View Full Version : Direct replacement BL motor for a 280?

11-11-2005, 02:10 AM
Hi all,

I've just recently got my feet wet with brushless power and am now convinced it is the way to go in many R/C applications. The big problem I have is understanding what motors are good for replacing their brushed counterparts.

More to the point, I have an airplane (Kavan Bloody Mary) that comes with an awful power set in the kit. I would like to replace the motor and gearbox with something brushless but need to use a gearbox (MP Jet 280 size) due to the way the front of the fuse is made. Is there a good replacement for the 280 in something brushless that will physically fit in an MP Jet gearbox? Obviously I seek a bit more power (or is it effiecency?) than the stock power set. I am also hoping to use a 3S ~700-900 lipo battery. AUW of the airplane is supposed to be 14 ounces and 325 sq/in.

Suggestions please.

11-11-2005, 12:12 PM
e-flite Park 370 outrunner, no gearbox needed and firewall mount.

11-11-2005, 01:08 PM

http://willstech.com/products/advanced_search_result.php?osCsid=baa5555d6e8e5d92 5d6427b9bab72918&keywords=eflm1915&osCsid=baa5555d6e8e5d925d6427b9bab72918

So you can see it.

Hope it helps.

Matt Kirsch
11-11-2005, 01:13 PM
Hang on, he said he MUST use the MPJet gearbox because of the design of the airframe. While I don't really buy that (airframes can always be modified), let's give him an option or two just in case it's true.

The MPJet gearbox is one of those "friction fit" boxes where you just shove the motor into the back of the box, correct? If it were a conventional open-gear box where the motor is held with two screws into the endbell, you've got many options because many of the motors are designed with 280/300 mounting holes.

Take a look at http://www.hobby-lobby.com/brushless_mpjet.htm, particularly the part #MJ20127. I believe that's EXACTLY what you're looking for as far as a bolt-in fit. Unfortunately there are no performance numbers with the motor so figuring out which prop to use with a 3S LiPoly is going to be a little bit of a challenge. Hopefully the instructions that come with the setup have more info, like the maximum current that the motor can handle.

According to HL, the battery ranges from 2-3 cell LiPoly, and 7x3 to 10x4.7 props. To keep the Amps in balance, you use the smaller prop with the higher-voltage battery, so a starting point would be the 7x3 with a 3S LiPoly. I'm hoping you can up-size to perhaps an 8x3.8 slo-flyer prop.

Dr Kiwi
11-11-2005, 01:39 PM
Hi Matt,
The MkI version was a good motor so I imagine this one (much lower Kv: 4100) will be too. He'll need to get the motor gearbox combo since that motor won't fit in his old 280 box.


11-11-2005, 03:55 PM
Thanks for the replies folks. And a tip of my hat to Matt who actually read my requirements ;)

The MP Jet brushless with gearbox looks like what I need.

It's kinda funny actually as I had one of these Bloody Mary's nearly 4 years ago and destroyed it on it's maiden flight due to the weak, ineffiecent power plant. So I recently found one NIB for a good price and snapped it up.

Yesterday my December copy of Quiet Flyer shows up in the mail. Surprise! The BM is one of the featured articles! The author flew his on the stock gear but after my previous experience I am not going there again. In the same article the author also reviews the Kavan Screwdriver which he powered with an MP Jet BL set 25/35-20 IIRC. So this might just be the right setup for Mary.

Also a short answer to a comment posted previously. Yes, modifications could be done to the airframe. No, I am not interested in such modifications when it should be (and is) trivial to find a suitable power plant per my requirements.