Mountain Models J-3 Cub Build

[Mojo]

Hey everybody!

So I've been meaning to get around to documenting the build of my Mountain Models J-3 Cub, but I've been pretty busy building it

So the first couple posts will be catch up (ketchup?)

Anyhow, this J3 is a 1/9 scale laser cut kit. The overall quality of the kit is astonishing.

It also comes with a printed manual but I suggest you also grab the pdfs: fuselage and wing. Some of the b&w pics are kinda hard to see.

[Mojo]

The J-3 Cub build continues...

I bought the servos, motor, esc, batteries, and what-not from Heads Up RC

Jeff was super helpful and shipped really fast.

I told him what I was building and he recommended this setup:

HURC 2712-12 motor
18A to 20A ESC
GWS 8040 prop
11.1v 860 30C to 11.1v 1500 30C Lipo battery

here's the full cart:

2 SERVO EXTENSION - 12" heavy duty - 22 gauge wire
1 SERVO "Y" HARNESS - 6" - heavy duty - 22 gauge wire
2 SERVO - 5g TP SG50 Ultra Micro Servo
2 SERVO - 9g TP SG90 Sub Micro Servo
1 HURC 2712-12 Outrunner Brushless Electric Motor
1 GWS 8040 Propeller - ONE BLACK 8" Direct Drive Prop
1 Power Up 20 Amp Brushless Motor ESC
1 11.1v 860mah 30C EZ Flite Lipo Battery

Like I said, I was surprised at how fast I got everything.

More to come shortly!

[Mojo]

Assembling the fuselage:

I rolled out the fuselage plans and covered them with parchment.
To be honest, I have no idea how you guys see thru that. I feel like I'm staring through a glass of milk.

The first steps involve gluing the side parts together. The parts are cut amazingly well. Unfortunately, they're not marked equally as well. Some of the parts have a faint etch on them, others aren't marked at all. If you take your time, you can figure out which part is called for. I recommend more patience and less glue

[Mojo]

The fuselage starts coming together pretty quick.

It's not quite at this point, but not too soon after, that I discovered that there is a left and a right hand side to the fuselage. They are identical, except for the push-rod exits. I think if they had been better marked, I wouldn't have swapped them, but then again it could have something to do with the beer. That said, I just needed to cut some new exits.

I am also apparently an ape (did I mention the beer?). After working on the giant Dynaflite Butterfly glider, these tiny parts are far too easy for my primate fingers to snap. I broke some formers and snapped a stick or two in the beginning. I think I have developed a lighter touch. I suppose that remains to be seen.

[Turbojoe]

Looking good so far. Glad to see you getting started on her. Remember, the LAST thing to do is glue the wing strut mounts to the wing and fuse AFTER you know the wing has NO warps and AFTER covering. If you aren't careful you can cause a warp when bolting down the struts if you don't glue them last. The struts are functional.

Joe

[Mojo]

Thanks TurboJoe! That's a good tip.

[Stevephoon]

Good start! Mountain Models kits are great. Don't force anything. You may need to sand the wood thickness down a touch or you have the wrong part if you can't get something to fit!

These kits are like 3D jigsaw puzzles. They almost build themselves.

Steve

[Turbojoe]

Originally Posted by Mojo

Thanks TurboJoe! That's a good tip.

You're welcome. I found this out the hard way. When I did the final assembly I put the plane on the washout jig and found the strut induced twist. Pulled the strut bolts and the wing was straight again. I ended up soaking the strut ends in acetone to loosen the CA then bolted everything back together with the struts causing no push or pull on the wing. Glued them in and all was golden. This is not a kit fault but more likely humidity causing wood shrink or swell that can't be factored into the cut.

It's amazing just how good this plane flys. It's worth it to make sure everything is just right on it. Don't make the mistake of overpowering it either. Most of my flying was at under 1/2 throttle. I'll bet she'll fly well on rubber band power.

Joe

[Mojo]

I mounted the firewall and added the side stringers.

I then did the hatches and false ribs.
The hatches are nice because they are magnetized.

You can see the bottom one in the attached photos.

If you look close, you can see where I had to reinforce the former after snapping it with my clumsy ape fingers. After "reinforcing" it, I realized the hatch wouldn't close and had to do some "skillful" cutting and sanding. Anyhow, the hatch works great now, but I won't win any awards for beauty on it.

[Turbojoe]

Mojo P.M.ed me with a question about the tail fillets. They are on page 18 step 3.4. When looking at the drawing the first two illustrations are looking down on the assembled parts (3 per side). The bottom illustration is a side view. Both of these views are before sanding and shaping. The illustrations are probably very close to the size of the actual parts.

I strayed from the instructions a bit and made a jig like you see in the picture I attached. I made this so I didn't have to worry about gluing the tail feathers in place by mistake. I slid my jig in place and glued the fillets in permanently. I then made another jig the same way and simply ground it away as I shaped the fillets. With the jig in place I didn't have to worry about deforming or breaking the fillets while I carved and sanded them to shape. When I was done shaping I simply slid what was left of the jig out and the tail feathers slid in and fit perfectly. Sometimes it's hard to type out how to do something. I hope what I did type makes a little sense. I'm checking other drives for pictures but I didn't take many of my Cub build and I don't remember if I took any of the fillet area.

Joe

[Mojo]

TurboJoe,

Thanks for the explanation. Unfortunately, I still don't quite get it.

I attached a pic of the fillet instructions. You said there are three per side, and the bottom one is a side view. Do you mean that each illustration is composed of three pieces? The top two illustrations appear to represent each side? But the bottom illustration (side view?) is strange. That's the one that trips me up.

That said, these are just fillets and I think I can go off script here if I have to. I need to noodle on your jig a bit more.

Thanks again for your help!

-M

[spad]

Or, use blue or similar foam rather than balsa....

[Turbojoe]

Originally Posted by Mojo

TurboJoe,

Thanks for the explanation. Unfortunately, I still don't quite get it.

I attached a pic of the fillet instructions. You said there are three per side, and the bottom one is a side view. Do you mean that each illustration is composed of three pieces? The top two illustrations appear to represent each side? But the bottom illustration (side view?) is strange. That's the one that trips me up.

That said, these are just fillets and I think I can go off script here if I have to. I need to noodle on your jig a bit more.

Thanks again for your help!

-M

Yes. Three pieces per side. They get laminated into one piece then carved and sanded to final shape. (I used wood glue to laminate as it sands easier and doesn't leave those hard ridges like CA does) Maybe these pictures will help. This is NOT a Cub tail and the fillet pieces are just scrap so they are for illustration only. Hopefully this helps. I searched my files and didn't find any pictures of this area from when I built my Cub. Sorry.

Joe

[Mojo]

Originally Posted by Turbojoe

Yes. Three pieces per side. They get laminated into one piece then carved and sanded to final shape. (I used wood glue to laminate as it sands easier and doesn't leave those hard ridges like CA does) Maybe these pictures will help. This is NOT a Cub tail and the fillet pieces are just scrap so they are for illustration only. Hopefully this helps. I searched my files and didn't find any pictures of this area from when I built my Cub. Sorry.

Joe

Oh, I get it now. Wow, I wish MM had included pics like that.
Thanks for your help TurboJoe!

[Mojo]

Landing Gear!

The kit comes with several pieces of 1/16 landing gear wire that you bend to shape over the plans.

The instructions say:

Be VERY careful and take your time. You want the landing gear to line up nicely, so measure 2-4 times and bend once.

I laid the wires onto the plans and then used t-pins to establish anchor points. I would mark a bend with masking tape and then bend it with vise-grips. Comparing the wire with the plans, I would then make any adjustments. Repeat over and over.

This was a slow process. If I was going to do it again, I might try tracing the wire pattern and then creating a jig using a 2x6 and some nails.

After you get the wires all bent correctly, you attach them to the landing gear mount using epoxy and some clamps. I think I got more epoxy on me than on the mount.

Finally you glue the fairings on to the sides of the gear.

All and all, I think I spent more time on the landing gear than anything else to date.

[spad]

I had an L-4 (hosed it due to my negligence) and flew it more than 600 times, repairing it several. It was my "go to" airplane, and I absolutely loved it-boohoo, but, maybe someday....

[Turbojoe]

Mojo,

I know you're done now but next time try bending gear wire with a vise. You get more precise bends that way. A cheap vise from Harbor Freight works fine and that's what I've been using for years. The Telemaster 40 is very close to this gear type but is 5/32"! A real nightmare bending that one up. I always destroyed more wire than I used each time I built one.

Theres a small amount of challenge but a ton of fun in building this kit. You're well on your way to completion. You'll be flying her in no time.

Joe

[Mojo]

Originally Posted by Turbojoe

Mojo,

I know you're done now but next time try bending gear wire with a vise. You get more precise bends that way. A cheap vise from Harbor Freight works fine and that's what I've been using for years. The Telemaster 40 is very close to this gear type but is 5/32"! A real nightmare bending that one up. I always destroyed more wire than I used each time I built one.

Theres a small amount of challenge but a ton of fun in building this kit. You're well on your way to completion. You'll be flying her in no time.

Joe

Joe, I'm not sure why I didn't do it that way. That sure would have been easier. Perhaps it was the glue fumes

Thanks for the tip!

[Mojo]

Time to catch up the build log...

The stab, elevator, fin, and rudder all go together easily.

I test fit the vertical fin and set the fillets (thanks to TurboJoe for the previous clarification).

In the last photo, you can the lines I marked on the fillets. These are for sanding the fillets down flush to the fuselage sides.

[Mojo]

The motor mount instructions are a bit of a head scratcher.

They really only go together one way, but the instructions indicate that the kit comes with 3 different motor mounts: a rear mount, a front mount, and a stick mount.

I wasn't sure of the the difference between the last two. I just built the one that seemed right and re-drilled two of the holes to fit the HURC 2712-12 mount.

[Mojo]

On to the wings...

Step 1 of the wing instructions say:

"Glue the 1/16" hard balsa spars together, as show in the image below..."

Unfortunately, the image below is a line drawing that only shows a single joint and not the two that are required. If you aren't careful you may end up gluing together the two end pieces. I did this, and then immediately smacked my head to my forehead.

I then had to debond the CA (yay!)

You can see the reglued spars. What a pretty joint!

[spad]

You can always call Brian and ask questions during the build.

[Turbojoe]

For me the head scratching is what makes building fun. If there was no challenge at all you may as well just slap an ARF together like everybody else. You're working through those little challenges and doing a great job so far. The satisfaction you feel when you maiden the plane that YOU built will make any frustrations now seem minor in comparison. You're in the home stretch now. Keep us posted as you go.

Joe

[Mojo]

Originally Posted by Turbojoe

For me the head scratching is what makes building fun. If there was no challenge at all you may as well just slap an ARF together like everybody else. ...
Joe

Oh, I agree. There needs to be some challenge. I don't understand why anybody wants a totally prebuilt plane.

I kinda feel like the point of these build posts is to document the challenges and gotchas. I hope nobody thinks I'm griping. This kit has been top-notch quality and a pretty easy build. Most of the problems I've run into were of my own creation

[Turbojoe]

Originally Posted by Mojo

I hope nobody thinks I'm griping.

Heck no! It's good to see posts where build problems can occur. I just hope you're not getting frustrated is all. Your posts are going to help someone else with their build because they'll see where a build step may be confusing and you clarified it for them.

Take a look in the fuselage manual at step 29 & 30 for the "false ribs". I had the plans and manual right in front of me and was well aware that there was a left and right side. I STILL managed to build, sand to shape and glue them very securely to the fuselage only to realize I did them backwards! ! I ended up cutting them off the fuse and scratch building new ones....... I made many other mistakes but this is the only one I'm going to 'fess up to here.

Joe