GURPS 3e: Making a duct-rotor helicopter

[thrash]

Originally Posted by RyanW

On the other hand, helicopter rotors generate lift like a wing when the helicopter is moving, requiring much less down thrust (and thus power) than when hovering.

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. This would apply to ducted fans as well. The benefit happens at relatively low airspeeds (~ 35 knots), and is called "effective translational lift."

A helicopter operating at the edge of its capacity can even stall...

It's called "retreating blade stall." It occurs when the forward airspeed gets close to the rotational velocity of rotor system. The advancing blade gets extra lift; the retreating blade has to increase its pitch to compensate for the loss of lift until it stalls. The helicopter pitches up (from gyroscopic precession -- the effect is felt 90 degrees out of phase) and rolls over. Note that if the rotor really acted like a fixed wing as high speeds, this wouldn't happen.

...(fully loaded Hinds in the high altitudes of Afghanistan had to make rolling takeoffs).

Same thing happened to Cobras during summer gunnery at Fort Carson, Colorado (5,874 ft above sea level at the airfield; higher at the range). This is just due to the reduction in effective lift at high altitude on hot days.

Another effect of high-hot conditions is loss of tail-rotor effectiveness at a hover. The Cobra operator's manual says that they aren't subject to this problem, but I can assure you that they are.