Main Rotor Systems

Question 1

Fully articulated

Answer 1

1. Requires 3 or more rotor blades
2. Flap, feather, lead and lag independently (due to hinges
   Advantages:
   - maneuverable (no mast bumping)
   - increased pay load (more blades = more lift)
   - less vibrations, more comfortable
   Disadvantages:
   - higher costs (initial + maintenance)
   - susceptible to ground resonance (HAZARD)
   - poor storage (multiple blades)

Question 2

Semi-rigid

Answer 2

1. 2 blades mounted rigidly to the rotor head
2. Flap as a unit (one blade moves up, opposite moves down)
3. Feather independently
4. Typically underslung to absorb lead/lag forces
   - the hub is attached to the rotor mast by an elevated trunnion hinge.
   - the entire rotor assembly is able to “teeter” about this pivot point so all the blades are able to flap as a single unit.
   - underslung to compensate for Coriolis effect. The center of pressure remains the same distance when coning.
   ADVANTAGES:
   - Less weight
   - Less expensive
   - Easy storage
   DISADVANTAGES:
   - More vibrations (rougher ride)
   - less maneuverable
   - Susceptible to low G mast bumping

Question 3

Rigid

Answer 3

1. Moves one way: mechanically simple (not many moving parts) but structurally complex
2. Only feathers: changes pitch in the blade as a unit
3. Can not flap or lead/lag
   - forces absorbed by flexing of the blades
4. Blades, rotor head (hub), and mast are rigidly mounted together
   ADVANTAGES:
   - very responsive and maneuverable
   - not susceptible to mast bumping or ground resonance
   - simple maintenance
   DISADVANTAGES:
   - high initial and high blade cost
   - limited slope capability
   - shorter blade life due to flexing
   - bumpy ride

Question 4

Tandem

Answer 4

1. Uses two counter rotating rotors
2. Three main designs
   - fore and aft (chinook)
   - side by side intermesh (k-max)
   - stacked common mast (k-A-226)
   ADVANTAGES:
   - more power available
   - does not need tail rotor
   DISADVANTAGES:
   - more complex
   - higher cost

Question 5

Anti-Torque

Answer 5

1. Helicopters with a single main rotor require anti-torque
2. Counteracts the torque produced by the main rotor
   - Newtons 3rd law: for every action there is an Equal and opposite reaction
3. Must be variable pitch for directional control
   - left pedal: nose moves left
   - right pedal: nose moves right

Question 6

Blade movements

Answer 6

Feathering- pitch change of the rotor blades
Flapping- up and down movement of the blade
Lead/lag- fore and aft movement of the blade along the plane of rotation