All,
Some further comments from my acquaintance at the scientific institute:
- A warm tank will swell more than a cold one. When exposing HDPE (high
density polyester) to gasoline, the swelling will continue until the
concentration of gasoline absorbed in the material is in equilibrium with
the gasoline outside it. Then the swelling stops. The equilibrium is
temperature dependent, i.e. higher temperature leads to higher absorbtion of
gasoline and hence more swelling.
- Also the chemical resistance of HDPE is temperature dependent. A typical
HDPE commonly used in gasoline cans is classed as "resistant" to high octane
gasoline at 20C (68F), while at a temperature of 60C (140F) it is classed
"not resistant".
- If the gasoline concentration (liquid or gas form) in the tank is reduced
to zero, then the gasoline will diffuse from the polymer and into the tank
atmosphere. In the ideal material, the swelling might then go back to zero,
but not in reality. If the concentration of gasoline in the tank (liquid or
gas form) is not reduced, the gasoline absorbed in the polymer will not
diffuse out, and the tank walls will remain swelled.
- His view is that it does not really matter whether our tanks are kept
topped up or not after each flight, or drained completely for long idle
period. Saturated gasoline fume in the tank over the liquid surface keeps
the swelling at the same stage as when the walls were washed with gasoline.
- So how come some have got cracks? The absorbtion of gasoline causes
swelling. This type of swelling can be compared to stretching of metal - if
stretched too much, the metal breaks. Same thing with HDPE that swells -
swells too much, it breaks. If local hard buckles develop for some reason,
this may overstress the material before it otherwise would. This is
independent of the softening effect that the gasoline absorption also has on
HDPE. Even though it gets softer, it does not take more stretch. Small
differences in tank wall thickness or other individual factors may be the
reason why some have experienced cracks, while most of us have not.
- Having seen the photo of Arnold's tank, he finds the buckling to be more
than might be expected. If one could measure the length along the most
pronounced buckling wave form from side to side, and compare it to the
original width of the tank, the amount of swelling can be estimated. If
this is less than 3 pct, the material is probably resistant to gasoline
according the European industry norm DIN ISO 175. If the swelling is more
than 8 pct, the material most probably is not suitable as its resistance to
gasoline at the prevailing temperature is limited. If the temperature has
been less than 20C (68F), it is not inconceivable that the material in
Arnold's tank is LDPE (low density polyester), which is not commonly used
for gasoline containers. He emphasiszes that he has not studied Arnold's
tank other than the photo, does not know which material has been used in our
tanks and does not wish to make any derogatory remarks or comments on the
Europa tanks.
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We can all make an easy estimate of the swelling of Arnold's tank, by
forming a paper strip to a similar wave form as his tank's and compare the
end-to-end distance with the flat paper's length. Does not take much
bulging before 8 pct is reached - - - -.
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So where do we go from here? I am sending this in copy to Roger at EA2004,
and encourage him to look into this issue with the aim of:
1. ascertaining what type of material is used in the tanks - older and newer
batches - and how does this compare with industry standards for gasoline
(mogas/avgas) containers. Provide this information to the Europa owners.
This might give us some reassurance that we can continue using the tanks "on
condition".
2. irrespective of the findings under 1, initiate a study to come up with a
replacement tank for those who need one and those who wish to replace
theirs. Check with suppliers of industrial containers, with automobile tank
manufacturers, ask this forum for ideas, present your preliminary findings
for comments by this forum (after all, we are the ones to fly around with
it!), test it thoroughly at extreme conditions, ask a fair price and I
guarantee you that you will sell many. While you are at it, why not
increase the height of the cockpit module to more than make up for the tank
volume lost by having to reduce the tank's width some to get it in (up there
you get lots of extra volume due to the full fore/aft space being
available), which also will give us a better angle for the shoulder belts.
Roger, would you kindly give some feedback on this forum as to what your
plans might be, so that we can rest easy that you will proceed with full
speed and we don't feel like we have to design our own, individual
replacement tanks of aluminium, fancy composites or what not, in order to
continue flying safely over water and across mountains - - - .
Regards,
Svein
LN-SKJ
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