It is indeed a PC case fan. They are remarkably well balanced and it certainly runs happily up to about 12000 rpm.
My design is very loosely based on the Optica simply because it is a good practical layout for a fan. When does a prop become fan? The Optica had a 5 blades.
My Ductfan park flyer will be 36" span and weigh no more than 12oz. It is intended to explore the feasibility of the power system and layout.
I have got this far.
The ailerons will be cut out and the servos, radio and motor added before the top skin goes on.