Stabilizer Bar

Stabilizer Bar

The stabilizer bar is a weighted, rotating unit mounted above and across the main rotor suspended in pivotal bearings of supports bolted to the rotor hub trunnion. The supports also contain the stops that limit bar travel. Each side of the bar frame is connected through a control tube to a damper on the mast. Stabilizer bar mixing levers are connected into main rotor control linkage by control tubes from the scissor levers and by pitch change links connected to the pitch horns on the blade grips.

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Stabilizer Bar Damper Assembly:

Two hydraulic (double acting, variable-rate, temperature-compensating, viscous) dampers are mounted on a pair of adapters which are attached on mast splines below the main rotor. The double acting lever is attached by a control tube to the stabilizer bar frame. As the bar is displaced during operation the cam displaces the pin. Movement of the pin and damper timing (5 ± 1 second ) determines stiffness (viscous) of action, therefore, the degree of stability the bar affords the helicopter. Damper rate and timing cannot be adjusted in the field but is factory preset for desired action. A window is provided through which fluid level and damper timing may be checked.

 

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Stabilizer Bar Operation:

The stabilizer bar is connected into the flight system in such a manner that the inherent inertia and gyroscopic action are induced into the rotor system to provide a measure of stability for all flight conditions. If, while hovering, the helicopter’s attitude is disturbed, the bar tends to remain in its present plane. The relative movement between the bar and the mast will cause the blades to feather and return the rotor to its original plane of rotation. When the rotor disc is displaced by cyclic action, initially a portion of the cyclic movement is removed, but because of pendulum action of the helicopter, the mast is displaced. Now, because of the restraining and damping action of the dampers, the bar possesses a mast-following characteristic. The time it requires the bar to follow the mast and return all the cyclic control to the rotor is determined by damper timing (5 ± 1 second). A compromise between gyroscopic action and damper timing is maintained during flight which allows the bar to provide a measure of stability, yet still affords the pilot complete control of the helicopter.

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Main Rotor & Tail Rotor Blade Terminology

M/R & T/R Blade Terminology

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Root – portion of blade attached to the hub

Span – distance from root to tip measured along center line

Tip – End of blade farthest from the hub

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Chord –  in a cross sectional view it is the imaginary line from the leading edge to the trailing edge

Trailing edge – aft edge of airfoil

Leading edge – forward edge of airfoil in terms of direction of rotation

Helicopter Flight Controls

The pilot uses the helicopter flight controls to achieve and maintain control of the Aircraft. Changes to the controls transmit mechanically to the rotor, producing aerodynamic effects on the rotor blades that make the helicopter move in a deliberate way.

There are 3 main controls used to fly a Helicopter. They are the:

The Cyclic Stick

  • The cyclic stick is used to control forward, backward and side-to-side movement.
  • It changes the pitch of the blade cyclically.

cycllic

The Collective or Collective Lever

  • The collective pitch control, or collective lever, is normally located on the left side of the pilot’s seat with an adjustable friction control to prevent inadvertent movement.
  • The collective lever is used to lower and lift the helicopter by changing the angle (pitch) of the main rotor blades collectively.
  • Therefore, if a collective input is made, all the blades change equally, and the result is the helicopter increases or decreases its total lift derived from the rotor.

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The Pedals

  • The pedals provide directional control for the helicopter and compensate for the torque cause by the main rotor that tries to rotate the fuselage in the opposite direction.

Helicopter_controls_diagram.svg

The Throttle

  • The throttle usually is mounted on the collective in the form of a twist grip just like on a motorcycle.
  • Helicopter engines maintain constant RPM of the rotor system. You do not increase the throttle to go faster. Changes in pitch of the rotor blades accomplish this. When you increase the pitch of the blades you increase the lift of the blade. Any time you increase lift you have a byproduct called drag. Because of this more power is required from the engine to maintain the rotor RPM.
  • In older Helicopters this was done manually by the pilot using the throttle.
  • Most  modern Helicopters have a automated system that does this for the pilot. The throttle is used for starting and shutting down the Helicopter.

In the photo below you can see the throttles mounted on the collective. Notice there are two. Why do you think that may be?

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25-06-2015 9-30-02 PM