A rotorcraft is a heavier-than-air flying machine that uses lift generated by wings, called rotor blades, that revolve around a mast. Several rotor blades mounted to a single mast is referred to as a rotor. Rotorcraft may also include the use of static lifting surfaces, but the primary distinguishing feature being lift provided by one or more rotors. Rotorcraft include helicopters, autogyros, gyrodynes and tiltrotors. In the United States, the Federal Aviation Administration places helicopters, autogyros (which it calls gyroplanes), and gyrodynes in the category Rotorcraft, and tiltrotors in the category Powered lift.
Classes of rotorcraft
A helicopter is a rotorcraft whose rotors are driven by the engine(s) throughout the flight, to allow the helicopter to take off vertically, hover, fly forwards, backwards and laterally, as well as to land vertically. Helicopters have several different configurations of one or more main rotors.
Helicopters with one driven main rotor require some sort of antitorque device such as a tail rotor, fantail, or NOTAR, except some rare examples of helicopters using tip jet propulsion which generates almost no torque.
Unlike a helicopter, the rotor of an autogyro (sometimes called gyrocopter or gyroplane) is driven by aerodynamic forces alone, and thrust is provided by an engine-powered propeller similar to that of a fixed-wing aircraft.
A Gyrodyne is a rotorcraft with a rotor system that is normally driven by its engine for takeoff, hovering and landing like a helicopter, but which also has an additional propulsion system that is independent of the rotor system. At higher speeds, the rotor system acts similar to that of an autogyro, no longer driving the aircraft but simply providing lift. A technology proposal by Carter Copter Technologies, as well as a research project funded by DARPA both refer to gyrodyne-type concepts as heliplanes.
Hybrids and compounds
Various kinds of hybrid between rotary and fixed-wing aircraft have been created:
• Some hybrid craft have separate sets of fixed and rotary wings. These are sometimes called hybrid or compound rotorcraft (although in the USA at least, the term "compound rotorcraft" officially refers to a mixed powerplant system).
• Some hybrid craft take off and land vertically in rotary wing configuration, then transition to a fixed-wing configuration for forward flight with the rotor now acting as a propeller. They may or may not have variable geometry:
o Tilt rotor - The rotors tilt forward after takeoff, to act as propellers in forward flight with a fixed wing providing lift. In practice, the theoretical advantage of high forward speed has not been realised.
o Tiltwing - The rotors/propellers are mounted on the fixed wing. The whole assembly tilts upwards for takeoff, hover and landing, and lies flat in forward flight.
o Coleopter - The rotor spins around the fuselage axis. Typically it is ducted inside an annular wing. The whole aircraft points vertically for takeoff and, in theory, then tilts horizontally so that the rotor becomes a propeller for forward flight. The transition to forward flight has never been achieved.
• Some hybrid craft take off and land vertically in rotary wing configuration, then transition to a fixed-wing configuration for forward flight with the rotor now acting as a fixed wing:
o X wing - For forward flight the rotor is stopped, but acts as a tandem wing to continue providing lift.
o Triebflügel - Similar to a coleopter, except that the rotor is not ducted and instead stops rotating to become a fixed wing during forward flight. The design has never been built, due to fundamental problems with flight during the transition.
There are also examples of hybrids between a helicopter and an airship:
• Piasecki PA-97 Helistat
• Boeing/SkyHook JHL-40
Number of blades
A rotary wing is characterised by the number of blades. Typically this is between two and six.
Number of rotors
A rotorcraft may have one or more rotors. Various rotor configurations have been used:
• Single rotor - One rotor disc.
• Twin rotor - Two rotor discs. These usually rotate in opposite directions, so that no tail rotor or other yaw stabiliser is needed:
o Tandem - One in front of the other.
o Transverse - Side by side.
o Coaxial - One rotor disc above the other, with concentric drive shafts.
• Multiple rotor - More than two rotor discs (sometimes indicates more than one): typically, one at each corner.
Where a rotorcraft has two or more rotors, the rotor discs may be arranged to pass through each other. The blades of the two rotors must be synchronised so that they intermesh without touching each other.
In a ducted rotor design, the rotor is surrounded by a large ring-shaped duct, to reduce tip losses. Typically, the rotor swings forward to act as a propeller in forward flight, and the duct is designed to act as a fixed wing in this mode. However the only aircraft of this design to fly, the SNECMA Coleopter, only ever flew in rotorcraft mode.
The difference between a ducted rotor and a ducted fan design (which is not a rotorcraft) is that when the rotor is stationary you can see through the rotor disc.
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