Brushless Motors: The Relationship between Input Volts & Output Torque - RC Groups
Thread Tools
Aug 30, 2012, 09:55 PM
Registered User
Question

Brushless Motors: The Relationship between Input Volts & Output Torque


Fellas, a q...

Take a given motor, say a Park 450. Flies on 3S/1000mah fine (10x4.7SF), but with a light model, the plane can be punchy in 3D.

Now say I wanted to soften the 3D a bit, and went down a cell to, say, a 2S/1300mah (about the same weight as the 3S/1000).

So given the Kv rating of the motor, obviously the rpm's are reduced- which is fine; ain't going for a speed record.

But here's the q: what happens to motor output torque (for a given throttle setting)?

Put another way: if I aim for softer/slower 3D, would dropping a cell make the power/weight ratio suffer too much? Make her a dog?

And what about prop? Prop up, down, stay the same?

Thx fellas.
Jeff
Sign up now
to remove ads between posts
Aug 30, 2012, 10:23 PM
I think I'm inverted. Maybe.
acetech09's Avatar
Normally dropping voltage increases max prop size. It's hard to predict what any one model will behave with a drop from 2S to 3S - there are calculators and test data available for that.

One thing I've been able to use to 'soften' the 3D power of a motor is to set dual rates/low trim/not max curve on the throttle. You need a decent computer radio to do that though.
Aug 30, 2012, 10:43 PM
Registered User
Quote:
Originally Posted by acetech09
You need a decent computer radio to do that though.
Thx...and therein lies the rub: I have a DX7S, no fixed-wing throttle curves (exc for my 700N).

It's like $12 shipped for a batt so no big deal; I just didn't want to launch the thing & find out she flies like a dog...hence the question about torque
Aug 31, 2012, 01:59 AM
Registered User
scirocco's Avatar
Quote:
Originally Posted by poiyt
Thx...and therein lies the rub: I have a DX7S, no fixed-wing throttle curves (exc for my 700N).

It's like $12 shipped for a batt so no big deal; I just didn't want to launch the thing & find out she flies like a dog...hence the question about torque
To "only" drop from around 100W on 3S/10x4.7 to about 70W, you'd need to go up to a 12x8. Note that you need to up the pitch a lot to keep adequate pitch speed at the reduced rpm.

With power for the same prop proportional to the cube of rpm, 2/3 rpm (2S vs 3S) represents only 30% power, hence the need for a big increase in prop size. It would barely fly on the same prop on 2S.

Re torque: for the same prop at any throttle setting on 2S vs 3S, less current is drawn as exponentially less power is required. Torque is directly proportional to current, so torque reduces too.
Sep 01, 2012, 09:29 AM
Registered User
Quote:
Originally Posted by scirocco
To "only" drop from around 100W on 3S/10x4.7 to about 70W, you'd need to go up to a 12x8. Note that you need to up the pitch a lot to keep adequate pitch speed at the reduced rpm.

With power for the same prop proportional to the cube of rpm, 2/3 rpm (2S vs 3S) represents only 30% power, hence the need for a big increase in prop size. It would barely fly on the same prop on 2S.

Re torque: for the same prop at any throttle setting on 2S vs 3S, less current is drawn as exponentially less power is required. Torque is directly proportional to current, so torque reduces too.
Thx bud...but Holy Cannoli!!! A 12" prop on this lil foamie...??? Guess I'll stay w/ 3#...appreciate the info, thx


Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Discussion Relationship between current and torque, how is energy conserved during PWM? Nereth Power Systems 28 Aug 20, 2012 02:17 AM
Discussion What Voltage Regulator can output 5Volts with 8 to 70 volts Input ? lazy-b DIY Electronics 4 Apr 03, 2012 05:45 PM
Discussion Relationship between power supply output and charge rate (?) KRProton Batteries and Chargers 13 Jun 23, 2011 10:28 AM
Question Relationship of Input Voltage to Output Power FlyingW Modeling Science 5 Sep 02, 2009 11:31 AM
Discussion relationship between volts and mAH mechmove Batteries and Chargers 26 Apr 09, 2008 06:26 PM