Center section - the finish line

This blog entry will bring us up-to-date as far as the center section is concerned.
We left with the center section innards more or less complete, but the whole top sking waiting to be completed.
The first step was to profile the hardwood parts that form the inside of the sandwich, since some areas around the fuel tanks have double skins. So around the fuel tank feeder and fuel quantity indicator this middle hardwood sections was profiled and the skins prepared.

 After this, the bottom skin of the sandwich was glued on, so that the bottom could be carefully sealed and varnished. Note that all the top skins to be used later are already roughly prepared and varnished.

This is how the first-right skin went on. Notice we yet had to remove the staples.

A rare image of the glue applied to aft portion of the right top skin.

The finished result for the right side - now to repeat the thing on the left, but with a little more complexity added due to the fuel tank filler and indiciator holes.

Not to bore you with the process, as it is the same as seen above, here is the right side more or less finished. Note the lovely geometrical shape evident around the holes.

Now, there are some details left to be added. Like a strenghtening shear plate seen during curing below. Quite a big task will be fitting all fairings with their nutplates to the center section. For that, we will need to install it once more to the fuselage. So this will probably be the next blog entry. After that, fabric covering of the center section, and then it is about time to get some paint on this thing. Have fun till then!

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).

Project overview - finishing up the fuselage

Some more details were finished up like replacing these nutplates, or actually nutblocks.

With newly made ones below:

Due to a leak from a brake master cylinder, some plywood under the fuselage was oiled up. These had to be replaced also. The below image shows one half of this area already replaced, but another half still waiting.

The most critical part of a wooden taildragger is of course the tail, since it has a very good chance of being damaged by water. At least this part of our aircraft failed to surprise us in a negative way. The tail section was absolutly dry and perfect, but to gain access to it, we took down a skin, so this had to be replaced. Small price to pay.

Some repairs also had to be performed elsewhere, but these are just minor things.

And this more or less finishes up the wood work on the fuselage. Through all of this construction phase, a very longlasting process was performed. This strenous thing was removing the revision layer of internal paint. The aircraft was painted internally with two coats of paint. The initial was very durable and a bit lighter in colour, while the second, that was obviously applied during overhaul was darker, but most importantly thicker and much worse in quality. The result was the image below.

So the remedy was to simply sand the paint away. Keep in mind that this is a huge area and it has a lot of nooks and edges. The right part of the picture is sanded, while the left is not.

A frame in the tail area showing the different shades of paint clearly.

Now the positive thing to this sanding business is that you literary go through every square centimeter of the aircraft and discover things that you could overlook. Like this method of how they used nails with a washer made of leather to keep the corner block in position during glueing.

See the sanded off paint on the fuselage bottom. Hard earned grams to take off the basic empty weight.

And this is how the final result looked like. New skins, new corner blocks and the revision paint completly removed.

The biggest effort went into the fusleage aft of the cockpit. Lots of area there and teribile working conditions.

To provide an illustration, this is an image I took of our friend Marko, who is woodmaker, turned historian, whose expertise lies in Slovenian military aviation operation in 1918 and 1919 skirmishes on the Austro-Slovenian border. Teribille work.

But in the end it all paid off. This is a structurally sound and complete fuselage.

After the work was completed, new filler was applied, but only in the areas that the original was present.

Before fabric work was started, some of the fittings had to be realigned to fit the new woodwork.

Here is an image that nicely shows the extent of work done on the fuselage....

But, after paint nothing is visibille. We went to great lenghts to get the shade of the paint matched up with the original layer of paint. It really went well, but more on this later.