All,
Preventing the tank from bulging too much forward will of course prevent it
touching any of the controls or the wing spar (in which case the strap on
the left spar might catch on the bulge during withdrawal/installation).
The inherent problem with high density polyetylene (HDPE) without any
barrier layer (which automobile tanks have, and the newer Europa tanks may
have - this is now being looked into) is that it is permeable and components
of the fuel get into the material and causes it to swell (and some gets
through the tak walls and gives the typical "Europa smell"). Where the
material is thick, as in the edges between the bottom and sides, sides and
top, and sides to sides, the swelling is apparently negligible. This
swelling cannot be stopped as such, so if the tank is prevented from
buckling forward at all, it will bulge only inwards. This will probably not
be one single large bulge in each panel, but smaller "wrinkles" that another
contributor reported yesterday from his findings along the top and the back
(where outward bulging is restricted).
The best, given this undesired but unavoidable swelling, would be if the
tank were of a simpler shape (say like a box without indentations and the
saddle) and if we had no spacers to keep it away from the controls. Typical
hard points as the saddle and the outlets at the bottom appears to be where
cracks are developing in some cases. Even though the fuel components
entering the permeable material also acts like softeners, local faults in
the material may cause overstressing (e.g. thinner than intended material
due to the fabrication process; maybe insufficient heating during the
moulding).
The spacers installed according to the Builders Manual could be typical
danger areas as far as cracks. However, both on Arnold's tank and mine
(which shows far less bulging - yet), the wave form of the bulging across
the width of the tank is such that there is an inward bulge right behind the
spacers and a forward bulge on either side. It is easy to envisage the
stress that would be introuced if a spacer were installed where the tank
naturally wants to bulge forward.
So, based on what I have observed and learned about the tank material in
these last few days, I would carefully evaluate where to stop the tank's
bulging. Newer tanks have a stiff rib or indentation running across the
front, which should help (one such installation will be inspected shortly by
another owner).
As for those still in the appropriate build stage, I would shape the BM
spacers differently: Build the plate of thicker material, oval instead of
rectangular (with long axis horizontal) and shape its thickness markedly
convex (thich in the center, thinning towards a well rounded circumference).
I am in contact with a polymer expert at a central Norwegian scientific
institute. His immediate comment is that HDPE does not "dry out" if the
tank is left empty, but I am awaiting his further comments to a host of
follow-up questions with relevance to our particular tanks and how we
operate our airplanes. Temperature definitely plays a part in the swelling.
The hotter, the faster it swells, but I am still awaiting to hear from him
whether HDPE swell more (higher maximum swell elongation) in hot weather
than in cold. He also mentioned that the particular fuel used may play a
role. When a major fuel supplier here introduced unleaded 98 octane mogas
several years ago, it was found to "attack" polymers more aggressively than
the previously used auto fuel did (we use unleaded 95 octane mogas in our
Europas here in Norway).
What I really hate about this issue is not knowing whether I have spent 10%
or 90% of my tank's safety margin with the present bulging, and what does
say 5mm more bulging translate into as far as approaching a crack somewhere?
Regards,
Svein
LN-SKJ
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