Helicopter rotor
From Wikipedia, the free encyclopedia
A rotor is the rotating part of a helicopter which generates lift, either vertically in the case of a main rotor, or horizontally in the case of a tail rotor.
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[edit] History and development
Before the development of powered helicopters in the mid 20th century, autogiro 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.
Some modern military helicopters employ a rigid rotor design, in which flexible materials are used in place of hinges.
[edit] Rotor head design
The rotor head is a robust hub with attachment points for the blades and mechanical linkages designed to control the pitch of the blades.
[edit] Parts and functions
The simple rotor of a Robinson R22 showing (from the top):
- The following are driven by the link rods from the rotating part of the swashplate.
- Pitch hinges, allowing the blades to 'twist', ie change pitch or roll.
- Teeter hinge, allowing one blade to rise while the other falls. Usually rise and fall is due to pitch or roll. There may be harmonics, it allows pitch and roll of the rotor to be independent of the fuselage, it disables negative G flights.
- Scissor link and counterweight, carries the main shaft rotation down to the upper swashplate
- Rubber covers protect moving and stationary shafts
- Swashplates, transmitting cyclic and collective pitch to the blades (the top one rotates)
- Three non-rotating control rods transmit pitch information to the lower swashplate
- Main mast leading down to main gearbox
[edit] Swash plate
Main article: Swashplate (helicopter).
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.
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.
[edit] Fully articulated rotors
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 asymmetry 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.
[edit] Two bladed rotors
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.
Arthur 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. Other names are Hiller panels, Hiller-system, Bell-Hiller-system, and flybar. In fly by wire helicopters or RC-models a computer with gyroscopes and a venturi sensor can replace the stabilizer. This flybarless design has the advantage of easy reconfiguration.
The two blades can flap as a unit and therefore do not require lag-lead hinges (the whole rotor slows down an 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.
[edit] Tail Rotors
Tail rotors are generally simpler than main rotors since they require only thrust control. A simplified swash plate is used to control collective pitch. Two bladed tail rotors include a teetering hinge to compensate for aysymmetry of lift.
[edit] Blade design
The blades of a helicopter are long, narrow aerofoil cross-sections 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.
[edit] Limitations and hazards
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.
[edit] Ring-protected rotors
There are serious dangers of rotor contact with a fixed object, the fuselage or people. It has been considered whether the rotors could have a ring fixed around the blades to protect them from contact-damage. Many radio-controlled model helicopters have this feature.
It has never been implemented in a full-size helicopter, even though blade strike accidents often have tragic consequences:
- A ring would add a significant amount of mass, and hence rotor inertia, where it is not wanted - at the blade tips
- A ring would prevent blades from flapping up and down as they face towards or away from the translational airflow
- A ring would prevent blades from leading and lagging, which is necessary on systems with more than two blades
- to provide a realistic degree of blade strike protection, such a ring would have to be massively strong and contribute a weight penalty
Such a device would seriously impair the helicopter's ability to achieve powered flight and lead to very poor flight characteristics and fuel economy. The practical approach, which is used for helicopters flown by the emergency services and private individuals, is to regulate the locations where helicopters are allowed to take off and land, always making the avoidance of human and property damage the highest priority.
