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 P: 20 Use whatever is cheapest and strong enough. The weight of the material is only confusing the matter. After the generator gets up to speed it has no effect whatsoever. At that point the only opposing forces are friction and the generator attached to the turbine. But the above post is a good point; when air speed is not constant the speed of the lighter material will adjust faster than the speed of the heavier material. Even so, if this is desirable you could always add a heavy flywheel (heavy wheel) to either setup. Air flow is a complicated problem, so you'll have to experiment to find the optimum turbine speed. Equal to the air speed would mean there is no resistance whatsoever, neither from friction nor the generator. And if the generator isn't pushing back then it's not drawing any power and thus not producing any either. Likewise if it the generator is completely stopped. The ideal is somewhere in between. Note that since the air is causing a force in one direction and the generator is causing an opposing force, whatever structure you use will have to handle that much torque without breaking.