Rotor Systems and Vibrations

Question 1

What is the role of the swash plate in a helicopter?

Answer 1

The swash plate converts pilot control inputs into rotor blade pitch changes, essential for lift and directional control.

Question 2

What are the two main parts of the swash plate?

Answer 2

The stationary plate and the rotating plate.

Question 3

What does the stationary plate do in a helicopter’s swash plate system?

Answer 3

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.

Question 4

How does the rotating plate function in a helicopter’s rotor system?

Answer 4

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.

Question 5

What happens when the pilot pulls up on the collective lever?

Answer 5

The stationary swash plate moves upward, causing the rotating plate to adjust the pitch of all rotor blades, increasing lift.

Question 6

What is blade pitch adjustment also known as?

Answer 6

Feathering.

Question 7

What does the collective control do in helicopter flight?

Answer 7

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.

Question 8

What does the cyclic control do in helicopter flight?

Answer 8

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.

Question 9

What is the main function of feathering in helicopters?

Answer 9

Feathering adjusts the pitch angle of each rotor blade, which influences the lift / thrust and its direction produced by the rotor system.

Question 10

How does the collective control affect feathering?

Answer 10

Raising the collective uniformly increases the pitch of all blades, increasing lift.

Question 11

Does the advancing blade flap up or down? What happens due to this flapping?

Answer 11

The advancing blade flaps up, LOWERING the angle of attack.

Question 12

What is the coning effect in helicopter rotor systems?

Answer 12

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.

Question 13

What affect can flapping up have on the center of mass of a rotor blade?

Answer 13

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.

Question 14

What is the difference between coning and flapping?

Answer 14

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.

Question 15

What does flapping compensate for?

Answer 15

Dissymmetry of lift between the advancing and retreating side of the helicopter.

Question 16

18. What is the Coriolis effect in helicopter rotor dynamics?

Answer 16

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!

Question 17

What does the lead / lag hinge do?

Answer 17

Allows each blade to move independently of the others, compensating for the Coriolis effect.

Question 18

What are the three main types of rotor systems in helicopters?

Answer 18

Fully articulated rotor systems, semi-rigid rotor systems, and rigid rotor systems.

Question 19

Advantages of a fully articulated rotor system?

Answer 19

-less prone to mast bumping
-more maneuverable
-blades less expensive since they are not absorbing as much aerodynamic forces

Question 20

Disadvantages of a fully articulated rotor system?

Answer 20

-more complicated rotor head therefore more maintenance and greater cost
-possible threat to ground resonance
-large hangar space required (due to 3+ blades)

Question 21

Characteristics of a fully articulated rotor system?

Answer 21

-hinges for feathering, flapping, leading / lagging
-3+ rotor blades

Question 22

Characteristics of a rigid rotor system?

Answer 22

-no flapping or lead-lag hinges, just feathering
-mechanically simple, structurally complex
-operating loads absorbed in bending rather than through hinges
-3+ blades

Question 23

Advantages of a rigid rotor system?

Answer 23

-very responsive
-usually not susceptible to mast bumping
-reduced oscillation
-reduced weight and drag in rotor hub
-increased reliability
-

Question 24

Disadvantages of a rigid rotor system?

Answer 24

-rougher ride in turbulent / gusty air
-needs larger hangar space

Question 25

Characteristics of low frequency vibrations?

Answer 25

-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

Question 26

Characteristics of medium frequency vibrations?

Answer 26

-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

Question 27

Characteristics of high frequency vibrations?

Answer 27

-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

Question 28

In turbine-powered helicopters, what is a common cause of medium-frequency vibrations?

Answer 28

Tail rotor imbalances.

Question 28

What is the source of medium-frequency vibration in reciprocating helicopters?

Answer 28

Engine vibrations.

Question 29

What is the cause of high-frequency vibrations in helicopters?

Answer 29

Tail rotor vibrations

Question 30

What type of helicopters are susceptible to ground resonance?

Answer 30

Helicopters with articulating rotors (typically with three or more main rotor blades) are susceptible to ground resonance.

Question 31

What causes ground resonance in helicopters?

Answer 31

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.

Question 32

What is the corrective action for ground resonance at low rotor RPM?

Answer 32

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.

Question 33

How should a pilot respond to ground resonance at normal operating RPM?

Answer 33

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.

Question 34

Why are rigid or semi-rigid rotor systems less prone to ground resonance?

Answer 34

Rigid or semi-rigid rotor systems lack a drag hinge, which prevents the blade movement that can lead to ground resonance.

Question 35

How do dampening systems help with resonance in helicopters?

Answer 35

They minimize vibrations.

Question 36

At what rpm is the R22 subject to sympathetic resonance?

Answer 36

60-70%– avoid this range

Question 37

What causes sympathetic resonance?

Answer 37

-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.