>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.
>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.