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
I worked this week on getting the elevator trim channel installed on the elevator skeleton. I went over the plans several times to visually ensure I was adding the trim to the correct side of the elevator (remember, I'm building it upside down to take advantage of the flat upper surface of the airfoil).
Again, the plans have to be interpreted correctly - in this case the position of the channel is determined relative to the trailing edge of the elevator. But, that can be difficult without the elevator skin installed as the fold of the skin at the trailing edge extends past the tail end of the elevator rear ribs.
To solve this, I made a small narrow strip of 020 aluminum and bent it exactly as the elevator skin would be - it looks rough but it is exactly the right length to simulate the trailing edge:
I placed the strip in position and clamped it with clecos to the spar as if it was a complete skin. Measuring back from this temporary trailing edge, gives me the position of where the elevator trim channel should be but gives me room to to see my work.
Even with the measurement confirmed, I was having a hard time getting the trim channel to fit correctly, until I got a look at the build pictures that come with the plans. Turns out the kit supplied channel has been joggled at the end, allowing it to sit inside the tip and inner elevator rib.
Once I joggled my channel (that sounds bad as I type it), it fit in the ribs where I needed it to. This automotive body panel air tool is very handy for this:
The middle elevator rib gets trimmed down to fit between the spar and the trim channel. It's attached to the trim channel by an appropriately sized L bracket.
With everything squared up thus far, a quick check of the elevator alignment to the horizontal stabilizer shows extremely close to the plans, so my measurements, cuts and bends are very good and accurate. Very, very happy.
Next step is to start cutting the skins. These are fairly large in size and the bench is pretty crowded at the moment, so I rolled out the 020 sheet and traced out the skin on the floor, leaving it a couple of millimetres wide and long - it can always be trimmed back once I have it fitted to the tail skeletons.
First up is the horizontal stab skin. Making it fit correctly is challenging as you have to make holes AND account for the curvature of the skin across the top (bottom) of the airfoil as well. A kit skin would already be trimmed and holes cut for the front and rear stab brackets. As a scratch builder, this isn't a luxury we benefit from, so we have to come up with a workaround. Time for a template!
First step I did was to mark the location of the front brackets on the spar:
In order to transfer these measurements to the skin, I made a template from scrap 020. I cut out the space needed for the rear bracket, keeping in mind the overlap that is required by the real skin past the spar (15mm):
I removed the front brackets and with the template now in place (clamped) where the skin will be including the curve, I drew a line with a straight edge to represent where the back of the spar is - the goal here is to simulate where the final skin will sit in relation to the brackets. It's better to make mistakes on the scrap than on the full skin!
Knowing where the brackets come through, I was able to measure-mark-create the matching holes in the template and gently open the holes a little at a time with a Dremel tool until the brackets can be reattached where they will protrude through the skin:
I'm very happy how this template fits and I'm very confident it will transfer the positions of the holes to the real skin. I'll use the template to cut the slot for the rear stab bracket before final fitting the skin, but for now I placed the skin across the stab skeleton to check the fit - perfect, nice and square with the outer tip ribs and has the correct overhang of the spar.
Now I've reached a decision point. Do I fit the skin on top first and tighten it down with straps across the flat bottom or vice versa? Both have advantages. I can work form the rear bracket at the spar, fit the skin over the front brackets and pull the skin tight across the nose. Or I can start at the spar on the flat side, secure it and draw the skin tight around the nose, over the curve of the top - essentially working in the opposite direction. I've read that drawing skins tight over a curve is easier, but that means fighting with the brackets.
Either way, the skin will need to be pre-bent at the line that defines the tightest curve first - at the nose with a 27mm radius. The plans show a 90 degree bend in the skin prior to wrapping it around, so I need to get that done first.
I flipped the skin over on the bench to mark the centre of the bend line as per the plans (checking very carefully to mark it in the right spot - right and square:
A long piece of factory edge aluminum clamped down with wood blocks makes a great straight edge:
With the scribed line, I slipped the sheet under the stab skeleton to where it overlaps behind the spar 15mm and the bend line coincides where it should.
The next step will be pre-bending the skin, but I'll need to obtain something close to 27mm radius and more than 8.5 feet long so I can clamp it to the bench. I was thinking a piece of 2 inch ABS plumbing pipe might work, but it may not be stiff enough laterally, so maybe a piece of steel pipe. Unfortunately, Ron doesn't have anything that long in house, so I guess I'm going shopping :)
Thanks for reading, more to come!
Time until takeoff
Husband, father and 911 dispatcher. Long time pilot with a licence that burns a hole in my pocket where my student loan money used to be. First time aircraft builder. Looking to fly my own airplane.