Skew angle in 3-phase AC induction outer rotor motor
If you have information about rotor design for a conventional induction motor, you should probably use that as a guide. There is a US patent application (20140246943) that claims that the optimum skew is the greater of the stator slot pitch or the rotor slot pitch.


  Is there a specific number of skews to be made on the rotor for my application?


Does "number of skews" refer to accomplishing skew by using groups of laminations without skew within the group, but skew from group to group, skewing in steps?" If that is done, it is probably best to have as many groups or steps as is manageable. I believe that I saw a recommendation of 5 to 7 steps.


  What kind of metal is preferable: copper or aluminum?


Copper rotor bars would increase the motor efficiency and allow the motor to run cooler. However copper rotor bars would be heavier, more expensive and make the motor more difficult to manufacture. It might be difficult to make the rotor bars tight in the slots. A lower resistance rotor would have lower locked rotor torque and higher starting current -- not concerns with VFD powered motors.