Tim,, so by way of accurate communications and clarity,,
you are saying I am essentially incorrect?
that a bigger motor (given the same ESC and Battery) will be MORE efficient than a smaller motor? or do you think the acceleration deceleration phase of operation would outweigh the gains with the bigger motor..
There's efficiency and efficiency. You are correct that at some point a bigger motor is going to show more non-resistive losses than a smaller one, and any time you go and plot efficiency vs. size (all else being equal), there will be a (thankfully fairly broad) sweet spot.
But then toss into the mix the fact that you're flying an airplane with this gizmo -- now, the most efficient motor on the bench may weigh things down so much that the
package isn't efficient (or pleasant to fly).
Motors for e-flight are pretty much rated for the power they can handle without burning up, not for their most efficient power. So yes, I would expect that to some extent you could gain a few efficiency points
at the motor by going to a bigger motor -- but you can't do that too much before you've weighed the plane down with motor.
how about some CLEAR DIRECT guidlines,,
Look at what provides success to others, and shamelessly copy. I would have laughed at that when I graduated from engineering school, but it's what about 90% of engineering boils down to. The next 1% is inventing entirely new stuff, and the 9% after that is fixing the mistakes you made when you invented entirely new stuff.
More seriously, look at the smallest motor that satisfies your required power level, and go one or two steps bigger in size. Not only does this make objective sense -- but it's what all the successful guys seem to be doing, so I'm just shamelessly (that's the key) quoting it as if it were my idea.
I get what Norm says,, and I agree, KV is the key for systems,, however I am really curious about the wattage, prop size to load, and motor size relationship and how it affects efficiency,, will it make a significant change,,
I would have to go do the math. Motors' wattage ratings are pretty slippery -- Hacker or eFlight wants you to come do business with them again next year, so they have an interest in not selling you a motor that burns up. Honest Wong's Quality Chinese Motors wants to get money quick, so they can buy off the local party boss before he comes and steals Wong's daughter for a concubine. So they have an interest in getting you to buy a motor
today. They rate the power of their motors accordingly.
Prop size is a way complex subject. Props are wings, and bigger props have less induced drag. Smaller props have less parasitic drag. Just like wings. Bigger props have more authority in controlling speed. And what makes the plane fly 'best' is subjective. So the best thing to do is to go out and do a ton of experimentation on what works best -- or to shamelessly copy what others have done. Read everything that's been said -- using big, low-pitch props at moderately high RPM makes oodles of sense to me, and it certainly works -- so why argue?
It's fairly easy to do the math on motor efficiency, if you know how much unloaded current the motor pulls and if you know its winding resistance. Power in is just volts * amps, power out is volts * amps - losses. The total losses are going to be quite close to the unloaded loss and the resistive losses, so losses = volts * (unloaded amps) + amps
2 * resistance. ESC efficiencies are unpublished (as far as I know), but are going to be between 90% and 97% from Honest Wong's, and (I'm guessing) 95% to 98% from a "name" brand. You're going to burn up a lot more power in the motor than in the ESC, unless you buy from
Dishonest Wong.
I very much doubt that there's an optimum lurking out there that hasn't been found already.