I usually ignore IC sizes completely, but recently I had some idle time & idle hands are the Devil's workshop!  While messing around with the motor chart I came on to this possible rule of thumb for electric versus IC equivalence.  Am offering it up here for review by the usual suspects.

I dare say ALL of us flying ECL have at some time been approached by an IC flyer who is curious about electrics.  The usual query goes something like this, " What size motor would  I need to replace my Belchfire 45?"  I usually ask a couple follow up questions like what size prop are you using, how big is your airplane, what does it weigh, etc. and within a few questions can usually recommend using this, that, & the other thing.  Admittedly that is like feeding  the man with a fish rather than - oh screw the metaphors.  Let's try some facts, factoids & swags.

We know that the motor power depends on the amount of copper & iron in the motor. We also know that larger diameter and/or longer motors are more powerful.  Thus it should not be too big a stretch to use the diameter and length of the motor to predict how strong it is - these are like the bore & stroke of an IC engine.  Let's calculate the "swept area" of the magnets over the commutator, something like this:

     Swept area = Commutator dia  x  pi  x  Magnet length.  (use mm)

The ROT that  I am suggesting  to convert the above into an equivalent IC displacement is to multiply the Swept Area (in square mm) by a convenient but thoroughly un-scientific factor of 0.0024:

     IC Equivalent = 0.00024 x Swept Area

Since I am most familiar with the Arrowind motors I have charted the IC equivalents in the attached file.

Like most (all?) rules of thumb the chart is a self-fulfilling prophecy.  In this case the chart correlates fairly well with what I developed for the Brodak power systems; systems that I know and have experience with.  It also needs the INTERNAL motor dimensions (in mm) and these are not always published by the motor manufacturers.  It does not account for differences in torque that occur when the motor diameter increases.  It also says nothing about kv, prop RPM and battery matching - but it does (I think) give a fair idea of the potential performance level for a particular motor when everything is aligned.

Please note: the 2215 motor shown on the chart is NOT part of the Brodak offerings but provides an interesting lesson - it can perform like a "25".  Bob Zambelli used one on his 25 sized "Cardinette" turning a 9x6 prop and was happy - for a couple flights.  Problem is the 2215 is built on a 3mm shaft & bearings and the motor simply was not stiff enough for cranking through square corners with that prop.  Bob soon experienced some hard magnet strikes due to motor flexing.  Bob switched to a 2810 with the 5mm shaft & bearings and he will have no further issues.  Similarly, comparing possible motors at the edges of what is available will lead to similar conundrums.

Whatcha think..?

Interestink! Verrrry interestink!
gratnted the 1/10 ~ 1/8th of the finished airplane weight rule works alright for outrunners, but I like the idea of what you are doing.
So how might we (what you mean WE, paleface?) extend this line of thinking to include geared down motors? Radius * gear ratio?

like I said, this sounds like a worthwhile line of thinking.

   Dean P.

Hi Brett,
You already know this: rated power numbers are often lies, and just as often, different kinds of lies.
If I was rating a motor intended for E-glider use, then 3 second motor runs would lead to fantasic power ratings.
These would be useless for us, but true in their own context.
Even the weight based rule-of-thumb is a bunch better than rated power.

Besides, we geeks have to overcomplicate everything! You in?

take care,
   Dean

You used to see all sorts of motor tests and battery efficiency curves in the electric power literature.  You don't any more, which is a shame.  (Of course, the 'zine what was most enthusiastic about that was S&E Modeler, and I don't buy it any more).

I wonder if FM or Stunt World would be interested in publishing a monthly motor & battery test column, and if you could get enough free stuff to test that you could do it without having to spend $$ on parts.

I suspect that the "so many watts/gram" rule (is that yours, Dean?) is pretty spot on -- motor construction is all pretty similar, neodymium magnets are pretty much a commodity item, and while there is some variation (insulation, bearings, general constructional features) in general that's not going to make too much difference.

I doubt that you want to routinely run your motor at it's maximum survivable power level, just because doing so is going to reduce efficiency quite a bit.  Best efficiency happens at a way lower current draw than highest power output, and you usually see brushless setups operated at closer to best efficiency than anything else.

In my always-humble opinion, a SWAG is the best you'll ever do -- after that, it's tweaking and tuning to get everything right.  RPM differences, vibration level differences and what that lets you leave out of the airframe, steadiness of power, etc., etc. -- these are all going to make exact comparisons impossible.

The important thing, in my mind, is something that's good enough to give someone confidence that they can buy motor, ESC, battery and timer, and at least have an airplane that'll do the pattern reasonably well.  I can't imagine not having to take any new thing and change stuff around to get it altogether right.

If you're not already doing it I would suggest that you slant things toward a heftier electric motor than you think is really absolutely necessary -- it's always easier to back off parts of an installation than it is to beef them up, and after someone gets mileage on the whole electric thing they're going to know what they like anyway.

And what's the matter with gears?  Just run a tube from the muffler to the gearbox so that you get adequate lubrication, and -- uh, wait...