So, what you're saying is that the tank "grows" as it swells and will do
so inwardly if not allowed to, outwardly. Hmmmm, that's something 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
information you should get...
Jeff - Baby Blue
Sidsel & Svein Johnsen wrote:
> <sidsel.svein@oslo.online.no>
>
> 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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