I honestly don't think it would work. It's an idea but as you go faster the air will tend to try to collapse the blades, thereby slowing the plane down. Which allows the blades to achieve a more realistic angle which gives more drive which causes more wind pressure which forces the blades back. I'd like to see one used and take readings and compare it to a straight prop. Then again the same blades in the normal front position is also fight the wind pressure so I'm stumped. y the way the previous answer should read "unfold" when needed. After you've tried it, post your findings if you would, it should prove interesting. Put a small camera on the prop to make a video of how they work.
The centrifugal force will hold the blades out, and when you back off the wind will push them back, it's too big for my bixler but it would fit my fpv-168 that I'm building, I've only been flying a couple of months so I won't fly the 168 till I'm more confident I wont trash it! I don't think these are as efficient as a normal prop but I will get one as a basket filler (after my wife cools down)
The thing that holds the prop axles is the yoke, normally. I don't know exactly what 'hub' is. Folders normally are determined by yoke (the part that holds the prop) and the spinner. Somebody with the product should measure this instead of us just speculating *)
The collet goes through the hub to tighten the assembly to the motor shaft. The Hub is the long piece which has the "axles" at each end to hold the prop blades. The spinner is the cone shaped piece which covers the lot except the axles and props and looks pretty. The hub can be neutral or positive or negative pitch. Mostly neutral. The hub can be longer than the cone is wide as in the case of the Radian Glider where the spinner is 50 mm and the hub 52 mm to allow the prop blades to fold tight against the cowling. I got stuck without a 50 mm spinner once and used a 50 mm hub with a 40 mm spinner. It solved a cooling problem but still kept it more aerodynamic than no spinner at all.
Sawdust: My full quote was "Folders are normally determined by yoke (parenthetical) AND THE SPINNER.". I was referring to the vocabulary one uses when talking about spinners, not actually the general diameter measure.
It's 9 inches tip to tip (that's why they call it "9 inch" and the spinner is 40mm so convert 9 inches to metric, subtract the 40mm for the hub then divide that by half and you'll know the answer to the second part of your question.