I bought an electric motor off Ebay. This is something special. It is a contra-rotating propellor electric motor. I had thought that it'd be just one electric motor with some kind of planetary gear based transmission to drive both props.

It is not.

It is two motors mounted coaxially, each motor driving a prop. The props are approximately 7 inches diameter.

I have one problem. The unit did not come with a spec sheet, and the seller says that they can't get one.

So, not having a lot of experience with electric flight motors for RC, I'm asking how I'd be able to determine not only the type of battery pack, but also what kind of electronic speed control this'd take, what kind of power requirements, that sort of thing.

When I saw this, I saw a golden opportunity to build something that's been tickling the back of my mind for years: a Fairey Gannet, although a Westland Wyvern comes in at a close second. The Gannet is a rather interesting looking tubby lady, while the Wyvern looks like either a Sea Fury or a Sea Fire on steroids.

But, due to the size of the unit, I doubt I'd be able to build a particularly large model, nor even a very detailed one. Mebbe a sport scale fun flyer, or even a stand way off scale model.

But, first I have to figure out how to power this little beastie.

The wires aren't even labelled, so I'm not sure which is the postive wire, negative wire, or the ground wire.

Maxwell
Welcome to RCG

If your contra system has 3 wires from each motor, it is brushless, and requires two brushless ESCs to operate it.
Does it look something like this?
http://www.hobbyking.com/hobbyking/s...dProduct=17150

Can you post a picture?

Unfortunately, I don't have a camera with which I can post a picture.

It is kinda like that, yet unlike, because the motors are a bit longer individually, and has three blade props.

But yes, each motor has three wire, but they aren't tagged or otherwise labelled. I'd at leat like to be able to find out which wire is the ground.

They are brushless.

How do I determine which esc to use(power output/voltage)?

I'm in the dark here, and am not sure how to grope around. I'm afraid that if I connect the wrong wire combination, I might fry the unit.

Brushless motors don't have a "ground" wire. All three wires are connected to the 3 phases of the windings. To connect them to a brushless ESC just connect in any order, see if the motor turns the right way and if it doesn't just swap over any two of the three connections. Sounds odd but that's how it works. Oh and before you ask, you can't really test anything without an ESC, a brushless motor will not run at all unless it's connected to a brushless ESC.

If you tell us the rough size and weight of each of the motors are we can make a reasonable guess at what sort of power they might want. But so far you've given us nothing to work on other than "approximately 7 inch prop"....from just that I'd guess at probably a 3S 11.1V battery and probably less than 25A...but I could be way off.

Steve

Well, the motors are .915" in outer diameter, and each one is approxinately .47" long, the whole unit being 2.75" long, including the props and spinner, and looks more compact than the one in the pic that was posted. The front prop has the legend EP-7035X3, the rear prop has on it EP-7035RX3.

Does this help?

I didn't know you could hook the wires up in any old way.

23mm = 0.9" which suggests that what you have is very close to the Hobbyking one. It uses 8x4 2-blade props which are equivalents to your 7x3.5 3-bladers so I'd guess that the specs are as near to identical as makes no difference. So 3S and a pair of 25A ESCs looks like a pretty good guess .

Steve

Two of these ESCs will do the job
http://www.hobbyking.com/hobbyking/s...C_3A_UBEC.html
Connect both ESCs to your receiver with a "y"cable
http://www.hobbyking.com/hobbyking/s...5pcs_bag_.html

Well, I'm kinda newly back into RC, as well as electric flight. Back in the early 80's, the best electric motor I could get was a rather large one that was most decidedly not brushless, and the only batteries available for any kind of power were NiCads. But at the time, I was designing my own scale flying wings, among others, then Life got in the way, and I dropped out a bit. Back then, there were no such things as electric ducted fans, as well, although with a bit of ingenuity, one could put together a peanut scale rubber powered ducted fan(such as an A7 Corsair II that flew indoors), and turbines for RC models were reserved for the financially very well off. If one could find an old Dynajet, and didn't get arrested flying it in the nearest school field, then that was the extent of fast flying jets.

But, having gotten married recently(coming up on two years, and we are both still acting like newlyweds), my wife asked me if I wouldn't want to take up a hobby of some sort, which I have, RC and scale plastic models.

Right now, while I'm putting together the plans for a scale Gannet(probably stand way off scale), I'll also be getting back into the air with a basic trainer, something that I'd be able to "kiss" the ground repeatedly as I learn to earn my wings back. Because, after all, the only way to graduate to the next step is to finally crash your model so many times that it can't possibly be put back together, and from what I've seen so far, the basic trainers available at the local hobby shops look pretty tough, relatively speaking.

Of course, having been able to purchase some acreage of my own helps, as well.

But, even when I wasn't building models, I was always doing aviation research, because I knew in the back of my mind that eventually I'd find more subjects for future modelling projects.

Read these:
https://sites.google.com/site/tjingu...arging-how-tos
http://scriptasylum.com/rc_speed/lipo.html
http://www.rcgroups.com/forums/attac...mentid=1690134
This diagram shows only one motor and ESC but your contra-rotating motor requires two ESC. The second one would just be in parallel with the one shown.

Glen

Yes, I have two sets of wires.

Originally Posted by maxwill

Well, the motors are .915" in outer diameter, and each one is approxinately .47" long, the whole unit being 2.75" long, including the props and spinner, and looks more compact than the one in the pic that was posted. The front prop has the legend EP-7035X3, the rear prop has on it EP-7035RX3.

Does this help?

I didn't know you could hook the wires up in any old way.

Maxwell;
Yes, it helps. More information is better than less.
Three wires from the ESC connect to three wires from the motor.
To reverse motor rotation, swap any two of the three wires (when power is removed).

Picture of two motors and two ESCs

Originally Posted by rick.benjamin

Maxwell;
Yes, it helps. More information is better than less.
Three wires from the ESC connect to three wires from the motor.
To reverse motor rotation, swap any two of the three wires (when power is removed).

I did not know that. The last electric motor I had, was back in the early 80's, and had only two wires, was 4 times bigger across than this one, and swung a 10 inch prop, and had no kind of speed control.