On Aug 29, 11:52*am, Michael Horowitz wrote:

...Figure 4-5 shows Rivet spacing for single-lap sheet splice. It
shows a double row of rivets in one case and the note "Strength
thru this section is 75% of sheet w/o holes". In the second
illustration it show more than two rows of rivets and a greater
overlap with the note "Strength thru this section is 83% of sheet
without holes".

My question is/was "How do you determine how much overlap there should
be?"

My tentative answer is "The more rivets, the stronger the joint. If
75% of the original sheet strength is fine, then overlap enough to
sustain *a double row of rivets. If 83% is needed, then go to an
overlap that will support three rows of rivets. "

Is my logic correct?

Also, is 83% as good as it gets? if so, why show more than 3 rows?

In my printed copy (AC 43.13-1A change three, circa 1990), that figure
is numbered 2.18. Note that the arrow points to a section line that
passes through a series of rivet holes. The way I read it, they're
talking about the strength of the aluminum sheet at that section line,
not the strength of the joint. My reasoning is that the strength seems
to be relative to the amount of material subtracted by the rivet
holes.

In the upper portion of the figure, the rivet spacing at the indicated
section is 4D, where D is the diameter of the rivet. So there is 3D of
material and then 1D of rivet hole where the material is missing.
Since 25% of the material along that section is absent on account of
having been drilled away, it stands to (mine at least) reason that the
sheet strength along that section line is 75% of what it was before
all that drilling happened.

In the lower portion of the figure, note that at the indicated section
line the rivet spacing is 6D. So you have 5D of material and then 1D
of rivet hole where the material is missing, with 1/6=16.7% of the
material drilled away leaving 100%-16.7%=about 83% of the original
material along that section line.

If this is about the Taylorcraft wing rib repairs you were asking
about in an earlier post, it looks to me like the kind of repair you
need is not well addressed in AC43.13. The section on metal wing and
tail rib repairs points you to a pretty looking figure, but then the
figure points you to paragraph 100e for the minimum number of rivets,
and 100e points you to figure 2.29 without showing how to determine
the width W that you need to make sense of the table.

My suggestion would be to find an actual A&P or IA who has actually
fixed one of these little puddle-jumpers, and ask them what they'd do.
Failing that, skip ahead a couple of figures to the one titled
"Typical metal rib repairs" (It's figure 2.22 in my old copy), and do
something similar to what they show there.

Thanks, Bob K.