doubler I'd offer the opinion that there is no way that a prosealed
doubler weakens the spar. In fact proseal excels in shear strength and
as long as the shear strength of the joint is not degraded, in general
separating the parts a bit will increase the spar strength (which does
increase fastener stress a tiny bit of course) One could argue some
minutae though. Different spar but the Rebel parts that are prosealed
are thick enough that the joint is not much thicker than primer anyway.
We only want enough to seal the joint and even thin brushable pro seal
would be adequate I think.
Now very thin doublers with a heavy coat of thick pro seal might be a
concern such as where the proseal causes an easily seen bulge in the
doubler between rivets such that buckling is encouraged. Such a thin
doubler would not be much use on a spar though. So while I believe that
I understand where this gentleman is coming from and why he might
question this, I would not give it a second thought when considering the
bigger picture. Encourage the questions though because that is how we
all learn. Probably a waste of effort to even run through calculations
in this case. Further I'd expect it to take several test samples to
establish a statistically significant change in strength either way. As
always the proof is in properly constructed testing. Any testing done
by MAM would have had proseal there I'd expect. I'd also suggest that
several other factors are more significant such as statistical variation
in material strengths of the parts and fasteners, not to mentions any
scratches or nicks.
Ken
Ted Waltman wrote:
The manual calls for proseal between several main spar sections as well as
between the rear spar and one of the rear spar doublers.
First allow me to digress a bit. I know, as I'm sure you do as well, that
there are terrible, so-so, good and even a few excellent A&P's out there.
I'm sure you know a few that you wouldn't trust putting a new windshield
wiper on your car let alone a part on your plane. Likewise there are those
few whose work is simply outstanding and whose advice is to be listened to.
We have a couple in the latter category at our airport. In fact there are
at least 3 that have built at least several experimental planes each as well
as worked on countless certified planes.
We were scratching our heads about using proseal between doublers on the
main and rear spar. We asked the opinion of one of our friends who is an
outstanding A&P. His comment/advice is to NOT put proseal between doublers
in these locations. Why? Because the doublers are designed to be as close
to each other as possible-in fact they are engineered with the idea that by
being sandwiched TOGETHER they are able to then effectively dissipate stress
across each other (or across multiple layers as in several instances on the
main spar). He commented that if they are apart any more than even a bit of
primer they lose some of this stress transfer ability. Furthermore, he
commented that by being even slightly apart (due to even a slight bit of
proseal) they would then be free to move against each other and act in a
shear fashion against the rivets holding them (close) together.
Anyone have any engineering experience with this? I emailed the above to
Jeff McMurrer at the factory to get his (and perhaps Darryl's) input.
Thank you,
Ted Waltman
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