Finishing rear bulkhead assembly

All but four of the 100-plus rivets needed to complete the rear bulkhead assembly can be squeezed using one of the two "C" yokes that came with the Cleaveland Main Squeeze rivet squeezer (a dozen or so are of the "pop"-rivet variety).  So I saved those for last, which, in retrospect, was kinda dumb.  Shoulda done 'em first.  The yokes have about a two-inch reach.  To get those last four rivets squeezed requires a yoke with a three-inch reach, which I don't have.  That means it was time to learn how to use a pneumatic rivet hammer and bucking bar.

Remember back in July I borrowed my neighbor's air compressor?  Well, six months later I finally fired it up and tested out the rivet hammer.  To my relief, both worked perfectly.  Here's what those items are:

By this time the rear bulkhead assembly had quite a number of hours invested in it and I didn't want to mess it up when it was so close to being finished so I asked the owner of nearly all of the tools that I've been using, Mr. B, for his assistance bucking those four rivets.  Mr. B was a great sport and came over to help.

First thing we did is select the right bucking bar out of the four or five that came with his rivet gun.  But even the one with the small end on it was slightly too large for the narrow space available to work with, now that all of the other "easy" rivets were in.  So Mr. B took a little bit off the corners of the bucking bar with the bench grinder.

And it worked!  Then Mr. B held the bucking bar and bulkhead steady and I "shot" the rivets with the rivet gun.  Due to my lack of experience (no dry run practice for me, thank you very much) a couple of rivets had to be drilled out, but fortunately we noticed that the top of the rivet wasn't sitting flush before getting too far along.  All part of the learning curve.  A bit later all of the rivets on the rear bulkhead were done.  Thank you Mr. B!

Before we finally get to the actual building of the tail cone skins, there's one last chance to test fit the bulkhead to the stabilator.  See those bearings?  The AN4 stabilator hinge bolts go through those with a washer on each side of each bearing.  AN4 hinge bolts are 4/16ths diameter (1/4" for the fractionally-challenged), which seems a bit wimpy.  But if you do the math you get a bolt shear plane of just under ¼ x ¼ x ¼ π = .05 square inches.  Multiply that by the shear strength of the bolts, about 100,000 psi, and you get almost 5,000 lbs of force required to shear the bolt.  Multiply that times four possible shear planes (two per bearing), and you can deduce that other parts would fail before those bolts will.

Vans has you choose whether to use a thick washer or a thin washer on each side of each bearing, as long as it's symmetrical.  So the choices are:  four thin washers, four thick washers, or two thin and two thick.  You determine which ones give the best fit in the stabilator brackets.  A good fit is snug (no lateral movement between the bulkhead and the stabilator), but not too snug.  Once you select the right washer combo they're super-glued in place onto the stabilator brackets.  That's 'cause, unless you have very small slender hands, those bolts and washers are difficult to place and even harder once the tailcone is assembled.  Having my eight year-old daughter help out really came in handy.  In my case the magic combination is two thin washers and two thick washers.  Perfect fit.  Just hope they don't fall off.

Note for RV builders who are married:  You may want to keep the fact that you're using super glue on the airplane to yourself.  Even though you're just using it to temporarily hold a couple washers in place, it doesn't help the confidence-building with your spouse, if you catch my drift.  Trust me on this one.

Merry Christmas everyone!

 

Skidplate Fabrication Part 2

I went a little bit out of order.  You're supposed to tap out the hole first, and then put it on the rear bulk head to drill the rivet holes in the skid plate.  But since I didn't have a tap handy, I went ahead and did the rivet holes drilling first.  In this case it didn't make any difference.  Next day I borrowed a 3/8" x 16 tpi tap and handle from a guy at work known for having a nice tool collection, drilled out the hole and started threading the hole with the tap.

As you can see, it gets harder as you go, until...

Uh-oh.  Broke the jaws on my friend's tap handle.  What are my options?

Vice grips can do a lot of things, but turning a tap isn't one of them (seemed worth a shot though).

This doesn't work either:

So I went to Home Depot to get a new beefier handle.  I'm sure I'll use it again sometime.

So with lots of turning, unturning (to clear out chips), and turning again, plus a few drops of bike chain oil, I eventually ended up with a nicely threaded one-inch hole in the bottom of the skid plate to screw the rear tie down eye bolt into.  Following day I returned the tap and asked my co-worker if he wanted me to replace the broken tap handle and he said he had extras and not to bother.  We'll see if he lets me borrow any more of his tools in the future.

With the skidplate fabrication done, we're ready to rivet together the bulkhead assembly, and here's what that looks like (well one side anyway):

Note that all 95 (or so) rivets are done except the two on the left and two on the right.  There's no way that the rivet squeezer I have with its two-inch "C" yoke was going to reach those two rivet holes, what with the "ears" for the main stabilator bearings on the reverse side being in the way.  I've mastered (sort of) the hand rivet squeezer.  Now it's time to learn to use the pneumatic rivet hammer.

Skidplate Fabrication Part 1

Page 10-3 has us fabricating a skid plate by taking a section of extruded aluminum, cutting several pieces off of the extrusion, and threading the center hole with a 3/8" tap.  Funny that they would call it a skid plate, since the part will never touch the ground.  But it will provide a place to screw in an eyelet bolt to be used as a rear tie-down point.  I suppose if you were to really over-rotate on landing or take off the eyelet bolt could scrape the runway, but airplanes aren't normally flown that way (sure would wake you up if you did).  Still, it's good to have because eyelet bolts are cheap and easy to replace, whereas runway rash on the rear bottom skins, well, I don't even want to think about what a PITA how difficult that would be to repair.

 Here's the [mostly] unmolested piece with a cut mark on it:

And here it is after a little band saw surgery:

The next couple steps use the drill press, band saw (again), and scotchbrite wheel on the bench grinder to remove additional material at the bottom (right), and to grind out a notch in the side, as shown:

A thin aluminum template is used to locate the notch and drill the first two of 16 rivet holes.  Here's an important builder tip for drilling those first two rivet holes:  After drilling the first rivet hole, cleco the template to the skid plate using that hole.  Guess what happens if you don't.  That's right, your second hole may end up in the wrong location.  Using the magic of Photoshop <not>, here's what that would look like ('cause I would never do anything that stupid):

So remember, cleco the template with that first hole.  You might even consider drilling just one of those two rivet holes now, and the rest when the plate is on the bulkhead.  Cleco each as you go.  That will allow you to precisely center the bottom part of the skid plate (with the threaded hole) between the doubler plate flanges on the bottom of the bulkhead.  Be sure the notch position is okay before you start drilling.

Back in the Saddle

I know.  It's been over a month since I last posted.  Truth is, I just haven't worked that much on the plane.  Lots of yard work and things like that have gotten done on the weekends (with body aches to prove it), but for some reason my enthusiasm for evening work on the plane has wained a bit with the shorter days, despite taking a day or two off to work on it.  I need to get on the ball though.  There'll probably be a scheduled price increase in February on the various kits that Van's sells and I'll want to have my order placed for the RV-12 wing kit before then.  Because of workspace issues I'll also want to have the tailcone finished and stowed away before the big crate full of airplane wing parts arrives at my door.

So what have I done in the last month?  Well, I
 * removed the blue plastic protecting the skins along all the rivet lines,
 * deburred all the rivet holes and edges,
 * bent the edges on the tailcone skins per the plans (not hard, once someone shows you how),
 * fluted the frames to straighten the rivet hole lines, and
 * fastened together the tailcone frame hoops shown in picture.

The large and medium hoops are easy - a few pop rivets and you're done.  The small hoop, the one that goes at the very aft of the tail cone, required the assistance of my neighbor Ken because there's a doubler plate and six-inch angle fastened with #4 rivets that need to be squeezed, and the pieces are too light to secure firmly while operating the rivet squeezer, which is also awkward to use (for me anyway).  In return I promised to help Ken clean out his second-story rain gutters this weekend.

As you can see, I did manage to mess up the first rivet (bottom row left) by oversqueezing it, I think.  Somehow there ended up being a small gap between the doubler plate and the frame after I squeezed the rivet.  So I drilled it out and, since the hole is now enlarged, ordered some #5 rivets from Aircraft Spruce to use in that hole.  They should arrive any day now.  The other 17 rivets went in pretty well because I took my own earlier advice and only gave each rivet a half-squeeze until all 17 were in, then went back and re-squeezed down to spec.  It also helped a lot having Ken hold the frame steady while I manned the rivet squeezer.  With the exception of the first bottom rivet (soon to be fixed), I'm satisfied with it.

December 18 update:  The # 5 replacement rivets came in the mail so I could finish the final rivet in the small bulkhead hoop.  You're not supposed to use a Cleaveland Main Squeeze rivet squeezer on 5/32" rivets but I used it anyway and didn't break anything.  Here's the # 5 "oops" rivet next to seventeen # 4s:

It should be okay.  It's deep in the tailcone and faces aft, so no-one will see it anyway.