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Balsa USA 1/4 Scale J-3 Cub

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Old 02-12-2011, 04:09 PM
  #1
PA BEN
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Default Balsa USA 1/4 Scale J-3 Cub

Just picked up this plane from a club member for free, it's a kit and he built it along time ago. The wings broke free at the fusalage other then that no damage. I will post on this thread of the rebulid and questions I have. I plan to recover the whole plane and paint it with normandy colors. I've never done this before so I will have lots of questions.
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Old 02-12-2011, 11:58 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

Oh nice -
some work to be done, but It should be able to be made into a really pretty model !

Gotta love free models....

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Old 02-13-2011, 06:12 AM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

Hi PA BEN
You mention it is a 1/4 scale. In my book that usually means it is large, about 80 inch wingspan. Is that about right? I notice that the wing was not removable. It looks like it was glued in place. How do you get the plane to the field? Most planes that size have a removable wing. Am I missing something? I guess it can fit into a truck or van. Just realized that since I have always used a compact car to carry my planes, fitting it into a car is more important to me than to others. Good luck with your build.
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Old 02-13-2011, 06:28 AM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

A VERY restorable plane, looks like.  Best of luck, I'll be watching with interest!
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Old 02-13-2011, 06:54 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

The wings bolt to the fuselage, and the struts bolt to the bottom of the fuselage. It’s a 108” wing.
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Old 02-13-2011, 07:28 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

Thanks, PA BEN. When do you think you will maiden it
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Old 02-14-2011, 06:02 AM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

It will be awhile. Going to take my time, I have never recovered a plane before. So I need to shop around for covering that won't break the bank that I can paint. My wife is really keeping an eye on the money I spend on this one. All the servos are in the front, and the wing uses one servo with rods working the alerons. After I get the old covering off I will wing mount the servos and see I want to do w/the others. The builder had a gas in it before, I have a supertirger G2300 to put up front.
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Old 02-14-2011, 03:57 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

Just picked one of these from a fellow club member wings are perfect fuse is pretty rough I need to get a hold of a set of plans so I can see how the fuse goes together. Picked up a used Zenoah 20 ei for this project. It may not get finished until next year got to many projects on the board
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Old 02-14-2011, 07:12 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

I might get the plans from the builder if he can fine them. Found this on the web today, looks to be close. http://manuals.hobbico.com/top/rc28-j3-cub-manual.pdf
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Old 02-17-2011, 05:52 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

Hey PA Ben
You have yourself one sweet flying Cub there, if your into Cubs and scale flying. Good luck with the rebuild.

Morgan
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Old 02-17-2011, 11:24 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

i cant wait to see more pics of this restoration!
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Old 02-18-2011, 05:13 AM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

He gave me the plans and enough covering to cover the hole plane. Still in the process of removering the old covering w/post more pict.s
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Old 02-18-2011, 05:36 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

Questions; I'm stripping the wings and finding dents and cracks. Do I CA the cracks and use filler? And what is a good sand-a-boul filler?

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Old 02-18-2011, 11:06 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

You could Ca the cracks but alittle PVA on them would be better and allow athin film of pva to spread over the crack so it gives it alittle extra strength and grab. As for the dents, try some wet toiletpaper and place it over the dent and try get the wood damp and leave it for 12 hours or so and keep wetting the paper till the dents lift up, you may also need to sand the area afterwards to level it all up, that should help raise the minor dents however any serious dents will need inspecting to see if they are going to structualy impare the model. but yes get the wood in the dent to absorb alittle water and it will bring it right back out, takes time but far better then adding weight on with fillers. hope this helps

incedently if you cut out the shear web where the damaged sheeting is it may be possible to put a thin sheet of balsa behind the cracks in the sheet to reinforce that area and that should work a treat.
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Old 02-19-2011, 06:09 AM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

 

Took the tail apart last night. It was weak and moved around w/the covering on. Found some repair needs and reinforcing to do. The rudder stab came right off and the elevator will come off w/a little work, the top half w/broke away already. Here’s some more picks. Question; what’s a good light wt filler. I’ve used hobbylite filler. I would think there’s better out there that’s EZ’er to work with. Also, on the planes I have it shows how to make shock absorbing landing gear using aluminum tubing. I was thinking of using carbon fiber and JB welding the metal ends. Here’s some more pictures. 






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Old 02-19-2011, 02:29 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

It looks like all clean fixes for you... it will be a different plane when you get through with it.

And, if you come across some epoxy joints that need to be unglued, try using a heat gun to soften the epoxy and you might get lucky with separating the pieces.

IMHO, for filler, I use what I find in the home center. I started out using hobby filler but, after someone suggested it, I did not find any difference in weight or sandability when using the stuff from the paint section of HD or Lowes. Just use the good stuff.

Just my $.02,

Bob
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Old 02-20-2011, 03:36 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

Finished removing the old covering on the tail feathers. The wood on all the hinge services are in ruff shape. Looks like someone replaced the old hinges once before. Unless someone has a better idea I will replace all the hinge wood. Here’s more pict.s









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Old 02-22-2011, 05:57 AM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

Anybody have a clue on what the wing dihedral angle should be for a 1/4 scale? The builder of this Cub gave me the plans but they are for a 1/3 scale. The plans say to lay a string tight across the wing tip to tip and you should have 3/4" from the top of fus. to the string.
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Old 02-22-2011, 06:53 AM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

Hi PA PEN
You gave the wing span as 108". If you divide 108 by 2 you get 54. Divide 3/4" by 54, then get the arc tangent you get .8 degrees. That is just under one degree. I would call that a straight wing. This is high school trigonometry which I learned around 1952. Love it. If I screwed up others will chime in. I just reread what you wrote; "from the top of the fus. to the string". I assume that fus. means fuselage. Frankly, I do not understand the directions now, so you can disregard everything I wrote, but I decided not to delete what I wrote. I mistakenly assumed that you were referring to "from the top of the wing center to the string". On your plane the wing center is a portion of the wing that is permanently attached, screwed, glued, etc., to the top of the fuselage. The two wing halves are then attached to this central wing portion. Please take the following with a grain of salt. I have built two 71" Sig J3-Cubs, and my own design of the same plane, built from Coroplast, so my plane is a SPAD. I have studied the method of construction of your plane, specifically how the two wing halves are attached, and I see a very weak condition. I am not an expert in this area. I suggest you contact others who have built this plane and ask them if the method of attaching the wing is correct and adequate, based on the photos that you have provided. I have been flying R/C since 1972, and based on what I have seen, I would not fly that plane. I want you to understand that I do not feel comfortable writing this, and urge you to contact others who know more about it. I would like you to let me know the outcome of this. I have been wrong many times in my life, and one more time will just make me grin, but I will have learned something else. I am a retire Mechanical Engineer, and I ask you not to let that influence you in any way.
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Old 03-04-2011, 10:16 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

I have all the covering off and working on the wings where they attach to the fuselage. Reinforced the wings and fuse. Starting on the tail fathers next, here are some pictures.   















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Old 03-05-2011, 01:07 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

Thats really starting to look pretty good.

Keep it up! I love these kind of projects, but it has been so long since I crashed anything, I have not done a rebuild in about 6 years.
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Old 03-13-2011, 12:25 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

Still working on it. Kinda busy doing other things. (Union VP and Shop Steward, going through contract negotiations), and trying to find time for the family.  Here are some pictures; the fuselage was very weak so I reinforced it with balsa wood.   



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Old 03-13-2011, 01:29 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

If you don't mind me chiming in about the dihedral, I agree that the ~1 degree would be about right. You need a little dihedral to make the wing look straight. If you were to build the wing truly flat, it would look like it's drooping on the plane. I did this once with an LT-40 wing and it always resembled a droopy hat when it was mounted to the fuse.

Just my $.02

Bob
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Old 03-14-2011, 09:13 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

Quote:
ORIGINAL: PA BEN

Anybody have a clue on what the wing dihedral angle should be for a 1/4 scale? The builder of this Cub gave me the plans but they are for a 1/3 scale. The plans say to lay a string tight across the wing tip to tip and you should have 3/4'' from the top of fus. to the string.
If 3/4" is correct for 1/3 scale then using a simple ratio, 3/4 over 1/3 = N over 1/4, comes out to 9/16" from the top of the fuselage to the string. Hope this helps.

Looking at your newest pictures, you don't say how thick it is, but that much balsa added to the fuse makes for a lot of added weight. Did you consider just replacing or reinforcing the weak stringers in the fuse? The portion of the fuse from behind the wing to the tail feathers doesn't really have much stress on it. Also, if the plane was balanced properly prior to your refurbishing, you'll need to add a lot of weight in the nose to counterbalance all the wood you're adding to the tail. I'm sure that the end result will be that the plane doesn't fly nearly as well as it could.
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Old 03-17-2011, 06:04 PM
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Default RE: Balsa USA 1/4 Scale J-3 Cub

Very thinn balsa. All the glue joints were old and lose. Not much wt. at all.
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