Barney,
Another member made a good point -- baffles. I didn't say anything because I assumed you'd employ them during design and fabrication. But since it came up, I recommend you consider open cell foam (Decacell is a leading manufacturer's brand name). Open cell foams are available in gasoline only, alcohol/gasoline, or multi-fuel tolerant formulations. These foams consume about 3% of their enclosed volume -- a small price to pay for 'infinite' baffling, and have the added advantage of explosion resistance if things get ugly. The additional cost is not significant, particularly if you use blocks positioned throughout the fuel cell rather than completely filling the tank.
Whether you use foam blocks or fill the tank(s), you should plan to add the foam after the cell is finished and pressure tested @ 7-8 psig. If you have a leak, use a spray bottle with slightly soapy water to pinpoint the source. Bubbles will immediately form at the leak site(s) and since the tank's internally pressurized, there is no opportunity for soap spray to contaminate the leak path. If porosity or lap-joint leaks are present, thoroughly clean the area with acetone or similar solvent and apply epoxy to the affected area while pulling a small vacuum on the inside of the tank. The vacuum will pull the epoxy into the porosity/lap-joint... forming a bond within the composite structure rather than merely on the outside surface. Repeat until the tank is pressure tight.
Open cell foam blocks are available from the following sources:
1 -
http://store.atechmotorsports.com/egnsearch.asp?N=400379+301310+115&autoview=sku2 -
http://store.summitracing.com/partdetail.asp?part=SUM-290190Installing safety foam after fabrication and pressure testing requires an inspection port large enough to accept the foam (which can easily be compressed but quickly expands to its original size). I've seen foam installed during and after fabrication, and both approaches work, but I prefer post-fabrication installation because foam can easily be removed if necessary in the future... without cutting the tank open. The inspection port must be large enough to get your hand and forearm through, and can be part of your filler port assembly or a separate end plate. The inspection port is usually a gasketed aluminum plate attached to the tank via a bonded backing-ring with sealed nutplates. This may sound complex but it's not.
As a final thought on composite or metal fuel tank construction, it's probably a good idea to coat the inside of the tank with a sealer designed for the task. The following website offers one alternative:
http://www.wicksaircraft.com/catalog/product_detail.php/pid=9129~subid=2651/index.htmlThe foregoing may seem like overkill, but when you've flown your airplane for many years without a fuel leak, you'll realize it was time and money well spent.
I hope this is useful.
Wayne