Power management with electrics

You are thinking of the old fashioned motor /speed controllers where they used a resistor. Today's modern units employ switching the turns the power on and off at a high rate, not much heating involved in this process.
Red S.

It depends. I tend to go for a motor that has enough power at full throttle to get the climbrate I want and enough efficiency in cruise to get the duration I want.
This actually means I end up with slightly larger motors than I need, running rather coarser pitch props than others do.
Too big a motor and the efficiency suffers in cruise. And you are carrying needless weight
Too small and you lose efficiency in WOT situations. Or in cruise..depending on how you set it up. That means extra battery weight to keep the duration...
And I don't like pushing kit further than needs be to get the performance I want. One hot day and an extra long flight and the thing may just overheat on you and bang goes the magnets.

| You are thinking of the old fashioned motor /speed controllers where | they used a resistor. Today's modern units employ switching the | turns the power on and off at a high rate, not much heating involved | in this process.
Well, what he said is still correct about a brushed ESC -- it's most efficient at 100% throttle, and less efficient at lower settings. Of course, the difference is like 99% vs 95% or so -- not very signifigant.
Brushless ESCs are always switching, even at full throttle, so I wouldn't expect them to become more efficient at 100% throttle. One reason for going brushless is that things overall tend to be more efficient at partial throttle values than brushed systems, however.
| > Since I'm fairly new to - and more than a bit ignorant about - electric | > flight, I'm curious about using more motor than one really needs. From | > what I've read, one wastes battery power, turning it into heat by the ESC, | > when at less than full power. Therefore, isn't it logical to use the | > smallest motor one can use, commensurate with desired performance
That's the key right there ... `commensurate with desired performance'.
You may be right -- a motor+ESC might be most efficient at 100% throttle (they often are, especially for brushed systems.) But if you need 100% throttle to fly around, then that doesn't leave you much extra power for playing around, and if your plane isn't fun to fly, why bother?
| > and run it at full power rather than installing a big motor and | > running it at part throttle?
If you're looking to break a world duration record, absolutely. If you're looking to just fly around and have fun, maybe not.
Also, that bigger motor weighs more, and that alone will make the plane require more power to stay airborne.
If you're really interested in maximizing power system (battery+ESC+motor) efficiency, get the `Electric Motor Handbook' by Robert Boucher.
Looks like it's been discontinued, so it might be hard to find.
It's a bit dated (for example, brushless motors only get a paragraph or two, though most of the rest of the book applies to them too) but it does a pretty good job of explaining things. It's also really `dry' -- Boucher doesn't seem to be the best writer, but he does know his stuff.

Motor tests by Steve Neu, published in QuietFlyer, bear out your observations. The better BL motors (e.g., Hacker, Neu) have higher eficiency overall and tend to efficiency curves that flatten out above 50% or so. Below the 50% power level they can fall off pretty steeply. For example, the Hacker B50XL/9/6.7@30V measured about 90% at 1300W, 80% at 650W, 65% at 325W, 40% at 160W. I think your rationale is sound, but the numbers from empirical tests seem to indicate that a good quality motor sized to run at 50-100% power doesn't give up much in efficiency, as compared to running full power all the time.
Abel

That is also a function of how they are run, and the controller. On brushed motors the low throttle efficiency seems very good..another reason I like them.
Out of interest, judging by my capacity/time measurements, half throttle on a futaba tranny with the controllers I use equates to about one quarter full power.