GURPS 3e: Making a duct-rotor helicopter

[thrash]

Originally Posted by Anthony

The main problem with ducted rotors for helicopters is the 'why' problem -- it's a bunch of weight that serves no purpose in normal flight.

Ducted fan blades are usually made of stiffer material and therefore able to run at higher RPMs than an equivalent rotor system, making them smaller for the same overall power. Done right, the blades should be lighter as well for being supported on both ends (and/or from not having to rotate some of the structural mass). Because they are self-contained, they can be vectored in ways that a conventional rotor system has trouble duplicating. I suspect there is also a gain in efficiency from eliminating rotor tip vortices, but I haven't seen the math on that.

That's not to say ducted fans don't involve a lot of additional weight and complexity, just that there are compensating gains.