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2. Advance ratio - Wikipedia

   en.wikipedia.org/wiki/Advance_ratio

   Single rotor helicopters are limited in forward speed by a combination of sonic tip speed and retreating blade stall.As the advance ratio increases, the relative velocity experienced by the retreating blade decreases so that the tip of the blade experiences zero velocity at an advance ratio of one.

3. Slowed rotor - Wikipedia

   en.wikipedia.org/wiki/Slowed_rotor

   The McDonnell XV-1 could slow its rotor from 410 to 180 RPM. The slowed rotor principle is used in the design of some helicopters.On a conventional helicopter the rotational speed of the rotor is constant; reducing it at lower flight speeds can reduce fuel consumption and enable the aircraft to fly more economically.

4. Contra-rotating - Wikipedia

   en.wikipedia.org/wiki/Contra-rotating

   A Mark 46 Mod 5A torpedo is inspected aboard a destroyer in April 2005. Contra-rotating propellers have benefits when providing thrust for boats for the same reasons. ABB provided an azimuth thruster for ShinNihonkai Ferries in form of the CRP Azipod, [1] claiming efficiency gains from the propeller (about 10% increase [2]) and a simpler hull design.

5. Intermeshing-rotor helicopter - Wikipedia

   en.wikipedia.org/wiki/Intermeshing-rotor_helicopter

   An intermeshing-rotor helicopter (or synchropter) is a helicopter with a set of two main rotors turning in opposite directions, with each rotor mast mounted with a slight angle to the other, in a transversely symmetrical manner, so that the blades intermesh without colliding.

6. Coaxial-rotor aircraft - Wikipedia

   en.wikipedia.org/wiki/Coaxial-rotor_aircraft

   A coaxial-rotor aircraft is an aircraft whose rotors are mounted one above the other on concentric shafts, with the same axis of rotation, but turning in opposite directions (contra-rotating). This rotor configuration is a feature of helicopters produced by the Russian Kamov helicopter design bureau .

7. BERP rotor - Wikipedia

   en.wikipedia.org/wiki/BERP_rotor

   Even when hovering, the rotor tips may be travelling at a significant fraction of the speed of sound. As the helicopter accelerates, its overall speed is added to that of the tips, meaning that the blades on the forward-moving side of the rotor sees significantly higher airspeed than the rearward-moving side, causing a dissymmetry of lift.