as well...

Ask me why your DX5e is doomed... and how to fix it.
CNY_Dave is offline  
  Reply With Quote
Old 06-28-2013, 06:51 PM   #11
JetPlaneFlyer
Super Contributor
 
JetPlaneFlyer's Avatar
 
Join Date: Mar 2009
Location: Aberdeen, Scotland
Posts: 4,492
Thanked 492 Times in 458 Posts
Awards Showcase

5kW  Outstanding Contributor Award  1kW 
iTrader: (0)
Friends: (8)
Default

Yeah, the whole spec looks to be all screwed up. It's far too big a prop for a 2200kv motor and 10A ESC limit. The motor would want to pull 30-40 Amps with that prop!

Do I smell burning?
JetPlaneFlyer is offline  
  Reply With Quote
Old 06-28-2013, 11:36 PM   #12
CNY_Dave
Member
 
Join Date: Aug 2011
Location: near Cortland NY
Posts: 774
Thanked 36 Times in 35 Posts
iTrader: (0)
Friends: (1)
Default

Originally Posted by JetPlaneFlyer View Post
Yeah, the whole spec looks to be all screwed up. It's far too big a prop for a 2200kv motor and 10A ESC limit. The motor would want to pull 30-40 Amps with that prop!

Do I smell burning?

Shhhh! No such thing as too many 'ESC burst into flames during the maiden' threads!

Ask me why your DX5e is doomed... and how to fix it.
CNY_Dave is offline  
  Reply With Quote
Old 06-29-2013, 12:00 AM   #13
Sir Crash-A-Lot
Crash Specialist
 
Sir Crash-A-Lot's Avatar
 
Join Date: Oct 2009
Posts: 294
Thanked 34 Times in 34 Posts
Club: AzWingNutz
iTrader: (0)
Friends: (2)
Default

Originally Posted by JetPlaneFlyer View Post
Yeah, the whole spec looks to be all screwed up. It's far too big a prop for a 2200kv motor and 10A ESC limit. The motor would want to pull 30-40 Amps with that prop!

Do I smell burning?
According to eCalc that setup would pull 12 amps. I would change to a 20 amp esc and go with it.

Mike

Sir Crash-A-Lot is offline  
  Reply With Quote
Old 06-29-2013, 12:11 AM   #14
fhhuber
Super Contributor
 
fhhuber's Avatar
 
Join Date: Mar 2013
Posts: 2,236
Thanked 219 Times in 210 Posts
iTrader: (0)
Friends: (1)
Default

Trying to equate watts to thrust has given people headaches ever since we started trying to put electric motors into models.

Change from an APC E series prop to a Xoar wood electric prop, keeping the same dia and pitch and you will change RPM, thrust AND watts.
(and demonstrate that the APC is more efficient) Then change to the APC SF series, again at the same dia and pitch (if within its RPM limits) and you'll get another set of numbers...

So... until you learn from experience you have to trust a calculator and/or the experience of others.

So keep "playing with" that logger... and writing down results.

And tie a fish scale to the tail for thrust at rpm, watts, volts A X B prop and Brand X "XXXXX" motor.

Turn in your data to the motocalc site. They like having actual numbers to compare with their calculations.

The data you log might help someone else some day.
fhhuber is online now  
  Reply With Quote
Old 06-29-2013, 12:12 AM   #15
JetPlaneFlyer
Super Contributor
 
JetPlaneFlyer's Avatar
 
Join Date: Mar 2009
Location: Aberdeen, Scotland
Posts: 4,492
Thanked 492 Times in 458 Posts
Awards Showcase

5kW  Outstanding Contributor Award  1kW 
iTrader: (0)
Friends: (8)
Default

Originally Posted by Sir Crash-A-Lot View Post
According to eCalc that setup would pull 12 amps. I would change to a 20 amp esc and go with it.

Mike
E-calc said 16.5A when i just tried it (see attached)

The reason i was so far out on my estimate is that the motor efficiency drops though the floor and the watts go into heating the motor rather than spinning the prop. I assumed 75% efficiency which is a reasonable number for a motor within it's normal working range, but with that prop e-calc predicts only 42%

But end result is the same, it's FAR too much prop for the motor it can hardly turn it all all, motor overheats and dies!

The prop is also stalled making it an extraordinarily bad choice for a quad.


Attached Thumbnails
Click image for larger version

Name:	ScreenHunter_01 Jun. 28 23.08.jpg
Views:	209
Size:	117.8 KB
ID:	169635  
JetPlaneFlyer is offline  
  Reply With Quote
Old 06-29-2013, 12:18 AM   #16
kyleservicetech
Dennis V
 
kyleservicetech's Avatar
 
Join Date: Apr 2008
Location: Wisconsin, USA
Posts: 8,232
Thanked 704 Times in 686 Posts
Club: www.racinercclub.com (I'm the newsletter editor)
Awards Showcase

Outstanding Contributor Award  3kW  2kW  100mph Speed Demon 
iTrader: (1)
Friends: (20)
Default

Originally Posted by fhhuber View Post
Trying to equate watts to thrust has given people headaches ever since we started trying to put electric motors into models.

Change from an APC E series prop to a Xoar wood electric prop, keeping the same dia and pitch and you will change RPM, thrust AND watts.
(and demonstrate that the APC is more efficient) Then change to the APC SF series, again at the same dia and pitch (if within its RPM limits) and you'll get another set of numbers...

So... until you learn from experience you have to trust a calculator and/or the experience of others.

So keep "playing with" that logger... and writing down results.

And tie a fish scale to the tail for thrust at rpm, watts, volts A X B prop and Brand X "XXXXX" motor.

Turn in your data to the motocalc site. They like having actual numbers to compare with their calculations.

The data you log might help someone else some day.
Here is a very useful web site provided by the APC prop suppliers:
http://www.apcprop.com/v/downloads/P...B/datalist.asp

One thing to keep in mind, those prop thrust figures don't allow for the net prop thrust, where you've got to subtract the effects of the prop blast hitting the wings, landing gear, fuse and so on. That effect can be substantial.

DennyV
Retired and the days are just too short, busier than ever!
kyleservicetech is offline  
  Reply With Quote
Old 06-29-2013, 02:48 AM   #17
Sir Crash-A-Lot
Crash Specialist
 
Sir Crash-A-Lot's Avatar
 
Join Date: Oct 2009
Posts: 294
Thanked 34 Times in 34 Posts
Club: AzWingNutz
iTrader: (0)
Friends: (2)
Default

Originally Posted by JetPlaneFlyer View Post
E-calc said 16.5A when i just tried it (see attached)

The reason i was so far out on my estimate is that the motor efficiency drops though the floor and the watts go into heating the motor rather than spinning the prop. I assumed 75% efficiency which is a reasonable number for a motor within it's normal working range, but with that prop e-calc predicts only 42%

But end result is the same, it's FAR too much prop for the motor it can hardly turn it all all, motor overheats and dies!

The prop is also stalled making it an extraordinarily bad choice for a quad.
He is using an 8x6 prop not 9x6.

Mike

Sir Crash-A-Lot is offline  
  Reply With Quote
Old 06-29-2013, 09:26 AM   #18
JetPlaneFlyer
Super Contributor
 
JetPlaneFlyer's Avatar
 
Join Date: Mar 2009
Location: Aberdeen, Scotland
Posts: 4,492
Thanked 492 Times in 458 Posts
Awards Showcase

5kW  Outstanding Contributor Award  1kW 
iTrader: (0)
Friends: (8)
Default

Originally Posted by Sir Crash-A-Lot View Post
He is using an 8x6 prop not 9x6.

Mike
Doh!.. that drops it to 14.5A (using 'full' battery)

Efficiency is still down the pan though, and e-calc predicts overheated motor
JetPlaneFlyer is offline  
  Reply With Quote
Old 06-30-2013, 01:55 AM   #19
hayofstacks
Super Contributor
 
hayofstacks's Avatar
 
Join Date: Jan 2012
Location: Utah
Posts: 1,562
Thanked 115 Times in 114 Posts
iTrader: (0)
Friends: (5)
Default

one reason I like heads up. no guess work and you know exactly how it will perform first time, every time, awas well as thrust numbers, even on different props and cells if the motor is rated for a highere cell count
hayofstacks is offline  
  Reply With Quote
Reply

  WattFlyer RC Electric Flight Forums - Discuss radio control eflight > R/C Electric Power - Batteries, Chargers, ESCs and More > Power Systems

« Previous Thread | Next Thread »

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is On

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Speed, Thrust, and Watts - Why do watts really matter? PerlAddict Power Systems 6 01-10-2008 06:25 AM
Amps X Volts = Watts ?? diverdon Power Systems 21 12-21-2007 05:34 AM
Kv Rpm Watts Pitch Speed Volts wattflyer6161 General Electric Discussions 1 06-19-2007 05:25 PM
Volts, Amps and Watts Johan Power Systems 41 03-03-2007 08:32 PM
Calculate Amps from C? RSS Feed RCU RSS Feed 0 09-28-2006 10:20 AM


All times are GMT +1. The time now is 03:47 AM.


Powered by: vBulletin
Copyright ©2000 - 2014, Jelsoft Enterprises Ltd.
Copyright 2005 WattfFlyer.com
RCU Eflight HQ

Charities we support Select: Yorkie Rescue  ::  Crohn's & Colitis Foundation



Page generated in 0.30933 seconds with 58 queries