Longerons

As previously mentioned in the Service Bulletin post, I decided to undertake the longeron assembly, while awaiting the outcome of the main landing gear alterations.  I was able to borrow a set of longeron dies from a fellow builder in Missouri.  Shown below are the dies clamping the longeron while tightening the assembly in the vise.

The dies are very efficient at setting a curve into the clamped section of longeron.  In fact, I was easily able to "over-bend" the longerons with only a couple of squeezes with the vice.  It was definitely a learning process that I am still uncertain I have mastered.

Once clamped, the longeron could be pulled or pushed to pre-load and set a significant bend.

After setting several bends, it was time to compare the curve in the longeron to the canopy deck or "template" as it is referred to.

Another issue for me with the longeron bending process was my uncanny ability to induce a twist into the cross section - very frustrating!  How can that possibly happen with use of dies?!  Most everything I had read on the subject of this project reflected the author's opinion of from "very easy" to "almost impossible".  Invariably, I approached this project with my natural optimism, but was quickly humbled by the daunting task.  I would have to say that I am currently in the latter camp....not impossible, but you can see it from here.

Pictured below is the most curved section of the longeron, exhibiting the battle scars of the dies.  Once I was mostly satisfied with the shape of the longerons, I proceeded to buff out as much of the surface abrasions as possible.  They actually came out looking pretty good after scuffing, priming and top coat.

Now as I approach the process of match drilling the holes in the canopy deck to the longeron, I am questioning the accuracy of the .025" overhang (lack of uniformity) along the curved edge of the assembly.  The overhang of the canopy deck is to serve as a butt joint for the upper edge of the fuselage side skins.  I have probably .035 - .045" inch variation at the worst points along the length of the assembly.  The issue then becomes will the inconsistent overhang affect the assembly. Obviously, I can rivet the longeron to the canopy deck as is and then file away any excess overhang of the canopy deck.  My greatest concern is that I induce a wrinkle in the fuselage side skins because my longeron curvature does not exactly match the original shape of the canopy deck.  As I compared the longeron curve to that of the arm rest member, which is parallel and located several inches below the canopy deck, it appeared that the differences in curvature were minimal and the risk of skin wrinkle or warp improbable...but that is definitely a guess!

Several days later: Okay, after finally positioning the canopy decks on to the longerons, it quickly became clear that my concerns over the variation in the overhang was a non-issue.  Once the match hole drilling/cleco-ing process began along the length of the longeron, slight thumb/index finger pressure applied to the longeron brought the overhang into a very consistent margin as prescribed by the manual.  Problem was way over-analyzed!

Though not possible to tell from these views, they do show the longeron cleco'ed into place with the canopy deck.  First picture shows the longeron/canopy deck temporarily cleco'd into position.

 
Final picture in this blog shows the longeron/canopy deck resting in place after the side skin holes were match drilled.  More to continue in the Mid-Fuselage Assembly Part 3 posts.

Seat Backs

While waiting for the dust to settle with the Main Landing gear service bulletin, I decided to try to remain productive by proceeding with the assembly of the seats.  Pictured below are the seatback brace and the various hinge assemblies.

The first seatback with its hinge assembly temporarily attached.

Close up of the hinge.

Hinge attachment to the seat back brace.  Notice in the following pictures that in the ensuing 3 weeks, the parts have now been primed and top coated prior to assembly.

The flimsy seatback is framed with all of the members which completely stiffen it up.

 Final assembly of the seatback frames.

The finished products.

Van's Aircraft RV-12 Service Bulletin

On September 26, 2012, Van's Aircraft issued service bulletin, SB 12-09-26, titled, "Loose U-1202 Attach Bolts", affecting all RV-12 model aircraft.

Several RV-12 pilot/builders had previously been discussing wrinkled fuselage side skins on the Vans Airforce web portal (www.vansairforce.com).  The wrinkled skins along with some crimped brake lines seemed to imply that the center section of the aircraft might be moving excessively upon hard landings of certain RV-12 aircraft.  Further investigation did not necessarily imply that simply hard landings or landings on unimproved airstrips were the exclusive symptoms of the airframes experiencing the problems.  Moreover, as more pilot/builders began to share their condition inspection reports, the problem of cracks in the center section channel near the attach bolt holes began to enter the discussion.

As more pilot builders checked into the torque of the attach bolts, most reported that they could turn an additional quarter to full turn on their bolts to achieve the required torque.  Someone finally surmised that the attach bolts were actually too long and that they were "bottoming out" in the nuts, resulting in perhaps proper torque but less than adequate compression on the gear leg, wear plates and related hardware between the head of the bolt and the nut.  This under-tightened bolt situation could allow the gear legs to develop too much play or slop under highly loaded conditions such as hard landings and/or landings and takeoffs on unimproved airstrips.  Hard braking and its effect was also conjectured as being a possible cause of exacerbating the gear leg problems.

After Van's Aircraft received enough pilot/builder reports of these incidents, they began their own internal investigation resulting in renewed testing of the landing gear system.  On October 3, Van's issued an addendum to the original Service Bulletin.  Its purpose was to "...supplements and clarifies the contents.." of the bulletin.  Specifically the addendum provided 2 options for the builder/pilot to properly comply with the bulletin's requirement of checking that the landing gear bolts were fully seated prior to torquing.  The first option was to drill four, 3/8 inch diameter holes into the F-1276 bottom skin to allow for the insertion of a brass drift in order to access the heads of the bolts.  The second option was to remove the F-1275G cover plates by drilling out their rivets, allowing greater access to the bolts.

Below are two pictures showing the cover plate.  My plan is to mount 12 nutplates on the 3 sides of each cover plate so they can be removable.

Following is a picture of the mid-fuselage inverted with the bottom skin partially cleco'd in place.  At this juncture, the factory has recommended that I await their parts manufacturing in support of the Service Bulletin.  Their estimate is that parts will be ready to ship by the end of November 2012.  They are stating that while the new parts will be capable of being installed in flying RV-12's, it will be much easier if a builder waits until the parts are ready before installing the bottom skin....so here I will wait.

Fast forward now to November 26, 2012 and Vans has begun shipping the parts for the service bulletin.  Officially, that is, Service Bulletin 12-11-09.

From a brief overview of one builder's post on Vansairforce.net, the parts will consist mainly of doublers to the lower fuselage, some additional wear plates and other goodies associated with the main landing gear attachment assemblies.  There are even 4 new quarter inch holes to be drilled into the center section channel to enhance the securing of the outboard gear attachment assembly.  The location of those holes appears to be accessible from the "interior" of the mid-fuselage, so I believe I could attach the fuselage bottom skin....but I think I will continue to wait.

It's not a total loss as I have begun work on the longerons and the seat backs as I wait on the parts for implementation of the service bulletin.

I had previously mentioned my intentions of installing nut plates and riv-nuts to the landing gear's F-1275G cover plates to facilitate easier inspection of the attachment assemblies.  I have cooled to that idea as I don't believe the implementation would be up to my personal standards with regard to asthetics and function.

Fast forward once again to early December and I have completely reversed my stance on the removeable cover plates as well as attaching the fuselage bottom skin.  The details on those assemblies can be found in the latest installments of the Mid-Fuselage Parts Assembly, Part 3 blog.

A Rivet Issue

I am fortunate enough that my former hangar mate and Sonex builder/pilot Mike Singleton is also one of our EAA Chapter's (774) Technical Counselors.  As it was approaching time to attach the bottom skin to the mid-fuselage section, I knew I had to get Mike to come inspect my work prior to closing up some of the fuselage structure.

Mike came by on October 25, 2012 and inspected my work.  He was complimentary of my project and asked a lot of good questions about my tools and my paint priming process.  During the inspection of the mid-fuselage, Mike noticed that the shop end of the top row of AN426AD4-8 and AN470AD4-8 rivets on the aft bulkhead, F-1204D, did not appear to protrude far enough through the F-1204 center section channel.  I had not noticed this until now.  A quick check with the rivet gauge suggested he was correct.

The picture below shows two rows of rivets with the shop end of the squeezed solid rivets exposed.  Mike referred to the bottom row of rivets as "overdriven".  The diameter of the shop head is within spec of the go-no-go rivet gauge for a number 4 rivet.  It is the thickness of the driven head that is lacking.  Note in the following 2 pictures that the fuselage is currently inverted as I am preparing to attach the bottom skin.

 The next picture is of the manufactured heads of the same AN426 and AN470 solid rivets. 

The head-driven thickness of the rivets is about .040 inches as measured in several places along the line of rivets with a micrometer.

The morning after Mike's inspection, I called Vans to get further information.  During the course of that conversation, the tech support rep never did address the possibility that the designated rivets might have been too short.  Instead, he acknowledged that drilling out those rivets would be a challenge, especially now, since the baggage ribs and floor skins were already in place, which significantly limits access with a drill.  His suggestion was to use a right angle air drill with a number 40 bit, followed by a number 30 bit if I elected to drill out the rivets.  He went on to cite a table on page 104 of the sixth edition of the Standard Aircraft Handbook which calls for a minimum upset height of 3/64 inches (.047) for a 1/8 inch solid rivet.  As noted above, the rivets are about .007 inches short by this standard.

As I further researched the issue on www.Vansairforce.com internet portal, I found a reference to the Mil-R-47196A, published September 6, 1974, which is the military's specification bible for all things structural.  In Table III, under the heading of Rivets, Buck Type, Preparation for and Installation Of, Standard Flat Driven,  Head Dimensions, the 1/8 inch rivet lists a Driven Head Thickness Minimum of 0.050 inches.

Finally, a search of the archives on Vansairforce yielded 2 relevant threads published by other RV-12 builders who had the same result in their F-1204D aft bulkhead rivets.  One thread in particular referenced a response from Ken Krueger, former Chief Engineer of Vans Aircraft with substantial knowledge of the RV-12 aircraft.   In that response Ken simply cited the Note in section 5D, entitled, "Riveting" of the builder's manual.  The note reads as follows:

"Note: There are times when the correct rivet length is not available.  Depending on the application a shorter rivet can be used or a longer rivet cut to the proper length.  Using a longer rivet, as is, can result in the shank being bent over like a nail. We have chosen to use a rivet that may seem too short in some places, but will do the job adequately."

It was also noted in this thread that the parts in question (F-1204D riveted to F-1204) are experiencing shear loads, which would either eliminate or minimize any tensile loads, in this area.  Tensile loads are normally the worst case for testing the head strength of a rivet (manufactured or shop).

Another reference also found in section 5D of the builder's manual pertaining to the subject of drilling out rivets is based on an excerpt from the Alcoa Aluminum Rivet Book, dated 1984. Without quoting the entire paragraph, the upshot of this reference is summarized in the following 3 points:

    1. Poorly set and cracked rivet heads tested in tension were still within 5% of the strongest properly set rivets indicating that minor deviations (.007 inches) are not cause for concern or replacement.

    2. A second rivet that is driven in any one hole is likely to be more defective than the first...and will be more likely to buckle and form an imperfect head.

    3. Tests have shown that very small rivet heads are sufficient to develop the strength of the rivet shank, even when the rivets are subject to a straight tensile pull.

It is my conclusion that the risk to reward of drilling out and replacing 73 rivets, (a combination of AN470 and AN426 rivets) in the center section structure is absolutely not merited.  There is sufficient evidence both from within and without the RV-12 community to support this position.

The previously cited 2 threads of relevant discussion on this subject can be found here:
    1. http://www.vansairforce.com/community/showthread.php?t=44303&highlight=20-04
    2. http://www.vansairforce.com/community/showthread.php?t=39106&highlight=20-04

Forward Lower Fuselage Assembly

One of the first steps of the forward lower fuselage assembly is to dimple and then rivet nutplates along the sloped edge of the left and right tunnel ribs.

Next, the first of two pair of engine mount brackets are installed on the tunnel ribs.

Now, the tunnel ribs are riveted to the forward fuselage (think floor) skin.

It is now time for the firewall bottom panel/skin to be attached to the tunnel rib/forward fuselage skin.

And finally, the left and right forward fuselage corner skins (curved sections) are riveted to the forward fuselage skins and firewall bottom.

The entire section is, of course, cleco'd together prior to riveting.  In this picture you will see a series of colored dots.  These are to mark locations that are NOT yet ready to receive rivets - a no rivet zone!

Mid-Fuselage Parts Assembly, Part 2

The mid-fuselage assembly continues with the final assembly of the two control column mount assemblies.  Cleco'd together and then rivet.

Next is a shot of the F-1203A bulkhead and the F-1203C and D wing spar stub receptacle components. The front wing spar stubs slip into the pocket created by these parts.  This bulkhead resides at the forward end of the seat ribs.

The "flip side" of the same bulkhead with its interior color coat and doubler, which also ties into the previously mentioned spar stub receptacle.  It has a combination of solid and blind rivets which secure the assembly to the bulkhead.  The entire doubler/spar stub receptacle assembly jigged together with clecos

With the first solid rivets of the assembly set, it is time to move on to some of the blind rivets.

All of the solid and blind rivets set.  Remaining clecos are there to remind me to NOT rivet in those holes until later.

Back on the primer side of the bulkhead, it is time to rivet the control column mounts and another rectangular doubler.

Just to the right of the control column mount is the other rectangular doubler previously mentioned.

The finished bulkhead now awaits its turn to be attached to the mid-fuselage assembly.

 

 
 The left-hand seat assembly is now attached to the mid-fuselage assembly.

The seat assemblies attach to the center section via the floor skins and the ribs.

Picture time!

Next step is to attach the bulkhead caps and rollers (not pictured) to the side assemblies.  The caps are cleco'd and then riveted to the side assemblies.

 

It is now time to attach the forward bulkhead to the front of the seat ribs.  As usual, jig in place with clecos and follow with rivets.

 

 

***********************Need to add forward floor ribs and skin pictures here.

Next comes the corner fuselage skins as they are cleco'd into place prior to riveting.

The fuselage bottom skin has had its edges formed or "broken" with a slight bend as well as the 20 nutplates attached to the perimeter of the inspection holes.

The inverted mid-fuselage section is supported in two places in preparation of the bottom skin being attached.

In light of service bulletin, SB 12-09-26, I am considering making the F-1275G landing gear cover plates removable via nutplates.  This would facilitate easier access to the landing gear attach fittings where improperly torqued bolts have caused structural issues with the center section.  It appears that at least part of the issue of improperly torqued bolts has been due to the use of bolts that were too long.  As a result, builders were not aware that nuts had bottomed out and not been fully tightened or torqued to specification.

It will be several more days before I can finish assessing the practicality of installing nutplates on the cover plates as I had to pre-order those parts from the Finish kit, which I do not currently have.  I have also ordered a mixture of miniature nutplates and Riv-nuts to further explore this option.  Stay tuned!