I recently finished and flew successfully a 30" span 1/2A powered DC flapping head gyro (see Gyrator thread) and it uses .032" sheet polypropylene as the hinge material. It works GREAT, contrary to all my fears. I'm pretty sure that .040" would be fine, if not too thin. See what floridagyro has to say; he's da man in my book! Especially on large rotors.