Custom Made Dimpling Tool

Those who know how fast an RV-12 can be built can tell things are not moving very fast at DOG Aviation. That said, there has been plenty of R&D going on behind the scenes. One example was the necessity of having a tool special made for the RV-12 project. Because the decision has been made to use flush rivets, that means all holes will need to be dimpled on the skins and ribs … a lot of extra work but the end result will be rewarding. There will be holes the C-frame can’t (or shouldn’t) reach so another approach is needed to make the necessary dimples at those locations. Those experienced with other RV’s that use pounded rivets may be familiar with the dimple dies that are used with a pop rivet puller for dimpling in tight places. The concept is simple, the dies are placed on either side of the skin hole to be dimpled and a nail is inserted through holes that are in the center of the dies. The nail is then placed into a hand pop rivet puller and the pop rivet puller is squeezed, pulling on the nail, thus compressing the dimple dies together making a dimple in the skin. Unfortunately, those specialty dies are for 3/32” rivets and are not readily available for the 1/8” holes which the RV-12 has.

                          The close quarter 120 degree dimple dies Bob Avery custom made for DOG Aviation.

Fortunately, Bob Avery from Avery Tools was willing to make the 120 degree dimple dies special for me. In fact, Bob told me he was wondering how long it was going to be before somebody building an RV-12 with flush rivets was going to ask for those specialty dimple dies.  Anyway, Bob quickly made them up and sent them right out. The close quarter dies were tested last week and they make perfect dimples … Thank you Bob!

The Compromise

The decision has been made to take the road less traveled and build the RV-12 as an EAB (Experimental Amateur Built) airplane as opposed to an E-LSA (Experimental Light Sport Aircraft). All that will be a topic for another post in the near future.

From the beginning there has been a desire to build the RV-12 using flush rivets as opposed to using the low profile rivets that come with the kit. Why? Mostly aesthetics … plus the ease of cleaning a smooth surface as opposed to a surface with thousands of bumps on it. Originally, there was hope of using solid rivets on all the skyward facing flight surfaces. However, that poses unique challenges because the materials used in all aluminum LSA’s is a rather thin .020 inch for skin and ribs typically. The RV-12 is no exception and the thinness of the materials is not conducive to riveting 1/8” solid rivets the conventional way (rivet gun and a bucking bar). That said, back riveting appears to be a viable option and I seriously looked into it. Unfortunately, even that option has compromises … as an example, the RV-12 stabilator utilizes an enclosed box spar which is assembled prior the skin being installed. There would be no way to back rivet the skin to spar rivets, thus necessitating using blind (pop) rivets at those locations. The results would be a hodge-podge of solid and countersunk blind rivets … functional and airworthy to be sure, but tough on the eyes.

Using solid rivets on the wings would pose an even greater challenge because the bottom skins are designed to go on first … they also wrap around forming the leading edge and continue back about a foot. In order to back rivet the wings, one would have to deal with keeping those large skins suspended overhead to allow access, plus build a solid gig that would have to be moved to each rivet to be back riveted without interfering with the Clecos holding the skins to the ribs. I still feel that it is doable, but it would be a daunting task for a single builder … and quite frankly, a daunting task for two builders. I feel the time involved would be immense and offer really no advantage to justify the time involved.

In light of the above issues and taking into consideration DOG Aviation has a staff of one, a compromise has been decided. Moving forward all rivets that make up the outer surfaces of the airplane will be countersunk blind rivets.

                            An example of low profile (dome rivets) compared to the countersunk (flush) version.

                           The low profile rivets are on the left and the flush rivets are on the right.

                        By clicking on the photo to enlarge it one can easily see how the countersunk

                             on the right sits flush as opposed to the low profile rivet on the left.

                                   Van’s LP-4 rivets on the left … Gesipa countersunk rivets on the right.

The LP-4 rivets Van’s uses are manufactured by Gesipa as one can clearly see on the left hand box. The Gesipa part number for what Van’s calls the LP-4 rivet is GSMD41-43APG. The D in the part number denotes a Dome or low profile rivet. The box on the right contains the countersunk or flush rivets I will be using on the RV-12 build. They are also manufactured by Gesipa and according to Gesipa, the countersunk version has the exact same specifications for shear and tensile strengths as well as the same grip range as the LP-4 rivets Van’s supplies. The Gespia part number for the countersunk version is GSMC41-43APG. As one can clearly see, the part numbers are identical except the D that denotes dome (low profile) is replaced with a C that denotes countersunk.

Wing Kit Arrives!!!

It came as somewhat of a shock when DOG Aviation received the call from FedEx saying they had two crates ready for delivery. Wasn’t expecting the wings for at least another two weeks. But it will be good having the parts here so when the empennage is completed, there will not be any down time waiting for parts to arrive.

                                               Ken (the FedEx delivery driver who delivered the empennage kit)
                                               amazed me once again with his skill at backing up that big rig to
                                               the DOG Aviation receiving bay.

                                 Moving the crate containing the wing spars to clear the way for the second crate.

                                                Crate containing the wing skins, ribs and miscellaneous wing
                                                components. Notice anyone with a big Cheshire cat grin?

                       After signing for the delivery, Ken and I took a few moments to catch up on how his family
                              was doing along with his expressed interest in following the construction of the airplane kit.

After the parts delivery, spent the next hour or so doing a little cleaning and rearranging things so all the crates had a home and out of the way (mostly). Having most everything on wheels makes for easy rearrangements.

Removing Protective Coating From Rivet Lines

There are two schools of thought regarding the protective thin plastic coating Van’s has on all the parts. One group says take it all off and be done with it and the other feels leaving it on parts as long as possible prevents the Alclad factory coating on the aluminum from getting scratched and possibly leading to corrosion. I lean towards the second group even though it requires a little more work on the skins.

                                 Using a straight edge and a soldering iron to melt a cut line into the film coating.

                                                        Pealing the protective film away is now a snap.

Builder Tip:

Don’t drag the point (tip) of the soldering iron down the skin ... you will put a scratch in the aluminum. It is better to use a cone shaped tip and drag the side of the cone down the plastic coating or if you have spare tips, just round off the tip altogether.

                                                        Skin with plastic coating removed from rivet lines.

Undersized Rib Flange Holes

When doing the test fit of the vertical stabilizer skin to the vertical stabilizer skeleton I discovered the undersized holes other builders have mentioned.  I remembered reading about a hole being undersized on the first flange hole at the forward end of the VS-1207 rib, but that was a long time ago and figured it would have been resolved by the time my kit was assembled … apparently not.

                                                The undersized holes on VS-1207 are the ones on the first flanges

                                                on the left … both upper and lower.

Thus far, all the parts from Van’s have fit together perfectly (prior experience with Van’s parts has shown if it doesn’t fit, the assembly is being putting together incorrectly) … so when I saw the hole in the skin was aligned with the hole in the rib but I couldn’t get the Cleco into the hole, I knew something was wrong. Then I had a flashback and remembered reading about this minor issue.  Drilled the two holes out with a #30 drill bit and now all is well.

Vertical Stabilizer Test Fit

Before metal prep, some dimpling and priming I wanted to do a test fit of the vertical stabilizer skin to the vertical stabilizer skeleton.  Sounds simple … right?  Well, yes and no.  It turns out I had to assemble and take things apart twice before the parts would fit together correctly. The plans call for trimming some metal off the narrow end of a few of the ribs. Not wanting to over do it, I did not remove a ton of material and as it turned out I had a clearance issue so had to take the ribs out and trim some more material off to get things to fit better.

                              Sliding the vertical stabilizer skeleton into the skin. It is a really TIGHT fit at the leading edge.

After trimming more material off the ribs, things were looking much better but the first rivet holes on the leading edges of the ribs are a bear to get aligned correctly. The taper of the skin on the leading edge wants to push the first rib tab on the ribs off to one side. The only way I could get a Cleco in the hole was to use a broom handle and slide it up through the lightening holes in the ribs and push on the leading edge of the rib to move the rib tab to the correct position.

                                                The completed test fit … yep, looks good … so on to dimpling,
                                                metal prep, primer and final riveting. To bad it needs to be taken
                                               apart for priming.

Machine Countersinking The Rear Spar

The plans call for machine countersinking the vertical stabilizer’s rear spar at six locations on the lower portion of the spar. The holes need to be countersunk to accept 3/32” flush rivets. In this case, the reason for countersinking rivets at this location is because the lower portion of rear spar will mate to the rear of the fuselage cone … therefore the mating surface needs to be flat with no rivet heads sticking up.

                                      Countersinking the rear spar using a 100 degree cutting bit in a countersink cage.

BUILDER TIP:

Based on prior experience I have found that countersink bits used with hand drills tend to make holes in thin metal larger. A good way to help minimize this is to first drill the proper sized hole into a piece of hardwood and place the wood under the piece of metal being countersunk. Align the wood piece so the tip of the countersink bit goes into the hole drilled in the wood. This really helps ... a lot. Give it a try on some scrap aluminum. Unfortunately, in the photo one can’t see the wood piece under the spar I was countersinking.

From left to right - countersink cage with a cutting bit installed, a cutting bit, a cage without a cutting bit installed.

Avery makes a nice countersink kit consisting of cutting bits and a high quality cage made with bearings as opposed to bushings. Most builders (myself included) will want an extra countersink cage (or two) because it takes time to get it adjusted correctly to cut at the proper depth. Once it is adjusted, you tend to want to leave it dedicated for that size cutting bit.

For those not familiar with how the countersink cage works, I’ll try to explain by making the simple sound complicated. The round shaft protruding from the cage assembly connects to a hand drill or drill press which rotates the shaft. The end of the shaft inside the cage has a threaded hole which the desired sized cutting bit screws into. The shaft is spring loaded and when downward pressure is applied, the shaft slides deeper into the cage allowing the cutting bit to go deeper into the hole to be countersunk. While the inner shaft is spinning, the outer portion of the cage does not spin as long as you hold onto it.

Now the time consuming adjustment I mentioned earlier ... there is a depth stop adjustment because one needs to limit the depth of the cut. Without a depth limit stop, it is very very  very easy to countersink much deeper than desired … thus there is a cutting depth  adjustment.  Unfortunately, the depth adjustment is all trial and error so one must make sure they start shallow and slowly adjust to the proper depth. The bits cut metal fast so it takes time to slowly get to the proper cutting depth. Once the tool is adjusted you are off to the races for it only takes a few seconds per hole.

The cage is on a threaded barrel and adjustment is made by loosening a locking ring at the top of the assembly then pulling back on the lock and screwing the cage in or out of the barrel. This essentially makes the cage longer or shorter thus allowing the cutting bit to go deeper or shallower. If you double click on the above photo it will come up larger.  Looking closely at the photo, you will see I have loosened the locking ring and pulled back the lock then wedged the mandrel of a rivet between the cage and the lock so one can see the locking teeth and also how small of an adjustment that can be made.

A close-up of the six countersunk holes on the rear spar.