sidsel.svein(at)oslo.onli
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 Posted: Wed May 14, 2008 2:33 am Post subject: Fuel tank expansion |
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I see that steps are taken by several builders to allow the fuel tank to expand and not be "glued" to the front and back of the cockpit module by the bid layups described in the Builder's Manual.
I do not wish to pass any judgement on the possible benefits, but there are some issues that I think should be considered:
- Judging from my own tank's bulging (not yet critical) and photos seen of others, I have no doubt that the thinner parts of the tank (e.g. tank walls) expand at least 15%.
- If the tank's overall dimensions also were to expand this much, there is far from enough space available for it to expand - it would begin pushing on the cockpit module and side/bottom skin and of the fuselage long before it had finished its expansion.
- So, on the installations made as per the BM, why has not this happened? Is it because the tank is "glued" to the bid layups? Most probably not - epoxy does not bind well to the tank material, we have learned.
- I think the reason is that the tank's overall dimensions do not expand much, if any. The material is permeable, but not porous to the extent that it leaks. Fuel (or certain components of the fuel) gets into the material, but if the material is thick enough, this ingress (for the lack of a better word) stops. Thick material therefore acts as a brake on the expansion - the thicker it is, the less it expands. All edges of the tank are considerably thicker than the panels, and my theory is that the tank therefore holds itself effectively within or very close to within its original dimensions. On my tank, there is no sign that the original edge between the tank bottom (including the saddle) and the tank front panel has changed shape at all, nor expanded.
- Making "non-glued" contact between the tank and the bid layups cannot cause any harm, I believe, provided the tank is not permitted to move outside its allocated "box" in the cockpit module. When I built my plane, I had heard that the tank would not be "glued" in place in the long term. I did not find the spacers per the BM to be sufficient to take up any considerable load (they would sit against a soft, thin tank panel only), should the tank slide forward at the bottom. I therefore installed additional spacers further down, going up against the bottom/front panel edge. I did not note what clearance I had originally (not many millimeters), but the tank edge is now touching the spacers on both sides (but not bulged around them). Whether this is caused by slippage of the bond that originally existed between the tank material and the epoxy in the bid layups, or tank expansion caused by the fuel, is unknown. These spacers are, however, a very effective stop against any forward movement of the lower part of he tank as a whole. See attached photos. It should be possible to fit something similar on existing aircraft - probably made of three parts: one glued up against the cockpit module in front, one held against the tank in the back while a thin wedge with glue is driven lightly in between.
As always when it does not come from an expert - take this with a healthy dose of salt!
Regards,
Svein
LN-SKJ
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