Fabric finishing

...is actually not only about fabric. In this blog post I will try to illustrate why.

First let's start with a very nice picture. It shows the fabric samples taken from the old fabric. The Aero, as you might recall was painted in olive drab/dull gull gray scheme, when it came to our shop. But then when removing, various pieces some original fabric started to reveal itself. This is nice, since now we prepared samples of the original colour, best of all, which had never been exposed to daylight.

Next stop, paint store. Expensive stop actually!

But the colours are nice, aren't they?

On the raw fabric several coats of varnish or dope must be applied. Some of them by brush and some of them with a spray gun. It is quite easy to get white blush on the parts when applying the varnish with the spray gun. It looks like this:

This is white blush. The fabric is a bit whiteish instead of being transparent. It occures when the varnish is evaporating so fast that it traps the moisture from the air. It must be corrected by a special procedure. (Yeah, it was!)

The last varnish crosscoat is applied using a mixture of varnish and aluminium powder. This gives the parts an UV protection. At least it did so in the past, but now it is used for authenticity.

Aluminum powder gives the parts a silvery finish.

Now, again, this moves at a fairly rapid pace. What eats away hours is the detailed fitting. I will try to illustrate this on example of the elevator control attachment.

The elavator is on the left.

During intial fitting of the elevator control horn, you can see that the fit was not all that hot. But acceptable?

So what is wrong above? If you zoom in, you will notice there is a gap between the flange and the fabric surface. This gap is actually unairworthy. Why? Below you can see the picture of the original horn that was removed from the aircraft. If you zoom in again on the flange, you will notice that there are several cracks around the flange. These cracks are circular and are the result of the fit that was simmillar to above. However when you tighten the bolts it tries to pull the flange upward and the flange cracks. If not immediatily, then eventually.

So having to rebuild the horn during the restoration, we didn't want to do it again, so we produced a plywood shim that enabled us to tighten the bolts, press the flange down to the fabric, but not do any damage  at the same time.

The plywood shim is installed between the horn and the fabric.

Of course this is not just a thick piece of plywood, since it must conform to the elevator radius.

You can judge the shape by the sanded off layer of the plywood.

And here you can see the elevator horn and the shim, correctly installed. It took about two hours to get this right. Longer then five minutes to install it improperly, but beats having the part fail!

Installed, view of the bottom.

...of the top.

Overview

And everything tucked away avaiting the very near painting. Strech foil is used to keep the finish layer somewhat checmically active to asist in bonding to the paint.

See you next time!

Fitting of the center section finished

Here is the more organised post that I promised you. Several parallel things are being worked on, so I will try to make posts more topic orientated.
This one is about the center section (again). Well it is now finished. All the nutplates are in place and fairings are not only fitted to them, but also primed and ready for painting.
Slowly now. This is an image of the leading edge fairing. Can you spot the difference?

No? Well, we are quite good at what we do. The difference is that on the right side the hinge is replaced - fabricated from sheet aluminium to quite close tolerances.
The hinges were mostly cracked also on the left side.

Like seen on this image - see inside marked circle.

So fabrication of new hinge, fitting and riveting it to the fairing took about 6 hours. This is why restorations take so much time. At least those done correctly.

Brighter aluminum is a give-away for the newly fabricated part.

And here it is in primer.

So our stack of restored parts is getting bigger.

The last thing before taking the center section off the airframe for fabric covering is one last check that everything really fits. Now is the time to correct that odd nutplate or bend the fairing a bit more. When paint and fabric will be applied, then it will be much harder to correct any mistakes.
So...

Looking good with all the screws installed.

Yeah baby, yeah!

Now wait a minute. How about this hole between the centersection and the fuselage?

I mean you can see the light through!

And this was the moment. Mistery solved. We were never really sure how so much water could accumulate in the centersection. Of course it could not leave the aircraft, since it had no drain holes, but how did it get there in the first place? Remember this? At some time there were at least several liters of water pooled up the rear spar. I always blamed a faulty windscreen seal, but missed the obvious. This hole, which was designed into the aircraft is to blame that we spent a year rebuilding the center section. See the image below.

The hole is between two arrows. All of the water that is collected in the shaded area then flows directly in the center section. No wonder that there were several liters of it there!

So, am I saying that the designers were complete idiots? Not really (however I am saying that the guy that didn't put the drain holes fore of the rear spar didn't earn his paycheck that day). Maybe it was originally designed with a sealing tape was placed in that hole, but none is mentioned in the Maintenance manuals. However, this hole was indeed recognised as a problem, and a cover over this hole was designed when storing the aircraft.

See how the cover goes over to the center section?

The only problem is that I had never ever seen such a cover used. However it is a valuable lesson in the future operation of this aircraft. I consider this now a fleet problem with all Aero-3 aircraft. More on this at some later date. The bad news now is that we have to take the center section off again. Does no good for motivation. See you next time with fabric work being done.

Little big things

Yeah, time flies (but so do we...)
Lots of work had been acomplished on the Aero project.
Biggies are that the canopy plexiglass is fitted to the frame, fuel tanks are now servicable after cleaning, pressure test and repainting.
There is a lot of paint stripping, fitting and priming going on on various metal parts and other stuff as well.

Fuel tanks and vent lines all ready to go.

Nicely primed! More or less metal parts from the center section, like fairings, D-boxes, flaps and some are from the windscreen also.

But here are two images just as a preview and I will try to put together a more concise post in the next ten days or so. Enjoy the summer!

Canopy frame

I actually managed to keep my promise to post more often. The following was done in the past week. We prepared the canopy frame. This involved restoring it, making some replacment parts and painting. It is going off to have the plexiglass fitted. Don't have enough courage to do that in our shop.
Here is the begining:

About three layers of paint had to go off first.

Same frame as above but with the plexiglass and metal fairing removed.

Here it is in primer. The original plating held up well so corrosion was not an issue. But we had to prime the inside for the new paint to adhere better.

And I always was a sucker for details:

Before and after shot:

The latch mechanism before.

and after.

I will come back soon hopefully with more nice images.

Have fun till then!

Bolts

Whoa. Time flies. Not our Aero, though.
Let me just say that in the first 50 days of the year, we reached our yearly plan for composite production. Enough said.
Somethings were still done on the Aero. This time I'd like to present the research work done on bolts. There were several bolts that were either damaged or corroded to such extent that they need to be replaced.
One thing that we constantly keep in mind regarding this aircraft is that it is heavy. H E A V Y.
Therefore all the primary structure must really be top notch and we have to be confident in it.
So we ran a tensile test on some samples of metal.

The image above shows three samples. One is a sample turned from an original bolt, the other is a material that I thought was suitable for new bolts, and the other is simply a 8.8 normal hardware bolt.
The result of the test was not so hot.

As you can see the normal hardware bolt peaked at 640 N/sq.mm as it should. The strongest bolt was the original one at 1000 N/sq.mm and the CrMo that was my best bet before the test reached a little over 800 N/sq.mm. Back to the drawing board, but this time it is much easier, since we know what we have to replace.
Another two samples are prepared. One again from the original bolt and another from the a different CrMo steel. So it is time for another test.

This kind of work is exacting and it does eat away at the time, but it has to be done. Imagine putting a normal hardware bolt replacing the high tensile old one and crossing your finger...
I won't promise to keep the blog more up to date, but I will certainly try to.
Have fun, enjoy the spring!

Windshield

Hey everybody,
First post this month - just in time actually. The reason is very simple. NO WORK had been done on the Aero. November and December were fully booked and no resources were left to work on this aircraft. A pricing policy change and some new help will get the project rolling a bit more rapidly in the first quarter of 2012.
Still, some stuff was outsourced - like the windshield.
The story goes that the bubble windshield was the first blown windshield produced locally. Was not all that great to start with. The unfortunate S5-MBB had a crystal clear windshield, but was not optically correct due to waviness in the plexiglass. When flying it, one had to constatlly weave around with the viewpoint to get the ''correct'' sight. The other problem was that our windscreen was full of microcrakcs as evident in this image. These are internal cracks that cannot be polished out, so the whole windscreen was unservicable.

Very, very big image, to show the cracks clearly

So this is the correct solution for the problem:

Looks nice, doesn't it? We will see how it is installed.

Thanks for stoping by. I wish all of you to stay healty in 2012! Happy new year.

A short introduction to Aero-3

Our workshop has a new roof. Now we have to tidy up a milion small details and this will take some time. So nothing new to report on the Aero, but I was thinking that this might be a good time to explain what exactly is an Utva Aero-3.

Utva Aero-3 in flight.

Utva Aero-3 is a primary trainer that was built in 1959 in Yugoslavia. As you might had noticed in the previous posts it is made of wood, but is powered by a Lycoming O-435 engine, which was pretty standard for that age.

This trainer was built primarily to serve in the Yugoslav Air Force Academy during the sixties, when the country was receiving a huge number of aircraft through the MDAP scheme. This needed a steady influx of novice pilots. Therefore the Academy devised a three year training programme. First years was basic training on the Aero-3, second year advanced flying on Soko 522 (picture is an older post), and third year operational training.

Most Aero-3 trainers wore trainer yellow paint scheme, but these two are camuflaged.

Now as I've written in a lot of previous posts, the aircraft is just a vehicle for people to communicate. Now, let us talk more about the people that flew Aeros.
This picture is from the personal archive of Mirko Anžel of Trbovlje. It was taken in 1965 during his training in the academy. Since he was the number one of his class, he opted for fast jets and therefore flew 10 hours of TV-2 (version of Lockheed T-33) and then solo on the F-86 Sabre.

Mirko Anžel before flying the TV-2.

Mirko always recals that this was amazing. His generation had almost no time on Soko 522, since they had a fleet problem of cracked propellors at that time. Therefore they flew about 120 hours on the Aero-3, a few hours on the Soko 522, 10 hours on the TV-2 and then soloed the Sabre.

Mirko Anžel, left. Notice that the Sabre is one formerly used by the 204.lap aerobatic team.

This image shows him in front on the F-86, with his instructor in the center. Mirko then went on to MiG-21s and graduated from EPNER at Istres, France as a full course graduate. Even now, he recalls that his greatest profesional achievment is the Sabre solo. Everything that came later, like the MiG-21, Mirage III or the Caravelle, was not that much of a jump then it was from the Aero-3 to the cockpit of the Sabre. Mirko is a constant source of knowledge and advice. If we were not restoring the Aero, we would not had met him.

Vojko Gantar in front of Lockheed TV-2, right.

The image above shows a very young Vojko Gantar from Idrija. Vojko was one of the top Yugoslavian Air Force fighter pilots. At age 23 he was assistant squadron CO on Sabre D fighters and then spent 22 years flying the supersonic MiG-21. He went through the same process Aero-3-Soko 522-TV-2-Sabre during his academy years. His last years before the breakup of Yugoslavia was in command of the MiG-21 training squadron at Pula, now Croatia. Again Vojko is one of the very best teachers and menthors you can imagine.
Perfectionist by nature, open minded and extremly willing to help out, I think he might be one of the most valuable forgoten pilots that we have right now.

So again, the aircraft does not matter. Sure, it is a nice aircraft that can teach tailwheel, basic aerobatics and actually has some post-stall flying characteristics (which is totally untrue for aircraft built under JAR/FAR-23, which just do not stall...), and with a supported Lycoming engine to boot, but it is the people that matter. What matters is that Vojko totally secluded from flying, worked as a mountain sheppard, but now he is back again lecturing on hand flown IFR and supersonic flying.

This is why we restore aircraft.

Till next time. Have fun, go beyond your ego, seek advice, talk to people, respect your elders.