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 P: 4,780 No, but seriously, go back and rework your numbers BrianC. You should get a rotor radius of 193.2' using momentum theory with 0.4HP HOGE (I think you have a units error somewhere). Solve for radius: $$R=\frac{T^{3/2}}{\sqrt{2 \rho \pi } P}$$ T = 300 lbs (Airplane plus pilot) $$\rho$$ = 0.002 slug/ft^3 P = 0.4 HP * 550