Center section - the main battlefield

Sooo. There we were. The first ribs were made new and more bottom skin was removed. At this point it was becoming apparent that all of the ribs and everything aft of the rear spar was just gone. When looking back at it, I think this is a problem with most Aero-3s, since all of them have scrafed in repairs at the aft spar, because they were rotting there. None of these repairs really solved anything. In 1984 the order was given to burn or othervise demolish these aircraft. They were about 25 year old at that time and most of them were out of the military and flying in various state run flying clubs. This is nowdays considered as an act of barbarism or socialistic dictation, but when looking at the center section I disagree - these wooden aircraft were burnt or demolished with safety in mind.
Slowly we came to the realisation that we have to build a completly new center section using only the original spars and fittings. The main problem was that we needed plans. The plans are stored in the aviation museum in Surcin, near Belgrade. They were very suportive of our project and after a visit copied the plans for us, but the whole process to arrange everything took quite a while, but again it was a necessary step to do it like that.
You can see the plans below the laminated capstrip for a rib.

 This is a sight you do not really see that often -  a core of a rib completed, but before it is a skinned with plywood. Try counting the parts and mulitply it with 12 ribs for the center section.

The spar was also reskined.

And the fittings refurbished with new plating.

One of the things that I regrett now is that we didn't build a jig for the centersection. In the end it would save us quite a bit of time. What we did to get the wing geometry correct is to bolt the two spars to the wings. Set the correct geometry on the wings and then glue in the ribs.

Here you can see all the ribs glued in, but the work is far from finished.

After the ribs were in, there were about a milion more corner blocks, stiffners, stringers and stuff to add to the structure.

Here you can see the centersection looking into the fuel tank bay. The cross stringers are nicely visibille.

Then we again put on the wings to get the halfribs which are located after the rear spar aligned and glued to the center section.

This is the semicompleted halfrib structure with the third spar on which flaps hinge to in place, ready to be glued to the center section.

So, after everything was more or less in place and with the structure still flexibille to some extent, we put it onto the fuselage to see how it fits and to get the rods for the flaps marked out.

View from the other side. Keep in mind you are looking at the bottom and the aircraft is on its back.

With this more or less completed, we begun stiffening up the center section with new skins. First the central went on.

At the same time as Janez was glueing on the skins I made the aft halfribs ready to accept the flap systems.

And this is more like it! The bottom skins are completed and flaps are fitted in! Big milestone.

The bottom received a very careful and complete coating with varnish. We don't want anything to ruin the new wood. Here you can see the billion of parts installed. They are sanded between two coats of clear varnish. Unsufficient varnishing did a lot to the detoriation of the original piece.

So, the final push will be desribed in the next post. Till then, have fun and keep water out of your aircraft!

Center section

With the fuselage more or less structuraly complete, this is a good time to present the work on the center section.
If you see a few post below, we removed the center section due to the water damage seen on the front side of the aft spar. So this is the thing removed and standing on its nose.

The first surprise was, that the whole center section was painted before being mated to the fuselage. This is evident by the original air force colour on the backside of the rear sspar. Looks like PR blue.

Of course the other thing is that this means that after the factory assembly, the airplane was never really taken apart...But anyway. The first thing was to remove the bottom plywood and see the extent of the damage. This is an image looking at the bottom of the center section and the rear spar is at the top. As you can see, this is the area where most of the damage is located.

Somewhere along the last 30 years the aircraft was for sure parked in the open and is probably leaked a bit through the cockpit seal. Therefore the water acumulated in the rear of the center section, where the design didn't have any drain holes at all. Therefore we started removing the rotten parts. The first face of the spar went off right away and it was also clear that the first pair of ribs will have to be removed.

The good news was that the hardwood of the spar was only discoloured and in some places the wood glue joint was delaminated, which is all in all, quite an easy fix.

The ribs were less fun to look at:

Another thing to notice on the image above - see how ''careful'' and ''good'' they were at factory when varnishing the plywood.
Now, the pair of No.1 ribs was made up quite quickly as seen from the sequence below.

And then, we ground to halt for more then a year on the center section. Why? Check back later.
Don't worry, the story has a happy ending (I think).