You fumbled the math. >doubling wind speeds gives ~8 times the power This describes a cubic relationship; 2 cubed is 8. >doubling the length of the blades gi

You fumbled the math. >doubling wind speeds gives ~8 times the power This describes a cubic relationship; 2 cubed is 8. >doubling the length of the blades gives something like 4 times the power output This describes a quadratic relationship; 2 squared is 4. 0.9^3 * 0.9^2 = 59% of the energy, much better than the 20% you claim.