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Fuel tank foam is a BAD idea.  As far as lessening the chance of a fire I'm convinced it will do its job and prevent gushing of fuel from the tanks.  However, had said that, the probability of having problems with water in the fuel goes WAY up.  All of the foams and I'm including the aluminum honeycombs have too much surface area for water.  In a typical Lancair wing there is on the order of a couple of square meters of exposed surface in the tanks for water.  If we figure that a very small layer of water forms on that surface and gets into the fuel there is no serious problem hence the occasional teaspoon or less of water.  The foams however, even if they are "poor" foams have on the order of 100's of meters of surface area. The same small layer of water is now magnified by a couple of orders of magnitude.  That teaspoon of water in the sump is now a quart or more!

LML website:   http://www.olsusa.com/Users/Mkaye/maillist.html
LML Builders' Bookstore:   http://www.buildersbooks.com/lancair

Please send your photos and drawings to marvkaye@olsusa.com.

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