Catalysis of the transfer of long-chain fatty acids from one side of the membrane to the other. A long-chain fatty acid is a fatty acid with a chain length between C13 and C22. This transporter is a porin and so enables the energy independent passage of substances, sized less than 1000 Da, across a membrane. The transmembrane portions of porins consist exclusively of beta-strands which form a beta-barrel. They are found in the outer membranes of Gram-negative bacteria, mitochondria, plastids and possibly acid-fast Gram-positive bacteria.