Centersection fabric - preparation

Good morning from a snowy(!) Slovenia,
During the past week I managed to get in some concentrated effort on the centersection. By working way afterhours for four days I am very happy to report that the center section is now fabric covered.
Proof:

This image shows the state of the work on Thursday evening. Friday was spent  doing the details.

So, to show you the process, I will probably post two blog entries. This one is about the preparation work involved before laying the fabric. Often overlooked and definitly not much talked about, the prep work is the basis of a masterclass fabric work.

First, we had some details to finish off. Like this big circular wood piece, you see in the center of the image. A fuel tee with a drain valve sits on it. It looks benign, but required some forethought.

The laminated wood block in the center with the through hole.

One of the challanges involved was blindly locating the three nutplates through the skin of the center section. A trick often used in sheet metal was used:

The other holes you see are drain holes.

Then we finally had the piece ready for fabric. One of the most important aspects in composite construction is the minimum bend radius. Depending on the specific weight of the fabric, there is always a certain radius that the fabric can follow. If the radius is smaller then this, you have problem in laying up the composite.

The same thing is actually true for fabric covering aircraft. Most of the prep work involved in preparing the piece for fabric is sanding it, to get off all the grime, dirt and oils, to enable the glue to set up a stronger chemical bond to the wood AND establishing a minimum bend radius on all the edges that the fabric will wrap around. This is actually a massive amount of work.

Most of the time these bend radiuses are made by sanding off the sharp edges, but sometimes filler is needed.

Like here.

No manual (that I am aware off) really states the minumum bend radius for fabric, but I estimate it to be about 2-3 mm for the normal grade aircraft fabric (cca 100g/sq.m). So the filler is then sanded accordingly.

Again, this is very simple, but requires a good amount of work (about 10 hours for the centersection) and is a step that will not only make laying up the fabric much easier, but also the quality improves drastically.
Do a good job on the fabric and you safeguard the structure (if you happen to remember to put drain holes in it!)

Next post is about actually doing the fabric. Before I sign off, I would like to put some faces to the names.
When in the blog I say ''we'' did something, that most probably means that one of the guys in the picture below did something.

On the left is Janez C., right is Igor M.

Remember that Janez is a partner in aereform and does all the wood. Igor is a hilarious kid that is excellent with the spray gun and quite good with the wrenches. While reading through the blog the other day he said, that: ''you know those are my blisters that you are posting to the internet!''. So, yes, it is about time to put some faces and names to our company and into worldwide fame and fortune.

Will post the rest of the fabric process in about a week. Go and build a snowman or something.

Windshield

Good morning from a rainy Slovenia!

Our shop is backed up with the usual autumn peak orders so there is not really a huge amount of free time to work on the Aero.

No time yet for the fabric covering of the centersection, altough some details were finalised on it. Fabric however takes a few days of concentrated effort that must be done in a stretch.

Anyway, we did (almost) finish a nice piece - the windshield.

In previous posts you can see that the metal part was in primer and interior gray was sprayed on the inside. Now it was time to fit the transparencies.

First a SolidEdge draft was made and printed out in 1:1 scale. Then this sheet of paper was cut and and test fitted to frame to check for accuracy. Then the files were sent out to the plexiglass shop for cutting.

After the plexiglass was delivered it was fitted to the frame. Slow and careful work is in order here.

This is the finished piece. Notice the system for cleaning the windshield on the bottom of the windscreen. This hooks up to the fuel system and sprays fuel on the glass.

The two remaining items are the leather seal, for which we are still waiting the material and the other is the handgrip for the student pilot when entering the aircraft. This is the original one.

To make it look like it just came out from the shop, we would have to replace the leather, and the bushing on the metal part. However then it occured to me that I really do not want this part replaced or restored. Every pilot that whenever flew this aircraft gripped this handle. So I really do not want to replace this. A bit of cleaning some touchups and now we have a legacy centerpiece in a most prominent place in the aircraft. Sometime one should not do everything one can.

See you!

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!