A helicopter rotor is the rotating part of a helicopter which generates an aerodynamic force. The helicopter rotor, also called the rotor system, usually refers to the helicopter's main rotor which is mounted on a vertical mast over the top of the helicopter, although it can refer to the helicopter's tail rotor as well. A helicopter's rotor is generally made up of two or more rotor blades, although several earlier helicopters had a rotor with a single main rotor blade. The main rotor provides both lift and thrust, while the tail rotor provides thrust to compensate for the main rotor's torque.
Before the development of powered helicopters in the mid 20th century, autogyro pioneer Juan de la Cierva researched and developed many of the fundamentals of the rotor. Cierva is credited with successful development of multi-bladed, fully articulated rotor systems. This type of system is widely used today in many multi-bladed helicopters.
In the 1930s, Arthur Young improved stability of two bladed rotor systems with the introduction of a stabilizer bar. This system was used in several Bell and Hiller helicopter models. It is also used in many remote control model helicopters.
The rotor head is a robust hub with attachment points for the blades and mechanical linkages designed to control the pitch of the blades.
The simple rotor of a Robinson R22 showing (from the top):
The pitch of main rotor blades is varied throughout its rotation in order to control the magnitude and direction of the thrust vector. Collective pitch is used to increase or decrease rotor thrust perpendicular to the axis of rotation. Collective pitch controls the magnitude of the thrust vector. Blade pitch is varied during rotation to effectively tilt the rotor disk and control the direction of the thrust vector. These blade pitch variations are controlled by the swash plate.
The swash plate is two concentric disks or plates, one plate rotates with the blades while the other does not rotate. The rotating plate is connected to individual blades through pitch links and pitch horns. The non-rotating plate is connected to links which are manipulated by pilot controls, specifically, the collective and cyclic controls. Rotors with more than two blades have two dedicated connections, which make the inner swash plate turn. In two bladed rotor systems the blades take over this task.
The swash plate can shift vertically and tilt to some degree. Through shifting and tilting, the non-rotating plate controls the rotating plate, which in turn controls the individual blade pitch.
During the development of the autogyro, Juan de la Cierva built scale models to test his designs. After promising results, he built full size models. Just prior to takeoff, his autogyro rolled unexpectedly and was destroyed. Believing this to have been caused by sudden wind gusts, Cierva rebuilt it only to suffer an almost identical accident. These setbacks caused Cierva to consider why his models flew successfully, while the full-sized aircraft did not.
Cierva realized that the advancing blade on one side created greater lift than on the retreating side due to increased airspeed on the advancing side which creates a rolling force. The scale model was constructed with flexible materials, specifically rattan, so the rolling force was absorbed as the blades flapped and compensated for dissymmetry of lift. Cierva concluded that the full size steel rotor hub was far too rigid and introduced flapping hinges at the rotor hub.
Flapping hinges solved the rolling problem, but introduced lateral hub stresses as the blade center of mass moved as the blades flapped. Due to conservation of angular momentum, the blades accelerate and decelerate as their center of mass moves inward and outward, like a twirling ice skater. Cierva added lag-lead, or delta hinges to reduce lateral stresses.
Arthur M. Young found that stability could be increased significantly with the addition of a stabilizer bar perpendicular to the two blades. The stabilizer bar has weighted ends which cause it to stay relatively stable in the plane of rotation. The stabilizer bar is linked with the swash plate in such a manner as to reduce the pitch rate. The two blades can flap as a unit and therefore do not require lag-lead hinges (the whole rotor slows down and accelerates per turn). Two bladed systems require a single teetering hinge and two coning hinges to permit modest coning of the rotor disk as thrust is increased. The configuration is known under multiple names, including Hiller panels, Hiller-system, Bell-Hiller-system, and flybar system.
Tail rotors are generally simpler than main rotors since they require only thrust control via change in pitch. A simplified swash plate is used to control collective pitch. Two bladed tail rotors include a teetering hinge to compensate for asymmetry of lift.
Most helicopters have a single, main rotor but require a separate rotor to overcome torque. This is accomplished through a variable pitch, antitorque rotor or tail rotor. This is the design that Igor Sikorsky settled on for his VS-300 helicopter and it has become the recognized convention for helicopter design, although designs do vary. When viewed from above, designs from Germany, United Kingdom and the United States are said to rotate counter-clockwise, all others are said to rotate clockwise. This can make it difficult when discussing aerodynamic effects on the main rotor between different designs, since the effects may manifest on opposite sides of each aircraft.
With a single main rotor helicopter, the creation of torque as the engine turns the rotor creates a torque effect that causes the body of the helicopter to turn in the opposite direction of the rotor. To eliminate this effect, some sort of antitorque control must be used, with a sufficient margin of power available to allow the helicopter to maintain its heading and provide yaw control. The three most common controls used today are the traditional tail rotor, Eurocopter's Fenestron (also called a fantail), and MD Helicopters' NOTAR.
Fenestron and FANTAIL are trademarks for a ducted fan mounted at the end of the tail boom of the helicopter and used in place of a tail rotor. Ducted fans have between eight and 18 blades arranged with irregular spacing, so that the noise is distributed over different frequencies. The housing is integral with the aircraft skin and allows a high rotational speed, therefore a ducted fan can have a smaller size than a conventional tail rotor.
The Fenestron was used for the first time at the end of the 1960s on the second experimental model of Sud Aviation's SA 340, and produced on the later model Aérospatiale SA 341 Gazelle. Besides Eurocopter and its predecessors, a ducted fan tail rotor was also used on the canceled military helicopter project, the United States Army's RAH-66 Comanche, as the FANTAIL.
NOTAR, an acronym for NO TAil Rotor, is a helicopter anti-torque system that eliminates the use of the tail rotor on a helicopter. Although the concept took some time to refine, the NOTAR system is simple in theory and works to provide antitorque the same way a wing develops lift using the Coandă effect. A variable pitch fan is enclosed in the aft fuselage section immediately forward of the tail boom and driven by the main rotor transmission. This fan forces low pressure air through two slots on the right side of the tailboom, causing the downwash from the main rotor to hug the tailboom, producing lift, and thus a measure of antitorque proportional to the amount of airflow from the rotorwash. This is augmented by a direct jet thruster (which also provides directional yaw control) and vertical stabilizers.
Development of the NOTAR system dates back to 1975 when engineers at Hughes Helicopters began concept development work. In December 1981 Hughes flew a OH-6A fitted with NOTAR for the first time. A more heavily modified prototype demonstrator first flew in March 1986 and successfully completed an advanced flight-test program, validating the system for future application in helicopter design. There are currently three production helicopters that incorporate the NOTAR design, all produced by MD Helicopters. This antitorque design also improves safety by eliminating the possibility of personnel walking into the tail rotor.
Tandem rotors are two horizontal main rotor assemblies mounted one behind the other. Tandem rotors achieve pitch attitude changes to accelerate and decelerate the helicopter through a process called differential collective pitch. To pitch forward and accelerate, the rear rotor increases collective pitch, raising the tail and the front rotor decreases collective pitch, simultaneously dipping the nose. To pitch upward while decelerating (or moving rearward), the front rotor increases collective pitch to raise the nose and the rear rotor decreases collective pitch to lower the tail. Yaw control is developed through opposing cyclic pitch in each rotor; to pivot right, the front rotor tilts right and the rear rotor tilts left, and to pivot left, the front rotor tilts left and the rear rotor tilts right.
Coaxial rotors are a pair of rotors turning in opposite directions on the same masthead. The advantage of the coaxial rotor is that, in forward flight, the lift provided by the advancing halves of each rotor compensates for the retreating half of the other, eliminating one of the key effects of dissymmetry of lift: retreating blade stall. However, other design considerations plague coaxial rotors. There is an increased mechanical complexity of the rotor system because it requires linkages and swashplates for two rotor systems. Add that each rotor system needs to be turned in opposite directions means that the mast itself is more complex, and provisions for making pitch changes to the upper rotor system must pass through the lower rotor system.
Intermeshing rotors on a helicopter are a set of two rotors turning in opposite directions, with each rotor mast mounted on the helicopter with a slight angle to the other so that the blades intermesh without colliding. This configuration is sometimes referred to as a synchropter. Intermeshing rotors have high stability and powerful lifting capability. The arrangement was successfully used in Nazi Germany for a small anti-submarine warfare helicopter, the Flettner Fl 282 Kolibri. During the Cold War, the American company, Kaman Aircraft produced the HH-43 Huskie for the USAF firefighting and rescue missions. The latest Kaman model, the Kaman K-MAX, is a dedicated sky crane design.
Transverse rotors are mounted on the end of wings or outriggers, perpendicular to the body of the aircraft. Similar to tandem rotors and intermeshing rotors, the transverse rotor also uses differential collective pitch. But like the intermeshing rotors, the transverse rotors use the concept for changes in the roll attitude of the rotorcraft. This configuration is found on two of the first viable helicopters, the Focke-Wulf Fw 61 and the Focke-Achgelis Fa 223, as well as the world's largest helicopter ever built, the Mil Mi-12. It is also the configuration found on tiltrotors, such as Bell's XV-15 and the newer V-22 Osprey.
The blades of a helicopter are long, narrow airfoils with a high aspect ratio, a shape which minimises drag from tip vortices (see the wings of a glider for comparison). They generally contain a degree of washout to reduce the lift generated at the tips, where the airflow is fastest and vortex generation would be a significant problem. Rotor blades are made out of various materials, including aluminium, composite structure and steel / Titanium erosion shields along the leading edge.
Helicopters with semi-rigid rotors, for example the two-bladed design seen on Robinson and some other light helicopters, must not be subjected to a low-g condition. Otherwise their rotors may move beyond the normal limits in a condition known as mast bumping which can cause the rotor droop stops to shear the mast and hence detach the whole system from the aircraft. In some conditons main rotor can produce so much lift that it detaches itself from the shaft and flies in the air.