It's done here and there... probably not to the extent you are suggesting. If you are saying you have blades, each 5-feet long, and attaching them to stubs on the hub that are each 3' long, then each resulting blade will sweep 8' from the hub's axis, and you have a 16 feet diameter rotor. This is a big step up from the 10-footer you started with, and the dead zone is 6 feet around. Lots of consequences.
Twist is wrong (adherence to the theoretical perfect twist is not very important).
TSR is determined more by the generator's loading than by the angle of the tip, but now that you have these tips way out there and hopefully on a generator large enough to load the big rotor properly, you are not going to see as high CP as before (unless the tips were cut at the wrong angle and somehow this corrects the situation!).
The dead zone could blanket the tail with turbulence.
Structural attachment may or may not be tricky. Guaranteed that the hub will be very heavy, though.
Net result...?
