elevator trim and stabilizer skin template

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.

Factory trim channel - note clean joggled end

My scratch built channel - no joggle stated in the plans!

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:

These little finger clamps are excellent for jobs like these. The channel position has been marked on the rib. Once the skin is added and the holes drilled on the spar, ribs and channel, everything will be taken apart again for deburring and prime before reassembly.

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):

Scrap aluminum from the 701 wing repair makes a great template!

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!