Warbird Information Exchange

Hi guys,
First post from me. I am a project lead in a small wood and fabric shop in Europe. What I would like to show you is our pet project - the full restoration of an Aero-3 primary trainer.
What is interesting about this one is to showcase what trouble you can get yourself in when rebuilding wooden structures.
So, let's start with the images.
The Aero-3 was a Yugoslavian design first flown in the fifties. It is a beefed up Aero-2 which can be roughly desribed as an wooden Chipmunk (it also performed similary)
This is the Aero-2.



And with the addition of the Lycoming O-435 and a change of the lead designer it became somewhat bigger, heavier and therefore called the Aero-3



Above is quite an interesting image since it shows the airplane in the RAF camuflage colours which the Canadair Sabers wore, but applied in a pattern simillar to the Thunderjets Yugoslavia acquired at that time from Greece, who upgraded to Thunderstreaks. But usually they wore trainer Yellow just like the images below:

Ok, since the images work, let's move on.



First order of business was to remove the fabric and the filler underneath.
This was done by sanding and was a big task.



As we went along a lot of cowlings and systems were stripped. Here you can see the aircraft is already on the jacks.



While doing, as always the history of the aircraft came showing through. First pieces of the trainer yellow fabric:



Then pieces of the underside PR blue (they had a thing for RAF colours at the time. They even painted a MiG-21PFM in Day fighter scheme).



And of course the serial data plate found on each main assembly. Produced in Utva, Pancevo (now Serbia as the 101st plane in the series).

And right there the structural work begun:



First the aircraft was put in our rotary jig and the centroplan was removed:



While the systems crew was doing this, the structural guys were hard at it on the tailplane.



They removed a few areas of plywood that seemed bad and this is what was underneath one of the sections:



They had to go as deep as the front spar box and found that some very sloopy repairs were made, but thankfully the beams of the spar are good.



At that time they also started to work on the now dismantled centroplan which we knew was almost a basket case. The first clue was the rusted bolts that connect the centroplane spar to the fuselage former. You can spot them to the right of the image:



The trailing edge was also shot:



And when we removed the bottom skin the tailplane story with the spar happened again. Delamination, discoluration, but fortunatly the wood itself is good. Therefore the wood crew had to just laminate and rebox the spar:

The ribs however were a different story. We have to replace six out of 12 ribs in the centre section.
You can again see the slopy original construction work.



This is the fabrication of the innermost ribs:





And a comparision:



Again this is the inner side of the spar. All good wood, but on the bottom strong discoluration, that doesn't effect the structural properties at all.



At that time there was a danger since we didn't really anticipate as much work on the centroplan that the system crew would stall. Therefore it was decided to move the woodworkers on the fuselage, where there is less work to enable the system guys to start installing various fittings back in.
The wooden crew had to remove and fabricate some new panels, most of them were oiled up.

The systems guys first tackled the fittings. The image below shows the tailplane attach fitting.




All of the holes were reamed and new bolts are being fabricated:



Also all the metal bits have been stripped of paint and inspected:





On the above image you can see in the back is the gyro directional indicator in a newly fabricated holder. The legend says it is a knock-off from the gyro the the F-47 had installed.

And there you have it. For about 14 days more we are fully commited for some museum stuff, but after that it is all about this project. If this is not completly boring (please say so) I can periodically show the problems and the solutions as they pop-up.
Kindest regards
Saso

Saso,
This looks like a fun airplane.
Excellent work on the wings!

Come help work on the B17 when you are done!

Don Price- Wing Leader
Commemorative Air Force- Gulf Coast Wing
Houston, Texas

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Hello Saso,

Many thanks for posting the info and pictures of the progress on your project. This is of great interest to me as I have a special affection for wooden primary trainers (the Fairchild PT's being most similar to your project for me), and I always appreciate seeing the restoration work on these type aircraft. It is getting harder and harder to find good wood craftsmen in my neck of the woods. It's good to see you have some quality people working on your team.

PLEASE keep posting updates.

Steve

Great work!!

Can you describe your acceptance criteria for the wood structure? How do you determine what is good enough and what is bad?

Thanks for the great photo look into your restoration. Nice photo work and careful documentation. Guess you really have to with a project like this, helps to be able to put it back together. Reminds me of my old balsa and paper kit days, but on a much bigger scale!
Chris

There is a nice Aero-3 on display at the air museum near here in Oakland, California. I had hopes of flying it at one time, as it is owned by a gentleman that I have flown other airplanes for (Funk B-85C, OXX-6 powered Waco 10, and Challenger powered Curtiss Robin), but he is a very busy man, and he has not activly flown the airplane in the time I have known him. I think it is in the correct markings. How many are out there flying?

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Hi,
Thanks for the very positive replies. I will continue with the images probably tomorow.

Don,
I don't really think that the B-17 is a wood and fabric aircraft!

Steve,
We are very fortunate for having the wood team that we have. Maybe in some future post I will write something more about it. The Fairchild is a nice aircraft. Doesn't really rank for beauty, but heck you are kinda used to that over there. Do you have any images on hand of a stripped example that shows the structure?

Bdk,
The acceptance criteria for wood comes down to plywood and hard wood. For plywood it is very simple: any serius discoluration or delamination is enough for replacing it. Replacing plywood is usually not difficult as it can be more or less convinently scarf jointed. We tend to replace any plywood panel or gusset that doesn't look good.
For hard wood is more difficult as we are almost always talking about primary structures. The difficult thing is that hard wood like spruce or douglas fir can take significant capilary water immersion and still not rot.
The difference between rot and discoluration goes on several criterias one is the apperance of microstructure, second is the measurment of moisture content (rotten wood is usually very dry), third is the general britelness of the structure - tested with a sharp needle, but with vintage airplanes the tip off is usually the glue. The spar beams for example usually delaminate way before the wood starts to rot. Therefore if the glue is ok, you have a fair for keeping the hardwood in place.

Holedigger:
In a deep project like this we end up usually at around 14-15 thousand images. (and always wish for more during assembly).

Eric,
Flying is exactly one. Slovenian registered S5-MBB (look it up). There are a few in France, but none airworthy. There are two under restoration in Slovenia. MMM OX powered Waco. Must come to the U.S. some day.

Great insight. Thank you!

Mr. Dennis: I do believe this fellow has just insulted American wimmens.

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It was a good idea, it just didn't work.

Hey,
As said here are the new images.
But before that: Dan, I was actually talking about aircradt, but since you started the discussion are you confident you can handle THOSE?

Back to wood:
The oiled up ventral decking was covered partially before we started workin on the replica for the museum:



And now work has progressed from here.
This is the decking completed with the inspection hole for the brake cylinder. The brake cylinder was the culprit for changing the whole plywood area. I must had leaked in the past and therefore destroyed the decking.