Refurbish of Carl Goldberg Sky Tiger (Build Log)

[TexMechsRobot]

I started tracing out the old tail surfaces so that I can rebuild them with the balsa. Next step is to cut, glue, and sand these down.



I also found a brass tube that will work to replace the one I had to cut away for the right aileron mechanism. I should be able to get all the wooden components ready for covering without too much trouble. Then I'll have to wait until I can buy the covering and tools which will probably be a while. I still intend to buy materials for and build a Flite Test foam board plane first.

 

[roversgonemad]

For covering, Ekonokote is great for the money. I think it's much easier to work with than than Monokote. It's also half the cost. The Towerkote is even less, and very similar to Ekonokote.

 

[TexMechsRobot]

Thanks for the advice! Do these alternatives have the same durability?

 

[roversgonemad]

Yes, in fact, I think they are a bit less brittle than MonoKote. I only buy Monokote if I want a color that is not available in Ekonokote. That's the biggest drawback to Ekonokote, not that great of a color selection.....

 

[TexMechsRobot]

It turns out, the brass rod I found was a bit to big internally for the hinge mechanism. But I was able to locate an aluminum tube with an OD of 3mm and an ID of 2.55mm. This was perfect for my 2.33 mm rod.



I decided to go ahead and replace the left tube as well so I cut the balsa wedge off of the back edge of the wing. Then straightened one bend on the hinge piece, cut two 6" lengths out of a 12" tube, slid them onto the hinges, and re-bent them back to their shape. Perfect fit! Now I just need to repair the wing which shouldn't be terribly difficult.



I also got the $30 worth of balsa that I needed (probably more than I needed but I can always use more balsa somewhere!) and got to work on the tail surfaces. They turned out nicely I think! They are sanded smooth and ready for covering.







Next steps for me are to repair sections of the wing (some of the wood needs to be built up and sanded smooth from when I was taking the covering off and some of the ribs are split in half along their length), build out the wing tips, and reattach the aileron hinges.

 

[roversgonemad]

Looks good!

 

[TexMechsRobot]

I got to work last night repairing some of the damage to the wing. I started with the 2 wing ribs that were split lengthwise. In truth, they probably were ok without support but I figured it was too easy to laminate them on one side to just let them be. I need to buy tiny clamps but these magnets seemed to work just fine





Next was repairing some of the wood that was damaged as I pulled it all apart. I carefully cut away what wasn't flat or was too soft and then added new balsa to the areas. I haven't sanded it yet. Figured I'd let it dry a while and let the glue cure before getting too aggressive with it. I'll probably finish that tonight.

 

[TexMechsRobot]

I finished sanding all of the patched wood and am happy with the results.



I also decided to sheet the top and bottom of the leading edge of the wing up to the spar. It was not originally designed this way so the ribs were flush with the top of the spar. I've gone through and carefully cut about 1/16" from the top and bottom of each rib and the shear web pieces. I've completed this on the right wing and have done the top of the left wing so far. I had to sand the ribs to maintain a good airfoil.



I also added a 1/4" riser to the root of the wing to help support the sheeting there.



Once I finish trimming the ribs and shear webs on the left wing, I'll decide whether or not to build out the wing tips or leave them flat. I'm not too concerned about drag since speed isn't really a goal here. But I also think that I would like the practice of building these and I don't think it would be too hard to do.

I've also noticed that the fiberglass is peeling off. I didn't think this was possible! I'm going to mess with it a little and if I think that I can get it off in one piece, I might try removing it and replacing it with fresh glass and resin. Or I might just try to glue down the edges that are peeling.

 

[TexMechsRobot]

I started sheeting the leading edge starting on the top of the right side.



A picture of the wing before I started.



Here's a shot of the 1/16" sheet glued to the spar.



I used the table to roll the wing and glue the sheet to the ribs and leading edge.



And here is a pic after doing a rough trim around the edges.



I'll still need to sheet the bottom and then sand any overhangs and fill any gaps.

 

[TexMechsRobot]

I finished sheeting the leading edge. Looks good in my opinion




I then tackled the hinges for the ailerons. The divots were full of epoxy and I had to enlarge the trench so that the hinge tubes would fit well.




The final product works well but I don't really like the way it looks. I guess it'll all look better once the covering is on it and the rough fiberglass job is hidden

 

[roversgonemad]

It looks good. That fiberglass is also under the fuselage. A little sanding will smooth it out quite a bit. Just make sure the epoxy is cured first. Gummy epoxy sands like crap!

 

[TexMechsRobot]

Yeah he basically wrapped a 3" strip all the way around the seam which I think was a good idea. I had to cut the back bit off though to get at the hinges. I don't think I'm going to worry about putting it back on. I did decide to bondo the seams though just because the perfectionist in me can take the aesthetics of it. I know it adds unnecessary weight but I don't think that weight is going to be too big of an issue with this air frame.

 

[TexMechsRobot]

The bondo turned out good. It actually looks worse because of the many color shades involved (balsa, aluminum, fiberglass, and bondo) but the profile is very smooth. All of this would have been invisible under the covering and fuselage anyway but now I feel better about knowing that it is smooth

I also began building the wing tips. I wanted to keep it close to the stock, plastic tips that came with the plane but not quite so large. I also wanted to keep it light and practice building tips this way since it seems to be a common technique. I'm nervous about how it'll go when I try to cover them but one thing at a time!

I started by tracing the profile of the airfoil onto 1/16" balsa and then drawing a tip shape on 1/4" and gluing these together. Then I cut and glued ribs onto the top and bottom to rough out the profile.








Then I sanded the structure to get closer to the actual wing shape.





The next step will be to create a mirror for the other side, glue them to the wing, and then give them a final sanding to contour perfectly with the wing.

 

[TexMechsRobot]

Last pictures before I go on a 7 day vacation to Colorado. First shot is of the bondo'd up middle section. Again, all of the colors make it look terrible but it really does have a super smooth finish so I'm happy with it.



Then we have the wing tips completed and on the wing. They were built off of the wing, glued on, and then sanded to fit perfectly. I'm happy with how those turned out as well.





Next steps is to cover and get all of the electronics and hardware installed. No particular order there I guess. I'll probably to the electronics first though so that if I need to make any adjustments to the frame, I can do that before covering. All of it will have to wait on my budget though. No idea when I'll be able to continue it

 

[roversgonemad]

The fiberglass is structural. It would be a good idea to put it back. Because of how the spars were designed, many of the older planes used fiberglass on the wing joint. You might get away without it, but it would be a real shame to see your wings do a high five over the top of the fuselage after you have put this much work into a plane.

If you only lost the fiberglass at the trailing edge, you'll probably be fine, but the underside is where I would be most concerned.

 

[TexMechsRobot]

I only removed it from the trailing edge. The rest of it is intact.

 

[TexMechsRobot]

So I got some more money. And then I bought the FT Bushwacker with Power Pack C, a hot glue gun, and some other miscellaneous supplies. I've maidened that one and it has a few issues (the pilot being one) but it's going well. I wanted to practice building and flying something a few times before I get this Sky Tiger up in the air. I imagine I'll buy and build another FT plane like the Spitfire or Mustang to practice flying low-wing. Might even do a Sig Four Star 20 EP as well since it's smaller than this but should have a similar flight characteristic. Anywho.

I got more money since then and bought some supplies and material for covering the balsa frame. I have watched a ton of YouTube videos and read a few articles on how to do this properly so I was well prepared. I practiced covering the original tail surface first. This tail surface isn't quite right and was reconstructed after I destroyed it getting it off of the fuselage. I knew I'd want to experiment with covering so I built it back up to allow it to be a test platform before I started working on the newly built tail surface and refurbished wing and fuselage.

I think it turned out rather well. I had some trouble on the inner curve of the vertical stabilizer where it goes to a point and blends with the body (no pictures) but I did learn a little. First, I think that a smaller iron (like the Monokote Trim Tool) would serve me well for this. Secondly, I was using too much heat when trying to set the adhesive so the film was shrinking at the same time. It wasn't a problem on the larger flat surfaces but it really hindered me on some of the curves. I turned down the heat and things went much smoother.

I also learned that using the iron to shrink the film (vs. a heat gun) cause the adhesive to stick to the ribs inside the tail surface. It isn't really a problem (and is probably stronger) but it doesn't look as nice. The blue side I used the iron to shrink and the white side I used the gun.

Overall, I'm happy with the results for a first attempt!





Next steps are to get the trim tool and then start covering the real parts using the lessons I learned. I'm looking for an airplane template that I can use to draw out my color patterns so I have something to work from than just a mental picture.

 

[TexMechsRobot]

So after practicing with the original horizontal and vertical stabs, I spent the weekend covering more of the actual surfaces. It was my first time buying monokote rolls so I didn't really know how much was on each roll and the one roll of each color I bought isn't going to be enough. I've ordered another roll of blue and another roll of white along with an econokote black (for the cockpit) and a roll of lime green for accent colors.

My wife is a seahawks fan so this one is going to sport their colors.

I did get all of the tail feathers covered along with the control surfaces. I got about half the wing done. I still need to cover the left wing with blue and then cover the wing tips with blue. I haven't done anything with the fuselage yet.

I should get the rolls I ordered by this weekend but I don't know if I'll have time to work on it much.

 

[TexMechsRobot]

So I finished covering the wing. I am thinking about putting neon green accents on it as well but I may not do that. I made quite a few mistakes putting on the Monokote but I learned a TON about how the stuff works and its limitations. That was sort of the point of this whole project anyway. I wanted to use the air frame to learn about balsa building techniques.

I still need to cover the fuselage and install the control surfaces. I'm waiting to cover the fuselage until I have the electronics so that I'm not worried about the cover when I'm adding wood and mounting hardware.

 

[TexMechsRobot]

I started gluing the Dubro pinned hinges into the tail control surfaces. I used their handy slotter tool and got all of that dry fitted well. I put a very thin bead of epoxy into the slot and then slid the hinge in being careful not to get the epoxy on the hinge itself.

This worked for 4 our of 5 hinges. I have since done research on ways to get a better bond while keeping the hinge safe but I can't seem to find anything about how to fix the one hinge I messed up. Worst case, I can cut the plastic flush with the control surface and then try again in a different spot.

Before I do that, is there a method I can use to fix it? I'm thinking some careful work with a razor blade could do the trick. It is only a very thin join between the rotation part of the hinge and the wood and I think I can make a few small slices and cut it free. Is there a chemical method that works to break down the epoxy without damaging the wood or plastic hinge?