Rotor Systems and Vibrations Flashcards
What is the role of the swash plate in a helicopter?
The swash plate converts pilot control inputs into rotor blade pitch changes, essential for lift and directional control.
What are the two main parts of the swash plate?
The stationary plate and the rotating plate.
What does the stationary plate do in a helicopter’s swash plate system?
Control rods from the collective and cyclic attach to the non-rotating lower swash plate and cause it to move. The collective raises and lowers the plate, while the cyclic causes the plate to tilt in the direction the cyclic was pushed.
How does the rotating plate function in a helicopter’s rotor system?
The upper rotating plate is connected to the rotor mast and rotates with the rotor blades. Pitch change rods from each rotor blade attach to the edge of the upper swash plate. As they travel around a 360 degree rotation, the pitch rods are pushed and pulled, decreasing or increasing the blade pitch over a rotation.
What happens when the pilot pulls up on the collective lever?
The stationary swash plate moves upward, causing the rotating plate to adjust the pitch of all rotor blades, increasing lift.
What is blade pitch adjustment also known as?
Feathering.
What does the collective control do in helicopter flight?
The collective changes the pitch angle of all the main rotor blades collectively and independently of their positions– so all
the blades change equally, increasing or decreasing total lift or thrust.
What does the cyclic control do in helicopter flight?
Can vary the pitch of the rotor blades throughout each revolution / cycle of the main
rotor system to develop unequal lift (thrust). This tilts the rotor disk in a particular direction, and then the helicopter moves in that
direction.
What is the main function of feathering in helicopters?
Feathering adjusts the pitch angle of each rotor blade, which influences the lift / thrust and its direction produced by the rotor system.
How does the collective control affect feathering?
Raising the collective uniformly increases the pitch of all blades, increasing lift.
Does the advancing blade flap up or down? What happens due to this flapping?
The advancing blade flaps up, LOWERING the angle of attack.
What is the coning effect in helicopter rotor systems?
The coning effect occurs when centrifugal force and the force of lift (as pitch is applied) cause blades to bend upwards, forming a coned shape.
What affect can flapping up have on the center of mass of a rotor blade?
Due to coriolis effect, flapping up can cause the center of mass to move inwards and the rpm to increase.
The semi-rigid, underslung rotor system neutralizes this.
What is the difference between coning and flapping?
Coning results from the combined forces of lift and centrifugal force when pitch is applied to the blades.
Flapping is how the blades compensate for dissymmetry of lift between the advancing and retreating sides of the aircraft.
What does flapping compensate for?
Dissymmetry of lift between the advancing and retreating side of the helicopter.
- What is the Coriolis effect in helicopter rotor dynamics?
As the blades flap up to compensate for dissymmetry of lift, the blade’s center of mass moves inward and therefore increases the rpm.
Think of a figure skater spinning- arms out, turning slower / arms in, turning faster!
What does the lead / lag hinge do?
Allows each blade to move independently of the others, compensating for the Coriolis effect.
What are the three main types of rotor systems in helicopters?
Fully articulated rotor systems, semi-rigid rotor systems, and rigid rotor systems.
Advantages of a fully articulated rotor system?
-less prone to mast bumping
-more maneuverable
-blades less expensive since they are not absorbing as much aerodynamic forces
Disadvantages of a fully articulated rotor system?
-more complicated rotor head therefore more maintenance and greater cost
-possible threat to ground resonance
-large hangar space required (due to 3+ blades)
Characteristics of a fully articulated rotor system?
-hinges for feathering, flapping, leading / lagging
-3+ rotor blades
Characteristics of a rigid rotor system?
-no flapping or lead-lag hinges, just feathering
-mechanically simple, structurally complex
-operating loads absorbed in bending rather than through hinges
-3+ blades
Advantages of a rigid rotor system?
-very responsive
-usually not susceptible to mast bumping
-reduced oscillation
-reduced weight and drag in rotor hub
-increased reliability
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Disadvantages of a rigid rotor system?
-rougher ride in turbulent / gusty air
-needs larger hangar space
Characteristics of low frequency vibrations?
-100–500 cycles per minute
-Originates from the main rotor, felt in controls or airframe with distinct movement direction (vertical, lateral, or horizontal)
-Possible Causes: Out-of-balance main rotor blades, worn bearings, dampers out of adjustment, or damaged parts
Characteristics of medium frequency vibrations?
-1,000–2,000 cycles per minute
-Felt throughout the airframe
-Possible causes: often due to components like engine or transmission cooling fans
-Effects: Can lead to increased pilot fatigue over time
Characteristics of high frequency vibrations?
-Above 2,100+ cycles per minute
-Felt in tail rotor pedals
-In piston engines- can damage components
-In turbine engines- can mask issues until failure
In turbine-powered helicopters, what is a common cause of medium-frequency vibrations?
Tail rotor imbalances.
What is the source of medium-frequency vibration in reciprocating helicopters?
Engine vibrations.
What is the cause of high-frequency vibrations in helicopters?
Tail rotor vibrations
What type of helicopters are susceptible to ground resonance?
Helicopters with articulating rotors (typically with three or more main rotor blades) are susceptible to ground resonance.
What causes ground resonance in helicopters?
Ground resonance is caused by an out-of-balance rotor disk vibrating at the natural frequency of the helicopter’s airframe, which is intensified by the engine power, leading to destructive vibrations.
What is the corrective action for ground resonance at low rotor RPM?
At low rotor RPM, the corrective action is to immediately close the throttle and fully lower the collective to reduce blade pitch and stop the vibrations.
How should a pilot respond to ground resonance at normal operating RPM?
At normal operating RPM, the pilot should lift the helicopter off the ground to allow the blades to rephase automatically before making a controlled touchdown.
Why are rigid or semi-rigid rotor systems less prone to ground resonance?
Rigid or semi-rigid rotor systems lack a drag hinge, which prevents the blade movement that can lead to ground resonance.
How do dampening systems help with resonance in helicopters?
They minimize vibrations.
At what rpm is the R22 subject to sympathetic resonance?
60-70%– avoid this range
What causes sympathetic resonance?
-Main rotor’s frequencies interact with frequencies of the tail rotor creating a state of harmonic oscillation where they can become additive.
-Results can cause the drive shaft in the tail boom to start vibrating and oscillating violently and can cause damage.
