This thread is privately moderated by fly_boy99, who may elect to delete unwanted replies.
 Apr 20, 2011, 08:41 AM Registered User Windsor, Canada, near Detroit Joined Jul 2007 5,518 Posts motor constants by Joachim Bergmeyer i think adding his formulas could help to increase the knowledge of our motors. i have been using them and learned a lot about motors. here is the link: http://www.rcgroups.com/forums/showthread.php?t=185271 he also has another formulas for a simplified method: http://flbeagle.rchomepage.com/resea...per_ver_02.pdf .
Apr 25, 2011, 12:11 AM
Registered User
Mexico, BC, Mexicali
Joined Aug 2004
5,519 Posts
I have this ecuations in Excel.

Manuel V,

# Files

 May 02, 2011, 02:36 PM T#0 Steamboat Springs, CO Joined Aug 2010 623 Posts very nice thanks for writing all of the formulas down, very helpful
 May 08, 2011, 06:02 AM Registered User Czech Rep. Joined Aug 2006 302 Posts Hi guys, I hope this is not much OT. For more than a year we have been using so called tax power formula for evaluating the input power in our local e-sailplanes competitions. The general formula is Pd=0.4*d*L*(kv*U*D/100000)^2 where Pd is tax power [W] kv is motor constant [RPM/V] U is battery nominal voltage [V] D is propeller diameter [inch] d is air gap diameter [mm] L is air gap length [mm] For "small outrunners" (with a bell diameter of 28 mm) it is slightly modified as Pd = 10*L*(kv*U*D/100000)^2 The calculation sheet is here: http://www.rcex.cz/kestazeni/taxpower_calc.xls. Examples: Axi 2814/10, 2S lipol, propeller 9.5x5: tax power is 0.4*28*14*(1640*7.4*9.5/1e5)^2 = 208 W, measured 210 W (this is what I fly in the AH Castor) TG 2826-1650, 2S lipol, 10x6 (this outer 2826 = inner 2209): tax power = 10*9*(1650*7.4*10/1e5)^2 = 134 W, measured 135 W (my "small outrunner" sailplane). Now, while the measured (static) values correspond to the calculated "tax power" values reasonably well, I am not able to get anything close to this formula theoretically. Would someone of you more educated experts be able to help? Thanks, Jan
 May 08, 2011, 09:14 AM Registered User Antony (France) Joined Sep 2003 3,118 Posts Hi Jan Normally the power at prop shaft is INDEPENDENT of the motor Power is a function of prop design + rpm Regarding the total power at the battery level 1) Power at prop shaft (could be around 80%) 2) + Ohmic losses (motor) (could be 10%) 3) + Iron losses (motor) (could be 6%) 4) + ESC losses (brushless controler) (could be 4%) Air gap dimensions are acting on 2) and 3) Louis
 May 08, 2011, 11:57 AM Registered User Czech Rep. Joined Aug 2006 302 Posts Louis, Thanks for your reply - it is clear to me. However, my problem is a different one . Using measured data for various combinations of motors/props/batteries and a "brutal computing force" I arrived in a relatively simple formula for calculating the input power of a drive unit. Well, it is not 100% accurate but it is, say, usable and we have been flying competitions based on it. The formula is "simple" with only linear and quadratic functions. I would guess it should be possible to derive it theoretically. However, here I have failed repeatedly. Very likely I have been using too simplified models, or even more likely I am not clever enough. So, I would appreciate any help in getting that formula just from the desk or notebook - I would sleep better then . Thanks, Jan
 Jul 17, 2011, 04:44 PM Registered User Joined Jul 2011 3 Posts No Receiver I am building an RC foam plane and trying to decide if I need to connect a receiver inside the plane if I already have a receiver in my Spectrum DX6i Radio. And if I do not connect a receiver inside the plane to connect the ESC to the Servo's can I connect the ESC directly to the Servo's? A simple question I cannot find an answer to.
 Jul 18, 2011, 12:39 AM Registered Aircraft Offender Carmichael, CA Joined Feb 2007 3,545 Posts Try asking over in the beginner threads, or better yet go by your local hobby shop and asking a few questions. They'll get you started in the right direction. This particular thread is not related to what you are looking for. Kev
 Sep 21, 2011, 11:33 AM Lost but making good time Joined Nov 2008 1,345 Posts To any Electricians, When you add voltage by going from a 2S pack to a 3S pack, do the amps increase along with the voltage? Somewhere in this section I seem to remember >Amps=(%>V)squared. Please advise. Regards, Hankg
Sep 23, 2011, 08:37 AM
Registered User
Antony (France)
Joined Sep 2003
3,118 Posts
Quote:
 Originally Posted by hankg To any Electricians, When you add voltage by going from a 2S pack to a 3S pack, do the amps increase along with the voltage? Somewhere in this section I seem to remember >Amps=(%>V)squared. Please advise. Regards, Hankg
Hi Hankg
Generally when voltage increases the rpm are also increased
Speed (rpm) higher means higher consumed power + higher motor losses.
If the load is a propeller you could have an exponent around 3.0 upto 3.3
See the picture (Power versus rpm)
For an e-car it could be different
Louis