HELICOPTER Multiple

Gyroplane

– Has no power to the main rotor except, in some cases, to start it spinning. It is turned in flight by an aerodynamic force called autorotational force.

Single-Rotor Helicopter

– Has a single main rotor that provides lift and thrust and a small vertical tail rotor that compensates for main rotor torque. The transmission drives the anti-torque rotor, and the pilot controls its pitch with foot pedals.

Dual-Rotor Helicopter

– Uses two rotors turning in opposite directions so torque cancels out. Rotors can be coaxial, tandem, lateral, or intermeshing.

Fully Articulated System

– Each rotor blade is attached to the hub through a series of hinges allowing independent blade movement.

Flapping Hinge (Horizontal Hinge)

– Allows the blade to move up and down (flapping) to compensate for dissymmetry of lift.

Lead-Lag / Drag Hinge (Vertical Hinge)

– Allows the blade to move back and forth. Dampers prevent excess movement and compensate for Coriolis effect.

Feathering (Pitch Change Axis)

– Blade rotates around its span-wise axis to change pitch angle which controls thrust and rotor disc direction.

Semi-Rigid Rotor System

– Two blades rigidly mounted to the hub; uses a teetering hinge allowing blades to flap together as one unit. Lead-lag forces are absorbed by blade bending.

Rigid Rotor System

– Blades, hub, and mast are rigid with no hinges; blades cannot flap or drag; blade bending absorbs forces.

Flexture

– Flexible hub (composite) allowing blade bending without hinges or bearings.

Elastomeric Bearings

– Rubber-type bearings with limited movement; no lubrication needed; absorb vibration and reduce fatigue.

Gravity

– Weight causes blades to droop when not turning. Droop stops prevent excessive drooping.

Centrifugal Force

– Pulls blades outward providing rigidity and strength to support helicopter weight; determines maximum rotor RPM due to structural limits.

Lift

– Low pressure on top of the airfoil creates downwash and pushes up the rotor, lifting the helicopter.

Coning

– Combined centrifugal force and lift during vertical takeoff form blades into a conical path.

Hovering Flight

– Helicopter stays in one position; lift and thrust act straight up equal to weight and drag.

Torque

– Main rotor turning counterclockwise causes fuselage to turn clockwise; amount of torque depends on engine power.

Translating Tendency / Drift

– Helicopter drifts in direction of tail rotor thrust; corrected by mast tilt or control rigging.

Density Altitude

– Less dense air requires more volume for the engine to release required energy; affects performance.

Ground Effect

– Hovering near the ground (less than one rotor diameter) increases lift and reduces induced drag due to restricted airflow.

Vertical Ascent

– Increase pitch of all blades with collective + add power; adjust tail rotor to prevent turning.

Vertical Descent

– Decrease pitch and power; adjust tail rotor.

Dissymmetry of Lift

– Advancing blade has faster airflow and more lift; retreating blade slower airflow and less lift.

Retreating Blade Stall

– High forward speed causes stall on retreating blade (high angle + slow airflow); causes nose pitch up, vibration, left roll; avoid by keeping speed below Vne.

Translational Lift

– Increased rotor efficiency when airspeed reaches about 16–24 knots as helicopter moves out of its own downwash.

Autorotation

– Rotor turns by relative wind instead of engine power; used for safe landing in engine failure; made possible by freewheeling unit.

Forward Hover Thrust Relationship

– If thrust is greater than weight, helicopter gains altitude. If thrust is less than weight, helicopter descends.

Dual-Rotor Examples

– Focke-Achgelis FW-61 (lateral rotors), Kaman helicopter (intermeshing rotors).

Direct Rotor Head Tilt

– Controlling a rotorcraft is done by tilting the lift produced by the rotor. Tilt forward = forward flight; backward = backward flight; sideways tilt = sideways flight. Requires light control forces.

Swash Plate Control System

– Transmits control inputs from cockpit to main rotor blades.

Stationary Swash Plate

– Mounted to the main rotor mast and connected to cyclic and collective pushrods. Cannot rotate but can tilt in all directions and move vertically.

Rotating Swash Plate

– Mounted to the stationary swash plate by a bearing; rotates with the mast. Tilts and moves up and down with the stationary plate. Connected to rotor grips by pitch links.

Collective Pitch Control

– Vertical movement of swash plate assembly changes pitch angle of all blades equally. Raising collective = increases pitch + increases lift. Lowering collective = decreases pitch + decreases lift.

Throttle Control

– Usually a twist grip on the collective lever. Regulates engine RPM when the collective is moved. If no correlator, must be moved manually to maintain RPM.

Cyclic Pitch Control

– Tilts the main rotor disc by tilting the swash plate to move helicopter in that direction. Pitch angle changes during rotation: increases and decreases 90° before direction of tilt due to gyroscopic procession.

Torque Compensation

– When rotor spins, fuselage wants to spin opposite direction. Torque must be counteracted for control about vertical axis. Multiple methods exist, but the universal system is the tail rotor (anti-torque rotor).

Tail Rotor Operation

– Driven by transmission; operational even in engine failure. Pedals control pitch; tail rotor thrust pushes nose left.

Fenestron (Fan-in-Tail)

– Shrouded anti-torque rotor system using a series of rotating blades inside the vertical tail.

NOTAR® System

– No tail rotor. Uses low pressure air blown into tail boom; air exits through slots and a rotating nozzle, generating anti-torque with Coanda effect and main rotor downwash.

Bell Stabilizer Bar System

– A gyro-like bar with weights mounted on a pivot. Maintains rigidity in space; if helicopter tilts, bar stays level, causing pitch changes to correct and right the aircraft.

Rotor Blade Tracking

– Ensures each blade follows same path as the blade ahead. Old methods: marking stick and flag with crayons (ground-only). Modern method: strobe or infrared light to check blade reflector pattern at operating RPM. In-track blades show aligned images; out-of-track blades show staggered images with direction and amount of correction identifiable.