Rotory Wing Theory of Flight CASA 12.1
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Questions and Answers

What does Newton's First Law state about bodies at rest or in uniform motion?

  • They have no inertia if they are not moving.
  • They will remain in that state unless acted upon by an outside force. (correct)
  • They will eventually change direction due to gravity.
  • They will always accelerate unless a force is applied.

    • How is inertia related to mass?

  • Inertia is constant regardless of mass.
  • Inertia has no relation to mass.
  • Inertia decreases as mass increases.
  • Inertia is directly proportional to mass. (correct)

    • Which statement correctly distinguishes between inertia and momentum?

  • Momentum can exist only for bodies in motion. (correct)
  • Inertia is calculated using velocity.
  • Momentum is irrelevant when a body is at rest.
  • Inertia depends on both mass and velocity.

    • What is the relationship between force, mass, and acceleration according to Newton's Second Law?

    <p>Force equals mass times acceleration.</p> Signup and view all the answers

    What happens to the inertia of a body if its mass increases?

    <p>Inertia increases.</p> Signup and view all the answers

    Which of these factors determines the momentum of a body?

    <p>Both mass and velocity of the body.</p> Signup and view all the answers

    If a helicopter's velocity remains the same but its mass increases, what happens to its momentum?

    <p>Momentum increases.</p> Signup and view all the answers

    What must be applied to change the state of motion of a body according to Newton's First Law?

    <p>A force that is proportional to the body's mass.</p> Signup and view all the answers

    How does the advancing blade's effective relative airflow compare to the retreating blade's effective relative airflow?

    <p>The advancing blade has greater effective relative airflow.</p> Signup and view all the answers

    What is the primary cause of dissymmetry of lift in a helicopter?

    <p>The difference in airflow caused by forward airspeed.</p> Signup and view all the answers

    What is the effect of blade flapping on lift distribution across the rotor disc?

    <p>It equalizes lift across the rotor disc.</p> Signup and view all the answers

    What is the role of gyroscopic procession in helicopter flight?

    <p>It causes downward movement to occur after the input is made.</p> Signup and view all the answers

    How does increasing helicopter speed affect dissymmetry of lift?

    <p>It increases dissymmetry of lift.</p> Signup and view all the answers

    What happens to the angle of attack on the advancing blade as it flaps up?

    <p>It decreases the angle of attack.</p> Signup and view all the answers

    What is the definition of advance angle in helicopter flight?

    <p>The angular difference between the swashplate and the rotor disc's position.</p> Signup and view all the answers

    How does the cyclic control affect the rotor disc during flight?

    <p>It impacts the rotor disc 90° earlier than the control input.</p> Signup and view all the answers

    What occurs if the helicopter does not address dissymmetry of lift?

    <p>The helicopter will roll over.</p> Signup and view all the answers

    What happens to the pitch angle of the retreating blade as it flaps down?

    <p>The pitch angle increases.</p> Signup and view all the answers

    Who pioneered the incorporation of the flapping hinge into rotor blades?

    <p>Juan de Cierva.</p> Signup and view all the answers

    In which direction is the effective airflow on the advancing blade increased?

    <p>By increasing the helicopter's forward speed.</p> Signup and view all the answers

    What is phase lag in the context of helicopter rotor operation?

    <p>The time delay in the pitch change response of the rotor.</p> Signup and view all the answers

    What effect does increased airflow have on the lift produced by the advancing blade?

    <p>It increases lift production.</p> Signup and view all the answers

    What is the tip path plane in a helicopter?

    <p>The path in which the tips of the rotor blades travel.</p> Signup and view all the answers

    What does collective pitch refer to in helicopter operations?

    <p>Simultaneous adjustment of rotor blade pitch.</p> Signup and view all the answers

    How does drag affect the performance of a helicopter?

    <p>It resists the movement of the helicopter through the air.</p> Signup and view all the answers

    What is gyroscopic precession?

    <p>The delay of a force applied to a rotating body.</p> Signup and view all the answers

    What can cause blade stall in a helicopter?

    <p>Increasing the angle of attack beyond a certain point.</p> Signup and view all the answers

    What is the effect of ground effect on helicopter performance?

    <p>It decreases the rotor speed needed for hover.</p> Signup and view all the answers

    What does the term 'Coriolis effect' refer to in rotor systems?

    <p>Variations in blade velocity due to flapping.</p> Signup and view all the answers

    What does total rotor thrust (TRT) accomplish?

    <p>Produces lift to counteract gravitational forces.</p> Signup and view all the answers

    What is a node in the context of standing waves?

    <p>A point of zero displacement in the wave.</p> Signup and view all the answers

    Which principle explains how lift is generated by a rotor blade?

    <p>Bernoulli's principle.</p> Signup and view all the answers

    Which scenario could lead to catastrophic results if blade stall occurs?

    <p>Close to the ground while hovering.</p> Signup and view all the answers

    What does cyclic pitch control allow a pilot to do?

    <p>Change the horizontal direction of flight.</p> Signup and view all the answers

    What is the consequence of increasing drag on a helicopter's rotor?

    <p>Decreases overall aerodynamic efficiency.</p> Signup and view all the answers

    What is the primary body affected by rotor vibrations in a helicopter?

    <p>The helicopter fuselage.</p> Signup and view all the answers

    What is the point in a vibration range where vibration is at its maximum called?

    <p>Anti-Node</p> Signup and view all the answers

    What must be true for a body to accelerate at a given rate?

    <p>The force used must be proportional to the mass.</p> Signup and view all the answers

    Which force opposes centrifugal force and acts toward the center of a circle?

    <p>Centripetal Force</p> Signup and view all the answers

    What happens to total rotor thrust during a banked turn?

    <p>It must increase to generate centripetal force.</p> Signup and view all the answers

    What does Newton's third law state?

    <p>For every action, there is an equal and opposite reaction.</p> Signup and view all the answers

    What is the main difference between hovering and forward flight in terms of drag?

    <p>There is no parasite drag when hovering.</p> Signup and view all the answers

    What does air density primarily depend on?

    <p>Atmospheric pressure, temperature, and moisture content.</p> Signup and view all the answers

    What is the angle of attack in relation to a helicopter blade?

    <p>The inclination of the blade's chordline to the relative airflow.</p> Signup and view all the answers

    How does parasite drag change with an increase in speed?

    <p>It increases with the square of the speed.</p> Signup and view all the answers

    What condition can lead to a vortex ring state?

    <p>Hovering at low airspeed with power in use.</p> Signup and view all the answers

    How is disc loading defined?

    <p>Gross weight of the helicopter divided by the disc area.</p> Signup and view all the answers

    What is one factor that encourages the occurrence of vortex ring state?

    <p>Low altitude and low density.</p> Signup and view all the answers

    What is solidity in relation to a helicopter rotor?

    <p>The ratio of total blade area to disk area.</p> Signup and view all the answers

    During a powered descent, what must be present for vortex ring state to develop?

    <p>An induced flow and some power in use.</p> Signup and view all the answers

    What determines the maximum operating altitude of a helicopter?

    <p>Rotor thrust and air density.</p> Signup and view all the answers

    What is required for total rotor thrust to counteract parasite drag?

    <p>Total rotor thrust tilted forward.</p> Signup and view all the answers

    What happens to rotor thrust as air density decreases?

    <p>It decreases and requires larger angles of attack.</p> Signup and view all the answers

    What is the relationship between blade angle and induced flow near the blade tip?

    <p>Higher blade angle increases induced flow at the root.</p> Signup and view all the answers

    What does the term 'flapping' refer to in helicopter blades?

    <p>The vertical movement of a blade in relation to the plane of rotation.</p> Signup and view all the answers

    Which of the following best describes blade loading?

    <p>Constant and based on total weight over blade area.</p> Signup and view all the answers

    What causes the necessity of increased total rotor thrust during acceleration?

    <p>Change in the disc attitude for weight opposition.</p> Signup and view all the answers

    What is the primary role of the main rotor disc in a helicopter?

    <p>To generate thrust and lift simultaneously.</p> Signup and view all the answers

    What happens when collective control is increased?

    <p>It may decrease rotor RPM.</p> Signup and view all the answers

    What does the term 'induced flow' refer to?

    <p>The mass of air forced down by rotor action.</p> Signup and view all the answers

    What is the blade angle associated with?

    <p>The angle controlled through collective pitch adjustment.</p> Signup and view all the answers

    What is the main function of the swashplate in a helicopter?

    <p>To transmit pilot control inputs to the rotor disc.</p> Signup and view all the answers

    What happens to the fuselage attitude before the helicopter gains forward flight?

    <p>It remains the same briefly after initial tilt.</p> Signup and view all the answers

    What does the cyclic control do in a helicopter?

    <p>It changes the tilt of the rotor disc.</p> Signup and view all the answers

    Which of the following conditions is likely to lead to vortex ring state?

    <p>Hovering with low altitude and zero airspeed.</p> Signup and view all the answers

    What does the inflow angle indicate?

    <p>The angle between the plane of rotation and the resultant airflow.</p> Signup and view all the answers

    What is a chord line in relation to a helicopter blade?

    <p>The straight line between the blade's chord and the plane of rotation.</p> Signup and view all the answers

    How does a correlating unit assist in maintaining rotor RPM?

    <p>By increasing power when collective is pulled up.</p> Signup and view all the answers

    What does dissymmetry of lift refer to?

    <p>The uneven production of rotor thrust across the rotor disk.</p> Signup and view all the answers

    What is the consequence of applying forward cyclic to a helicopter in flight?

    <p>The plane of rotation will tilt forward.</p> Signup and view all the answers

    What maintains the required engine RPM in modern helicopters?

    <p>A governor.</p> Signup and view all the answers

    Which components vary in design across different helicopters?

    <p>The cyclic control and swashplate arrangement.</p> Signup and view all the answers

    What limits the downward movement of the rotor blade over the nose of the aircraft?

    <p>Swashplate orientation.</p> Signup and view all the answers

    How does the twist-grip type throttle on piston engine helicopters primarily function?

    <p>To fine-tune engine RPM.</p> Signup and view all the answers

    What happens to the rotor thrust orientation when collective control is changed?

    <p>It does not alter thrust orientation.</p> Signup and view all the answers

    At which position does the blade's rate of downward movement become zero over the nose?

    <p>Position C.</p> Signup and view all the answers

    What aspect of helicopter flight does the term 'collective control' refer to?

    <p>The simultaneous change of blade angles.</p> Signup and view all the answers

    What mechanism enables horizontal movement between the stationary and rotating plates of the swashplate?

    <p>A ball bearing arrangement.</p> Signup and view all the answers

    What are the symptoms of vortex ring state?

    <p>Aircraft vibration and cyclic stick shake</p> Signup and view all the answers

    Which action is part of the recommended recovery technique for vortex ring state?

    <p>Cyclic forward to increase airspeed</p> Signup and view all the answers

    What defines over-pitching in rotorcraft operation?

    <p>Loss of rotor thrust due to low engine power</p> Signup and view all the answers

    What is the main recovery action from over-pitching?

    <p>Rolling on throttle and lowering the collective lever</p> Signup and view all the answers

    How does the NOTAR system achieve controlled flight without a tail rotor?

    <p>Employing a fan to direct high-pressure air over the tailboom</p> Signup and view all the answers

    What effect does over-pitching have on rotor RPM?

    <p>It decreases rotor RPM, leading to rotor thrust loss</p> Signup and view all the answers

    What role does the tail rotor play in a single rotor helicopter?

    <p>It compensates for torque reaction and maintains directional control</p> Signup and view all the answers

    What mechanism does the cyclic control utilize to affect rotor disc movement?

    <p>Gyroscopic precession</p> Signup and view all the answers

    What does the collective control do in helicopter operation?

    <p>Controls total rotor thrust for altitude changes</p> Signup and view all the answers

    What is the benefit of using a Fenestron design in helicopters?

    <p>Reduces the rotor diameter requirement for equivalent thrust</p> Signup and view all the answers

    In twin rotor designs, what influences directional stability?

    <p>Thrust produced by each rotor in a given direction</p> Signup and view all the answers

    What happens to rotor thrust in a helicopter during excessive coning angles?

    <p>Thrust is diminished as it points inward</p> Signup and view all the answers

    What is the main consequence of pulling up the collective during a descent?

    <p>It can induce a rotor stall if RPM decreases</p> Signup and view all the answers

    What does cyclic control adjustment do to the rotor thrust?

    <p>Points the rotor thrust in different directions without changing its total amount</p> Signup and view all the answers

    What does the advance angle measure in relation to the swashplate and rotor blades?

    <p>The difference between the attachment point of the pitch link and the blade</p> Signup and view all the answers

    What effect does hovering in ground effect have on the angle of attack compared to hovering out of ground effect?

    <p>It is slightly less.</p> Signup and view all the answers

    What happens when the cyclic stick movement is not in line with the swashplate tilt?

    <p>The advance angle must be adjusted</p> Signup and view all the answers

    How does conservation of angular momentum affect a rotor blade when it flaps up?

    <p>It increases the rotational velocity of the blade</p> Signup and view all the answers

    What new force appears when the helicopter gains forward speed after hovering?

    <p>Translational lift</p> Signup and view all the answers

    What happens to translational lift when a helicopter moves horizontally?

    <p>It increases with speed and flow.</p> Signup and view all the answers

    What is the primary force maintaining rotor RPM above a minimum value to avoid excessive coning?

    <p>Centrifugal force</p> Signup and view all the answers

    How does the addition of mass to a rotor blade affect the coning angle?

    <p>It decreases the coning angle</p> Signup and view all the answers

    What causes translating tendency in a helicopter?

    <p>Uneven rotor torque forces</p> Signup and view all the answers

    What describes the underslung rotor system?

    <p>Blades attached below the rotor mast</p> Signup and view all the answers

    How should a pilot correct translating tendency during flight?

    <p>By moving the cyclic to the left.</p> Signup and view all the answers

    What is the primary reason for incorporating blade twist into rotor blades?

    <p>To improve lift characteristics along the blade.</p> Signup and view all the answers

    What effect does ground effect have on a helicopter flying close to the ground?

    <p>It restricts rotor downwash from escaping</p> Signup and view all the answers

    What occurs when a rotor blade reaches a stall condition?

    <p>Loss of lift primarily at the tip.</p> Signup and view all the answers

    What is the primary function of the pitch link in relation to the rotor blades?

    <p>It adjusts the blade angle of attack</p> Signup and view all the answers

    At what airspeed is effective translational lift most noticeable?

    <p>Between 15 to 20 miles per hour</p> Signup and view all the answers

    What happens to a blade's center of gravity when it flaps down?

    <p>It moves away from the axis of rotation</p> Signup and view all the answers

    What happens to tail rotor drift during a maximum performance takeoff from a confined area?

    <p>Drift to the right can occur.</p> Signup and view all the answers

    Which statement is true about the relationship between rotor thrust and centrifugal force?

    <p>Increasing rotor thrust decreases coning angle if centrifugal force remains the same</p> Signup and view all the answers

    What is the consequence of exceeding rotor RPM limits?

    <p>Potential blade stall</p> Signup and view all the answers

    What factor increases the likelihood of rotor blade stall in forward flight?

    <p>Heavy wing loading.</p> Signup and view all the answers

    What is a common method used to automatically correct for translating tendency?

    <p>Incorporation of a slight tilt in the main rotor mast.</p> Signup and view all the answers

    What role does the Coriolis Effect play in the behavior of rotor blades?

    <p>It influences the angular momentum of a blade during flapping</p> Signup and view all the answers

    What happens during a rapid throttle closure in a hover situation?

    <p>Drift to the left occurs if not corrected.</p> Signup and view all the answers

    How does the coning angle affect the rotor disc area?

    <p>Lower coning angles decrease rotor disc area</p> Signup and view all the answers

    How does the speed of rotor blade tips compare to roots during hovering?

    <p>Tip speed is greater than root speed.</p> Signup and view all the answers

    Which force tries to increase the coning angle of a rotor blade?

    <p>Rotor thrust</p> Signup and view all the answers

    What must be considered alongside forward speed to predict rotor stall accurately?

    <p>Altitude and temperature conditions.</p> Signup and view all the answers

    What initial indication is observed when a helicopter rotor enters a stall condition?

    <p>Vibration as each blade passes through the stall region</p> Signup and view all the answers

    What corrective action should be taken when a stall occurs in a helicopter?

    <p>Reduce pitch and increase rotor speed if possible</p> Signup and view all the answers

    Which region of the rotor blade is responsible for lifting during vertical autorotation?

    <p>Driven region</p> Signup and view all the answers

    What happens to the angle of attack of a rotor blade during vertical autorotation?

    <p>It can increase due to up-flow of air</p> Signup and view all the answers

    What is the primary factor determining the TAF direction in the driven region of the rotor blade?

    <p>The TAF is inclined slightly behind the vertical</p> Signup and view all the answers

    In the event of a power loss, what action can help the rotor blades begin to autorotate forward?

    <p>Reduce the collective to its minimum</p> Signup and view all the answers

    What is indicated if all autorotative regions shift towards the tip of the rotor blade?

    <p>Increased collective pitch</p> Signup and view all the answers

    What is a consequence of allowing the rotor to decelerate to a stationary position?

    <p>It may start to rotate backwards and complicate autorotation</p> Signup and view all the answers

    How does gliding autorotation differ from vertical autorotation?

    <p>The inflow of air is changed slightly during forward flight</p> Signup and view all the answers

    Which factor does NOT affect the angle of attack of a helicopter rotor blade?

    <p>Weight of the helicopter</p> Signup and view all the answers

    What leads to the helicopter pitching up during a stall condition?

    <p>Gyroscopic precession effects</p> Signup and view all the answers

    Which region of the rotor consists of high drag that tends to slow the rotor down?

    <p>Stall region</p> Signup and view all the answers

    What is the main purpose of designing the center part of the blade as the 'driving section'?

    <p>To provide a safety factor for cyclic steering and gusts</p> Signup and view all the answers

    What occurs after a sudden gust causes yaw in a helicopter?

    <p>The aircraft yaws back the other way due to unbalanced forces.</p> Signup and view all the answers

    How does airspeed affect the directional stability of a helicopter?

    <p>Positive static and dynamic stability is achieved at higher airspeeds.</p> Signup and view all the answers

    Which is a cause of dynamic instability in helicopters?

    <p>Rotor tilt with changes in airspeed.</p> Signup and view all the answers

    What effect does a stabiliser bar have when connected to pitch control links?

    <p>It stabilizes disturbances of the main rotor blades.</p> Signup and view all the answers

    What is a significant characteristic of the horizontal stabiliser on a helicopter?

    <p>It counteracts pitch disturbances with an opposite force.</p> Signup and view all the answers

    What function do vertical stabilisers serve in a helicopter?

    <p>They enhance the weather-cocking stability.</p> Signup and view all the answers

    What is the first phase of autorotation following an engine failure?

    <p>The Entry</p> Signup and view all the answers

    What happens to the stability of a helicopter if the centre of gravity is not aligned with the rotor disc centre?

    <p>It leads to fuselage pitching with rotor RPM changes.</p> Signup and view all the answers

    What should a pilot do immediately after losing engine power to avoid rotor RPM decay?

    <p>Reduce the collective pitch</p> Signup and view all the answers

    During which phase of autorotation does the helicopter convert potential energy into kinetic energy?

    <p>Steady State Descent</p> Signup and view all the answers

    What is the main advantage of adding fixed and adjustable stabilisers to a helicopter?

    <p>They contribute to achieving positive static stability.</p> Signup and view all the answers

    What happens to the airflow through the rotor disk during the Steady State Descent phase?

    <p>It flows upwards</p> Signup and view all the answers

    How do mini wings on a stabiliser bar influence rotor blade disturbances?

    <p>They provide a force to restore main blades to their original position.</p> Signup and view all the answers

    What technique is generally recommended for touchdown in autorotation?

    <p>Increase collective pitch rapidly just before touchdown</p> Signup and view all the answers

    What role does gyroscopic rigidity play in helicopter stability?

    <p>It contributes to correcting disturbances in the main rotor blades.</p> Signup and view all the answers

    How does the lack of engine torque affect the helicopter's fuselage during autorotation?

    <p>It introduces a tendency to rotate with the rotor</p> Signup and view all the answers

    What is an example of positive static stability in a helicopter?

    <p>The helicopter attempts to return to its initial attitude</p> Signup and view all the answers

    Which type of stability is indicated when an aircraft remains in its displaced attitude?

    <p>Neutral static stability</p> Signup and view all the answers

    What happens to an aircraft when negative dynamic stability is present?

    <p>The aircraft starts to oscillate with increasing magnitude</p> Signup and view all the answers

    What typically occurs during hovering stability when a helicopter experiences a disturbance?

    <p>The helicopter experiences blowback and tilts in the direction of the disturbance</p> Signup and view all the answers

    During a forward flight gust, what is the primary effect on the helicopter's stability?

    <p>It induces a cycle of pitch changes and speed variations</p> Signup and view all the answers

    What is the typical response of a helicopter to lateral disturbances regarding stability?

    <p>It may demonstrate static stability but with dynamic instability</p> Signup and view all the answers

    Study Notes

    Newton’s First Law

    • Objects at rest or moving in a straight line remain in that state unless acted upon by an external force.
    • Inertia is a bodies tendency to resist change in motion.
    • Mass is the amount of matter in a body, it is directly proportional to inertia.
    • Greater mass means larger inertia and requires greater force to change state of motion.
    • Momentum is the product of mass and velocity.
    • Momentum increases with velocity or mass.
    • Greater momentum requires a greater force to bring it to a stop.

    Newton’s Second Law

    • Force is proportional to mass multiplied by acceleration (F = M x A)
    • Increasing force leads to a greater acceleration for a given mass
    • Greater mass requires a larger force for acceleration.
    • The accelerated air through a rotor system is an example of this law in action.

    Newton's Third Law

    • For every action, there is an equal and opposite reaction.
    • When forces are equal and opposite no acceleration occurs.
    • In hovering flight, forces are equal and opposite as long as there is no acceleration.

    Air Density

    • Air density is the number of air molecules per unit volume.
    • Air density is influenced by atmospheric pressure, air temperature, and moisture content.
    • Low air density requires larger angles of attack and more power to achieve required lift.

    Angle of Attack

    • The angle between the chord of an aerofoil and the relative airflow.
    • Angle of attack and induced flow are inversely proportional for a given blade section and rotor RPM.

    Inflow Angle

    • The angle between the plane of rotation and the resultant airflow.

    Axis of Rotation

    • The line through the rotor head at right angles to the Plane of Rotation (POR).
    • The blade rotates around this axis.

    Shaft Axis

    • The centerline through the rotor shaft (mast).
    • The shaft axis and axis of rotation align when the plane of rotation is perpendicular to the shaft axis.

    Flapping

    • Movement of a blade in the vertical sense relative to the plane of rotation.

    Feathering

    • Movement of the blade about its axis, resulting in pitch angle changes.

    Disc Loading

    • The gross weight of the helicopter divided by the disc area, expressed as lb./sq.inch or kg/m2.
    • Disc loading in flight.

    Blade Loading

    • The gross weight of the helicopter divided by the combined area of the helicopter blades, expressed as above.
    • Blade loading is constant in flight.

    Solidity

    • The ratio of total blade area to disk area.
    • Solidity affects power absorption from the engine and rotor thrust production.

    Lead-Lagging (Dragging)

    • Movement of the blade forward or aft in the plane of rotation.

    Coning Angle

    • The angle between the spanwise axis and the plane of rotation.
    • Also defined as the angular difference between the feathering axis and the tip plane path.

    Feathering Axis

    • The straight-line axis between the root of the blade and its tip about which the blade can alter its blade angle.

    Chord Line

    • The straight line between the chord of the blade and the plane of rotation.
    • This angle can be altered by the pilot through movement of the collective lever or cyclic control.

    Blade Angle (Pitch Angle)

    • The angle between the chord of the blade and the plane of rotation.
    • This angle is controlled by the pilot using the collective lever and cyclic control.

    Relative Airflow

    • The resultant airflow to each blade due to its speed/direction and the induced flow downwards.

    Induced Flow

    • The mass of air that is forced down by the rotor action.

    Blade Angle and Angle of Attack

    • The angle between the chord of the blade and the plane of rotation is known as the blade angle or pitch angle.
    • Blade angle is also the angle of attack when no other airflows interfere.
    • Blade angle does not affect the direction from which rotational speed (VR) occurs.

    Tip Path

    • The circular path described by the tips of the rotor blades.
    • Tip path plane is parallel to the plane of rotation.
    • A pilot can alter the tip path plane through movement of the cyclic control.

    Tip Path Plane

    • The path in which the tips of the rotor blades travel.
    • It is parallel to the plane of rotation.
    • A pilot can alter this plane through movement of the cyclic control.

    Disc Area

    • The area contained within the tip path plane.
    • Disc area is not constant in flight due to coning angle variations.

    Collective Pitch

    • Pitch angle of all main rotor blades is varied equally and simultaneously.

    Cyclic Pitch

    • Pitch angle of the main rotor blades is varied individually during a cycle of revolution of the rotor disc.

    Ground Effect

    • A gain in lifting power when operating near the ground.

    Gyroscopic Precession

    • A characteristic of all rotating bodies.
    • When a force is applied to the outside of a rotating body parallel to its plane of rotation, the effect of that force is felt 90° later in the direction of rotation.

    Cyclic Pitch Angle (Blade Angle)

    • Varied by the pilot movement of the cyclic control.
    • The cyclic control controls the tilt of the rotor disc and therefore the direction of flight.

    Coriolis Effect

    • Rotor systems with individual flapping hinges are subjected to the Coriolis effect.
    • Coriolis effect is the change in blade velocity to compensate for the change in distance between the centre of mass of the blade to the centre of the axis of rotation as the blade flaps.

    Lift

    • The force produced by the aerofoil that is perpendicular to the relative wind and opposes gravity.
    • Based on Bernoulli's Principle, lift is developed by creating a low pressure at the top of the rotor blade and a high pressure at the bottom.

    Drag

    • The force which tends to resist the aerofoil's passage through the air.
    • Drag is always parallel to the relative wind and perpendicular to lift.
    • Drag increases as the square of velocity.
    • Drag increases with angle of attack.

    Blade Stall

    • Occurs when the streamlined flow of air separates from the camber of the blade and reverse flow occurs, resulting in an almost complete loss of lift.
    • Blade stall angle is reached as the angle of attack increases with constant velocity.
    • Corrected by reducing collective pitch and increasing engine/rotor RPM simultaneously.

    Total Rotor Thrust Vector Resultant

    • Rotor thrust is dependent on lift production and is the force produced by each blade section to overcome part of the helicopter weight.
    • Total Rotor Thrust (TRT) provides the force that overcomes the helicopter's weight.

    Node

    • A point of minimum amplitude or no displacement on a standing wave.
    • Vibration can affect both aircraft and crew.
    • Human bodies tolerate up-down vibrations more poorly than side-to-side and fore-aft vibrations.

    Anti-Node

    • The point in a vibration range where vibration is at its maximum.

    Centrifugal Force

    • The force that pulls a spinning object away from its centre of rotation.
    • Centrifugal force, determined by rotor RPM acts in line with the plane of rotation.

    Centripetal Force

    • The force that opposes centrifugal force and acts toward the centre of a circle.
    • Centrifugal force is equal to centripetal force for a constant radius.

    Turning

    • An object travelling on a curve requires a force pulling it towards the centre of the curve, known as centripetal force.
    • Tilted total rotor thrust provides both the vertical component to oppose weight and centripetal force.
    • Total rotor thrust must increase during turns to provide CPF.

    Acceleration

    • Parasite drag is negligible during hovering flight, but increases with the square of speed.
    • Total rotor thrust overcomes parasite drag by tilting forward and increasing.
    • The fuselage attitude changes to maintain the total rotor thrust and the weight/parasite drag resultant acting through the helicopter's centre of gravity.

    Vortex Ring State

    • Also known as settling with power.
    • A potentially hazardous condition where the helicopter descends at an increasing rate.
    • Recovery may involve even more loss of height.
    • Develops with a low or zero airspeed, some power in use, and a rate of descent that is too fast.

    Vortex Ring State

    • Induced flow is required for flight conditions like powered descent, loss of height, quick stop, downwind approach, squashing recovery.
    • High collective settings, including high gross weight, high altitude, high density altitude, and maneuvers, increase the risk of vortex ring state.
    • Vortex ring state is characterized by aircraft vibration, cyclic stick shake, random yawing, rolling, and pitching, increasing the rate of descent, and reduced cyclic stick effectiveness.
    • The recommended recovery technique for vortex ring state is to cyclic forward, pause, and apply power.
    • Early recognition of vortex ring state is crucial for recovery as it can lead to catastrophic consequences if not corrected.

    Over-Pitching

    • Over-pitching occurs when rotor RPM decreases during hover or slow flight, and the pilot attempts to restore thrust by pulling up collective.
    • This results in a further decay of rotor RPM and increased coning angles leading to a decrease in total rotor thrust.
    • The recovery action for over-pitching is to increase throttle while simultaneously lowering the collective lever.
    • Over-pitching is more likely near a hover or during a hover and can occur at various flight stages.

    Anti-Torque Systems

    • A single rotor helicopter needs a tail rotor to counter the torque reaction from the main rotor.
    • Tail rotors provide directional control during flight and autorotation.
    • NOTAR system eliminates the tail rotor by using a fan and slots to create a boundary layer flow, generating thrust to counter torque.
    • Directional yaw control for NOTAR is achieved through a direct jet thruster.
    • Fenestron is a ducted tail rotor system that matures the wake in the duct, reducing contraction and increasing thrust effectiveness.
    • Fenestron always integrates with a vertical stabilizer, reducing loads and extending system life.

    Twin Rotor Designs

    • Twin rotor designs feature two rotors rotating in opposite directions to cancel out torque.
    • Tandem rotors, contra-rotating rotors, and intermeshing rotors are examples of twin rotor configurations.
    • Yaw control on tandem rotors is achieved through pilot input on the aft rotor.

    Helicopter Control

    • Cyclic control is responsible for roll and pitch control.
      • Cyclic motion tilts the rotor disc by changing blade angles on individual blades.
      • Cyclic control does not affect total rotor thrust, only its direction.
    • Collective control manages total rotor thrust.
      • Upwards movement of the collective lever increases all blade angles, resulting in higher thrust.
      • Downwards movement decreases blade angles, reducing thrust.
      • Collective control can include anticipator, twist grip, and collective control features.
      • A correlating unit automatically increases engine power while pulling collective, maintaining rotor RPM.
    • Yaw control uses foot pedals to adjust tail rotor blade angles, providing yaw control.

    Swashplate

    • The swashplate is a mechanism that transmits cyclic and collective inputs to the rotor disc.
    • The lower plate, or stationary plate, is fixed and moves vertically and tilts according to inputs.
    • The rotating plate follows movements of the stationary plate and connects to each blade via pitch links.
    • Cyclic input changes the vertical position of the stationary plate, altering blade angles and tilting the rotor disc.
    • Collective input changes the tilt of the stationary plate, affecting overall blade angles and total rotor thrust.

    Phase Lag

    • Gyroscopic precession causes a 90° phase lag between cyclic input and rotor disc tilting.
    • This means that the maximum change in blade angle occurs 90° before the disc reaches its maximum tilt.
    • To counteract phase lag. the swashplate's geometry and attachment points are adjusted to maintain a direct relationship between cyclic input and rotor disc tilting.
    • This is achieved through a concept called "advance angle," the angular difference between the pitch link attachment point and the blade it affects.

    Coriolis Effect

    • The conservation of angular momentum, known as the Coriolis effect, affects a blade's rotational velocity when it flaps up or down.
    • As a blade flaps, its angular momentum remains constant causing changes in its rotational velocity.

    Coriolis Effect

    • When a helicopter blade flaps up or down, its center of gravity moves closer or further from the axis of rotation, respectively.
    • As a result of conservation of angular momentum, the blade's rotational velocity increases when flapping up and decreases when flapping down.
    • This change in rotational velocity due to changes in the radius of rotation is known as the Coriolis effect.

    Coning

    • Coning is the upward movement of helicopter blades due to combined lift and centrifugal forces.
    • The coning angle is determined by the balance of rotor thrust and centrifugal force.
    • Increased rotor thrust increases the coning angle, reducing the rotor disc area. Conversely, increased centrifugal force decreases the coning angle, increasing the rotor disc area.
    • It is crucial to maintain rotor RPM above a minimum value to avoid excessive coning.

    Underslung Rotor

    • An underslung rotor system involves attaching the blades to the rotor head lower than the top of the rotor mast.
    • This design reduces vibrations and minimizes lead-lag tendencies, reducing stress on the rotor head components.

    Ground Effect

    • Ground effect occurs when a helicopter flies close to the earth's surface, restricting the escape of rotor downwash.
    • In ground effect, the angle of attack is reduced, requiring less power to maintain a hover.
    • Translational lift is an additional lift force generated during horizontal flight due to increased airflow efficiency.

    Translating Tendency

    • Translating tendency is the drift of a helicopter to the right during takeoff due to the imbalance of forces caused by the main rotor torque and tail rotor thrust.
    • This tendency is counteracted by tilting the rotor disc slightly to the left using the cyclic control.
    • Some designs incorporate automatic corrections for translating tendency, such as tilting the main rotor mast or using a bias in the cyclic control mechanism.

    Blade Twist

    • To improve lift characteristics throughout the blade, helicopter blades are often twisted, with a greater twist angle at the root than at the tip.
    • This twist increases the angle of attack of the slower portions of the blade, improving lift distribution.

    Blade Tip Stall

    • The advancing blade on a helicopter moves at a faster speed than the retreating blade, leading to a higher angle of attack on the retreating blade.
    • This can lead to blade tip stall at high speeds.
    • Contributing factors include insufficient airspeed, a high angle of attack, and heavy wing loading.
    • Blade tip stall is indicated by vibrations and may cause a pitch-up of the helicopter due to gyroscopic precession.
    • Correction involves reducing speed, pitch, and increasing rotor speed if possible.

    Autorotation

    • Autorotation is the process of generating lift using aerodynamic forces created by the upward flow of air through freely rotating blades.
    • Autorotation can be divided into vertical autorotation (hovering descent) and gliding autorotation (forward descent).

    Aerodynamics of Autorotation

    • In autorotation, the rotor disk is divided into three regions: a driven region, a driving region, and a stall region.
    • The driven region near the blade tips creates drag, reducing the rotor speed but generating lift.
    • The driving region near the mid-section provides both lift and forward momentum, driving the rotor.
    • The stall region near the root operates above the stall angle of attack, generating mainly drag.
    • The pilot can control the autorotation regions by adjusting the collective pitch.

    The Three Phases of Autorotation

    • The three phases of autorotation are entry, steady-state descent, and deceleration and touchdown.

    The Tail Rotor in Autorotation

    • With engine failure, the tail rotor needs to be adjusted to a negative pitch to counteract the rotation of the fuselage caused by friction in the transmission system.

    Helicopter Axes of Stability

    • The main rotor provides lateral and longitudinal stability, while the tail rotor provides directional stability.

    Positive Static and Dynamic Stability

    • Positive static stability occurs when an aircraft returns to its initial attitude following a displacement.
    • Positive dynamic stability involves the aircraft returning to its original attitude through a series of decreasing oscillations.

    Static and Dynamic Stability

    • Positive Static Stability - Aircraft returns to its original attitude after a disturbance.
    • Negative Static Stability - Aircraft continues to move away from its original attitude after a disturbance.
    • Positive Dynamic Stability - Oscillations decrease in magnitude over time.
    • Negative Dynamic Stability - Oscillations increase in magnitude over time.
    • Neutral Static Stability - The aircraft attempts to return to its original state of equilibrium, but the oscillations neither increase nor decrease in magnitude as time passes.
    • Neutral Dynamic Stability - The aircraft remains in its displaced attitude after a disturbance

    Helicopter Stability

    • Hovering Stability-

      • Gusts cause blowback, tilting the rotor disc and increasing relative air flow.
      • This creates positive static stability, but the airframe lags behind.
      • Continued gusts fuel oscillations, resulting in negative dynamic stability.

    • Longitudinal Stability -

      • Gusts of wind encountered in forward flight affect helicopter stability.
      • A gust from ahead of the helicopter causes blow-back to tilt the disc and the Total Rotor Thrust (TRT) back, and the airspeed reduces.
      • The inertia of the airframe causes it to pitch nose-up. This pushes the disc forward, and the airframe swings back nose down, the disc tilts forward.
      • The airframe swings back nose down, eventually repeating the whole cycle.
      • The cycle results in positive static stability, but negative dynamic stability.

    • Lateral Stability -

      • Lateral gusts create a similar sequence of events to pitch, but affect roll, resulting in positive static stability and negative dynamic stability.
      • The sideways flight creates a cross coupling with the normal axis, causing the aircraft to yaw in the same direction as the roll.

    • Directional Stability (Hovering) -

      • Gusts from the side cause the helicopter to translate and yaw.
      • The change in relative air flow affects the tail rotor angle of attack and anti-torque force.
      • This creates a divergent oscillation resulting in positive static stability and negative dynamic stability.

    • Directional Stability (Forward Flight) -

      • Similar to hovering, but increased airflow from ahead enhances the "weather-cocking" effect at higher airspeeds.
      • This creates positive static and dynamic stability.
      • Can be enhanced by adding a vertical stabilizer.

    Instability Causes

    • Rotor Tilt with Airspeed Changes (Blowback)
    • Rotor Disc Following the Fuselage's Pendulum Effect
    • Fuselage Pitching with Rotor RPM Changes (when center of gravity is not under the disc)

    Methods to Reduce Instability

    • Gyroscopic Rigidity
    • Reduce Airspeed and RPM Effects of Flapping

    Stabilizer Systems

    • Stabilizer Bars

      • Short, end-weighted bars placed at right-angles to the main rotor
      • Possess gyroscopic rigidity, providing correcting input to stabilize disturbances of the main rotor blades.
      • Reduces "blowback".

    • Mini Wings on Stabilizer Bars

      • Provide a force to restore the main blades to their original position.
      • Minimize "blowback".

    • Fixed and Adjustable Stabilizers

      • Horizontal and Vertical stabilizers function similar to fixed-wing aircraft.

    • Horizontal Stabilizer

      • Provides static stability by inducing a force opposite to any pitch disturbance.
      • Effectiveness depends on distance from the aircraft's center of gravity.
      • Can be inverted depending on the helicopter design

    • Vertical Stabilizer

      • Increases "weather-cocking" stability by providing surface area behind the aircraft's center of gravity.
      • Important for maintaining longitudinal stability.

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