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Jun 20, 2005, 02:54 AM
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My first (semi) scale electric project - J3 Cub

I've had my Black Horse semi scale J3 Cub for a couple years now. It is the same ARF plane that Hobby Lobby sells under the Graupner name and is featured in a review in Ezone. I *finally* am taking the plunge and starting to work on this project.

I am pretty much using the same set up as Hobby Lobby recommends. I'm going to incorporate a AXI 4120/14 motor for motivation and use a Hacker Master 77 - 3P Opto controller with a UBEC. I have two CP2400SCR 8 cell battery packs from that I will wire in series to give me a total of 16 cells. I was thinking of also experimenting with a AXI 4120/18 motor to see if there would be any performance changes. I will be using a APC 13 x 8 electric flight prop. Servos are Hitec high torque HS-635HB servos. I was thinking of going with lipo batteries *but* they are expensive compared to nicads.

Since this plane will be my wireless video camera platform (video camera, transmitter, receiver, and monitor from I wanted to go sixteen cells to have enough power to carry the extra payload. I've seen a thread that has a lipo battery set up of the same plane here on RCGroups so that has bolstered my confidence in setting up this project and getting if "off the ground" .

I was hoping that other experienced builders/pilots would give me some input and feedback on my project and setup. I was hoping that I would not have to make a hole through the firewall for setting up the batteries but it seems like others have gone the same route. I guess it is easier to cut out the firewall and install a battery tray then slide it into the plane then put the cowl back on then mount the prop (whew). I was hoping that I could keep it a bit more easier than that but no because it seems like it would be difficult to install the battery tray through the fuselage opening where the wing mounts.

Mahalo for looking and hope to get some input/feedback.
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Jun 21, 2005, 05:35 AM
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glide's Avatar
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Jun 21, 2005, 03:45 PM
Glide, I donīt know what the expected AUW of the Graupner Cub is with your conversion, but I know it is around 180cm (71") wingspan and 2800gram (6,2lbs)recomended AUW for glow.
Let me put it like this:
I have converted the Thunder Tiger 1/5th scale (81" ws) J3 Cub to electric power. Iīm using an AXI 2826-10 spinning an APC 12X8"E" prop powered from a 4s (14,8V) 6600mAh Lipopack. Iīm using a Jeti 45A ESC, Ultimate BEC, MZK Quatra RX and four Bluebird/Waypoint BMS 380 Max servos (bb/metalgeared micro/mini servos). The AUW of my Cub is 3,2kg (7lbs) (the recomended AUW for glow is 6,6-7,7lbs).

My Cub, which is significantly larger than yours (and probably heavier also) flies very well on this lighter setup with a much smaller/less powerful motor than what youīre planning to use. I get short takeoff from our grassfield (30' or less), strong climb (much stronger than scale) and enough vertical for loops from level flight and stallturns plus other non-scale manouvres. Iīm pulling 14A inflight average, and get around 25 minutes of flight (28minutes theoretically, but I donīt want to risk it) out of each of my 2 batterypacks, so duration is seriously good!

I would seriously consider changing a couple of things if I were you.
First and foremost, I would be to go with Lipos if possible to save wheight. Not because your plane wouldnīt FLY with the extra wheight, but because it may not fly very "Cub-like" due to the high wheight and lower wingarea. You would not need a pack as large as mine (6600mAh and 595grams or 21oz)), a smaller 4s pack would save you considerable weight. But even the large pack that Iīm using would save some 150grams (5,3oz) compared to the packs you intend to use. If you went with a lighter 4s Lipopack (say somewhere around 4-4,5A), you could easily save 330grams (12oz) compared to the packs you are proposing.

Also, youīd save about 140grams (5oz) by using the lighter AXI 2826-10 (or -12) compared to the 4120 series motor, and I can tell you that you will not need the power from the 4120! Not if you intend for the plane to fly like a Cub anyway.

That is almost a 17oz (1lbs) wheightloss on a 6lbs plane!

What you are proposing will surely fly, but it will fly rather fast, and it will have a lot more power than what is really neccessary, but will also use a lot more power than neccessary while pulling around a lot more wheight than neccessary.

Hope this helps!

Anders O
Last edited by Zeroaltitude; Jun 21, 2005 at 04:04 PM.
Jun 21, 2005, 04:10 PM
Visitor from Reality
Do a search on the Cub in this forum. It's possibly the most popular conversion type - what a surprise! - and about every BARF 'kit' around has been electrocuted at some point.

If you're going to haul cameras and suchlike around, you might want to consider something like a Sig Kadet Senior, or Hobby Lobby's "Telemaster". Your Cub will involve you in a lot of compromises,being as that's the nature of any model that gets close to 'scale', whereas a Kadet or Telemaster has a lot more fuselage volume, easier structure to work around and are generally more forgiving of extra weight than a 'scale' model.

Hope that helps

Jun 21, 2005, 06:07 PM
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glide's Avatar
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Mahalo zeroaltitude and Dereck for responding to my post.


Looks like you are knowledgeable about the plane that I am working with because all the specs (AUW, etc.) you mention are correct. I would have gotten lipos but those batteries are more that what I want to spend for them. I already have the nicad packs as I mentioned in my initial post so I will going with them. In regards to my motor, I figure this is what Hobby Lobby reccomends and I already have it (including the 4120/18) so again I will incorporate it in the project. I could sell those items off (at a loss) but I'm already headlong into the project so I just have to make due with what I've got. I so it looks like I do have a lot of power with the motor(s) I have at the moment. It just builds my confidence that I will be able to get it flying despite the weight factor. I appreciate you adding to my confidence with your critique of my setup. If it doesn't fly scale (a.k.a. "fast") that will be okay just as long I am able to get it off the ground with a not too long take off distance.


Thanks again for responding to my posts. I did do a bit of searching and saw a great video of a similar plane with the same motor taking off and landing - awesome! I am hoping that I will have the same results even with the heavier payload. I did consider a Senior Telemaster for carrying my camera but I got a great deal on the ARF Cub plus I didn't have to build it and cover it like I would with a telemaster.
Last edited by glide; Jun 21, 2005 at 11:18 PM.
Jun 21, 2005, 07:17 PM
Glide, best of luck with the conversion!

Regarding the cost of Lipos, I hear you. However, the cost has dropped significantly here in Sweden after a new brand became available. For the Cub, Iīm using E-Power Lipo packs. The fact that these lowpriced packs became available here is one of the main reasons I decided to convert the 1/5th Thunder Tiger Cub in the first place. The cost for one E-Power 6600mAh 14,8V pack is half of that for a comparable E-Tec Lipopack, making larger conversions a lot more feasible. They will not provide quite as much Amps as the E-Tecs either in peak or sustained use, but it sure is enough for most users (the 6600mAh I have provides 55A sustained and 99A in peak).
It might be worthwile looking into these packs, perhaps itīll be cheaper than you expect.

The 41 series motor will provide plenty of power, and Iīm sure the plane will fly with authority on the round cells you have planned for it. If it doesnīt, it wonīt be because of a lack of power, thatīs for sure! If the plane becomes a pig with the extra wheight from the round cells, you could allways switch to Lipos later on.

Anders O
Jun 21, 2005, 11:29 PM
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I checked out a lipo pack at Tanic website (I like their lipos because they have the taps built in). A 7500 mAH 3 cell lipo pack is $350!!! WOW. This is almost three time the cost of the ARF Cub I'm working on! Much too much to deal with at the moment. I'll see how it goes with my setup and if I could shake the money tree a bit more, I'll invest into the lipos because I know it is lighter and flight times will last longer.
Jul 09, 2005, 06:13 AM
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Just an update:

I finally got the miscellaneous parts I needed to complete this project. The UBEC came in as with a few other connectors and odds and ends. I got the aileron servos installed and now the rudder and elevator servos are going in. I was able to mark off on the elevator where it will be mounted on the fuse as with the rudder. The AXI motor and mount was hung on the front about a week ago. I should be finished before the weekend is over and hope to have a flight report and maybe some onboard video in the very near future.

BTW, the battery packs fitted in nicely where the fuel tank would normally mount. I just have to set something up so that the batteries will stay in place which should be easy enough. I'll get some photos posted when I'm all done. I hope to keep my weight less than 7.5 pounds so I hope that where the batteries are will allow me to balance it close to the proper flight CG. I will also have the onboard video camera power pack to help in balancing the plane.
Jul 09, 2005, 05:01 PM
Looking forward to the maiden report and some pictures!

Anders O
Aug 21, 2005, 05:43 PM
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Today was the day for the J3 maiden flight. I got in touch with one of the field marshals at the local AMA sanctioned flying field and got all the info about flying there earlier last week. So I was up early this morning and packed my car with all what I needed to fly.

I'll just cut to the chase and say it wasn't a good day for the J3 Cub because right off the get go I had problems. As I throttled up the motor the Cub pulled to the right as it lifted off. I tried to correct, but it I couldn't get it straightened out. So I cut the motor and it rolled off to the right of the runway into a fence. No major damage other than the cowl being scuffed by the fence.

So I gave the plane a good once over and found no other damage to the plane. Great, I've got another chance to get this plane off the ground. I waited until the air was unoccupied and gave it another whirl. I throttled up again and again the plane rolled to the right. This time I was not so lucky as the first attempt. The Cub kept on rolling to the right and into the runway. I may have been about three feet off the ground. I just couldn't correct it. I didn't have my rudder coupled to the ailerons and used the aileron control to get it to come back to the left. No dice.

Some at the field mentioned that I may not have it balanced laterally. I'm also going to see if the wing is not twisted. I'm going to check that after I repair the damage. BTW, the damage was slight -the prop was broken, the Axi motor mount got bent, the cowl was slightly cracked. But that was the end of my flying day.

Oh well, things could have been worse.
Aug 21, 2005, 06:10 PM
Lateral balance could be an issue. However, if this Cub flies anything like every other reasonably sized J3 Cub Iīve flown (including my 1:5th scale TT ARF), then it might also be a matter of crosswind.
The J3 Cub may look like an innocent little plane, judging by the looks alone, it would make the perfect ailerontrainer. However, it has a few surprises up itīs sleeve. It will groundloop like crazy, and if it starts to veer off to one side, thereīs really nothing you can do except cut the throttle and hope for the best. Even a small amount of crosswind can cause a groundloop, and I do meen small. My plane wheighs 7lbs, and it will take wind very well. Crosswind however nomatter how small is a BIG no-no!

Trying to correct with the ailerons will lead to disaster, the ailerons have a serious case of adverse yaw, which meens that they may move the wings allright in the roll axis, but they will also yaw the nose of the plane in the opposite direction (in other words, INTO the allready occurring groundloop).

The groundloop can only be corrected with the use of rudder, and even then the outcome is a gamble once the Cub is starting to veer.

So, for ROG, allways set her up straight into the wind, and if a gust changes the direction of the wind, then wait until it calms down.

Itīs really the same in the air. Rudder is needed for coordinated turns, aileron will bank the plane just fine, but it wonīt turn. Instead, you may end up in a 45 degree nose-up attitude with the wings pointed vertically! Use the rudder and ailerons in coordination.

The way I fly my J3 Cub is with differentiated ailerons to reduce the adverse yaw. My aileons will only travel 30% down of the up-travel. This helps a lot with adverse yaw, but I still need to use rudder more than ailerons to turn the plane. I do not use a mixing between aileron and rudder, as silly as it sounds, I feel that this is "cheating" (for ME, I donīt judge anyone else who wants to make use of the technology we have today). When I want to turn my plane it usually goes something like this:
-Rudder to initiate the turn
-Coordinated ailerons to bank the plane into the turn
-Up-elevator to keep the altitude level through the turn
-Slight amount of opposite aileron to keep the bank from getting too steep
-Let go of the rudder to exit the turn, while keeping a small amount of opposite aileron until the plane has leveled and is flying straight

Iīve let a few people fly my Cub (reasonably experienced pilots, but with no "Cub-experience"), who couldnīt understand what I was talking about when I said it was no trainer, afterall, itīs cute so how hard can it be? Let me assure you they were quite aware of what I was talking about after having testflown it.

Itīs NOT a hard plane to fly, but one needs to be aware of itīs "personality".

Anders O
Last edited by Zeroaltitude; Aug 21, 2005 at 06:15 PM.
Aug 25, 2005, 03:53 AM
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Thanks ZA for posting your thoughts and insight with the issue I am having with the J3 Cub. I must admit, I was taking off slightly crosswind so your observations may have merit. I double checked my wing and there was a slight twist to it and I took that out with the heat gun. I don't think that would have been the problem though because I just couldn't get the plane to straighten out even with all the aileron control I gave it when I was airborne. Could it also that I should have some offset thrust on the motor? At the moment, I have it at no offset. I notice that the Axi has quite a bit of torque.

Another aspect worth mentioning is that I did roll on the throttle quite quickly at take off. Would that affect the plane so much that it would torque violently to the right and not allow me to have much roll control? I'll ease in the throttle control the next time I am out to the flying field after I have what I can squared away like the lateral balance, CG, etc. A long time R/C pilot mentioned to me to try gliding the plane first to make sure that there was not off power issues. When that is accomplished, he said to go out and see if there is still any on power issues.

Wish me luck!
Last edited by glide; Aug 25, 2005 at 11:50 AM.
Aug 25, 2005, 11:50 AM
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Aug 25, 2005, 05:35 PM
Regarding rolling RIGHT on throttle, that is not a symptom of torque, unless you have the motor running in reverse and a "pusher" prop on it. If torque was the problem, it should roll to the left, not right.

Iīd say that unless you had a MAJOR warp in the wing, the real problem was crosswind, AND correcting the groundloop with aileron instead of rudder before the plane had accelerated above stallspeed.
If you use aileron before the wing is flying, it will drop the wing into the bank, it will drop like a rock!
Use rudder instead, while it may not be as effective a low speeds, it is the only way to correct a situation like this since it will not agrevate the stall.

Iīd be very afraid of "gliding" a plane like this. How did he propose youīd do that? Throwing a 5-6lbs plane fast enough to get it flying without the help of the motor is NOT an easy task.

Best of luck!
Anders O
Aug 27, 2005, 11:44 PM
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Thanks again Anders O. I've checked the wing with a couple incidence meters and there is no problems with warpage on the wing. I finished making the repairs on the plane which were relatively minor. As you may assume, I am a bit ignorant on flying this large tail dragger. I thought it was a trainer!

In any case, the place where I would have tried gliding it has a some slope lift and a gradual grade. It would have glided there well but I am kind of concern about doing that so I decided not to chance it. As you mentioned, it is quite a heavy plane to chuck out.

I'll give the plane another whirl again soon. Thanks again.

BTW, I emailed Tanic Packs on getting an equivalent lipo pack for the Cub. A 5S 2 P 6500mah pack is about $300. I will need to shake the money tree a bit before I could get one of those packs.
Last edited by glide; Aug 27, 2005 at 11:51 PM.

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