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Build Log -- Mountain Models J-3 Cub beta

Old 09-06-2006, 04:29 PM
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Default Build Log -- Mountain Models J-3 Cub beta

Brian Eberwein at Mountain Models/Laser Arts was kind enough to offer me a beta kit of his new J-3 Cub. I'm always excited to have a new project, and even better, a new scale project, to tackle.

This one is a little larger than the usual MM kit, with a 48" wingspan, and 332 sq in. Weight on Brian's alpha is 12.6 - 13.4 oz depending on the battery, with a wingloading in the 5.5 - 5.8 oz/sq ft range. The model is obviously light enough to be flown with a lot of different power systems, from small outrunners to geared inrunners. I haven't picked a power system yet, but have a couple of GWS Pico servos for the ailerons, and a couple of Hitec 55's for rudder and elevator.

Brian has been terrific about getting me started, and answering a ton of emails. Keep in mind that this is a beta, being built without plans or a manual. I'm sure that production kits will have some changes as Brian tweaks the design, so future builders should note that their kit may vary slightly from my build pics.

The first pic is the obligatory shot of the contents of the kit.

Mark
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Old 09-06-2006, 04:32 PM
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Because nothing screams "J-3 Cub" to me like the tail feathers, and because I knew they'd be easy to do, that's where I started. Everything, of course, is laser cut, out of quality wood. Part's fit was smooth and tight.

Mark
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Old 09-06-2006, 04:37 PM
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The fuselage was next. The center area formers are tabbed/slotted into one of the sides, glued (hopefully perpendicular to the fuselage side), and then the other side added. The cabin top is also inserted at this point, creating a sturdy "box" in the center of the fuselage. The other formers are then put in place. The very tail area is sanded, using laser-scribed lines, so that the fuselage sides mate nicely. Everything is tabs, slots, and laser cut joints, so building a nice straight fuselage should be an easy task.

More to come later!

Mark
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Old 09-07-2006, 01:30 AM
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Darn it! Now I will have to buy yet another kit! (when available)

Nice, I'll be watching this thread.
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Old 09-07-2006, 02:18 AM
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Mike,
Buy one. You won't be disappointed. Even though it is a pre-production kit, it's a lot of fun to put together.

Mark
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Old 09-07-2006, 10:01 PM
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The fuselage build continues with the rest of the formers, sanding the rearward tail area, and mating the ends of the fuselage. Here's a pic of the rest of the formers installed.

Mark
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Old 09-07-2006, 10:04 PM
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Here's a pic of the basic fuselage completed, from the top. Looks pretty good to me!

Mark
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Old 09-07-2006, 10:06 PM
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Next the cabin sides are installed. Easy to do with everything tabbed, slotted and laser-cut joints. It's starting to look like a J-3!

Mark
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Old 09-07-2006, 10:08 PM
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To finish up the basic fuselage construction, the tailpieces are installed along with the fuselage longerons on both sides and the top. There are also side longerons that stick out from the side of the fuselage, to give the appearance of the original's structure. The second picture shows the detail of the tail area. If you enlarge the picture, I think you can see the side longerons.

I forgot to take a picture of it, but the nose area forward of the dashboard is sheeted with some 1/32" balsa.

Mark
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Old 09-11-2006, 12:36 PM
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Default Wing Construction

The wing consists of a three piece main spar and a two piece rearward/trailing edge spar. Probably the most complicated part of the whole build (according to Brian) is the center section of the wing, which is built up first on the main spar. A very small amount of dihedral is built into the main spar. The wing is built as a one piece unit.

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Old 09-11-2006, 12:37 PM
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After the leading edge area of the wing is built, the center trailing edge section is built and added.

Mark
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Old 09-11-2006, 12:39 PM
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Now the rest of the ribs are added to the wingtip. The horizontal part of the leading edge is now installed. Other than the center section, I haven't glued anything else together yet.

Mark
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Old 09-11-2006, 12:41 PM
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The vertical part of the leading edge has been added, as has the trailing edge piece between the aileron bay and the center section, and the wing tip.
If you look at the picture, you can see that the third rib from the tip is a little thicker just aft of the main spar. That's two pieces of 1/64" ply that are glued on either side of the rib to provide the aileron servo mounting area.
The aileron servos are hidden inside the wing. These pieces should be glued onto the rib prior to installing that rib. Also, cap strips top and bottom complete the main spar, giving you a very sturdy wing. The wingtip consists of two pieces which slot into the outermost rib, the main spar and the rearward spar that also makes up the aileron bay trailing edge. Piece of cake!

At this point I glued up all the ribs, wing tip, trailing edge, etc.

Mark
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Old 09-11-2006, 12:46 PM
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Here's a pic of the wing with the aileron servo installed, a second pic of the ailerons built up, and a pic of the wing with an aileron in place. The ailerons have a ">" leading edge, so they will be CA hinged. Because I used GWS Pico's for the aileron servos (with their short leads), it looks like I will be using 24" servo extensions through the wings.

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Old 09-11-2006, 12:46 PM
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I am not a particularly quick builder. While I did not keep close track of my build time, up to this point I would say I am at 8-9 hours, for all of the major subassemblies (fuselage, tail feathers, wing). The wing struts consist of just three laser cut pieces, so that should be a five minute job. Overall, a very fast and straightforward (even easy) build.

Mark
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Old 09-13-2006, 12:44 PM
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A llittle more progress to report. The first is the landing gear and the rudder with the tailwheel wire installed. Brian provided the full size pattern for the scale gear. It was bent up from 1/16" wire, and secured with lashings of Spiderwire and thin CA. I have used this technique on models weighing 2 1/2 to 3 lbs, and have yet to have the gear fail. The tailwheel wire is simply epoxied in place.

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Old 09-13-2006, 12:45 PM
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The wing struts were assembled in about two minutes.

The rudder and elevator servos were installed, based on the location provided by Brian and his experience with the prototype. I used a Dubro Parkflyer guidetube/wire set as I had it on hand.

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Old 09-13-2006, 12:49 PM
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With all of the major subassemblies done, I thought it would be time to see what the airframe weighs. The weight shown includes all the balsa plus the four servos (two Picos and two HS55's). 6.125 ounces. I think Brian's weight of 13-14 oz. is very reasonable, depending on the motor chosen and the rest of the scale bits. At 332 sq in, I think this model could easily be converted into rubber free flight.

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Old 09-13-2006, 12:51 PM
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Because no build thread is complete without the "in the bones" pictures, here are two of the assembled model. Time to start covering. It certainly looks like a Cub! Brian told me over the weekend that he hopes to be able to get me a prototype cowl this week.

Mark
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Old 09-13-2006, 03:24 PM
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Really cool build thread! Can't wait to see what it looks like once it's all finished up. =D
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Old 09-13-2006, 04:45 PM
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Originally Posted by PerlAddict View Post
Can't wait to see what it looks like once it's all finished up. =D
Hi PerlAddict,
Me too! I'm anxious to get this one finished up and in the air. It really has a nice presence, due to the 48" wingspan.

Thanks for posting, btw. I was beginning to think that no one was looking at this thread.

Mark
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Old 09-13-2006, 05:45 PM
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Oh no, I'm definitely keeping up. I just am new to the hobby and have absolutely no idea where to even start to talk shop on kit building. But being new, it's nice to see a detailed journal of a build with lots of photos along the way to get a better idea of what goes into a project like this rather than just, "Here's what it looked like when I got the box, here's what it looks like all finished." I certainly appreciate all the time and effort so far in your updates. Keep up the good work! =)

- Dave
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Old 09-13-2006, 09:44 PM
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Default nice looking model

i love the j3's always have do you have a link for this kit??
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Old 09-13-2006, 10:40 PM
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Dave, thanks again. This one is a pretty easy, straightforward build. It could be a first balsa build if you took your time. Because this is a pre-production kit, I don't have instructions, etc. I built the MM/LA P-51, and the instructions were detailed and excellent, so I have no doubt that the instructions for this kit will be of the same caliber.

dmmalish, the kit is not yet available. But it should be very soon. You will be able to find it at mountainmodels.com when it is in production.

Mark
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Old 09-13-2006, 10:45 PM
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Default found it

look like reasonable prices on there kits how does the p51 fly and what are you using for a power system on that? thanks
dave
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