Tailcone - Bottom Skin Riveting

It is way too cold to finish priming the stringers, so I decided to go ahead and try to get my riveting muscles back in shape by riveting a couple of bulkheads to the bottom skin.  In an ideal world I would have clecoed the entire tailcone together before riveting anything, but riveting these bottom portions alone would be next to impossible (no physical way to reach over the side skins and buck rivets while also using the gun underneath).  I don't want to lock the structure in place until it's all clecoed together and I can verify there's no twist, but I don't think there's any harm to riveting a few pieces together prior to doing that.  I laid the skin upside down and let the bulkheads hang down underneath it.  Aside from the fact that it's obviously been a while since I've riveted, this worked fine.

I was going to cleco on the baggage bulkhead so I could also rivet the bellcrank rib to the bottom skin, but I realized that dimpling the holes in the rib chipped off all of the primer around every hole.  This is the rib that I tried the Rustoleum primer on.  I guess the verdict is in - it does not adhere anywhere near as well as the Ekoprime.  I sprayed a quick shot of the Rustoleum back on the rib to cover up the bare aluminum.  I'll see what it looks like tomorrow, but given the temp, I'll probably have to hit it with a heat gun to make it to where I can rivet without just rubbing it off.

I was just going to quickly spray the stringers with the rattle can primer, but not anymore.  That stuff is probably fine if it's sprayed out after all work is done and it's not handled much.  Otherwise, it's just too fragile.  I'll figure out a way to spray the long stringers with the Ekoprime.  Granted, there's already one layer of primer on the skins, so realistically I could probably leave the stringers bare alclad and not worry about it.

Tailcone Deburring & Dimpling

The last couple of afternoons have mostly been filled with hole deburring and dimpling.  I just have one side skin to finish.  Before I can start riveting it all together I do still have to prime the stringers though.  We have a couple of weeks of snow/rain in the forecast, so I'm back to the sub-par priming in the garage at 35 degrees.

Tailcone Disassembly

Time to take all of my hard work apart!  I flipped the tailcone over just to make it easier to pull clecos out.  My work area looks like a disaster - so many parts laying around.  As soon as I get to the point where the central part of the fuselage is ready to join to the tailcone, I'll have to reorganize again, move tables, and break down the crate that I still have parts stored in.  Once the fuselage is ready to be joined, it grows quickly and won't fit in the narrow and short space I have it in now.

I had a few odds and ends I had to take care of as well.  The aft canopy deck rails that I drilled to the longerons a while ago still needed to be modified since I'm building a tip-up canopy.  I laid everything out, checked my measurements about 100 times, then finally made the cuts.  I have come to prefer using the pneumatic die grinder with a cutoff wheel for work like this.  You have to be careful to not let it get away from you, but I actually find it easier than the bandsaw for cuts like this.

I still hadn't fabricated the tie down bracket for the tail, mostly because the instructions never called it out.  I just realized I still needed to do it after looking at the plans.  I cut the aluminum blank to shape, drilled and countersunk the holes and match drilled them to the aft bulkhead (only 4 of the holes are countersunk - the others get bolts when the tail is attached), and cut threads for the tie down ring.

There was one oops the other day that I need to figure out a solution for.  When the J-stringers were match drilled to the skin, I created an extra hole in one of the flange tabs of the 710 bulkhead.  The design of this is such that I'm kind of surprised I only messed up once.  The skin has a pre-punched hole, then behind the skin sits the blank stringer (no holes), then behind the stringer sits the bulkhead flange with a pre-punched hole.  So there is no way to accurately line up the two pre-punched holes since there is a solid piece of aluminum between them.  In hindsight, I should have taken the stringer out, bent the tab as best as I could to line up the hole with the skin hole, then put the stringer back in place and drilled the hole.  I didn't realize the tab had a hole already until it was too late though (a lot of the other bulkhead tabs don't have holes in them to allow you to simply drill your own wherever it needs to land).  I emailed Van's to ask what I should do.  I'm guessing they'll have me just back the tab up with another piece of aluminum that is riveted to the bulkhead web - basically sandwiching the tab with the two holes.  I also asked them why in the world they pre-punched a hole in the tab instead of just leaving it blank so it could be drilled to the skin without having to blindly hope the holes were lined up.

Tailcone Stringers

There's not much exciting to show from the day, but I made quite a bit of progress.  I clecoed the entire tailcone together for what felt like the 30th time, this time so I could move on to drilling the J stringers.  A few of the stringers are just a tad long, but they fit well enough that I could get them drilled to the skins, and then later I will sand off a little of the ends (they just stick out beyond the skins slightly).  I had already drawn a line down the center of the stringers, so lining them up for drilling was as simple as keeping the line visible through the skin holes.  Once the middle and lower stringers were done (the upper stringers don't get put in place until the top skin goes on), I moved on to final size drilling all of the other holes.  I didn't quite get done before dog walking duties interrupted me, but am close.  I just have to finish reaming the bottom skin to side skin holes and match drill some new holes into the aft baffles.

As usual, by looking at the plans and reading through other builder blogs, I discovered a few other things I need to do that aren't mentioned in the instructions.  I need to prep and at least drill the tail tiedown bracket to the aft baffle (it won't be riveted on until after the skins, since it sounds like it gets in the way if you do it first), as well as make some modifications to the aft canopy deck rails that I drilled to the longerons.  The aft end of those get chopped and shaped differently to accommodate the tip-up canopy vs slider.

Longeron Bend & Twist

With the help of the bending dies, I finished the main curves in both longerons.  The dies definitely made the process much easier simply because of the fact that the out-of-plane bends weren't as bad.  They still occurred, but to a far less extent and were much easier to fix.  Van's provides a full scale drawing of the bend, but when I compared it with the actual part that the bend is to fit, the drawings were off enough to cause problems.  I instead made the curve fit the aft canopy deck.  That is, after all, the part that will eventually get riveted to the longeron.

Next up the aft canopy deck had to be match drilled to each longeron.  To get the edge alignment for the canopy deck correct, I taped a couple of aluminum sheets the same thickness as the fuselage skins to the outside edge of the longeron.  The canopy deck sits on top of the longeron and on top of the skin (so overhanging the outside edge of the longeron itself, hence the need for a spacer to imitate the skin).

Then it was just a matter of drilling and clecoing the canopy deck to the longeron.

The longerons still require more bending at this stage.  At the front portion where the front of the aft canopy deck ends, the longerons have to dive down to match the angle of the side skins.  The plans provide both the measurement for the distance the end has to drop as well as the actual angle (5 degrees).  I put the longeron in the vice right where the bend starts and did the ol' bend and whack it with a rubber mallet approach.  It actually bent quite easily and didn't bend out of plane too terribly much.  Lastly, from this mark forward, the longeron needs a twist.  I still haven't quite pictured in my mind how a twisted longeron gets attached to the skin, but I'm sure that'll become clear once it all goes together.  The twist was a bit nerve wracking.  There needs to be a 17 degree twist from horizontal.  I put a crescent wrench on the end and gave it a turn.  No twist.  I tried again.  No twist.  Turns out you REALLY have to twist the longeron to get it to maintain a permanent twist.  I was sure somehow it was going to snap in half, but eventually I got the twist in.  I used a simply digital level app on my phone to check the down angle as well as the twist, then made a cardboard template of the side fuselage skin and held that up against the bend to verify it was good.  At least from what I can tell, it looks like it's right.  I won't know if it's accurate enough until it's time to assemble.

There are two J stringers for the top of the tailcone that don't get put in place for quite a while (the top skin is left off as long as possible to make access to the tail easier).  I decided to go ahead and cut them to length and shape the ends just to be done with them though.

All of the skins and bulkheads are in place now, so I inserted the J stringers (all but the top ones).  Next up will be leveling the whole tailcone as much as possible and match drilling the stringers to the skins, followed by final size drilling all of the other skin/bulkhead holes.

Aft Tailcone & Longeron Dies

I drove into Seattle yesterday and visited with another RV builder (he's building a RV7).  It was nice to be able to take a look at a project that is a little further along (the RV7 fuselage is actually the same as the RV9).  He is letting me borrow his longeron bending dies, which I put to use today.

First things first, I went back to the bottom aft skin to try to get it and the bulkheads shaped appropriately.  This skin is very thick and it took quite a bit of bending, but I finally got it to the correct angle.  After seeing the RV7 tailcone, I figured out that the two aft bulkheads I have are simply severely under bent, so I also bent those flanges so they match the skin angle better.  I slid the J stringers out of the way and clecoed the skin into place.

After the bottom skin was in place, the 710 bulkhead was next.  I looked at it and thought "this will just slip into place."  Boy was I wrong.  For being such a simple piece, somehow it was a bear to get in.

The aft two bulkheads, which I thought would be the tougher ones to get in, actually went in pretty easily.  I did have to put them in and take them out a few times to get the flange angle bent to the skin correctly though. 

I decided to give the longeron dies a shot with the second longeron (I'll still need to correct the first one I already started).  The dies do exactly what my previous method did, but the difference is that they capture the top portion of the angle.  This keeps the angle from bending out of plane so much when a curve is put in.  It was 100x easier than the method Van's uses!  I finished up with the main bend of the longeron in about 15 minutes.  I still need to correct a tiny bit of out-of-plane bending that happened, but it was very minimal compared to doing it without the dies.  The first longeron that I already started needs to be corrected yet.  I'll try that tomorrow when I'm fresh on patience.  Since it's already been bent in multiple planes, I'm still going to have to do the back and forth approach to inch in on the correct bend.

Tailcone J Stringers

Tonight was one of those nights where I stood around and scratched my head a lot and assembled and disassembled things over and over to figure out the right order to put it all together so pieces didn't conflict with each other.  I was trying to think through the best order for eventual riveting.  While the tailcone structure is pretty simple, the aft end is hard to fit together unless the stringers are left out until the very end (as opposed to what the instructions say).  I did also get a tiny bit of actual construction done as well.  All of the stringers had to be trimmed on the fore and aft ends to allow one portion to slide in between the end bulkheads and side skins.  It was a simple task, but took a while, since of course none of the cuts called out in the plans are just 90 degree cuts.

The thing I really puzzled over for quite a while and eventually gave up on was the most aft skin and aft bulkheads fit.  The flanges on the bulkheads are all a perfect 90 degrees from the web, which means none of them fit the pretty dramatic skin angle.  That's easy enough to remedy by just changing the flange angle, but they are so far off that it has me wondering if I've done something wrong.  The plans don't really show anything very clearly unfortunately.  As luck would have it, I'm going to go visit another builder on Friday to borrow his longeron bending dies, so I'll be able to take a look at his project and hopefully figure out how things should fit.

The pictures don't really show how far off the flanges are from the surface of the skin, but it's pretty significant - probably over 1/8" in the front sides.

Tailcone

I haven't had much time in the shop this past week, but I finally got back to it today.  I'm putting the longeron bending aside for now.  I may have found a local guy who has bending dies that I can borrow to make the longeron bends easier, so I'm going to move on to the tailcone for now.  I clecoed the aft two bulkheads to the tail skin.  This skin is very thick and stiff.  The flanges on the bulkheads don't seem like they'll line up with the outside of a skin, but I'm going to leave them as is for now until I see how everything goes together.

The tailcone has to be laid onto saw horses to allow access for clecoing the bottom skins.  I went to Harbor Freight and picked up three cheap saw horses.  I set the bottom skin on the saw horses and clecoed the 707 and 708 bulkheads to it.  The plans technically show how to cut the ends of the J stringers to mate up with the aft baggage bulkhead, but it's convoluted to say the least, so I didn't cut the stringers to length or shape the ends yet.  I just slid them into place for now and will cut them later.

Next up was the side skins, which got clecoed to the bulkheads.

The baggage bulkhead was the last one to cleco on today.  It never ceases to amaze me how much super flimsy material stiffens up once assembled.

Longeron Bending & Center Bulkhead

I've worked in spurts the last few days.  I marked off the various lengths on the longerons that show where the main fuselage bend starts and ends, and I made some marks every inch inside the bend area just to help bend with consistency.

Van's method is to simply put the longeron in the vice, bend it a bit, then whack it with a mallet to set the bend - over and over and over.  I've been told by many that this is easier said than done, so I'm trying a different technique.  I took 3 stacks of 2 nickels and taped them to the vice.  Two stacks on one side and one stack in the middle of the other side.  By cranking down lightly on the vice, it puts a bend in the longeron.  No hammer necessary.  The main problem is that by putting a bend in the horizontal direction, the angle inherently also bends in the vertical direction.  Once you flip the longeron 90 degrees to take out the vertical bend, it also moves the horizontal bend again, so there is a ton of back and forth and back and forth.  I still haven't quite gotten the horizontal bend correct because of that.  Van's says both dimensions need to be within 1/16" (horizontal 1/16" within the template line and the vertical component laying flat on a table within 1/16").

Van's provides a paper template for the curve, but for a big part of it you can actually test it against the actual curved rail cap that it'll be riveted to (more accurate than the paper template).

I'll come back to bending later.  It's one of those things that is better to do in small chunks so the frustration of two steps forward one step backwards doesn't become too much.

Van's finally sent the right rivets and material that I was missing for the center bulkhead, so I jumped back to that to finish it off.  I made the spacers that go on the back of the fore bulkhead, primed, riveted nut plates on, and riveted the whole thing to the bulkhead.

With the spacer on, I could screw on the remaining two outside ribs to the front side of the bulkhead.

The last step for these bulkheads was to temporarily bolt them together to check that everything lines up, before taking it all apart and storing it for later.  I made some wood spacers that perfectly (as perfect as I could get wood to be anyway) fit in between the bulkheads, taking up the space that will ultimately be filled by the wing spars.  I bought some bolts at Home Depot to line the holes up.  The final bolts that are used are close tolerance bolts.  They are extremely difficult to get in, so aren't used until the wings are ready for the final mounting.  Apparently you often have to shrink the bolts by freezing them, otherwise they are difficult to fit.  The hardware store bolts are slightly smaller, so slide right in.  I also checked that the two long bolts on the inside fit (you can see them just outside of the control rod bases).  I had to drill a reference hole to 1/4".  I'm not sure if I got that just slightly off or what, because one of the bolts was a nightmare to get in.  I finally got everything together by doing them all little by little.  So in the end it all fits okay.  I dismantled everything, bagged the bolts and spacers for later, and stored the bulkheads.