Model Tech J3 Cub build

[CHAMM128]

Well I just have about everything to start conversion of my cub. It was meant for a .25 nitro engine, but I have become tired of the nitro slime cleanup after every flight and have been wanting to try my hand at electrics. Anyway, I have an AXI 2820/12 for the motor and have three 4000mah lipo cells to build an 11.1 volt pack. As soon as I get enough time i'll get started. It will probably have to wait till spring to fly though. It'll probably take me that long anyway.

Chuck

[tioli]

Hi Chuck,

I have a Hobby Lobby Bonnie 20, with the Axi 2820 motor and three 2000mAh packs (6000mAh altogether). It flies like a dream and for a long time. I think you're going to end up with a really nice cub...

d

[Zeroaltitude]

I help out in a hobbystore, and we received this Cub a couple of weeks ago. My first thought was "wow, this would convert to electric easily".
I have a softspot for the J3 Cub, and have converted the Thunder Tiger 1:5th scale (81"ws) J3 Cub to electric with an Axi 2826/10. It wheighs in at 7lbs (at the lower end of the recomended AUW for glow) with a 4s 6600mAh batterypack. With that combination spinning a APC 13X6"E" prop I get 28 minute flights.

I do think that the 4000mAh packs you intend to use are "overkill" on this plane, at least if they have a reasonably good ability to withstand 10-12C continous dischargerates as most packs do today, and possibly a bit on the heavy side. With a 2820/12 spinning a 10x5"E" prop you´ll have around 2,5lbs of static thrust (on a plane that will probably come in at around 3lbs, possibly less if you use lightwheight servos, rx and so on) and over 65W/lbs which is way more than scale, at an Ampdraw of only 17A!
You could probably easily come in over 1:1 in thrust to wheight with this motor and batterypack if you choose for instance an 12X6-ish prop. That said, if you´ve allready got the Lipos then use them. This is a Cub, it will cruise at 1:3rd throttle if you set her up with the right prop (high enough pitchspeed to cruise well at part throttle), you should be looking at well over 40 minute flights if I´m correct (and you don´t fly her like a 3D plane of corse! ;) ).

The 4s 6600mAh packs I use in my Cub are definitely overkill and unneccessarily heavy, but the plane still came in well within the recomended specs. Plus, I enjoy haveing the ability to do loong flights! My bigger Cub is of corse more forgiving to any extra wheight, as smaller planes react more severly to any extra wheight.

To do your own calculations for the setup, take a look at the charts on this page: http://www.flyingmodels.org/
I´ve been using them for almost all my planes for a couple of years, they are measured figures, not off some calculator, and they have proven very correct when compared to the figures I have been able to measure after a setup has been finished.

Good luck with the conversion, it should turn out just great!

Edit: I mistook the motor for the 2820/10, but have changed the figures to reflect the /12 motor you have instead.

Anders O

[CHAMM128]

I kind of figured the batteries would be overkill, but I also like long flights and hopefully it won't come off to heavy. I am new to all the electric stuff and know nothing of figuring the amp draw and so on. I'll have to check out that link on the calculations. Thanks. All the help I can get is appreciated.

Chuck

[Zeroaltitude]

Chuck, I want to make clear that I´m no expert by any meens. The E-Zone and its members have been very helpful during the last 3 years since I entered the hobby, and I´m starting to get a grasp on what works and what doesn´t thanks to this site, the chart I posted a link to and trial and error with my own setups. I still have a hard time understanding most of the theory behind it all, but I´m slowly getting there too. At least I´ve got to a point where I can "visualise" how a setup will perform, and be right more often than not. Hopefully, you aren´t as slow a learner as I am, but even if (against all odds) you are, it´s no big problem! :D
If you have any questions, there are several REAL experts on these forums that are very willing to help out.

Anders O

[CHAMM128]

That's good, because i'm gonna need the help. Building planes from kits and ARF's is great, but this is my first at doing something different from what the manual recommends for the motor/engine. I am sure my setup will be on the powerful side, but I like to make sure I have enough. Electric motors are costly enough and if a more powerful one is the same price, i'll go for it.

Anyway, hopefully i'll get started on this project this friday or Saturday so i'll have some more pictures to post. Just been so busy these days with other stuff.

[tusk91]

I have purchased the .25 Nitro cub myself and I am curiuos to know if anyone has had a chance to build and fly this kit yet?

I purchased my Cub at:

http://www.raidentech.com/blj3picuniga.html

My setup will be:

MPJett AC 28/20-7 Brushless Motor
http://www.hobby-lobby.com/brushless-mpjet.htm#AC

Thunder Power 2100 2nd Gen battery

10X4.3 prop

This setup flies my 2lb 10ounce Combat corsair like a dream!

[scb28]

I found this old thread when I was searching for info on the Model Tech J3 Cub. I got one from a LHS. It was on closeout and I couldn't pass it up. Of course my first thought was that it would be a good candidate for conversion to electric.

The process was pretty easy. As you can see from the attached pictures, I simply cut away the wood motor mounts and used aluminum stand-off type mounts for an E-Flite park 480 motor I had sitting around. I used an E-Flite 40amp esc, GWS 'park' servos(light weight but plenty of power, 35oz/in), and a Corona 6channel Rx. Also shown in an attached picture, I installed a 1/16" ply mount for a 3s 2100mah lipo. I put it where the fuel tank was to be installed. That's about it for changes. I do have to remove the wing to change the battery, but better that than having to cut an access hole get to the battery. I was concerned that I might have trouble with CG but after a mockup to check, the CG was literally right at the recommended position. This was a very simple conversion.

This plane has a 48" wingspan and RTF weight is 41oz. I was afraid that the park 480 might not be powerful enough. However the first flight proved that not to be the case. It took off in about 20 feet and climbed smartly. I flew around for maybe ten minutes and then attempted a landing. I was flying off of a ball diamond and my final came up a little short and I landed on the grass and nosed over. No damage at all. Two more landings ended the same way. The Cub is a real floater and the ball diamond just wasn't big enough for me to land. But given a good runway, landings would not be an issue.

Overall, I'm happy with the outcome of this little project. I'm looking forward to many enjoyable flights with this Cub

scb28

Pictures are attached

[CHAMM128]

Finally, I looked at the date I started this and it has been almost three years from the time I started this plane. Other things came and went and I fianlly had the urge to finish the build. Heres a couple pics. Hopefully saturday this week it will maiden successfully. Of course I have to have a buddy present to take pics other wise it would be flying right now, there is like no wind at all. And he absolutely hates J3's. I think he wants to see it crash and burn is all.

Chuck.

[scb28]

Chuck,

my only input would be to make the hole in the bottom of the cowl bigger. I made mine maybe twice as big. I like the battery access. Is that the way you mount the battery, vertical? What motor are you using? And what size battery?

Good luck with the maiden. Let me know how it goes. Mine flies very well, kind of scale or a little better.

scb28

[CHAMM128]

The motor is an AXI 2820/12 and the battery may be small, a TP 3s 1320, but was all I have right now. I have some bigger cells, just have to make the pack. As far as the battery being vertical, we'll see how that works.

[scb28]

CHAMM128

Just curious, did you maiden your Cub and if yes, how did it go?

scb28

[CHAMM128]

I tried, but didn't get off the ground. Combination of too tall grass and probably low batteries. Since then I have sold all the electronics out of it and will probably sell the airframe.

Chuck.

[scb28]

Sorry to here that. Mine flies very well with an E-flite 480 and 10x6 prop. I'm using a no name 2100 3s battery.

scb28

[rc4stroker]

I know that this is an old thread, but I was wondering If anyone had any comments on if any of these combos would fly this cub?
http://www3.towerhobbies.com/cgi-bin/wti0091p?&P=SM&C=GHH&V=GPM

Thanks in advance.

[scb28]

rc4stroker,
This one would work fine: http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXRYN9&P=SM
I'm using a very similar setup. I have a Suppo 2814-1100kv motor and a Suppo 40amp esc. I use 2100 3s batteries also. FYO you can get the Suppo motor and esc from lightflight.com. It would be less expensive and their products and customer service are very good.

Regards, scb28

[rc4stroker]

scb28, Thanks for the info.
I had already ordered the exact combo from Tower that you linked, I figured that was the best fit too. I had some coupons and they ran out this month.
I will check out your link on the next model.
The airplane that I got had a 25 2 stroke on it sometime way back. It does not look like it is fuel soaked, so I think it will be a good electric flyer?

[scb28]

rc4...

Let us/me know how the conversion goes. I'd be interested.

scb28

[rc4stroker]

If I keep the stock cowling as, I will have a spacing problem.
Do you guys use something like this
http://www3.towerhobbies.com/cgi-bin/wti0001p?&I=LXLCL4&P=M
, or make your own?

I was thinking about hanging the motor on the firewall as is for now and give it a flight just to see if the combo is a match.

[scb28]

rc4...
Look at post #8 of this thread for a picture of the way I did it. The standoffs are 6-32 aluminum spacers I got from Ace Hardware, and 6-32 screws with nuts on the back side of the firewall. The top ones are a bit difficult to get in but do-able. I put a small amount of right and down thrust in and it has worked okay.

If you have any other questions, don't hesitate to ask.

scb28

[rc4stroker]

I did notice your post 8 before and it looks like a good way to do it and keep the weight down too.
I am going to try to mount the battery like post 9 so I do not have to pull the wing to change battery. It may be too much stuff up front?
Thanks for the help.

[scb28]

Yeah, I am going to get around to cutting a battery access in the bottom so I don't have to remove the wing to get at the battery. I was just lazy and wanted to get the cub in the air, so I took the quick/easy way out.

scb28

[rc4stroker]

Starting to put the stuff in the cub, hope to get a flight out of it this weekend.

[scb28]

rc4stroker

Have you maidened the Cub yet? If so, how did it go?

scb28

[rc4stroker]

Sorry I have not given you an update, but I have not been working on the cub at all. I will try to do better.

[scb28]

I know how it is. I always have multiple iron in the fire, as it were....or I just get lazy.

scb28

[rc4stroker]

I flew the Cub today without the cowl. The motor still will have to be out a few more inches to fit the cowl on.

It was a rocket ship, way overpowered with the combo.
It was pitchy, squirrely, and a little tail heavy.
My guess tail heavy is the answer to the above problems.


All of the problems can be overcome with some fine tuning.
All and all, It was a blast!!!