I have recently added a rotor with no magnets to one and used it with 5' 6" prop with tsr 5 for use in a very low wind area. It worked exceptionally well as a 4 ft machine, but changing to dual rotor with larger blades has made a dramatic difference in this case ( it really is a hopeless wind area) . I would set it to furl at about 250W to keep it safe.
If you use 2 rotors with magnets and change the winding it will very easilly cope with 6ft blades at tsr 6 and be running well towards stall so it should easily cope with 7ft but if 8ft is too big I would think perhaps you want to stop at 6 ft.
With the metric magnets (46 x 30 x 10) I have used 100 turns per coil of #15 for a 24 v machine. If you want 12v use 50t of 2 in hand #15 or single #12. Stator increased to 1/2" thick.
The same winding should do for the 2 x 1 x 1/2 magnets, you have plenty in hand and may need to open the gap and use a higher line resistance especially at 24v.
Should be good for 300 to 400 W.
Remember that if you uprate to a larger prop you must increase the furling offset and tail size and length. If you don't then it won't stay in the wind properly and almost certainly won't furl. You must always change everything.
Flux
