Main Rotor Heads (MRH) CASA 12.2.2

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Questions and Answers

What is the primary function of the rotor head in a helicopter?

To reduce weight and increase efficiency

To control the helicopter’s speed

To transfer the pilot’s flight control movement into changes of blade pitch (correct)

To stabilize the helicopter during flight

Which rotor system only allows for pitch change between blades and mast?

Dynamic rotor systems

Variable pitch rotor systems

Rigid rotor systems (correct)

Fully articulated rotor systems

Which of the following rotor systems incorporates hinge mechanisms for individual blade movement?

Fully articulated rotor systems (correct)

Single fixed rotor systems

Semi-rigid rotor systems

Rigid rotor systems

What material is often used in rigid rotor systems for the blades due to its flexibility?

Reinforced fiberglass Signup and view all the answers

Which rotor system type is most widely used in modern helicopters?

Semi-rigid and fully articulated rotor systems Signup and view all the answers

What aspect of rotor head design must accommodate stresses from flight?

Vibration, twisting, and flexing Signup and view all the answers

Which component may be part of a rigid rotor hub to allow pitch change?

Feathering bearing Signup and view all the answers

What kind of rotor system allows for a flapping motion as well as lead-lag properties through the flexibility of its blades?

Rigid rotor system Signup and view all the answers

Why is the support of the blades critical during removal?

Because of the blade height and potential damage Signup and view all the answers

What should be done with blades after they are removed from the helicopter?

Store them on blade racks to prevent damage Signup and view all the answers

What is necessary before reassembling rotor head components?

Cleaning parts and checking their dimensions Signup and view all the answers

What should be done with the rotor heads after disassembly?

Keep them on a suitable stand to avoid damage Signup and view all the answers

Which of the following is NOT part of the maintenance procedures after reassembling the rotor head?

Lubrication of the housing Signup and view all the answers

What is the primary purpose of elastomeric bearings?

To absorb oscillating loads without full rotation Signup and view all the answers

Which type of elastomeric bearing absorbs heavy torsional loads?

Spherical elastomeric bearing Signup and view all the answers

What is included in the construction of the semi-rigid rotor head of the Bell 206?

Lubricated bearings Signup and view all the answers

In the Sikorsky S-58 rotor head, what mechanism controls the lead-lag of the rotor blades?

Hydraulic dampers Signup and view all the answers

What is one major advantage of bearing-less hubs in rigid rotors?

Reduced weight and cost Signup and view all the answers

What feature simplifies the rotor head construction in the Sikorsky S-76?

Use of elastomeric bearings Signup and view all the answers

What is the function of the flap restraint in the S-58 MRH?

To limit flapping action at high RPM Signup and view all the answers

What is a disadvantage of rigid rotor systems?

High control loadings Signup and view all the answers

What does the bifilar vibration absorber do in the S-76 rotor head?

Absorbs vibrations and reduces stress Signup and view all the answers

How does a control gyro assist in rigid rotor systems?

By stabilizing and controlling the rotor head Signup and view all the answers

Which assembly in the Bell 206 MRH is responsible for pitch change?

Grip assembly Signup and view all the answers

What movement does a semi-rigid rotor allow in addition to pitch change?

Flapping as a unit Signup and view all the answers

What does the tension-torsion strap in the Bell 206 MRH facilitate?

Managing the pitch change mechanism Signup and view all the answers

What is a characteristic feature of the underslung rotor system?

Small distance between blades and rotation axis Signup and view all the answers

What mechanism limits the flapping and drooping action in the S-58 rotor?

Flap and droop stops Signup and view all the answers

What does the flapping hinge in a fully articulated rotor primarily help to do?

Relieve bending forces at the blade root Signup and view all the answers

What is NOT a characteristic of conical elastomeric bearings?

Allow complete rotary motion Signup and view all the answers

How does the semi-rigid rotor construction help reduce complexity?

Through elimination of flapping and drag hinges Signup and view all the answers

What component is included in the main rotor head design of the S-76?

Swashplate Signup and view all the answers

What is the role of dampers in rotor systems?

To control the rate of dampening Signup and view all the answers

What is a significant consideration regarding fully articulated rotors?

They can result in geometric imbalance. Signup and view all the answers

What is the main principle behind elastomeric dampers?

Hysteresis Signup and view all the answers

Why are lead-lag forces absorbed in semi-rigid rotors?

By the blade construction itself Signup and view all the answers

What is one effect of the 'see-saw' principle in semi-rigid rotors?

Increased wind gust sensitivity Signup and view all the answers

What feature allows for simplified construction in semi-rigid rotor systems?

Elimination of flapping hinges Signup and view all the answers

What is the purpose of tracking in rotor head maintenance?

To adjust blade pushrods and/or tabs for matching characteristics Signup and view all the answers

Which factor can lead to an objectionable one-per-rev vibration in a helicopter?

Imbalanced forces due to manufacturing tolerances Signup and view all the answers

What must be done if the main rotor head exceeds its speed limit?

Remove and send components to a Maintenance, Overhaul and Repair (MRO) facility Signup and view all the answers

What is a likely consequence of a sudden stoppage of the main rotor?

Secondary damage in the root area of the blade Signup and view all the answers

Which of the following inspections is necessary after an overspeed or sudden stoppage occurs?

Complete disassembly of the rotor head Signup and view all the answers

What does proper lubrication of the rotor head help prevent?

Bearing damage caused by excessive leakage Signup and view all the answers

What should be inspected immediately after a sudden stoppage beyond the rotor blades?

Tail rotor drive assembly and mast bearings Signup and view all the answers

What is the first step in the removal of any rotor system?

Disconnect the flight controls from the rotor head Signup and view all the answers

What kind of environment necessitates more frequent lubrication for the rotor head?

Sandy environments Signup and view all the answers

What must be done with the mast nut after rotor head removal?

Discard it and fit a new nut Signup and view all the answers

What could excessive leakage from the rotor head lead to?

Bearing damage Signup and view all the answers

Which rotor head component may require replacement without inspection?

Certain rotor head parts in specific designs Signup and view all the answers

What is the outcome of maintaining lubrication intervals as specified by the manufacturer?

Extended lifetime of rotor head components Signup and view all the answers

What is required when a helicopter experiences an over-torque event?

Inspection and special inspections per maintenance manual Signup and view all the answers

What component connects the blade to the hub in the S-76 MRH assembly?

Elastomeric bearing Signup and view all the answers

What type of vibrations cannot be completely eliminated in helicopters?

None-correctable vibrations Signup and view all the answers

Vertical vibrations in helicopters are primarily caused by which factor?

Blade tracking discrepancies Signup and view all the answers

What is the primary indicator of vertical vibrations a pilot can observe?

Up and down movement of the hand Signup and view all the answers

Which of the following is a cause of lateral vibrations?

Faulty drag dampers Signup and view all the answers

What type of vibrations are associated with the number of blades in the rotor system?

Non-correctable vibrations Signup and view all the answers

What maintenance practice can help manage non-correctable vibrations?

Regular inspections and maintenance Signup and view all the answers

What adjustment is made to correct out-of-track vertical vibrations?

Adjusting pitch links and trim tabs Signup and view all the answers

What is a common cause of uneven mass distribution that leads to lateral vibrations?

Incorrect paint application Signup and view all the answers

How is the thrust load carried from the spindle in the S-76 MRH assembly?

Through another elastomeric bearing Signup and view all the answers

What characteristic defines correctable vibrations in helicopters?

They result from incorrect blade tracking or balance Signup and view all the answers

What type of damper is used for the dampening action of blades in the MRH system?

Hydraulic damper Signup and view all the answers

What problem can cause a malfunction in blade tracking?

Worn pitch link bearings Signup and view all the answers

What is one way to check the tracking of helicopter blades during flight?

Using strobe lights and blade markers Signup and view all the answers

Study Notes

Rotor Heads

Rotor heads transmit pilot flight control inputs into changes in blade pitch
This changes blade lift, controlling the main rotor disc attitude
Rotor heads must be designed to handle vibration, twisting, and flexing forces

Rotor Head Types

Rigid:
- Blades are fixed to the mast
- Only allows for pitch change
- Relies on blade flex for flapping and lead-lag motion
- Common in smaller helicopters
Semi-Rigid:
- Most popular rotor system
- Blades move as a unit (flapping)
- Can have additional movement about the chordwise axis to compensate for the Coriolis Effect
- Underslung configuration, where the rotor is mounted below the mast
Fully Articulated:
- Incorporates individual hinges for flapping and lead-lag
- Allows the rotor disc to tilt without tilting the mast
- Can be more complex than semi-rigid systems

Advantages of Rigid Rotor Systems

Simplicity
Fewer components
Improved reliability and maintainability
Reduced weight, cost, and drag
Composite materials offer long fatigue life

Disadvantages of Rigid Rotor Systems

High control loadings, which can be mitigated by using a control gyro
Higher control loadings and stability issues in earlier designs

Advantages of Semi-Rigid Rotor Systems

Simpler construction due to fewer hinges
Blades are fixed to the hub and do not rely on centrifugal force for rigidity
See-saw flapping motion

Disadvantages of Semi-Rigid Rotor Systems

More susceptible to wind gusts
Higher bending forces on the blade roots
Requires underslung rotor configuration

Advantages of Fully Articulated Rotor Systems

Allows for individual blade flapping, reducing gust sensitivity
Relieves bending forces at the blade roots
Smoother operation

Disadvantages of Fully Articulated Rotor Systems

More complex design
Heavier due to more components
Requires additional drag hinges

Dampers

Stabilizer dampers restrict the travel of the stabilizer in both directions
Multi-disc dampers utilize friction to control damping, submerged in hydraulic fluid for cooling.
Hydraulic dampers use fluid flow to control dampening, newer designs are sealed.
Elastomeric dampers use elastomeric materials to absorb and release energy

Elastomeric Bearings

Use layers of elastomeric materials sandwiched between metal laminates
Used for oscillating loads, rather than complete rotation
Types:
- Cylindrical: Absorbs radial loads and allows for radial oscillating
- Spherical: Absorbs torsional loads and allows for movement about three axes
- Conical: Absorbs radial and axial loads with movement in both directions

Bell 206 Semi-Rigid Rotor

Uses a yoke as the main structural member
Tension-torsion strap retains the blade to the yoke
Uses hollow blade bolts to adjust spanwise balance
Flap restraint on the trunnion assembly limits flapping at low RPM

Sikorsky S-58 Fully Articulated Rotor

Features an upper and lower plate assembly that support the lead-lag hinges
Uses dampers to control lead-lag rate
Sleeve spindle assembly provides feathering, pitch change is accommodated by a thrust bearing stack
Flap and droop stops limit flapping and drooping

Sikorsky S-76 Fully Articulated Rotor

Features a bifilar vibration absorber, a one-piece hub, elastomeric bearings, and a swashplate
The bifilar absorber is located at the top of the rotor, reducing vibration
Spindle assembly is attached to each hub arm
The blade has an integral spindle, elastomeric bearings, a pitch control horn, and anti-flap restraints

Helicopter Vibrations

Caused by out-of-balance forces, aerodynamic imbalances, and unbalanced components
Can be categorized as non-correctable or correctable

Non-Correctable Vibrations

Related to the number of blades in the rotor system (e.g., 5-blade rotor system will have a 5-per-rev vibration)
Caused by aerodynamic forces that excite natural resonance
Can be managed through maintenance but not eliminated

Correctable Vibrations

Caused by out-of-track blades, uneven mass distribution, and mechanical malfunctions
Vertical vibrations are caused by out-of-track blades
Lateral vibrations are caused by uneven mass distribution

Main Rotor Head Inspection and Adjustment

One-per-rev vibrations caused by mass or aerodynamic unbalance are corrected through tracking and balancing
Ideally, all forces from the rotor head components cancel each other out due to rotor symmetry

Rotor Head Unbalance

Rotor head unbalance can occur due to manufacturing tolerances or wear and tear on components.
This results in a one-per-rev vibration felt in the helicopter.
Tracking adjusts blade pushrods and/or tabs to match the flight characteristics of the blades.
Balancing involves adding weights to the rotor head.

Rotor Head Overspeed

Overspeeds above the specified limit require the removal of the main rotor head, blades, and swashplate.
These components should be sent to an MRO facility and tagged with details of the exceeded speed.
If engine overspeed also occurred, remove the engine drive shafts and send them to an MRO facility.
Troubleshoot the engine overspeed protection system for malfunctions.
Inspect the main gearbox chip detector/strainers and oil filters for contamination.

Sudden Stoppage

Sudden stoppage of the main rotor can occur after contact with an object.
Damage to the blades may be evident at the impact point, but secondary damage can also occur in the root area.
This can affect the rotor head, requiring a complete disassembly and overhaul inspection.
The extent of the inspection is determined by the severity of damage found.
Sudden stoppage of the tail rotor can lead to damage in the tail boom structure, hanger bearing mounts, and attachment points.

MRH Servicing

Rotor head servicing primarily involves lubrication using manufacturer-specified grease.
Frequent greasing is recommended in sandy environments to remove sand from greased areas.
Excessive leakage from wet head systems can lead to bearing damage.
Visual inspections of the head check for loose items and cracks.
Periodic inspections may include dye checks, partial disassembly, and bearing rotation.

Removal and Installation of MRH

Removal and installation procedures vary depending on rotor head design.
Some heads can be removed with blades installed, while others require blade removal first.
The first step is to disconnect flight controls from the rotor head.
The mast nut holding the head to the mast assembly requires a special wrench for installation and removal.
Removed rotors require suitable stands for support to prevent damage.
After blade removal, they should be stored on blade racks to prevent damage.

MRH Reassembly

Following inspection and cleaning, reassemble the rotor head using proper tools.
Reinstallation of blades requires the same precautions as disassembly.
A series of maintenance procedures follow reassembly, including:
- Blade alignment
- Static main rotor balance
- Vibration check
- Tracking
- Spanwise dynamic balance of the rotor
- Blade sweeping
- Electronic balancing
- Damper maintenance
- Counterweight adjustment
- Autorotation adjustments

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Description

Explore the intricacies of helicopter rotor heads and their types in this quiz. Learn about rigid, semi-rigid, and fully articulated rotor systems, their functions, and advantages. This quiz is essential for understanding how rotor heads transmit pilot inputs and control lift.