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        Discussion Repairing an RTF wing - my Hobbistar 60 trainer

#1 grosbeak Aug 07, 2012 02:26 PM

Repairing an RTF wing - my Hobbistar 60 trainer
 
This project is complete!

SUMMARY

Cost


In terms of cash alone it would have been a lot cheaper to buy the replacement wings. A LOT. In terms of time, the ratio is virtually incalculable However, there are several mitigating factors.
1. The purchases for this project were either supplies or tools. I still have a lot of balsa, covering and filler left. The tools, of course, I now own for other uses.
2. I've learned a lot about working with balsa and covering and about wing structure and I added to my pool of knowledge on shaping wood by hand
3. I've got a finished result that I'm very proud of
4. I had a ton of fun in the process

If I had to make the decision over again, I would still opt to do the repair myself.

Lessons learned

Mark your balsa stock for purpose before you start cutting -OR- you might make a complete set of leading edge ribs out of wing sheeting stock
Pay attention to the direction of the grain in the balsa -OR- your leading edge ribs may not have the strength they otherwise might
Find an intact piece to use as a template for replacement pieces of the same size and shape -OR- you might cut a whole set of leading edge ribs that don't match the rest of the wing
Use some dust filtration / extraction and a mask when you cut or sand balsa -OR- you're going to do a lot of coughing

Photos

I took, let's see, 301 pictures during the repair process. Lots of them are in this thread. All of them are in my Flickr set, which is titled Hobbistar 60 - Wing repair

- - - - - - - - - - - - -

The original thread starts here

I learned to fly last year with a Hobbico Hobbistar 60 Mk III select - an RTF. This spring while I was flying I dumb-thumbed it into a tree. I was lucky enough to get it back, and just the wing was damaged.

http://farm8.staticflickr.com/7047/6...25156af2_z.jpg

The starboard wing took a hit on the leading edge and cracked near the root. The port wing left its leading edge and tip in the top of the tree.

http://farm7.staticflickr.com/6058/6...095f553d_z.jpg

A replacement wing is a reasonable $90. I decided to rebuild it myself for the experience and to save money*. The purpose of this thread is to follow my progress.

*Ha!

#2 grosbeak Aug 07, 2012 02:38 PM

I found some information on wing repair online, but there were definitely gaps. I figure a good first step was to remove any torn covering. In so doing I got my first look at the structure of the wing.

http://farm9.staticflickr.com/8028/7...2b24fd03_z.jpg

http://farm8.staticflickr.com/7122/7...731d4851_z.jpg

After I'd removed as much covering as I felt was necessary, I removed the broken sheeting and severed rib parts from the leading edge of the starboard wing. I left the stumps of the ribs to serve as gluing points for the new ribs.

http://farm8.staticflickr.com/7254/7...21bf0d02_z.jpg

I made a cardboard template, then cut out four replacement ribs. I glued and clamped those, then glued and clamped the wing root damage to be reinforced with fibreglass later.

http://farm8.staticflickr.com/7257/7...ab5f6025_z.jpg

It wasn't until later that I realized I got the rib shapes wrong. They should be flat on the leading edge, but I think I can fix those on the wing.

#3 SeamusG Aug 07, 2012 02:51 PM

Nice work.

Next thing you're gonna say is that you have a scratch build planned.

#4 grosbeak Aug 07, 2012 07:21 PM

Quote:

Originally Posted by SeamusG (Post 22382476)
Nice work.

Next thing you're gonna say is that you have a scratch build planned.

Baby steps, Seamus. Baby steps. My last plane was an ARF but I did put about 100 hours into the "assembly". ;)

#5 grosbeak Aug 07, 2012 07:34 PM

So - onwards and upwards. I set the starboard wing aside and turned my attention to port.

None of the rib stumps were big enough to provide support for the new ribs, so they all came off. Afterwards I prepped the main spar, planing off all of the remaining stubs and any glue.

http://farm9.staticflickr.com/8287/7...73757630_z.jpg

After a couple of not-so-great attempts at making a cardboard template for the ribs, I removed one of the two intact ribs to use instead. That did the trick.

http://farm9.staticflickr.com/8288/7...6f93e289_z.jpg

Shortly afterwards all the ribs were in place, supported at the base by tri-stock. I made a mistake, though...

http://farm8.staticflickr.com/7251/7...c85fd405_z.jpg

... I cut the ribs in the wrong grain. I fixed that later.

#6 grosbeak Aug 07, 2012 07:48 PM

After the ribs were on, it was time for the leading edge. The piece I chose was a little (read: WAY) too thick so I brought the whole thing over to the band saw for a trim.

http://farm9.staticflickr.com/8282/7...b0937816_z.jpg

There - that's better. But still square.

http://farm8.staticflickr.com/7268/7...63ca5860_z.jpg

I tried using my large block plane to round the corners but it's way too big for such delicate work. Enter Lee Valley Tools and the miniature block plane.

http://farm9.staticflickr.com/8289/7...f69d20da_z.jpg

After a long time spent planing and sanding, the proper shape of the leading edge emerged.

http://farm8.staticflickr.com/7115/7...9eea973c_z.jpg

#7 grosbeak Aug 07, 2012 07:54 PM

I mentioned a post or two ago that I cut the ribs in the wrong grain. My solution was to make stiffeners out of 3/32" balsa, 1/2" wide. Cut in the correct grain this time. Luckily, they don't weigh too much.

http://farm9.staticflickr.com/8430/7...6220bfd2_z.jpg

One went on each side of each rib. Now they're much more rigid.

http://farm9.staticflickr.com/8425/7...7f1aa2c9_z.jpg

#8 BMatthews Aug 08, 2012 12:06 PM

You're learning a LOT from all this.

The only real hint I can offer is that you really did not need the tri stock. If you had done the rib noses with the grain going the right way first off you could easily have given them enough stiffness to resist the usual handling loads with a simple flat sheet stock gusset triangle off one side per rib to the spar and no gussets at the leading edge. Even then the gussets would be there solely for some toughness for handling until the leading edge sheeting was applied. At that point the sheeting will take all the loads nicely.

But this is a relatively small point to ponder for next time.

Once you're all done be sure to balance the wing out by putting a small weight on the heavy side. Your repairs will have added some weight to the badly busted up side compared to the relatively lightly damaged side. Although considering it's an ARF if you used a lighter wood for the sheeting on the badly damaged side for the repairs it might come out the same weight as the original wing. Balsa has such a wide variation in density from soft and light to hard and heavy that this can easily happen.

#9 Curare Aug 09, 2012 01:03 AM

Yep, what bruce says, it's all a little overbuilt.

Still, a start like this is a good one, you'll soon learn that that wing is bulletproof, but heavy.

Now you can start to play the weight game like the rest of us.

Is this strong enough?
yes
Is this too heavy?
yes
Can I lighten it without losing strength?
yes.
Ok, lets go.

I guarantee that after a while you'll start to look and structures and see in your minds eye that they can be made lighter, stronger and easier all the time.

#10 Thermalin Aug 13, 2012 08:44 AM

looks good... be ready to add some ozs. to the opposite panel at the tip..

#11 EBurkart3rd Aug 13, 2012 10:09 AM

Looks great Grosbeak! I had similar damage to my hobbistar 60 wing last Fall. I did the opposite of what you did, and shelled out the $90. After reading about your repair, I regret buying my new wing. :o You made it look easy.

Well the bright side is that if I repair the original wing I will have a spare. :D

#12 grosbeak Aug 15, 2012 08:46 AM

Thanks for the positive feedback, folks.

Turning back to the starboard wing, I cut the leading edge to length and glued it into place. During the process I used a pair of end cutters to trim the leading edge ribs flush.

http://farm9.staticflickr.com/8301/7...37bfd91c_z.jpg

Once the leading edge was shaped, I started the sheeting process. I figured it was better to start with the starboard wing because the leading edge repair is small. I hope the experience I gain will serve me well when I undertake the sheeting for the port wing.

I didn't find much online about sheeting but I figured it was better to start at the base and work towards the leading edge.

http://farm8.staticflickr.com/7123/7...67a56d53_z.jpg

#13 grosbeak Aug 15, 2012 08:57 AM

While the glue for the sheeting cured, I took a look at a hole in the sheeting that was closer to the wing root. I cleaned out the splintered pieces, gave it a simple shape and beveled the edges with a razor blade.

http://farm9.staticflickr.com/8441/7...7a17be9a_z.jpg

I used a piece of broken sheeting from the port wing to make the patch, and beveled the back edges.

http://farm9.staticflickr.com/8429/7...178aaaca_z.jpg

Then it was just a matter of adding some glue and securing it in place while the glue cured.

http://farm9.staticflickr.com/8433/7...de240b32_z.jpg

Later, it will be sanded smooth.

#14 BMatthews Aug 15, 2012 10:29 AM

All good again.

A handy tool for shaping ribs in place instead of the cutters would have been some 80 grit sandpaper glued to a flat stick such as a paint mixing stick.

#15 grosbeak Aug 15, 2012 11:17 AM

Agreed - but I did have to remove a substantial chunk so in this case I thing the end cutters were a better choice.


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