I have a friend who's expressed some real interest in joining me in this madness. This evening he asked me a question I simply have no idea how to answer. Not sure if this is the best forum for this, mods plz move if it is not:

Friend wants to know if there is a 'formula' to allow one to compare a glow engine to an electric motor. E.G. a formula to calculate, say, lbs of thrust from a glow motor, and a seperate formula to do so for electric motors, such that we can compare the two in similar units.

For that matter, is there a simple 'rule of thumb', like, say, 'every X volts of electric motor are = Y inches of glow motor displacement'?

Obviously, I realize prop size, brushed vs brushless, several other factors would all come into play here...and we're not looking for a precise mathematical formula (though if one exists it would certainly be interesting to know), but just trying to get a good basic set of rules/guidelines by which one could say 'this electric motor will do about the same as this glow motor'.

TIA,

you can always fall back on 746 watts of output equal 1 horsepower output, and using that, calculate your effective horsepower via the rpm and loading of the prop.

that said, there is no easy formula to give you drop in equivilents for glow motors, and it is further complicated by the fact that e planes can be built quite a bit differently then glow. (no extreme vibrations trying to destroy your plane, and your weight generally is further back)

Watts of power in an electric is a good way to guess what the performance will be like. 100 watts per pound of weight will give you a good flying electric with decent vertical and good aerobatic capability.

So I'd say use that as a comparison to glow. 100 watts per pound would give you power equivalent to a 46 glow engine on a 40 size plane.
Dennis-

All I can say is that the Hobby Lobby ad on AXI motor is just nonsense, slightly misleading, they are talking about nitro RPM in the absurd range when they talk about USEABLE RPM (RPM too high as the plane's air frame will not meet the speed). That is pure hogwash, since when do we run a 46 2C at 15K-16K RPM?

There is no real formula to compare, motors are not the same as engines and EP props are not the same as our GP props. How many cells (volts) being used and what sort of battery (capacity and discharge rate) makes a lot of difference. Also, there is type of use, what sort of plane. What sort of gear ratio is being used. A lot of variables for EP. Then in EP it does not mean that the hotter the motor the bigger a prop it will spin (we do that in nitro engines), the amount of current flow is an issue, it can mean burning motors and ESC.

I am moving into EP a lot now but have kept it within the speed 300 and speed 400 sized motors and its really good fun. I find the power to weight ratio for size of plane in EP is more fun than nitro. However, the planes are made of foam and not balsa, and even if it is balsa its very flimsy.

I would say that your:

AXI 4130 with 6S lipoly is about the same as a Saito 100
AXI 2808 with 3S lipoly is about the same as 20 sized 2C
Speed 400 motor on 3S lipoly is about the same as 10 sized 2C

For me I find that the guide used by Hobby Lobby for AXI is a good one. Tells you what sort of weight the motor will haul. Of course I take the high end number as for the likes of a Piper Cub etc.

I have half-heartedly investigated this aspect of the hobby (glo vs elec) & concluded that in order to get the same performance in similar airframes (not the normal elec "flimsies"), you have to spend a LOT of money on electric models.

It is reasonably cheap (similar to glo) to do electric with light and fragile airframes (also, usually smaller airframes). However, size & substance really cost big bucks in the "live better electrically" world. So much so that I'm not going there until the price comes down to believable levels for power to match the glo models.

For what it is worth, the following setup will provide about 10 minutes of semi - aggressive flying time with performance equivelant to a 40 size plane. This plane will also take off from a grass field with no problem at all.

Model Electrics Corp http://www.modelelectronicscorp.com/ or call 1-866-507-9956 and ask for Pete. The plane is a Freedom 3D with a Mega 20-22-2 motor, gear box, prop, etc. It includes everything you need except the battery pack (10 cell 3300 nimh pack) , servos and receiver. Price is ABOUT $435 for the plane/motor etc.

I have this plane and love it. It is not a beginners plane. It is for intermediate (on low rates ) flyers or better.

Thanks for the input, guys....I think the Watts/HP is about as close as he's going to come to some sort of 'comparison', though as mentioned, types of prop, airframe, etc, are still going to make a world of difference.

So, one must wonder then...

If, through the means above, we decide that a given Electric motor is generating X horsepower at Y RPM with Z prop, and that a given glow motor is ALSO generating X HP at Y RPM with Z prop, is it safe to say the same plane will perform the same way with either motor, or are there still other differences between motors that would make that untrue?

I'm with Britbrat on this one. For many of us, its about cost. For example, take a look at a H9 Twist. $99 for the arf, you still need an rx and servos. To power via electric, you'll need a half decent sized brushless motor. At the very least, cost ya $100. Then, your gonna need a brushless esc. Probably close to another 100 bux. Electrics are all about keeping the weight down. So, you use lipoly battery packs. These sure as heck aint cheap!!! So, I'll give electric the benefit of the doubt, $80 on the pack. Oh, and don't forget you are gonna need a lipoly charger too. So, your at the field with your new twist with a Hacker or AXI or Plettenberg motor. You fly your plane and land. The guy next to you also has a twist but it is powered by an OS .46. While your waiting for you batt to recharge,
he re-fuels in about 1 or 2 minutes and is back in the air. You figure, man I'm gonna need some more lipoly packs!!! More $$$$.
The point here is once you get to a certain size, its not economical to run electric unless your related to Bill Gates. Power for power,
there's not a brushless system in the world for $109 that matches an OS .46. I also don't buy the "I fly electric now because I don't like the smell or mess of glow engines and, the engines are too hard to tune etc. etc." which is what the electric zealots tout. Get a good named brand engine and you won't have tuning problems. As far as the mess, takes me about 2 minutes to clean my planes.
I fly both, electric and glow and I also run brushless on my 1 pound and under planes. I get as much enjoyment out of them as I do on my glow aircraft. Cost savings = more planes!!!! And as far as I'm concerned, lipoly packs are 40 to 50% overpriced. And lets not forget the safety factor when charging them!

Dave...

I regularly run thundertiger .40s (basic $70 motor) at 16500-17000 with APC 9x6 props. 15-16000 is not an unreasonable range for many .40ish engines; some are fine at 11-13000 and others run better with lighter props. It's worth checking the manufacturer's recommended RPM range when you choose your prop, as some run really poorly when overpropped.

tIANci - i think you AGREE with hobby lobby (ad in model airplane magazine). the whole ad is about how glow engines are actually much less powerful than their "claimed HP" while electrics don't lie. so hobby lobby is saying exactly what you're saying, if max hp is achieved at 17k rpm for a glow engine, you'll never get that much power running a normal prop on a normal plane. hence, while glow engines may "claim" to have more horsepower, they actually produce LESS horsepower in their useable range (11-13k).

it's the same thing as two strokes and fourstrokes. even though a surpass 91 has 1.6hp, 0.5hp less than the 46AX's 1.65hp, no one would argue that a 90 four is significantly stronger than the 46AX. because the 1.6hp is developed at a useable 11k rpm, while AX needs 16k to reach max hp.