[thrash]

Originally Posted by The Colonel

However, you could, as I understand it, drive a Harrier perfectly well without knowing how to hover as long as you didn't try any vertical manoeuvres and treated it as an STOL aircraft

I suggest that this is the realm of skill defaults, with no (or extremely poor) defaults allowed for completely novel flight modes.

I get the impression that the same cannot be said of tilt-rotor machines (so far limited to the Osprey AFAIK) and helicopters.

Osprey can take off and land like conventional airplanes. There's a bit of jiggery required because the prop/rotors are wider than the ground clearance, but that should be in the checklist.

Helicopters, not so much.

Actually, speaking of helicopters, quadcopters and similar things, is driving a twin rotor (like a Chinook) a different skill from a single rotor? How about Kamov style stack rotors? Based on my (limited) understanding of how these things fly, they should behave very differently...

More of a familiarity penalty than a different skill altogether. The aerodynamics are fundamentally the same -- it's the applications that vary.

Features that can produce different handling characteristics even in a single main rotor helicopter include clockwise vs. counter-clockwise rotation, fixed vs. semi-rigid vs. fully articulated rotor system, high vs. low inertia rotors, conventional tail rotor vs. ducted fan vs. NOTAR, nose-high vs. skids-level hover attitude, and degree of automation and hydraulic assist in the flight controls.

I was surprised and pleased as a senior aviator to sit down in my first unfamiliar helicopter type in almost ten years (fifth overall) and pick it up to a hover with nary a bobble. Learning all the systems and how to respond to specific emergencies took considerably longer -- but that's what familiarity penalties represent.