Why do you need more power?

The power needed to move the model is given by the models characteristics, and the effiency of the prop..
If you want a given speed on a given model you need a given amount of power, split that power on the efficiency of the prop and you have the needed power from your motor. There is no reason to claim that a bigger prop is less efficient, on the contrary, bigger props can move air more efficient than smaller ones.

What it does require is more torque, both to drive the size of the prop and the higher pitch of the prop. I did this with a larger electric motor, but it might aswell be done with a reduction gear. To try to use fairly the same pitch as the small prop and drive it at the same RPM would make the large one less efficient and you would require more power.

If the power needed would increase with a bigger prop, how come my Spitfire with 4 blade 24" Solo and a Spitfire in Britain with a 2 blade 20" APC have the same current draw in flight? If any difference they where to my advantage..

Stian