Empennage: Horizontal stabilizer. Factory error on skin? Must redo entire stab.

Update 13-Dec-14:  The reason why the spar was mis-punched has been identified.  Scroll to the bottom or click here.

So again, Van's has been very reasonable with me on this issue.  They continue to demonstrate their focus on the customer.  They're fantastic to work with and knowing that I will have one of their airplanes gives me great comfort.  Now to the story...

My original horizontal stabilizer had what I (not Van's) determined to be a factory error on the rear spar.  You can read about it here.  In summary, there were four places on the spar with 0.05" too great of spacing between holes, so the spar couldn't line up with the skins.  It took a lot of time to diagnose but, no problem:  Van's kindly sent me a new rear spar without issue after I explained the issue.

I couldn't test-fit that new rear spar until it was countersunk in order for it to fit the skin dimples.  So, after fully prepping the new spar, prior to priming it, I cleco'd it up into the stab assembly and it fit perfectly on the right side, top and bottom.  It fit perfectly on the left bottom side.  But on the left top side...it wouldn't fit!  And where did the alignment problem begin?  At one the exact same places as before:  Between the 26th and 27th hole from outboard on the top of the left skin (location "B" below).  So, this was definitive evidence that old spar was indeed mis-punched, as my previous measurements showed as the new spar fit in places where the old one didn't.  But why wouldn't the new spar fit on the top of the left skin?

Dumfounded, I measured the hole spacing on my new rear spar with my caliper, afraid of another mis-punched set of holes.  Spacing was fine:  Exactly 1.125".  I then measured the hole spacing on the skin.  It too looked fine to me.  But clearly, there was a problem either on the spar or the skin (see first image below).  So, to ensure I had an accurate measurement, I seated unset rivets into the spar when it wasn't on the stab assembly.  Separately, I did the same on the skin when it didn't have the spar in it.  This way I could use the dimples in the AN426AD3 rivets as a good reference.  Turns out, the skin had two holes with spacings 0.02" too great!

 

 

Even though 0.02" isn't a big deal, two of them naturally add up to 0.04".  This creates a skewing of holes outboard.  Simply drilling these out up to a #30 wouldn't solve the problem because the dimples still wouldn't fit in the countersink for all holes outboard of the misalignment.

As another sanity check, a fellow -14 builder who completed a -6 and a tech counselor A&P who worked for United and the original Frontier and has also built several kit planes, to include a -12, both came to look at my stab to see if I was missing something.  They agreed with my assessment, though the tech counselor felt I could just drill out the holes to a #30 and use NAS1097 rivets in their place (my other tech counselor advised against this approach when the original problem surfaced).  I didn't believe that was feasible given that the dimples didn't seat into the countersinks due to the misalignment.  It would also greatly increase the risk of the dimples cracking due to the knife-edge cut from the drill.

I cut off the implicated pieces (spar and skin sections) from my original stab and sent them to Van's for examination (the image below may not be suitable for all audiences).  When they received my parts they told me the next day that they couldn't find anything wrong with them (click here for an update on this outcome).  They stated that they found the spacing to be correct and that my spar countersinks were a bit under-countersunk.  They felt the latter may account for the less than optimal layup.  To their credit, they involved several relevant people internally to investigate possibilities including looking at their CAD files, checking inventory parts and even cleco'd up a few stab mock-ups.  They took this issue very seriously even though I'm the only -14 builder to report this issue with these parts to-date.  They sent me images showing the skin portion riveted to the spar portion.  For sure, the mis-punched skin and spar portions would fit fine, but this is because they were both mis-punched in such a way that the two misaligned parts align fairly well.  However, that skin won't fit properly on a good spar and neither will the bad spar portions fit good skins (I wonder if they've misunderstood the problem).

Their assessment perplexes me.  Obviously I disagree with their conclusion, as precise measurements illustrate incorrect hole spacings on my original rear spar and left top skin and the fact that the new spar fit in 3 of the four places the old one didn't.  Even slightly deep countersinks couldn't account for these observations.  At this point it's moot.  Despite the different conclusions that we've both reached, they've agreed to send me parts to rebuild a new stab.  That represents a significant cost on their part and that is highly appreciated on my part.

My original horizontal stab was probably one of the best pieces in the build:  Only one very minor dent on the skin that no one would have known was there.  I hope that can be repeated on stab #2.

I guess I now have some hangar art in the form of a grounded, handicapped horizontal stabilizer.

Update 13-Dec-14:  Another builder ran into the same problem as I, posting on VAF.  In his communications with Van's, they found that the spar is too large to chuck up once, so it has to be rechucked in the machine at about the locus of where mine and his mis-punches occurred.  In case other builders run into the same issue, Van's is manufacturing several rear stab spars without holes punched so builders can simply match-drill the unpunched spar to their skins directly.  I'm not sure why a simple replacement, properly punched, spar isn't the better approach.  But at least Van's identified the problem and I'm no longer some lone builder relegated to Crazy Town.

Empennage: Horizontal stabilizer. Factory error on rear spar?

First, let me extoll the dedication of Van's.  They went beyond what I would have expected in looking in to this issue for me, even though their diagnosis wasn't correct (let's be fair, they can't actually get their eyes and hands on my parts).  They are going to send me a replacement rear spar and doubler.  They are a great company to work with and are customer focused!

Previously, I had indicated that the rear spar on the horizontal stabilizer was grossly misaligned with the stab assembly.  In fact, it was simply not possible to rivet it in.  It seems it wasn't related to the bow in the spar.  Other -14 builders I consulted with indicated that their rear spars also had a prominent bow and it wasn't an issue in the final assembly for them.

An example of what I was up against is shown below.  Notice how the trajectories of the unset rivets are off (in fact the lower left-most one can't even fit through the hole) and the skin pushing away from the spar maximally at the red line.  The associated dimples won't sit in their countersinks either.  What's interesting was that the misalignment was not progressive, but rather discontinuous at the red line and happened at the same four (symmetric) places along the rear spar:  Between the 26th and 27th rivets when counting from outboard on all four corners.  

After spending several hours trying to determine the cause of the problem, including a visit from a tech counselor, a RV-9A builder, a call to Van's and several emails with the latter, everyone concluded the problem was most likely a twist induced in the assembly when it was riveted together.  However, I wasn't convinced that the stab was twisted.  Since the issue presented itself as a discontinuity in rivet alignment, I couldn't wrap my mind around the problem being my assembly having a twist.  After all, wouldn't a twist manifest with a progressive misalignment?  So, I was quite certain that the spar was mis-drilled at the factory, but I approached the problem with an open mind since I'm often (well, frequently...actually, just about all the time) wrong.

So, to chase down the alleged twist, I tried 

1. Cleco'ing in various orders to see if the misalignment could be "chased" in one direction or another.  It couldn't.
2. Applying forces to the assembly at various points to slip rivets in with a straight trajectory.  That wasn't successful (and having rivets in sheer is not good)
3. Pulling a string line through the rib tooling holes in the assembly to check for twist.  It looked pretty good
4. Van's suggested seeing if anything changes when the spar is inserted backwards (top-to-bottom).  Problem persisted
5. The great folks at Van's even tried a few mockups with parts from different batches and couldn't reproduce my problem.  They continued to feel it was a twist and suggested removing some rivets and reworking the assembly

At each point in the above diagnosis progression, I found no evidence of a twisted assembly.  Ultimately, I requested the rivet spacing for the parts.  The standard spacing is 1.125".  Sure enough, right where I identified the misalignments on my spar (each of the four places I marked), I found one pair of misdrilled holes at 1.175", 0.05" too big.  The corresponding holes in the skin did not have this additional 0.05" spacing (see this later post which expands on that statement).  This explains the issue I had and why it manifested as a discontinuous affair rather than a progressive one.  Images below illustrate my findings.

 

In retrospect, I should have just measured the holes as soon I suspected that was the problem.  However, I had it in my mind that the holes had different spacings, like the J-stiffeners on the wings and that thought locked me out of just measuring the holes.  Had I just made the measurements, I would have saved a lot of time.

If you're curious why the spar's countersinks appear not to have been primed like the rest of the part, it's because I realized that my countersinks were slightly shallow, so I shaved off a few thousandths to bring them to the right depth.  I had planned to spot prime them if the spar could align to the skins.

Visit this link for the next entry, 8-May-14, on this issue.  It includes the reason why the spar was mis-punched at the factory.

Empennage: Horizontal stabilizer. Ribs and front spar riveted.

The horizontal stabilizer is at the point where only the rear spar needs to be attached.  That won't happen yet.  There is a gross misalignment that I will elaborate on more when the fix is determined.  Until then, here's where things are at.  Update:  See here for misalignment details.

Nose ribs went in without a hitch.

Looks clean inside with the ribs and stringers.

Full stab without aft spar riveted in.

Empennage: Horizontal stabilizer. Ribs and stringers riveted to front spar.

The ribs and stringers are riveted to the front horizontal stabilizer spar.  Was able to squeeze every rivet save four.

Where the spar attaches to the empennage's tail cone, there is a lot of reinforcement.  This suggests a lot of loading occurs here.  And there probably is.  However, the horizontal stabilizer from the -14 comes from the -9 (except the 4 inspar ribs, which are from the -10) and yet is not attached to the aft fuselage in the same way as in the -9.  This is why there are those square patches of 9 rivets on both sides of the doubler:  In the -9, these holes are to hold attach brackets.  Because the -14 has forward attach bars located on the aft empennage bulkheads rather than on the stab spars, those pair of 9 holes do not secure the attachment elements.  Instead, 1/4" holes are drilled into the doubler on each side of both spars so the attach bars on the bulkhead can grasp on to the horizontal stabilizer with AN4 bolts (a total of six, four aft and two fore).  This approach is unique to the -14.  Even the -10 has forward attach brackets on the spars.

Attachment approach used in the -14.

 

Here is the shop head side of all that.
 

The two inside "inspar" ribs each have four AN470AD4-4 rivets fastening them to the stringer web.  Because those two ribs are at a 12.5 degree angle, access to those rivets is a challenge.  I ultimately settled on the following approach and it worked well for me

Empennage: Horizontal stabilizer. Spar doublers and brackets riveted.

Both horizontal spars are ready for ribs.  The aft spar has brackets (left) in addition to the doubler (center and right).  I'm showing both sides of the riveted doubler below.

The forward spar includes the spar caps.  I cleco'd the heck out of those when riveting to make sure they fit good and tight.

I did make a mistake and riveted in a universal head where a flush head rivet was called for.  Removing that was interesting since that countersink could cause trouble.  So I used my pin punch to start the hole, then drilled down flush to the spar with the following sequence of bits:  1/8", #42, #40, #36, #33 and #30.  I took that cautious approach to be sure my hole was centered.  When I popped off the manufactured head, I was greeted with a perfect sight (left).  I wacked the shank a few times with my 1/8" pin punch and the thing wasn't coming out.  I figured, since there were three thick parts being held together, the shank may have a non-uniform diameter.  So I decided to snap off the shop head too, using the same sequence of drill bits as before.  That was the first time I've removed a shop head.  Nothing to it.  Turns out, still couldn't get that shank out.  So I started drilling out the shank's center to relieve the pressure.  It took up to a #33 followed by the punch to get it out.  Notice the shank's non-uniform diameter (right).  This is why it was so hard to get out!

Forward spar with its caps and doubler, both sides.

Empennage: Priming. Rudder and horizontal stabilizer.

I decided to redo my rudder due to the trailing edge.  Here is the post outlining the new rudder.

Nothing exciting here.  Just routine priming with Stewart Systems.  Got the remaining rudder parts and skins completed.  As well as the rear horizontal spar, caps, doubler and skins. 

Still need to flute, drill, etc. then prep and prime the horizontal spar ribs.  I will group those in with some priming of elevator parts.

Empennage: Rudder, vertical and horizontal stabilizers priming.

I decided to redo my rudder due to the trailing edge.  Here is the post outlining the new rudder.

When priming for the empennage, being a smaller portion of the aircraft as opposed to the wings, it makes sense to try to aggregate many parts together.  This helps minimize the amount of time spent cleaning, etching, spraying primer and on the wait for curing.  Thus, I jumped ahead to get many of the rudder and horizontal stabilizer match drilled, countersunk, etc. so they can be primed in the same batch as the vertical stabilizer parts.

Primed:  All vertical stabilizer parts.  Front horizontal spar, doubler and stringers.  Rudder ribs and spar.

I use Stewart Systems for priming.  Being water-based, it's far less toxic to use though I still wear my respirator.  The vast majority of aerosol doesn't get airborne.  It just falls and collects as dust.  Do not, however, use water-based primers for steel.

Empennage: Horizontal stabilizer. Match drilling and deburring.

To make better use of my time priming, I've jumped ahead to prepare more parts for the event.

Both horizontal stabilizer spars have doubler plates in their centers.  Deburring can be time-consuming without the right tools.  For the intersection of the forks, I used a small flap wheel (left).  It makes for a quick, great job in those tight corners (right).

My 3M Scotch-Brite fine-finishing sander perfectly smoothed out the lightening holes in both spars.

The rear spar has "caps" that must be both cut on an angle and then match drilled to the spar.  I'm guessing these caps help stiffen the rear spar to prevent distortion when the elevators are engaging in significant force-creating AOAs.

When match-drilling the second cap, I kept the first cap on the spar thinking it will help stiffen things up better.

After all that's done, both spars get their #40 holes countersunk for the skin dimples.  Of course,  there are 18 holes in the rear spar that are cut for flush rivets instead.