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Old Mar 14, 2010, 02:42 PM
Stuart Warne is online now
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How do I work out how many Watts I'll get from my set-up...

Just a quick general question here. Its mostly likely been asked before and I'm most likely to have been told already. But my mind doesn't hold onto equations too well at the best of times. So, can somebody outline the basic maths behind this problem so I/we can have it for a point of reference in the future when I/we inevitably forget it again!

Lets say I have a 50", 5lbs model that I'm looking to power. I know roughly that I'd need a motor with a max rating (for 15sec) of 600watts to achieve 100+ w/lb, but how do I figure out how many watts I'd be getting constantly and not just at this Max peak? I'd also like to know how I can figure out what amp rate speed controller I'd need to use, based on using a 3cell 3300mah lipo. I believe this is how I work out the basic amp draw :

600w ų 12.5v (3 cell lipo) = 48amps

But my issue is that this is only the stated max watts for the motor for 15seconds. Its the constant output I need to figure out, that way I can get a better estimate of my amp draw and thus what size speed controller to use. Of course prop size comes into it too, but how and at what point in the equation?

Any clear help on this would be most welcome, maybe it can even become a sticky....unless there is already a sticky showing how to work out all these things (watts, amp draw, estimated duration etc) and I've just missed it.

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Old Mar 14, 2010, 03:01 PM
Dr Kiwi is offline
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You can't expect more than 10.5v-11v under load, so to get 600W-in you'd need to prop for 55A-57A. However you really shouldn't try to run your motor at much more than 80% of max... so that is 480W-in... that means you should prop for 44A-46A.

Depending on the efficiency of the motor at this level of input (probably 70%-80% at best) you can expect ~380W-out.

If you really want 600W-in, as a constant power level, you probably need a motor rated realistically for at least 800W.
Old Mar 14, 2010, 06:20 PM
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If it is a Hobby King motor read the reviews and you may be able to find some has posted some data that will help figure it out.

The "down rate everything to 75% of the advertised values" rule is a good idea at Hobby King. If you really want to use a motor at 600 watts look for one that is rated as an 800 Watt motor.

And you are getting up into planes of the weight where 4S or 5S batteries might help get the power you want. Don't forget to down rate the C rating on bargain batteries to 75% of the stated C rating too. A 20C 3300 mAh battery won't last long at 66A, but if you use it at 45A or so it might.

Old Mar 15, 2010, 06:16 AM
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Originally Posted by stuart warne View Post
But my issue is that this is only the stated max watts for the motor for 15seconds. Its the constant output I need to figure out, that way I can get a better estimate of my amp draw and thus what size speed controller to use.
Wrong. Your speed controller should always be sized according to the maximum current you'll ever use....which you've already (sort of) worked out.

Better to assume the battery will be giving around 11V. Though for those sort of rough calculations I often just call it 10V because the numbers are easier. So 600W will need around 60A.

For flight duration you'd need to know what your AVERAGE current is and that depends on what sort of plane it is and how you will be flying it. E.g. I know people who enjoy tearing round the place flat out all the time so their average current is close to the maximum current. Many of my planes I enjoy best when I'm just stooging round slowly so my average current is well under half the maximum current (giving me over twice the duration ).


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