1/4 Scale Cubs - Sig and Balsa USA, Rescue and Re-Build

[BobK]

I think heat will be your friend with removing the silk. A covering iron on a hotter setting might just do the trick.

I have tried heat both from a gun and an iron, it worked pretty well on the LE sheeting of the wing but apparently they used some unknown atomic super fantastic glue on the fuse..it literally needs to burn before it comes off and by then the wood is burnt. I tried solvents like paint thinner without much luck, soaked a rag and let it sit.

I think at least a third of the planes weight is covering alone.

-Bob

 

[Joker 53150]

Sand it smooth and cover over it with a good dark covering that won't show the old stuff, and call it done!

 

[Joker 53150]

Here's my "little" Cub, a 62" span J-3 from BH Hobbies. The reconstruction is far on the back burner for now.




But back to the big Cub... As I'm going through the wing making some updates I'm finding some additional problems before they cause me grief later on. This is certainly helping me justify the work involved with tearing off all of the original covering. One rib has a split in it, which you don't see unless the wing is flexed. The big brace by the aileron has a bad glue joint, and the tape by the front center of the wing is where another rib is coming loose. That one could have been the biggest problem since the main bolt which holds the wings together is located there. Some sheeting will be removed to inspect the internal structure before gluing it up securely.




The wings are normally held together by a single small bolt near the leading edge of the wing, while additional strength comes from the struts. However, I noticed that if you push the right wing forward and back a little the wings will separate at the trailing edge. It seems like a weird design flaw to me unless you only fly in a slow, scale manner. Near the rear of the wing is a big locating tab & slot which only keeps the wing lined up, it doesn't keep them together. My plan was to make it more functional by building a larger "slot" structure for the tab, and then running a bolt through the entire piece into a captured nut. I didn't spend a lot of time making it pretty since it will be covered by sheeting soon, but here is the result. Once covered the only indication it's there will be a small hole in the bottom of the wing for inserting the bolt.

 

[Spastickitten]

Woo! I love to hear about the process of building these planes. I hope more balsa builders will start posting their builds too.

 

[BobK]

Woo! I love to hear about the process of building these planes. I hope more balsa builders will start posting their builds too.

I love building with wood, nothing else like it

The little Cub looks nasty but shouldn't be too bad

With the silk covering I really want to get as much off as possible since it is sooo heavy..it is the main source of completely dead weight on the plane. The plane is old and most of the fuse is 1/4" sheet (structurally framed then sheeted over) so I think I am going to sand it all off then cut my lightening holes, any imperfections I can fix with thinner sheet balsa and lightweight spackle.

The coolest thing about this old plane is I found a name and address inside, I assume it was the original builder and he was a Dr. from Wis., how it ended up here in central MN is beyond me. I did some google searching and I believe this fellow passed away over 20 years ago. I always thought it would be fun to rebuild it and see if I can contact a son or daughter and ask if they recognize the plane.

The big Cub is looking great, your ideas sound like good ones. That center section might be tricky but I think it will work.

-Bob

 

[Tench745]

Did a quick google search on removing silk and found some advice from another forum. Looks like you're on the right track with sanding it off.
http://www.rcuniverse.com/forum/questions-answers/1527910-help-please-removing-silkspan-dope-print.html

Cubs are looking good Joker.

 

[BobK]

Did a quick google search on removing silk and found some advice from another forum. Looks like you're on the right track with sanding it off.
http://www.rcuniverse.com/forum/questions-answers/1527910-help-please-removing-silkspan-dope-print.html

Cubs are looking good Joker.

Thank you for the link, I tried solvents a little but not a lot and really do not want to go that direction. Now that I have checked out this Cub rebuild I am really itching to work on the old girl again.

I really want a big Cub like this, I have a pond that covers about 4 acres right behinf my house to do some float flying

Joker any vids of your clipped wing on floats?
-Bob

 

[Joker 53150]

Bob, Neither of my big Cubs has flown since I've got them over the last few months. The cream-colored one that came with floats needs a motor, electronics, and thorough inspection before it is ready, and the Cub for this build-thread needed lots of attention before it flies. By Spring I hope both are ready to go.

 

[aeroncalover]

About my cub wing struts-
I doubt I need to worry about splitting the wood at the wing end of the struts. I did several tests last night to test the strength of the wood. I drilled a hole into the end of the strut scrap just like on the actual strut, clamped the screw into my vice and pulled as hard as a could. The screw bent and pulled out of the vice. After I straightened the screw, I inserted it into a 2x4 block and clamped just the 2x4 into the vice. I drilled another hole into the opposite end of the strut and hooked a fish weighing scale into it. I bottomed out the scale at 28 pounds and lifted the table before the hook of the scale bent. Finally, just to really assure myself that the strut was strong enough, I clamped the screw back into the vice with the threads clamped, and was able to lift the table again. The hole for the screw did not elongate at all. The grain of the wood was also running along the strut scrap, which should have encouraged it to split.

 

[BobK]

About my cub wing struts-
I doubt I need to worry about splitting the wood at the wing end of the struts. I did several tests last night to test the strength of the wood. I drilled a hole into the end of the strut scrap just like on the actual strut, clamped the screw into my vice and pulled as hard as a could. The screw bent and pulled out of the vice. After I straightened the screw, I inserted it into a 2x4 block and clamped just the 2x4 into the vice. I drilled another hole into the opposite end of the strut and hooked a fish weighing scale into it. I bottomed out the scale at 28 pounds and lifted the table before the hook of the scale bent. Finally, just to really assure myself that the strut was strong enough, I clamped the screw back into the vice with the threads clamped, and was able to lift the table again. The hole for the screw did not elongate at all. The grain of the wood was also running along the strut scrap, which should have encouraged it to split.

Is this a one or two piece wing on your 1/6th scale?

Joker I caught that they haven't flown and was going to mention it but then the forum went down, I have a feeling they are both going to be a real treat to fly

When I get working more on my Skylane I will update with a new thread and quit taking away from your project here, that might be soon the way the weather is looking for my area. I would like to mention that I do have the full size plans for the old Goldberg Skylane 2 so if a complete rebuild is neccesary it I have them, if anyone else wants the plans I can point them to them and also explain how to tile them if need be. I guess it is a great flying plane, I plan to find out.

-Bob

 

[aeroncalover]

My cub wing is 1 piece, but they make very clear on the plans that the struts are functional and not to try and fly without them.

 

[BobK]

Go to Walmart spend $3.50 on a carbon fiber hunting arrow, remove the fletching and install it inside the wing. Make the struts look pretty..you will never have to worry about that wing folding ever.

-Bob

 

[Joker 53150]

Turning the attention to the aileron servos I ran into a problem. The first picture shows how the ailerons are done on the Clipped Wing Cub, a method I like. The servo is mounted to a removable "hatch" which gives you easy access to the servo and a nice straight pushrod. I couldn't figure out why the builder of this plane did the servos like this.




So that's what I did on the first wing. A hole for the control arm was roughed out, mounting blocks were added to the servo hatch, and everything fit nice and neat.




...except I didn't take the wing struts into consideration. On the Clipped Wing Cub the struts aren't in the way so that method works well. Here, I run into clearance issues and the pushrod can end up rubbing on the strut. It turns out the plans were designed around this problem, and if I only bothered to look at the details before changing things... The picture doesn't show the problem well because this pushrod has a bend in it, which makes it look like there is more clearance than there really is.




So back to how things were! I'll adjust the servo placement a little, but can re-use the original hardware now. A little structure needs to be made where the pushrod exits the wing, which will be quick & easy to do.

 

[BobK]

Wow that is close

 

[Joker 53150]

Yeah, but looking at the bright side, this will look better not having the servo arm and entire pushrod sticking out of the wing. I do have to cut a new hatch since I modified the old one, but I've got enough light ply on-hand so it's not a big deal.

 

[Joker 53150]

The Cub originally had CA hinges, which I don't care for on a model this size. They were stiff so the surfaces didn't move smoothly. The servos are all pretty large and it worked, but a regular pinned hinge moves much easier so there is less strain on the components. The first thought was to use flat hinges, but the CA hinges were installed with so much adhesive that the wood is very hard to slot for the new hinges. I don't have any tools to make it easier, so I went with Plan B, which is a round pin-hinge. These are very easy to use, as you simply drill matching holes in the aileron and wing to put the pin in. The hole needs to be wided up a little at the pin since they flare out, but it's still a very easy process. I also like how the small mechanical hinges look. None of these will be glued in place until covering starts.




The wing strut ends and holes for the jury struts all call for a little fiberglass reinforcement. A little spray adhesive keeps the fiberglass in place while the epoxy resin was brushed on. After drying it'll get a light sanding before final prep for paint. New hardware is also needed so it all matches. Right now it's a mix of Phillips, slotted, and hex head screws, and it'd be a pain to take all the different sizes/styles to the field for assembling before flight!

 

[aeroncalover]

Looking good!

 

[BobK]

Nice work! I was wondering if you could post a pic of how the aileron control rod connects to the aileron, the third pic back shows a slot in the TE edge that the control arm passes through

 

[Joker 53150]

A little clean-up is needed before covering, but this is the general layout. The servo is completely hidden and all you see is about 1/3 of the pushrod as it comes out the slot by the aileron. The slot cut into the wing (right under the pushrod in these pics) gives clearance for the pushrod as it moves in an arc when pushing the aileron up.

 

[BobK]

Ok I see now I was confused about the slot in the TE. When you switched the mounting to the way you wanted and then realized it was too close to the strut did you use the bent pushrod? It is really hard to see but it looks like a straight pushrod would clear that strut.