Are they unducted fans, are they high tech propellers, or are they, as one wag put it, a bunch of bananas whirling around.

I don’t know, and it appears that regulatory authorities are in a similar conundrum.

The regulations if they are classed as turbofan blades are stricter, and thus the would add more weight, and the regulations if they were classed as a propeller.

It’s a non-trivial issue that needs to be resolved.