T is the thrust, M is the figure of merit for the design, take between 0.5 and 0.8 for a ductless rotor, rho is the air density, R is the radius of the rotor and P is the power required.
From this you'll see that your duct would have to be very efficient indeed. You'll probably need a gearbox as well, but I can't be bothered to work the speeds out.
