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        Discussion Goldberg Monster Pitts rebuild

#1 kenh3497 Jan 31, 2013 08:29 PM

Goldberg Monster Pitts rebuild
 
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This was a "gimme" after it was crashed last summer. I put it up in the rafters of my garage and dug it out just a week ago. One of the lower wings is in kind of tough shape with 1/2 of the ribs missing or broken. The spruce spars somehow survived except for about two inches of the bottom spar at the tip. I can graft on easy enough and there is not much stress that far out on the wing. The only other wing damage is a slightly crushed tip on the opposite top wing.

The fuse is another story. There is previous damage to the landing gear / motor mount. There was copious amounts of epoxy and glass cloth trying to shore up the damage. I started to try and salvage the remains but decided that new was the way to go. My new parts weigh the same as the old busted up assembly. The new is made from thicker ply in may spots and doublers are added to the LG mounts. I've already done some reconstruction on the ply pieces of the fuse itself. The motor box will be installed next and finished adding the ply parts to complete the assembly. It kind of has to go back together "in order" so some parts will wait until the basic assembly is glued in place. I think I'll use Gorilla Glue for the assembly to take advantage of it's foaming action and gap filling abilities. Not everything fits exactly as it should. :p I'll also add some small tri stock to help tie everything together. If the original construction had use ply half again as thick the structure would be very much stronger with little penalty in weight. Of course you don't sell as many planes if they are strong to start with.;) The biggest thing that would have helped this plane is to actually use glue that bonded things together. It appears to me there is no, or darn little, glue in the joints. It seems most of it is next to the joint in fillet fashion.

I know it wont do any good, but I emailed Carl Goldberg INC with a suggestion to release or sell CAD files for discontinued airframes. It would really help those of us that like to repair these old neglected birds.

#2 kenh3497 Feb 01, 2013 03:05 PM

I've been thinking about this today and I torn if I should remove all the sheeting from the front half and just replace it or just patch it with new. I think the sheeting adds a lot of strength to the fuse???? I'll leave the turtle deck alone behind the canopy and maybe even in front of the canopy though it would not be that much extra to do it all.

I'll post a pic or two when I get home tonight. Post your opinions!!

Ken

#3 loNslo Feb 01, 2013 03:21 PM

Where are the before photos? :)

#4 kenh3497 Feb 01, 2013 09:42 PM

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Quote:

Originally Posted by loNslo (Post 24003533)
Where are the before photos? :)

The "mishap" happened at our annual fall fun fly. That's the previous owner with the plane on the starting stand.

A couple of shots of the front with the new motor box temporarily installed. As I'm typing the GG glue is doing it's thing and foaming up nicely. I'm thinking I'll strip the sheeting off. I might also just replace the 3/16 stringers in the process. There are three short ones on the top and one long one on each side. I think it would be easier in the long run than trying to clean off the glue and excess wood. I think I may be giving up some strength by trying to splice where the sheeting is cut. I had to cut the sheeting to be able to gain access to make a decent repairs. Besides with the sheeting off I can get a really good look see at the rest of the ply structure and find any other damage or broken glue joints.

Ken

#5 kenh3497 Feb 06, 2013 08:49 PM

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One step forward and two steps backwards.....

I ripped the sheeting off the forward part of the fuse last night. It seemed to me the best way to go and gain access for further repairs. It's a good thing I did as I found more damage. Nothing terrible but a little medium CA was needed. I'm amazed at how brittle the glue used in the original construction is. It pops right off buy scraping with a #11blade. You have to remember I got out of the hobby about the time ARF's were coming of age. This is my only second experience with an ARF. My first was assembling a H9 CAP 232.

#6 loNslo Feb 06, 2013 09:01 PM

Good photos. Nice project.

#7 kenh3497 Feb 09, 2013 10:24 PM

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I did get a tiny bit further with the Pitts. All the blocking and reinforcement are in place and and the glue on the stringers is drying as we speak.... So to speak:p

Some of the blocking and reinforcement is not the same as the original, but should be just as strong if not stronger. I've been using a Polyurethane glue for the reassembly. I would not recommend building with the PU glue, but for rebuilding it seems to be a good choice. Not everything fits perfectly together and the PU glue foams up and fills the gaps and also gives a bit of a fillet, hopefully reinforcing the joint a bit. I've noticed the PU glue seems to be a tiny bit flexible. Even after several days the fillets seem to stretch as you dig at them. I had to remove a couple to add doublers in key spots. It was very tough to get off the wood.

Hopefully I can sheet it tomorrow. That will regain the two steps I lost. It will also finish the fuse so I can start on the lower wing. I do have a little issue with the horizontal stab. Miss rigging cause some warpage. I'll post a photo when I get to that point.

I am also in the midst of building a new wing for the CAP in my avatar. I broke the wing in flight but saved the plane. The first rebuild/scratch build here, and photos of the broken wing. http://www.rcuniverse.com/forum/m_10804273/tm.htm The new wing will have a full depth spar to reinforce where the old wing broke. This is the first foam wing I ever broke. But, I've never had a wing with the cutout for the fuse. That is a huge stress riser in that corner.

Ken

#8 kenh3497 Feb 12, 2013 08:55 PM

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A little more progress.....

I made up some wide sheets to cover the side of the fuse. I didn't realize there were some compound curves involved so I had to make the large sheets small again:o Both sides done and now I can do the top and a couple of small pieces along the sides to form the sides of the cockpit.

The other small problem is the rigging of the tail caused the horizontal stab to warp somewhat. I don't know if you can see it in the photos or not. The center of the bow is at the flying wire attachment point. Too much tension on the bottom wires also pulled the back of the stab down in effect giving a little positive incidence. Or maybe a little wash-in would be more appropriate term.

Some of the photos are a bit blurry but I couldn't get my camera to take in the whole thing without focusing on the wrong spot.:o

#9 loNslo Feb 12, 2013 09:38 PM

You might have to rebuild the horizontal stab.

#10 kenh3497 Feb 12, 2013 10:01 PM

I think I might try the Windex trick and see what happens. That or just tension the upper wires to pull it back into shape. I tried to straiten the bow in the trailing edge of the stab. It comes right out but of course comes back when I release the tension. I THINK if I soak it up and hold it until it is dry with the flying wires and some sticks to take out the bow I can kill two birds with one stone. Maybe......:confused:

Ken

#11 loNslo Feb 12, 2013 11:29 PM

It would be a miracle if it worked.

#12 kenh3497 Feb 16, 2013 09:33 PM

To good of a miracle......
 
I was able to take the bow out of the trailing edge of the stab. I soaked the wood with some Windex with ammonia. I places a 1/8 spacer at the fuse and a stick and clamp at the center of the bow. One side went just a tad to far and the other side is right on the money. I re-soaked the wood and use a stick on the tips of stab and leaned it up against the freezer and twisted the tips slightly past ideal. Now both tips are twisted to far:eek::o:( I'm going to let it set and see if mother nature will allow them to relax back to normal.

The good news is I'm working on my CAP wing so I have some time. I'll try to get some pictures posted.

Ken

#13 kenh3497 Feb 19, 2013 08:48 PM

Stab Photos
 
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Finally, here are the promised photos of the half strait and over strait stab on my Pitts. I did both at the same time so I have no idea why one came out "too good".

#14 kenh3497 Feb 23, 2013 11:45 AM

The last photo in the previous post shows how I straitened the twist in the stab. The next morning the fuse was laying on the floor and the stab was broken at the hard point, where it had been repaired before. I think the soaking allowed the glue to soften and let go from a previous repair. That was bad that it broke but good that I could do a proper glue job. I found little to no glue in the joint so I spread it as far as possible and injected copious amounts of Tightbond II. I worked the joint back and forth (carefully) to work the glue into the joint, clamped it all together and wound up with a perfect repair. That stab half is now strait and strong.

Ken

#15 loNslo Feb 23, 2013 11:49 AM

Perseverance furthers.


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