HobbyKing.com New Products Flash Sale
Reply
Thread Tools
Old Apr 25, 2010, 04:28 PM
Registered User
Joined Mar 2010
15 Posts
Question
How to calculate the values for multiple motors based on Drive Calculators results?

Hey!

Sorry for staring a new thread if there's an answer to my question elsewhere already.

We are starting a student project with a quadrotor: http://www.rcgroups.com/forums/showthread.php?t=1202361

and we are trying to define the right components for the quad.

The Drive Calculator doesn't have an option for multiple motors/rotors, so can somebody tell me how is it possible to calculate the change of flying time when adding three more actuators on the system?

Oh, and how does the thrust change? I don't think it's just 4 x thrust of one actuator, cause the battery load changes.

Thanks a lot!
Finrotor is offline Find More Posts by Finrotor
Reply With Quote
Sign up now
to remove ads between posts
Old Apr 26, 2010, 12:30 AM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
5,182 Posts
It won't be quite 4 times the thrust because (as you guessed) the battery's voltage drops more. So... just choose a battery with the same C rating but 1/4 capacity, then Drivecalc will give you a more accurate value.

If that's too confusing then download Motocalc, which does handle multiple motors.
Bruce Abbott is offline Find More Posts by Bruce Abbott
Reply With Quote
Old Apr 26, 2010, 07:14 AM
Registered User
Joined Mar 2010
15 Posts
Quote:
Originally Posted by Bruce Abbott View Post
It won't be quite 4 times the thrust because (as you guessed) the battery's voltage drops more. So... just choose a battery with the same C rating but 1/4 capacity, then Drivecalc will give you a more accurate value.

If that's too confusing then download Motocalc, which does handle multiple motors.
So if I pick a battery from the database and make a new version based on it, I'll just change the capasity by 1/4 and the battery weight, for example:


Original battery: XCell 5000 (30C) 3S

Imax = 100A
Imax short = 150A
Capacity = 5000 mAh
Weight = 375 g
Ri = 2,5 mOhm
Cell voltage = 3,7 V

Modified battery: XCell 5000/4 (30C) 3S

Imax = 100A
Imax short = 150A
Capacity = 1250 mAh
Weight = 145 g (approximation)
Ri = 2,5 mOhm
Cell voltage = 3,7 V

Like so? With this modified battery the max thrust value is exactly the same as it was with the 5000 mAh battery, so is it only dependent of the C number?
Finrotor is offline Find More Posts by Finrotor
Reply With Quote
Old Apr 26, 2010, 07:57 AM
Registered User
Staffs, UK
Joined Nov 2003
10,428 Posts
Changing the capacity also changes max current and Ri. A 20-30C 1250mAh battery would only have Imax of 25A/37.5A. Ri will also be higher, likely around 10 mOhm.

Steve
slipstick is offline Find More Posts by slipstick
Reply With Quote
Old Apr 27, 2010, 05:21 AM
Registered User
Joined Mar 2010
15 Posts
Thank you both very much!

I'm allowing myself to go offtopic on my own topic.

I also tried the MotoCalc to compare the results and I ran into this problem: the calculated flight time doesn't change when I add or reduce the weight on the airframe, how come? Does it have something to do with the fact that I didn't add any values to the wing span or the wing area fields ('cause it's a quad)?
Finrotor is offline Find More Posts by Finrotor
Last edited by Finrotor; Apr 27, 2010 at 07:07 AM.
Reply With Quote
Old Apr 27, 2010, 07:27 AM
Registered User
Staffs, UK
Joined Nov 2003
10,428 Posts
The flight time won't change because the weight changes. It's still the same motor, prop and battery drawing the same current so why would the run time change ?

What changes are the flight characteristics, speed, climb rate etc.

Steve
slipstick is offline Find More Posts by slipstick
Reply With Quote
Old Apr 27, 2010, 08:44 AM
Registered User
Joined Mar 2010
15 Posts
Yes of course, if the program calculated the hovering time, then the weight would matter.
Finrotor is offline Find More Posts by Finrotor
Reply With Quote
Old Apr 27, 2010, 11:14 PM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
5,182 Posts
To calculate hovering time, back the simulated throttle off until static thrust is 10% greater than weight (the extra 10% is to account for propwash pushing the model down).
Bruce Abbott is offline Find More Posts by Bruce Abbott
Reply With Quote
Old May 05, 2010, 03:45 AM
Registered User
Joined Mar 2010
15 Posts
Quote:
Originally Posted by slipstick View Post
Changing the capacity also changes max current and Ri. A 20-30C 1250mAh battery would only have Imax of 25A/37.5A. Ri will also be higher, likely around 10 mOhm.

Steve
Ri is the internal resistance of the battery, right?
Is it getting bigger only because the Ohm's law: R=V/I, so if the current drops to 1/4 then the resistance rises to 4xR?
Finrotor is offline Find More Posts by Finrotor
Last edited by Finrotor; May 05, 2010 at 05:09 AM. Reason: correction of a term
Reply With Quote
Old May 05, 2010, 07:49 AM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
5,182 Posts
Quote:
Originally Posted by Finrotor View Post
Ri is the internal resistance of the battery, right?
Yes.
Quote:
Is it getting bigger only because the Ohm's law: R=V/I, so if the current drops to 1/4 then the resistance rises to 4xR?
No. Ohm's law works on the principle that resistance is a constant, ie. not affected by current. The internal resistance of a battery is largely a function of electrode size (surface area, thickness) so a smaller battery will have higher internal resistance.

If it were possible to cut a Lipo battery in half to make two smaller batteries, the internal resistance of each half would be double that of the original battery. If you then wired these two halves in parallel, the combined resistance would once again be equal to that of the original battery.
Bruce Abbott is offline Find More Posts by Bruce Abbott
Reply With Quote
Old May 05, 2010, 09:13 AM
Registered User
Joined Mar 2010
15 Posts
Quote:
Originally Posted by Bruce Abbott View Post
If it were possible to cut a Lipo battery in half to make two smaller batteries, the internal resistance of each half would be double that of the original battery. If you then wired these two halves in parallel, the combined resistance would once again be equal to that of the original battery.
So if I'm trying to simulate the situation where the Drive Calculator uses four motors insead of one, I'll just divide the capacity, Imax, Imax short and the weight of the battery I've chosen by four:

Original battery: XCell 5000 (30C) 3S using one motor

Imax = 100A
Imax short = 150A
Capacity = 5000 mAh
Weight = 375 g
Ri = 2,5 mOhm
Cell voltage = 3,7 V

Modified battery: XCell 5000/4 (30C) 3S using four motors

Imax = 25A
Imax short = 37,5A
Capacity = 1250 mAh
Weight = 93,75
Ri = 2,5 mOhm
Cell voltage = 3,7 V

Is this correct? (Please say yes)
Finrotor is offline Find More Posts by Finrotor
Reply With Quote
Old May 05, 2010, 01:14 PM
Registered User
Staffs, UK
Joined Nov 2003
10,428 Posts
What do you think ? Bruce says if the battery was half the size the internal resistance would be double. I said (as the equivalent battery is 1/4 the size) "Ri will also be higher, likely around 10 mOhm".

It would be a lot easier to just use Motocalc.

Steve
slipstick is offline Find More Posts by slipstick
Reply With Quote
Old May 05, 2010, 03:50 PM
The 6 P principle works for me
elecfryer's Avatar
Ventura CA
Joined Aug 2007
961 Posts
To the best of my knowledge drivecalc is more accurate (real world) than motocalc (I usually use motocalc) however, both will get you in the ballpark, i.e. within 5%-10%. Not all props work the same, different brands of batteries = different resistance, different connectors and different gage wires, so on and so forth. Get close and go for it! If you have the equipment, check amps/volts/watts in prior to flying. Sometimes just changing the prop (or 4 in this case) can make a big difference. The secret to electric flying is "Matched Components". Thats my story and I'm a stickin to it!

Michael (if your not fryin, your not tryin!)
elecfryer is offline Find More Posts by elecfryer
Reply With Quote
Old May 05, 2010, 04:17 PM
We want... Information!
Bruce Abbott's Avatar
Hastings, New Zealand
Joined Jan 2001
5,182 Posts
Quote:
Originally Posted by Finrotor View Post
Imax = 25A
Imax short = 37,5A
Capacity = 1250 mAh
Weight = 93,75
Ri = 2,5 mOhm
Cell voltage = 3,7 V

Is this correct? (Please say yes)
No. The battery is now 1/4 the size, so Ri must be 4 times higher, ie. 10 mOhm.
Bruce Abbott is offline Find More Posts by Bruce Abbott
Reply With Quote
Old May 06, 2010, 02:45 AM
Registered User
Joined Mar 2010
15 Posts
Quote:
Originally Posted by Bruce Abbott View Post
No. The battery is now 1/4 the size, so Ri must be 4 times higher, ie. 10 mOhm.
But the battery is not 1/4 the size, but it's the same old size battery simulating the use of four motors. Why would the resistance change in that situation?

slipstick:

This is not about what would be the easiest way, It's about understanding this whole thing.
Finrotor is offline Find More Posts by Finrotor
Reply With Quote
Reply


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion How to Calculate a set up for F5J and FXJ (heihgt limited) Edwinzea Electric Sailplanes 17 Mar 17, 2010 09:23 AM
Question How to Calculate a set up for F5J and FXJ (heihgt limited) Edwinzea High Performance 1 Mar 14, 2010 03:34 PM
Discussion how to calculate lift on airfoil b-29er Modeling Science 4 Oct 08, 2009 06:28 PM
Cell values for motor calculators KatManDEW Batteries and Chargers 2 Mar 12, 2004 08:17 AM
How to calculate the gear ratio for a gear box with teeth on the inside. sn1973 Foamies (Kits) 5 Oct 02, 2002 08:04 PM