Guys,
I'm getting close to selecting a motor for my Olympic which should come in around 44oz ready to fly. With the latest trend in the IC world being toward very large power packages I was wondering if in electric we might benefit from the same approach. So instead of using say a "35" size Eflite 15 running a 10x5 which would pull this ship fine, could there be an advantage to using say a Eflite 25 or 32 (close to a 45 - 55 IC) lightly loaded on an 11x6 and just loafing but with lots of reserve power for the wind? Anyone try this kind of comparison?

Best,           DennisT

In my opinion it is more important to have a sufficient and properly sized battery to handle the demands of the governor.  As long as your motor is not overheating it is more than likely giving you equal power as any larger motor would.  The governor will only allow the motor to reach a specified RPM.  Having plenty of battery overhead will keep this rpm more constant and ensure the rpm never falls below the set point rpm.

Thanks,
Jason 

While I have a marginal understanding,, ( there are those who know a lot more) I believe that if you oversize the motor, you will be running it outside its max efficiency range and will be burning electrons with no real gain. another observation,, the POWER we use to fly is the Watts,, and if you underprop a motor, it will not generate the watts we want for performance, so in a lot of ways it is similar,, you need to match the prop to the power system for max performance.
That said,, Personally I am targeting motors that generate a known  watt output about 15% in excess of what I expect the plane to require ( all based upon observation of other peoples setups,, )

Just read Norm's post,, what he said!

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..

how about some CLEAR DIRECT guidlines,,
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,,

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) + amps² * 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.

Tim, I know and follow the principles of copy what others do,, However one thing that I am very curious about is the relationship of the work done,, as related to watts,, and the work needed , as related to performance of the airplane.. I may be barking up a dead tree, but I know that say a glow powered plane that flies with say a 10x6 prop, convert it to a 10x6 electric system, and you may not pull enough watts to create the POWER to perform, this makes me wonder if that is why we see a lot of 12 inch and 13 inch diameter props on systems that typically would be 11 inch props on a glow system,, because we need the diameter to generate the Wattage

Ah.  I get it.

I think it's more a matter of prop efficiency vs. motor efficiency.  IC engines (in general) like to spin fast.  E-flight electric motors are specifically built to be efficient at lower speeds.  At the speeds we fly, large props are more efficient and have better speed regulation.  If you've been flying RC for a while, you'll remember the 2nd wave of electrics, where most everyone was flying geared brushed "can" motors, and guys with $$ were flying brushed, geared, but purpose-built motors from Astro Flight.  The gearing was because a 'regular old' electric motor wants to spin even faster than a typical IC engine, and were overloaded by anything other than a tiny prop -- that tiny prop was hugely inefficient in anything other than a racer, so we used gearing to get the speed well below typical IC engine numbers, which allowed us to use bigger props, which let us get enough prop efficiency to make up for the abysmal efficiency of a can motor and the weight of NiCd batteries.

Outrunner brushless motors are specifically designed to deliver torque at relatively low speeds -- that's why they're large diameter, and why they have so many more poles than 2.  They are (or were, according to things I've read) heavier and less efficient than the motor in a motor/gearbox -- but they're lighter and more efficient than a motor/gearbox combo, and they're certainly a sight less complicated.  Basically, in coming up with outrunners, the electric motor folks purposely leapfrogged the IC engine folks for a motor that is efficient at lower speeds and can therefore use a larger and more efficient prop.

You see the same thing in airplane engines, by the way: compared to automotive engine crank speeds, full-sized airplane props turn really, really slowly.  If you ever get into a race with a Cessna, your engine may be turning 5000 RPM while the Cessna prop is turning 1800 or so.  Airplane engine manufacturers don't do this because they like it: they do it because if they amp up the horsepower by turning the crank faster, the prop gets small and inefficient.  So ever since the 1920's, there have been successful, high-performance airplane engines that have been small, high-revving power plants with some sort of a gearbox to turn the prop at significantly lower speeds than the crank.  This continues on today in the homebuilt airplane world, where a great many of the automotive engine conversions feature a gear- or belt-driven speed reducer from crankshaft to prop.

So what the "bigger prop than IC" really boils down to is that you're getting higher efficiency through increased prop size, at the same time as you're getting better speed control.  If someone wanted to build an IC engine specifically for stunt, they'd make it relatively huge for the airplane it's designed to fly, they'd make it to turn at much lower speeds than a typical Schnuerle ported "RC engine with venturi" that most of us can afford, and because it was designed to turn slowly, they'd take advantage of the lower stresses to make it more spindly in an effort to keep the weight down.  And, indeed, if you look at the purpose-built CLPA stunt engines, they are (to my knowledge) huge, and light, and they turn way slow.  (and they cost a lot).

You could (in theory) get a motor that turns that 10-6 prop at the same RPM as a given IC engine would.  It would require the same power at the prop as the IC engine delivers.  This would be more power than you'd need to deliver the same thrust at the same airspeed if you used a bigger prop, and your small-prop/high-speed combination lose some speed regulation.  So your whole package would be somewhat heavier (I'm not sure how much -- very possibly not much), but even if it weren't, you'd lose some or much of the advantage in 'feel' that one gets from electric.

Guys,
Well some of these points are a little off the mark. There are several purpose built IC stunt engines that run at around 9000 rpm. Our electrics use very thin blades, the IC engines turn props that are three times our blade thickness. The thick blades pull really good but on an electric they pull lots of amps. To get pulling power we add thin blade diameter. The question is if we used a motor with a little more reserve power could use props with a little more pulling power that would help in the wind and still pull reasonable amps.

Best,     DennisT

Guys,
Well some of these points are a little off the mark. There are several purpose built IC stunt engines that run at around 9000 rpm. Our electrics use very thin blades, the IC engines turn props that are three times our blade thickness. The thick blades pull really good but on an electric they pull lots of amps. To get pulling power we add thin blade diameter. The question is if we used a motor with a little more reserve power could use props with a little more pulling power that would help in the wind and still pull reasonable amps.

Best,     DennisT

I am ont so sure that you would gain anything by running a thicker blade prop, in fact I am almost certain that you would LOOSE efficiency and performance... I have done tests with electric setups using massaged wooden props,, thicker blades, but the exact same outline as the E prop,, they created less aparant thrust,, used a far greater amount of battery, and ultimatly did not fly the plane as well as the stock E APC prop,,
I think you will find that blade thickness on IC engines is there more for strength than for actual performance. Look at CF IC props, they are much thinner than wood props, at least standard wood props,, When I rework props for IC use, I tend to thin the blade significantly, it makes a better prop from my experience,,
of course this is all seat of the pants observation, no mathematical prowess available here to prove by the numbers,,

Dennis A,
I agree and understand that the electrics on governor control will run at the set rpm. My thinking was not that they unload but that the larger motor would possibly react a little sooner to load and maintain rpm when we need it gong vertical. Your data seems to indicate that also.

On the prop issue, again I agree that if you make a copy of the APCE P props and add thickness it will likely alone it will have very little impact on thrust and will pull more amps. But with a different plan form i.e. one like the BY&O's or rev-up they can pull better on certain ships. The APCE's have undercamber and that seems to work well for pulling out of a corner for both electrics and IC piped ships (Bolly CF props also have undercamber). I guess the answer is to test different props on a particular ship and use which one works best.

Best,         DennisT

Bob

Excellent input. Thanks for sharing your experience. Good information which proves bigger is not always better. 

Guys,
Lots of interesting information and experience. One thing that would be interesting and I think DennisA and JimW may be close is to take the same ship install different class motors, adjust the CG by moving the battery so it remains at the same location, maintain the same lap time. A comparison of how it flies in dead claim and highish wind would be interesting. It seems in "dream stunt wind" the lightest ship will have the edge most of the time. When the wind comes up and you climb to the top on the vertical eight, hourglass, overheads and clover is were the rubber meets the road and we need reserve pulling power.

I agree that in electric the prop is key. Since we don't 4-2-4 we need a prop that pulls as the load increases. There seems to be lots of room to work here. I do a lot of work with Wind Turbines, they make max power when the pitch is lowest. For us low pitch would cause the motor to be pushed and would back off the power just when we need it most. Maybe we could use high pitch props with wide blades at lower rpm. The high pitch would not push the motor as much in the wind and keep it pulling stronger. Just something to think about and test.

Best,    DennisT