Cyclical plateau (helicopter)

The swashplate is the essential element for flying a helicopter . It allows to transmit to the blades of the main rotor the pilot commands (requests) given by the pilot by means of the control levers (collective and cyclic) or by the autopilot which acts directly at the wheelhouse and the jacks or servocontrols .

Principle

The difficulty of this transmission comes from the fact that the movement must pass from a lower system “immobile” (the helicopter cell) to a higher rotating system (the main rotor ).

To do this, the two systems are in contact with each other only at the level of the cyclic trays, one of the trays is integral with the cell and the other of the so-called “dynamic” elements.

The “plateau” assembly is composed of two superimposed half-plates sliding and swiveling around the axis of the main rotor mast , which is rotated by the engine (s) through the main gearbox , and ends with a hub on which are fixed the blades.

On the upper half plate (mobile) are fixed pitch rods each of which is connected to a blade to vary its angle of incidence (see also propeller ) according to the cyclic commands, and mobile compasses (they cause the plate in rotation as they are attached to the rotor hub). On the lower half plate are fixed servocontrols and a “fixed” compass (preventing rotation around the rotor axis). This plate is articulated on the rotor axis by means of a bearing ball allowing the swiveling and the displacement of the plates with respect to the axis rotor

A vertical displacement of the set trays will change the angle of incidence of all the blades at the same time and the same value. This movement is controlled by the lever called “not collective” actuated by the left hand of the pilot and is intended to raise or lower the device.

If the plate is tilted to one side, the angle of incidence of the blades on the side to be raised is increased and, conversely, that of the blades on the side which is to be lowered is reduced. As the angle of incidence of one blade increases and that of the opposite blade decreases, the lift increases on one side and decreases on the other. This has the effect of tilting the plane described by the rotation of the blades (called the “rotor disk”) and to switch the component of the overall lift force which then tends to “pull” the device. This means you can steer a helicopter like with a ” joystick” “(Pulling towards you to climb, pushing down, etc.) with this characteristic that can be printed on the helicopter a translational movement in any direction. However, the speed will be very low in the other directions than in the forward direction for many reasons: interaction of the main rotor blast with that of the anti-torque rotor , limited need for maneuvers in reduced space (see hovering ), aerodynamics of the cell (outer shells), visibility, etc.

These movements are generated using the so-called “cyclic” control lever that the pilot operates with his right hand.

Because of the gyroscopic effect, the blade must have the required bearing 90 ° before the point of application of the desired reaction (max incidence in the left azimuth so that the reaction is done on the front azimuth)

The cyclic and collective flight controls are independent of each other but they must be able to act together on the wing. For this, the links of the chains of the flight controls are joined at the level of the combinator which makes it possible to superimpose the cyclic and collective corrections, in order to attack the three main servo-controls. There are always three servocontrols to orient the swashplate: indeed, it must be considered that to orient a plane, it takes three points of support (static system in mechanics). Note: the servocontrols can be placed at 120 ° from each other (in the case of heavy helicos, in order to distribute the mechanical stresses applied on the swashplate) or 90 ° in the case of light helicopters. The 120 ° placement of the servo controls requires a special system on the rods of the cyclic flight controls, the phase shifter. To summarize, it allows to distribute the controls side and longitudinal three output rods corresponding to the attack rods of the main servocontrols.

History

The principle of the swashplate is invented by the Russian Boris Yuriev in 1911 1 . Later, Pescara helicopters with two coaxial rotors (1919-1931) will have a precursor system of cyclic trays, controlled by a joystick with a steering wheel 2 .

The , the French engineer Étienne Œhmichen files a cyclical plateau patent in France and thein the United States 3 .

The swashplate as it is used today was developed by the German company Focke-Achgelis during the Second World War on the Focke-Achgelis Fa 223 Drachen .

Notes and references

  1. ↑ ( in ) J. Gordon Leishman, Principles of Helicopter Aerodynamics , Cambridge University Press, 2002  [ archive ]
  2. ↑ Pateras-Pescara Patent FR 553 304 of 12 April 1921
  3. ↑ ( in ) E. Œhmichen: Lifting Device  [ archive ] – Google Patents

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