It’s been a while since I posted but we’ve been making lots of progress with the horizontal stabilizer. Also known as the tailplane, its purpose is to keep the plane flying straight and prevent it from pitching up and down.
This is quite a complicated piece. The internal structure consists of a large front and rear spar, each of which has a doubler (large thick plate across the middle) and caps (long thin reinforcement strips that sit inside the edges) for extra strength, and a series of ribs that sit between the spars. Finally there are some pretty impressively-sized skins to go around the internal structure and finish it off.
There is a lot of drilling to do on these pieces – some holes are common to the spar, doubler and the caps and they have to be drilled through all three pieces at once to line them up exactly. That means labelling is key afterwards, as the parts only match in one direction (front to back and top to bottom) and therefore eventually have to be riveted in the exact configuration they’ve been drilled in. We’ve had to keep writing on them in sharpie pens and remembering to refresh it ever time we accidentally rub it off with sandpaper. We’ve also – unsurprisingly – had to replace our first drill bit as it got completely blunt!
The hole density on some of these pieces is incredible – I guess it’s not surprising as the middle of the spars will be an area of high stress, but there will be more rivets per square inch than we’ve seen on any of the other parts we’ve done so far.
After the drilling, many of the holes along the edges of the spars have to be countersunk to allow the flush head (flat) rivets to sit neatly in the hole. This is a very long-winded and somewhat frustrating task as the holes are all differently spaced and the drill bit has a tendency to wander when countersinking a shallow hole.
J discovered that making a drill guide out of some old aluminium angle was a good way of keeping the drill bit still, but the spacing differences between the holes meant it took ages to make the guide as it had to have all possible configurations. We later realised that the drill could be kept in place by running it slower – takes a few more attempts to get each hole perfect, but it means you don’t need the guide so overall the process takes less time. We’re learning!
The spars are the longest we’ve dealt with so far and a huge deburring challenge – lots of holes of different sizes, and long edges which have been machine cut and shaped so they have to be smoothed right down with a large file and several grades of sandpaper. It’s a mammoth task which just takes an awful lot of patience! Oh, and most of the skin off your fingertips.
For the large lightning holes in the spars, we’ve been using fine sandpaper pads, which normally gets them to the right level – for stray sharp edges we’ve been using a coarser grade of paper first and finishing off with the pads.
For the drilled holes, we’ve been removing large burrs with a deburring tool (basically a drill bit on the end of a metal stick) and then using fine grade sandpaper to tidy them up.
The parts that take the most time are the edges – these first need evening out with a large vixen file, then smoothing with a finer grade of file, normally the edges then need trimming to remove the sharp overhangs created by all the filing…then finally at least two grades of sandpaper are needed to smooth out all of the marks.
Once everything has been deburred, the task of washing all the pieces begins – they have to be completely clean of dust, grease and any finger marks before priming. We wash them in alkaline soap (wearing gloves so we don’t add finger marks after they’re rinsed!) and this weekend we’ve been hosing down the larger parts in the garden to rinse off the soap. Thank goodness for the sunshine, as we flooded the kitchen several times doing the same task in winter…
Here are the spars and associated pieces – drilled, countersunk, deburred, washed and ready for priming…phew!
We think it’s taken us about 25 hours of work on these pieces to get to this point, and it will be very satisfying when we finally come to rivet them all together!