When I inspected my tank a couple of years ago I noticed that the back wall
had buckled inward, probably reducing its capacity somewhat. This may well
have happened on other Europas, but unless you have a large opening at the
top you would never see it. I have an opening for the fuel level probe,
which allows me to get my whole arm into the tank. I was considering constr
ucting some sort of internal framing, but couldn't come up with a good solu
tion.
Karl
lock@cox.net>> > So, what you're saying is that the tank "grows" as it swel
ls and will do > so inwardly if not allowed to, outwardly. Hmmmm, that's so
mething I > hadn't considered. I'll have to rethink my solution. Thanks for
the > information, Svein. Please keep us up to speed on any additional > i
nformation you should get...> > Jeff - Baby Blue> > Sidsel & Svein Johnsen
sidsel.svein@oslo.online.no>> > > > All,> > > > Preventing the tank from bu
lging too much forward will of course prevent > > it touching any of the co
ntrols or the wing spar (in which case the > > strap on the left spar might
catch on the bulge during > > withdrawal/installation).> > > > The inheren
t problem with high density polyetylene (HDPE) without any > > barrier laye
r (which automobile tanks have, and the newer Europa tanks > > may have - t
his 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 t
hrough the tak walls and gives the typical "Europa smell"). > > Where the m
aterial is thick, as in the edges between the bottom and > > sides, sides a
nd top, and sides to sides, the swelling is apparently > > negligible. This
swelling cannot be stopped as such, so if the tank is > > prevented from b
uckling forward at all, it will bulge only inwards. > > This will probably
not be one single large bulge in each panel, but > > smaller "wrinkles" tha
t another contributor reported yesterday from his > > findings along the to
p and the back (where outward bulging is restricted).> > > > The best, give
n this undesired but unavoidable swelling, would be if the > > tank were of
a simpler shape (say like a box without indentations and > > the saddle) a
nd 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 e
ntering the permeable material also acts like softeners, > > local faults i
n the material may cause overstressing (e.g. thinner than > > intended mate
rial due to the fabrication process; maybe insufficient > > heating during
the moulding).> > > > The spacers installed according to the Builders Manua
l could be typical > > danger areas as far as cracks. However, both on Arno
ld'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 in
ward 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 bui
ld stage, I would shape the BM > > spacers differently: Build the plate of
thicker material, oval instead > > of rectangular (with long axis horizonta
l) and shape its thickness > > markedly convex (thich in the center, thinni
ng towards a well rounded > > circumference).> > > > I am in contact with a
polymer expert at a central Norwegian scientific > > institute. His immedi
ate 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 airplane
s. Temperature definitely plays a part in the > > swelling. The hotter, the
faster it swells, but I am still awaiting to > > hear from him whether HDP
E swell more (higher maximum swell elongation) > > in hot weather than in c
old. He also mentioned that the particular fuel > > used may play a role. W
hen a major fuel supplier here introduced > > unleaded 98 octane mogas seve
ral 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>
===============> > >
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