GURPS 3e: Making a duct-rotor helicopter

[Anthony]

Originally Posted by thrash

They don't, really, except for specialized configurations (like X-wings). What is happening is that the rotor in forward flight is working on undisturbed air, rather than eating it's own downwash at a hover.

While it's not precisely the same mechanic, the math actually works out the same -- either a wing or a helicopter rotor has the net effect of causing downward deflection of an incoming stream of air, generating lift and causing drag. The equations mostly diverge at less than stall speed for a fixed wing of equal wingspan.

Which presumably would happen with a ducted fan as well, but you'd want to make sure that the ducting isn't interfering with the airstream, which I suspect it would do for designs that look like the SA-2.