David,
I have a lot of respect for this forum and it's members. I didn't throw that out as an "off-the-cuff" opinion. My foundation in sheet metal and riveting began with a 15 year mentoring relationship with John Thorp - augmented by regular discussions with other Structural Engineers and research related to Forums given at OSH and Sun'nFun. The forums I gave at Sun'nFun qualified for Continuing Education Credit to IA certificate holders.
The following is from a Boeing structural engineer, discussing just this question, posted in an Aerospace Engineering Forum:
"" I suspect that the mechanics want to reverse drive to ensure max gun-energy directed to deform the tail. If this is the reason, then stop the presses... as You indicate there will be hole-fill issues galore.""
They will be mushrooming the tail without swelling the shank.
reinforced in another paragraph:
"IF this is the case, then what has happened is that the shop is circumventing rivet conventional installation protocol in-favor-of "just getting the job done [any-way they can]". This is a shabby way to run an aerospace company."
point expanded by the following:
"Hole (or hole/csk) preparation???
As usual, hole preparation [IE: straightness, perpendicularity, roundness, roughness, deburring, inner-laminar chipping, +plus all countersink issues, etc] can make a BIG difference... grossly affecting installation soundness.
[end quotes]
I will add that last quote went on to estimate up to 10% less strength and much lower fatigue life (most important to RV builders). This shows the problem lies more with the substrate than the means used to form the rivet. The quality of the rivet installation depends on the execution of the maker of the parts to be assembled and the greatest variability lies with dimpling/countersinking for installation of flush rivets. I do not feel the instruction on riveting currently offered in the E/AB community adequate to sufficiently inform a builder.
I'm not acting as the harbinger of doom in making these statements.
My guess is that there is more than enough "margin" in the RV rivet spacing that the 10% loss of strength is of minimal concern. What I do feel is more significant are fatigue life issues - The recent SB on the rear wing spar is not directly related but it shows the consequence of the point I stress.
RV builders expend a tremendous, admirable, level of effort and detail in the construction of their aircraft. Their airframes, more than likely, will be around long enough to manifest these fatigue related problems if the considerations I raise aren't addressed.
FWIW
mjb