Aerodynamics 2: Helicopter Flight Dynamics A2QA #2

Forces on a Helicopter in Hover

• Lift acts upward, generated by the rotor blades.
• Weight acts downward, due to gravity.
• Thrust has forward and rearward components depending on the helicopter's pitch and yaw.
• Drag opposes the forward motion of the blades, directing rearward.
• Centrifugal force acts outward from the center of rotation of the blades.
• Tail rotor thrust counteracts the torque produced by the main rotor, directed sideways.

Forces on a Helicopter in Climb/Ascent

• Lift acts upward and is greater than weight to achieve climb.
• Weight acts downward, opposing the climb.
• Thrust acts forward, providing horizontal motion.
• Drag acts rearward, opposing motion.

Forces on a Helicopter in Descent

• Lift acts upward, but is less than weight, creating a downward motion.
• Weight acts downward, causing the descent.
• Thrust is less than in level flight or zero, directed forward or neutral.
• Drag acts rearward, opposing forward motion.

Forces on a Helicopter in Forward Flight

• Lift acts upward, balancing weight.
• Weight acts downward, opposing lift.
• Thrust acts forward, provided by the main rotor.
• Drag acts rearward, opposing forward motion.
• Centrifugal force acts outward from the center of the rotor's rotation.
• Tail rotor thrust counteracts torque produced by the main rotor, directed sideways.

Blade Terminology

• Angle of Attack (AoA) is the angle between the relative wind vector and the chord line of the blade.
• The tip path when blades are straight out from the mast, generating no lift, is called the no-lift plane or trajectory.
• Pitch angle or blade pitch is the number of degrees of rotation of the airfoil between the plane of rotation and the chord line of the blade.
• The feathering axis is the axis on which the blade rotates to change the angle of the chord line of the blade.
• Blades moving through the same point in space as they pass through the same point in 360 degrees of rotation are said to be coning.

Centrifugal Forces

• Centrifugal force acting on the main rotor blade tends to send it straight out from the center of rotation as it turns.
• The upward flexing of the blades caused by a combination of lift and centrifugal forces is coning.

Hazards

• Rotor wash, or the downwash airflow from the main rotor, is a hazard to equipment and personnel during start-up and shutdown of the main rotor system.

Main Rotor Disk Tilt

• To tilt the main rotor disk to the left (counter-clockwise rotation, viewed from above), the highest blade pitch angle should be at the 3 o'clock position (90 degrees from the front).
• To tilt the main rotor disk forward on a rotor system rotating counterclockwise, the highest blade pitch angle should be at the 12 o'clock position.
• To tilt the main rotor disk to the back, the highest blade pitch angle should be at the 6 o'clock position.
• To tilt the main rotor disk to the right, the highest blade pitch angle should be at the 9 o'clock position.

Lift

• Due to flapping in forward flight caused by dissymmetry of lift, the angle of attack of the blade is greatest at the advancing blade (moving forward).
• A key factor limiting the maximum forward speed of the helicopter is retreating blade stall or aerodynamic stall of the retreating blade.
• Ground effect is a flight condition when the helicopter is in a hover less than one-half the diameter of the rotor system from the ground, generating increased lift.
• Translational lift is a flight condition when moving forward at approximately 16 to 24 knots, generating increased lift.

Flight Conditions

• Ground resonance is a flight condition when the helicopter has landed and self-excited vibration occurs uncontrollably.

Stabilisation

• A stabilator or horizontal stabilizer is a stabilizing measure similar to pre-coned blades in fixed-wing design.
• A gimballed rotor system is a stabilizing system using rigidity in space of a rotating mass.
• Autopilot stabilization is a stabilizing system using an autopilot system.

Autorotation

• "Nr" in autorotation means rotor speed (revolutions per minute).
• "Deadman’s Curve" in autorotation is the relationship between rotor speed and airspeed, indicating the minimum speed for safe autorotation.
• Upward airflow through the blades due to gravity and downward motion causes blades to continue rotating during an autorotation.
• Torque is the equal and opposite reaction caused by engine power applied to drive the rotor system, which the tail rotor counteracts.

Rotor Systems

• A tandem rotor system counteracts the equal and opposite reaction with two counter-rotating rotors placed one in front of the other.
• A coaxial rotor system has two counter-rotating rotors placed side by side.
• An intermeshing rotor system has two counter-rotating rotors placed one above the other.
• A tail rotor system has a smaller rotor mounted vertically at the back to counteract torque.
• Translating tendency is the tendency of the helicopter to drift sideways due to the tail rotor’s thrust.

Axes of Movement

• The three axes of movement that every main rotor head and blade design needs to take into account are the longitudinal axis (pitch), lateral axis (roll), and vertical axis (yaw).

Rotary Wing Concepts

• Dissymmetry of lift is a rotary wing concept causing a difference in flight forces between advancing and retreating blades and also a rotary wing concept caused by unequal lift across a rotor disc in forward flight.
• The effect caused by the change in blade speed when the blade flaps up is Centripetal force changes, altering the blade's aerodynamic properties.
• Gyroscopic precession is a rotary wing concept stating that a change in blade angle results in a resultant action 90 degrees later in the direction of rotation.

Rotor Head Design

• A rigid rotor head has only one axis of movement.
• A fully articulated rotor head has three axes of rotation.
• A semi-rigid rotor head has only two axes of movement.