View Full Version : Motor size confustion

05-31-2008, 01:37 AM
I have a GreatPlanes Slinger which was lacking power, so I bought a brushless motor and ESC (http://www.hobbycity.com/hobbycity/store/uh_viewItem.asp?idProduct=5353), but when I went to put it on my motor mount, I found that the screw holes didn't line up. Not only that, but the shaft is the wrong size for the propellers included with the Slinger. So, now I'm totally baffled by how stupid I am, and I can't find any information online about motor sizes. Do Speed 400 size motors have a specific shaft size? Also, I need to buy a prop adapter, but I'm not sure which adapters are correct. My new motor is listed as having a 3mm shaft. Does that mean I want a 3mm prop adapter?

I'm so confused. Thanks!

05-31-2008, 02:42 AM
Good learning opportunity....
Speed 400 is a descriptive term and tells you very little. Unfortunately a LOT of the motor information tells you little. Learn to dig before you make purchases.

Brushed 400's have 2.3mm shafts. If you have a 3mm shaft on the new motor then you have to get a 3mm adapter. Guess what? It will NOT fit your prop. Not only that but since it's a different motor you should NOT use the original prop UNLESS for some coincidence it happens to be appropriate for the NEW motor. Not only that, the hole won't be the right size most likely. And you'll find there are way more propellers sizes and types than you ever imagined

Electric motors have a narrow power operating range. That range is defined by the voltage in (battery) and the prop (load). Each motor will have a manufactured suggested power range (usually defined as Amps or Watts). If you are lucky they'll suggest a prop size and type for a few different batteries. The cheaper the motor the less likely you are to get this info. If it's a Chinese cheap motor it is likely inflated and not to be believed, but it's a starting point.

Go back to your link, look carefully at the motor specifications, start reading the reviews for the motor. A bunch of people have done some testing for you. Their results won't make sense at first. Keep reading. Have a beer. Go back tomorrow. Keep at it until it starts to click. It's information that you have to be able to deal with if you want to put unknown motors on planes and be successful with it.

BTW-Hobby City removes 'bad' review. Unless you stumble upon one before it's removed you'll rarely see a poor review. It's their web site, they can do as they like.

Thought it would be simple didn't you? The question you asked I've been trying to explain to a buddy for about 6 months. Takes a LOT of digging. Hope you get it a little quicker.

05-31-2008, 03:58 AM
Wow, that's so nice of them to delete the helpful reviews. Not buying from them ever again.

Luckily, the Slinger props are in the recommended size for the new motor, being 5x4.3. I think that's the only luck I've had with this project.

Why won't the prop adapter fit my prop though? Any suggestions on an engine mount for a delta wing? I'm using a piece of aluminum bent at a 90 degree angle with holes drilled in it. I figured I'd just drill a new one for the new engine.

05-31-2008, 06:28 AM
Not personally familiar with the the Slinger, only seen one a few times.
LOTs and LOTs of planes were and still are designed for 300 and 400 size brushed motors. These are essentially 'inrunners'. When you put an outrunner on them you end up with a mounting puzzle. Here's where you end up having to dig through forums, web sites, and hope to find some ideas to fix the problem. You would have been better off buying an appropriate inrunner. If you do that make sure you do some research on inrunner motors, KV (RPM/volt), and get an appropriate size and KV. In your case likely something around 2000KV. DO NOT buy a 3000KV (or more) motor and think that will be faster-it will be too much.

Here's some threads on the Slinger and one on the Wild Wing. The Slinger looks like it needs reinforcement if you go brushless or it'll do bad things.

The WW comes designed for a brushed 400 and is often converted to brushless so there are numerous discussions on how to do this. Looooonnnnngggg thread on it.


And a review with an upgraded inrunner power set:

It appears the original motor 'might' have been 3mm shaft. It says your new motor is also. If so, should fit. OTOH this is a cheap kit and a cheap replacement motor. Who knows what you really got. LIke I said, most 400's are 2.3mm but not necessarily all. If your's is just a tad tiny for the outrunner shaft that's likely the issue.

Good luck.