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How do I work out how many Watts I'll get from my setup...
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 ÷ 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. Stuart. 
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You can't expect more than 10.5v11v under load, so to get 600Win you'd need to prop for 55A57A. However you really shouldn't try to run your motor at much more than 80% of max... so that is 480Win... that means you should prop for 44A46A.
Depending on the efficiency of the motor at this level of input (probably 70%80% at best) you can expect ~380Wout. If you really want 600Win, as a constant power level, you probably need a motor rated realistically for at least 800W. 


USA, ME, Ellsworth
Joined May 2008
19,105 Posts

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. Jack 


Staffs, UK
Joined Nov 2003
12,176 Posts

Quote:
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 ). Steve 

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