Originally Posted by jgracco

I just got off the phone with an engineer I know in military aircraft research who works with full scale engines. He took one look at the pics I sent him from this forum with the curvature of the blades (inward) and suggested that this could be flutter. Apparently there is a resonant frequency for every motor and in full scale motors there are controllers designed to avoid whatever that rpm range is altogether. Wren could either experiment with different length intakes or calculate the length of an intake needed to shift the flutter to an RPM range that is not problematic.