Warbird Information Exchange

Hi guys,
Here is a bit of a mamoth post. But as I was browsing the forum I've came to the realisation that the threads that are most interesting for me show HOW are things being done.
So here is an effort to do the same. I've prepared a photostory about the wood work involved in renovation the vertical fin and also another one in making an aluminium doubler for a crack in a fitting.
So here we go.
First we had to patch up a few cracks in the fin plywood.
Here is one:



and here is another:



Note also that the bottom end rib is removed, because it was also damaged. More on that later.
The first task is to shape up the patch. Make a pencil outline directly on the plywood. Notice also that the grain of the patch must corespond with the grain of the original ply.



And remove the excess material. This tool is an interesting thing. It is a circural file that can be used in a battery powered drill bit. It is perfect for shaping plywood as indicated.



A few minuted later the patch is finished.



Next up, we have to prepare the fin for the gluing of the end (bottom) rib. The patched repairs must accept the rail to which the vertical-to-horizontal stabilizator fairing attaches to. Also notice another big hole to patch up.



Here are the new holes drilled and countersunk.



and here is how the rail fits to the new holes:



As you could on an image above the end rib bay had a lot of oversprayed paint. This was cleaned up, the bay varnished and with the metal rail riveted the thing was ready to accept the end rib.



But the end rib had to be fabricated before:



I didn't take any pictures of the gluing or clamping process, but on the following image you can see it already installed in the background.
The standard practice is to make new fillets under highly stressed fittings. In this case the plywood doublers are for forward attachment point to the fuselage.



And this is the result. Also notice that after removing the paint coats a handwritten ''4'' is evident just above the fitting. As one of our woodworkers is a history graduate he is maintaining a very strict historical standard regarding the preservation of such things. So the ''4'' was penciled over again and it will be varnished and top-coat-painted over.





The gray spots on the fin is actually old filler.



which was removed.



The filler was used mostly to cover substandard work when removing the fabric during one of the major overhauls the aircraft had in it's history. There are a lot of nicks to the plywood, where the knife went too deep and this was then covered with filler.



So, as the last stage of preparation of the fin before fabric covering we reaplied the srcaped and sanded filler to some areas. I know some guys that can do amazing things on the first pass, but I was always content with the I'll-sand-it-down-later approach.





Next up fabric covering the fin.
Kindest regards
Saso

Thanks for the great update, Saso! Keep 'em coming.

Scott

Yeah, interesting stuff. I've not done much wood work in the past, so this is educational.

Gary

Hiya there,
Scott and Gary - thanks for your comments. I am glad you like the (for me) new approach of documenting the process.

TriangleP,
The patch is installed via scrafjointing. I do not have any images of that at hand, but I will make a short photostory of it as soon as some patchwork comes in the process.
I could also try a textual explanation, but that can easily lead to the subject of large wimmen.
Kindest regards
Saso

Yep, the WIX family can be a little predictable. I'd personally rather see the scarf cuts and repair as a photo essay. Keep up the great work! If you make woodworking look really easy I might trick my finance officer (wife) into a PQ-14 project.

Scott

Likewise Saso,
I was wondering about HOW the patch was fit in securely.
Scarfjointing?
I know I could look it up, but I would love to see how you do it when you get a chance to show pictures.

I didn't do so good building a Spice Rack in 8th Grade woodshop!
Loving seeing this project come together.

SPANNER

_________________
Support Your Local Warbirds! KBO!
The only reasons the airplanes matter is what the veterans did with them... and why.

Hey Scott and Spanner,
Thanks for your comments. I will post a photostory about the scarf-jointed-patch-repair technique. I searched a while in my archives, but I had never taken images during the process. I just have a bunch of before and after images.
But the fabric on the fin is finished. Take a look:
I had left off the explanation when the filler was in place. After a few days some sanding followed to shape the filler. After that the whole fin was coated with glue. We use aceton based glue, therefore the glue is dry in a matter of ten minutes. Here you can see how the wood takes a darker hue with the glue. Also the reinforcing patch around the fairing rail is already attached:



After that a blanket of fabric is laid over the fin and the edges are tacked down. This is where the magic of aceton glue takes place. One just has to brush the aceton over the fabric, it soakes through the fabric and activates the glue underneath. First you do this with the outside edges, then you heat-shrink the inside warps and then secure the inside area like this lad is doing right now.



After this was completed we rough trimmed the edges and also tacked the leading edge overlap on a few places.



The image below shows the other side of the fin, where the overlap finishes. To do a good job on the fabric work use plenty of rulers, curvatures and draw liberaly. Here I am marking the fine edge of the overlap. For marking the radius line we have an indispensibile set of plastic rules for various diameters. Originaly it was a road engineer's set for drafting road turn radiuses.



Then you do the whole thing on the other side. However, don't forget to draw the overlap line on the new fabric and to coat it with glue.



Here is the same leading edge with both sides of the fabric on and a new coat of glue ready for the finishing tape.





And when the bias cut finishing tape was secured the fin was proclaimed ready for painting. Of course we did a motivational dry-fit on the fuselage.



I must apologise again since my DSLR is unavailable and I had to use this sugar workshop camera, so the detail shots of how the tape and corners are finished will be supplied later.
Kindest regards
Saso

Sure, however If I might suggest you should maybe just omit the part that you will spend the time and the money on a drone.

Her main hang-up isn't the drone aspect--she doesn't like the single-seat concept as she enjoys sharing the things we spend too much money on!

Scott

Just have her get a pilot's license, so she can go fly it herself.

...and then use it as a drone for AAA training?
Still doesn't sound right to me.
Saso

Hi guys,
This is the other photo-essay, I was talking about:
It is regarding a repair in a large riveted 2024 fitting that carries flap acutators and rudder hinges.
It is the big piece in the back of the fuselage in this sketch:



The task was to fix a developed crack in this fitting.
See if you can spot it:



When you spot a crack it is a good practice to chech the symetrical portion of the part. If a crack develops on the left, it is very likely it will also develop in the right side.
Here was no diferent:



First the rivets were drilled out:



Then, the cracks were stop drilled to prevent them developing further:



And then a doubler was fabricated, no need to get fancy here:



And the stuff was wash primed and riveted together:



There is also an underline to this story. I really wanted to match the washprimer colour to the original part. Therefore I tracked down the original company that produced the wash primer. In this case it is Chromos in Zagreb, Croatia. They sent a sample of this wash primer to Chromos Slovenia, who did a great job consulting us. This whole process took about three weeks to obtain the wash primer and one afternoon to repair the cracks. BUT you cannot beat working with original materials.
Kindest regards
Saso

Hi guys,
It seem like the maintanance section of the site kinda died off or turned into a flea market.
So here is my attempt to liven things up. Here is a quic-nasty-pictures-looking tutorial to try and explain what is scraf-jointing all about.
As you might remember we had a bit of a problem with our tailplane. What we had done is opened up the spar and replaced the front spar cap, as it was rotten. This also meant creating a new set of noseribs. After this was done, we had to close the darn thing.



This is where I tried to take a few images to show you the basic things of scarf jointing.

First you have to tailor the plywood. Here you can see the side of the plywood panel that will actually be the inside of the tailplane. You cannot varnish the whole wood, since that would make the glue less adherent to the surface, so this is a bit of a tedious job.



The point when gluing wood is that you cannot but-joint it. There must be an overlap - the scarf. Basically you cut a slope to the existing plywood and the oposite slope to the new plywood panel. This is the gluing surface and this is how it looks when cut:



To see how this two slopes look like when they are glued together I am reposting a picture from a few pages back where you can clearly see how the two slopes meet up:



Then you proceed with each new panel cutting the slope to the new panels as you go along.
Here is the final result:



This technique is used for all glue joints. Sometimes this can get a bit hairy.
For example take these repairs on the leading edge of this wing. Imagine cutting the correct and matching slope on the irregular shape of both the new plywood panel and the existing D-box plywood. This is where currency and mileage of the craftsman comes into play.



Have fun,
Saso

It is kind of quiet in the MX Hangar, isn't it? I guess everyone else is either flying what they fixed or raising money to do more fixing. Glad to see your update. Keep them coming!

Scott

Saso,
Sorry I haven't been keeping up. I got distracted...
Thanks for posting the scarffing technique.

The project looks like it going to be a beautiful airplane.

Please keep posting updates.
Thanks,
SPANNER

_________________
Support Your Local Warbirds! KBO!
The only reasons the airplanes matter is what the veterans did with them... and why.