Mountain Models J-3 Cub Build - RC Groups
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Mar 09, 2013, 09:02 PM
Ivan Dubrov
Build Log

Mountain Models J-3 Cub Build

Finally I got my MM J-3 Cub shipped and delivered!

This would be my second-and-a-half balsa build (first one is House of Balsa Acro Cub 52", first-and-a-half is PuddleBug from Stevens Aero). I will build two wings for it, regular and clipped.

I want it to be a replacement for my HOB Cub which isn't perfect (well, I guess, this one would not be either!).

I plan to put the following electronics into it:
  • HS-5045HB servos for tail and ailerons (want to play with digital servos).
  • Scorpion SII-2205-1900 motor
  • CastleCreations Thunderbird 18 ESC
  • AR600 DSM2 receiver
  • 2S 1300mAh LiPo

I will cover it with AeroFILM (I want more opaque colors).
Last edited by WFrag; Mar 11, 2013 at 12:19 PM.
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Mar 09, 2013, 09:11 PM
Ivan Dubrov
Now some building stuff!

Well, it turned out that self-jigging puzzle-style technology works really well. I took me about 6 hours total to glue all balsa parts together, hence only one photo at the end. However, you still have to check everything is straight. I made few mistakes (did not notice that part was bent by tension), so had to make few adjustments.

Comparing with my HOB kit building experience I found the following pluses and minuses:

  • Builds very quickly
  • Easier to build straight and square
  • With CA glue you don't have to spend 30 minutes to invent a jig for keeping parts together while glue is setting up.
  • Broken balsa parts are easy to repair — just drop thin CA into it.

Minuses (so far):
  • Builds very quickly
  • I find documentation slightly weaker than HOB one

Overall, I have very good impression of the kit so far.

I can't evaluate balsa quality due to lack of experience, but it seems to be fine so far. One part (1/32" balsa) was broken, but it was easy to repair. Laser cuts are good, parts fell off after little bit of flexing the sheet. No over-burnt parts.

Documentation is pretty clear, however it is not ideal. It is sad (to me) how few tiny things make kit less perfect than it could be. Like two parts named the same. Or parts that do not have names at all and you should guess which one you need (true, it is not a toy for 3-year old kids, but I like to have extra confidence while building). Or images I can't see the details in (or even missing the part it supposed to display!).
Last edited by WFrag; Mar 09, 2013 at 09:19 PM.
Mar 10, 2013, 12:14 AM
Ivan Dubrov
Landing gear waiting for epoxy to cure. Was not too hard to assemble them, but required some patience. Easy to bend in a slightly offset location. Also 1/16" wires are quite hard to cut without special tools, too.
Mar 10, 2013, 12:18 AM
Ivan Dubrov
Tried fillets blocks fit and found they are slightly smaller than former in front of them (F11). Also, the smaller fillet block have is wider than other two for some reason.

Does that mean I did something wrong or I simply need bigger blocks from scrap balsa? Need to check instruction and plan carefully...
Mar 10, 2013, 07:34 PM
Visitor from Reality
Bit of a guess here, and maybe only useful if those fillet blocks are not glued in place. The middle block - take it out, turn it 180 degrees end for end and re- place with it's top front edge lifted up where you can start to shape it. The slope on the end that's at the rear now suggests this, as that slope looks to match that end for end swap.

Hope that helps

Mar 10, 2013, 10:29 PM
Ivan Dubrov
I don't know what was wrong with the filler blocks. Everything seem to match the plans. I added piece of scrap balsa to them.

Now to the fun part — covering! I decided to cover tail feathers before assembling the wing. Fuselage will be covered after the wing, as suggested in the instructions.

It took me about two hours to cover one of the simplest parts — stabilizer. Even little shrinking of Solarfilm warps the part. After multiple tries to make it straight, it is still not ideally straight, covering is not super-tight, but should be good enough.
Mar 16, 2013, 12:15 PM
Ivan Dubrov
After few evenings of work tail surfaces are covered. This time I won't make mistake and cover it in "Cub Yellow", as the previous one! Also, for this build I am not using trim covering.

I added two triangles to both sides of rudder to reinforce area where tail wheel is attached, there is a visible bump in covering in that area.

I am happy with SolarFilm so far, although I can't say it is absolutely superior to Monokote (which I used in my first build).
Mar 16, 2013, 01:13 PM
Registered User
Nice progress! Keep the pics coming.

I built one of these a couple of years ago (clipped-wing version) and they fly amazingly well. Unfortunately, I lost it in a dramatic mid-air that crumpled the fuse back to about mid-cabin! The battery plank/hatch design is really nice...the twin hatches make it super easy to position the battery. For that piano wire, I recommend linesman at Harbor Freight, and they'll go through 1/16 wire without even breathing hard.

Mar 16, 2013, 09:32 PM
Ivan Dubrov

Today I managed to assemble the whole wing! This kit assembles really quick. Seems like the major part of the build is actually sanding and covering.

On the first picture, wing is assembled without a single drop of glue! I can't tell if I like this technology or not. It builds quick and straightens itself, but all these small parts and tabs are really fragile! I broke few of them.

Also, it seems like dihedral is not 100% symmetric. Not much, but still. I will try to fix this with covering.
Mar 16, 2013, 10:52 PM
Registered User
Joker 53150's Avatar
Originally Posted by WFrag
On the first picture, wing is assembled without a single drop of glue! I can't tell if I like this technology or not. It builds quick and straightens itself, but all these small parts and tabs are really fragile! I broke few of them.
I've built the MM Lucky ACE and Switchback and the Lucky ACE is exactly like you describe above. The Switchback needed a little extra work to make sure it was aligned properly before gluing, but nothing compared to a Chinese "kit" I recently built. If you simply enjoy creating a quality finished balsa model then certain MM kits like this are a great choice. For those who like to problem-solve and really put some effort into it there are better choices.

I, however, love how well they go together.

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