gimme some skin

Got a chance for a few hours at the shop last night.

The plans call for the stabilizer skin to be pre-bent to 90 degrees prior to attaching it to the skeleton for drilling.  Conventional wisdom from other scratch builders is that this can be a challenge to do right.... bending this incorrectly, either crooked or in the wrong spot on the skin can at minimum lead to challenges getting the skin to lay flat on the skeleton and at worst render this very large piece of 020 aluminum useless (kinks are easy to make in such thin skins).  I been nervous approaching this step, but with a bit of prep and double/tripple checking everything it worked out well.

First step was to mark the skin with the bend line which represents the middle of nose curvature (done previously) and lay the stab skeleton in position to confirm alignment:

I had to find a long enough (10ft) length of hard wall pipe to use as the bending form, as Ron doesn't have anything long enough in the shop.  I originally thought I'd need some schedule 40 iron pipe, but I managed to find this mild steel electrical conduit at Home Depot that will work fine:

​To prevent the sheet from slipping, I used wooden blocks as clamps, screwed down to the table:

Some thought went into the placement of the pipe in relation to the line.  The overall goal here is to pre-curve the aluminum leaving room to tighten the aluminum down, not to make the curve exactly the same as the nose ribs.  The pipe gives the curved surface to bend against.  So the pipe is not directly on the bend line, but where the bend begins.  Once exactly straight, the pipe is secured at both ends with blocking screwed down and a strip placed in the end and screwed down to the table to prevent it from lifting:

It's hard to capture when both of us are lifting the skin and forming it around the pipe.  We use a long piece of rigid steel tube to place even pressure across the width.  It's bent a little at a time, readjusted under the pipe, bent again, shifted, bent until the 90 degree bend is complete.  If you look closely at the end you can see the incremental marks we worked to:

The size/width of what will be the horizontal tail on my 750STOL is really evident when I stood the skin upright.  At almost 9 feet across it almost touches the ceiling!

Placing the skin back on the bench and adding the stab skeleton confirms the bend is very close (enough) to make assembly move on to the next step.

In a previous post, I mentioned making a decision having to be made about which surface to fit first.  While researching how to bend the skin, it became apparent most builders suggest easiest is attaching the flat side first and drawing the skin across the curved side, so that will be my plan too.

Another challenge faced by scratch builders is that nothing made by the builder comes pre-drilled.  In my case this becomes compounded by the fact my factory spars have rivet holes already - so how am I going to match those blindly from outside the assembly?

First step was to drill though the existing spar holes to match the holes on the doubler from the outside - no problem, that part is easy.

Now how to proceed?  I can't drill from inside the skeleton out through the skin, there isn't room for a drill inside the spar web that is big enough to drill straight and the strap duplicator won't reach that far.

I placed the stab skeleton back on the skin all squared up and marked some of the points on the skin where holes will have to be drilled.  I really don't like this method as there is too much room for error if the skeleton shifts even the slightest over the length of the skin and there is no easy way to clamp it down.

Before drilling more than a couple of holes, I flipped things over on the bench, moved it to the edge where the top side can hang over the edge and secured the skin where the holes matched up at the one corner:

The trailing edge holes I can rivet from the outside and matching them up is easy with the duplicator.  There isn't a good way to blindly drill holes in the forward spar that guarantees a tight fit of the skin and correct hole alignment - and no second chances.

This got me to thinking about a way to drill from inside like I want to.  The only thing that I can come up with is buying a tool like this that will fit inside the spar, but man they are expensive and most require specialized drillbits:

I'll continue to ponder this until the next time at the shop and maybe Ron and I can come up with an alternative.  Happy with how this stab is coming together so far and really glad the bending of the nose worked out so nicely.

Another thing that got done for each of the airplanes being built in the shop was the elevator hinge pins.  Made form plate steel and a modified AN bolt, Ron put several hours into each of these.  What you can't see from the picture is the tiny cotter pin hole drilled in the pin or the finite welding he did making these - excellent work.  The black paint is a first coat of anti-corrosion primer:

Back to the shop this weekend for a whole day hopefully on Sunday.  Got to keep moving this project forward :)

Thanks for reading