Some papers have been analyzed, and I will give you a picture.
Therefore, ducted propellers are mostly used for light helicopters, not for medium and heavy ones, because both of them have many hovering tasks, such as lifting things, lowering ropes and so on.
Another troublesome problem is that the blade diameter of the ducted impeller is relatively small, and the thrust generated is the same as that of the conventional impeller, with much higher rotation speed and higher noise. The assault helicopter is better to be quiet, after all, it sometimes goes deep behind enemy lines. (The noise of ducted propeller is less than that of ordinary tail rotor only at the same speed, and the slower the speed is under the same thrust, the smaller the noise is. )