Term
| How does the design of a fully articulated main rotor system compensate for conservation of angular momentum (so-called Coriolis Effect)? |
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Definition
Lead/lag hinges and dampers or 2-way elastomeric hinges damp and absorb the "hunting" forces.
[image] |
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Term
| How does the design of a semi-rigid main rotor system compensate for conservation of angular momentum (so called Coriolis Effect)? |
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Definition
The two blades are rigidly mounted to the hub. The while assembly is free to tilt via a teetering hinge which is underslung - the key to minimizing changes in the center of mass.
[image] |
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Term
| What are some of the ways that a helicopter's design can compensate for translating tendency? |
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Definition
- Main rotor transmission and mast tilted to left
- Rigging cyclic so centered with small opposing rotor disc tilt
- Mounting tailrotor on top of vertical fin or pylon
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Term
| How does translational lift affect the main rotor? |
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Definition
| When a single rotor helicopter flies through translational lift, the air flowing through the main rotor and over the tail rotor becomes more horizontal less turbulent, resulting in greater efficiency of both rotor systems. |
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Term
| How does translational lift affect the tail rotor? |
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Definition
| As forward speed increases, the airflow through the main rotor smooths out, causing an increase in tail rotor efficiency and thrust, causing a left yaw. |
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Term
| How does translational thrust affect the tail rotor? |
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Definition
| Similar to translational liift in the main rotor, as forward speed increases, the tail rotor outruns its turbulence and becomes more efficient. This will cause left yaw during takeoff. |
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Term
| How does translational lift affect the helicopter's flight characteristics? |
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Definition
| If no corrections are made the helicopter will pitch up, roll right, and yaw left. |
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Term
| What causes the effects of translational lift on the helicopter's flight characteristics? |
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Definition
- Dissymmetry of lift
- Transverse flow effect
- Smoother air around tail rotor
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Term
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Definition
| The lateral or longitudinal oscillation of the fuselage due to its suspension from the rotor system. |
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Term
| What is the main cause of excessive pendular action? |
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Definition
| Over-controlling of the helicopter. Control movements should be smooth and not exaggerated. |
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Term
| What is cyclic feathering? |
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Definition
| The mechanical change of the angle of incidence, or pitch, of individual rotor blades, independent of other blades in the system. |
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Term
| What is the chordwise axis? |
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Definition
| For semirigid rotors, a term used to describe the flapping or teetering axis of the rotor. |
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Term
| What is the chort or chord-line? |
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Definition
| An imaginary straight line between the leading and trailing edges of an airfoil section. |
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Term
| What is centrifugal force? |
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Definition
| The apparent force that an object moving along a circular path exerts on the body constraining the object and that acts outwardly away from the center of rotation. |
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Term
| What is centripetal force? |
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Definition
| The force that attracts a body toward its axis of rotation (opposite centrifugal force). |
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Term
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Definition
| The area swept by the blades of the rotor. It is a circle with its center at the hub and has a radius of one blade length. |
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Term
| What is the disc loading of a helicopter? |
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Definition
| The total helicopter weight divided by the rotor disk area. |
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Term
| What is a flapping hinge? |
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Definition
| A hinge that permits the rotor blade to flap up and down and thus balance the dissymmetry of lift generated by the advancing and retreating blades. |
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Term
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Definition
| Movement of a blade with respect to the other blades in the plane of rotation, also called leading/lagging or dragging. |
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Term
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Definition
| The property of matter by which it will remain at rest or in a state of uniform motion in the same direction unless acted upon by some external force. |
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Term
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Definition
| The tendency of the rotor disk to tilt aft in transition to forward flight as a result of unequal airflow. |
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Term
| What is blade twist and why is it used? |
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Definition
| The variation in the angle of incidence of a blade between the root and the tip. Used to even out lift versus wind velocity spanwise along the rotor blade. |
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Term
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Definition
| The relationship of the blade tips in the plane of rotation. Blades that are in track will move through the same plane of rotation. |
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Term
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Definition
| The furthermost part of the blade from the hub of the rotor. |
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Term
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Definition
| The condition of the rotor blade when it is operating at an angle of attack greater than the maximum angle of lift. |
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Term
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Definition
| The length of a blade from its tip to its root. |
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Term
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Definition
| The part of the blade that attaches to the blade grip (part of the hub) |
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Term
| What is a helicopter's blade loading? |
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Definition
| The load imposed on the main rotor blades, determined by dividing the total weight of the helicopter by the combined area of all the main rotor blades. |
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Term
| What is blade lead and lag? |
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Definition
| The fore and aft movement of the blade in the plane of rotation. It is sometimes called hunting or dragging. |
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Term
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Definition
| The ability of the rotor blade to move in a vertical direction. Blades may flap independently or in unison. |
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Term
| What is blade feathering? |
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Definition
The rotation of the blade around
the spanwise (pitch change) axis. |
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Term
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Definition
| The condition of flight during which the main rotor is driven only by aerodynamic forces with no power from the engine. |
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Term
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Definition
| The angle between the airfoil’s chord line and the relative wind. |
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Term
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Definition
| The movement of the aircraft about its longitudinal axis. Movement of the cyclic right or left causes the helicopter to tilt in that direction. |
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Term
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Definition
| The movement of the aircraft about its lateral, or pitch, axis. Movement of the cyclic forward or aft causes the nose of the helicopter to pitch up or down. |
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Term
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Definition
| Any surface designed to obtain a useful reaction of lift, or negative lift, as it moves through the air. |
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Term
| What is the advancing blade? |
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Definition
| The blade moving in the same direction as the helicopter. |
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Term
| What are the two forces acting on an airfoil that create lift? |
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Definition
- Bernoulli's Principle (major)
- Newton's Third Law (minor)
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Term
| Explain Bernoulli's Principle and its application to lift generation |
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Definition
| Conservation of energy - Airflow through or over restriction increases speed (kinetic energy), and decreases pressure (potential energy). This pressure differential over an airfoil is the major cause of lift. |
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Term
| Explain Newton's Third Law and its application to lift generation |
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Definition
| For every force/action, there is an equal and opposite force/action. Air striking the underside of the blade is deflected downward, resulting in an upward force on the blade. This is a minor factor in lift generation. |
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Term
| What are the four dynamic forces that act on a helicopter during all maneuvers? |
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Definition
- Lift - Up
- Gravity or weight - Down
- Thrust - Forward
- Drag - Backward
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Term
| How is feathering accomplished in a semi-rigid rotor system? |
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Definition
A separate feathering hinge is provided for each blade.
[image] |
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Term
| How is each blade attached to a fully articulated rotor hub? |
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Definition
By a horizontal hinge (flapping hinge) as well as a lead/lag hinge, or an elastomeric hinge and bearing that provides both flapping and lead-lag motion and damping.
[image] |
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Term
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Definition
A flapping hinge with an axis skewed so that the flapping motion introduces a component of feathering that would result in a restoring force in the flap-wise direction. Most often used in semi-rigid tail rotors with teetering mounting.
[image] |
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Term
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Definition
| That part of the total drag that is created by the production of lift. |
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Term
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Definition
| The component of air flowing vertically through the rotor system resulting from the production of lift. |
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Term
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Definition
| The maximum ratio between total lift (L) and total drag (D). This point provides the best glide speed. Any deviation from the best glide speed increases drag and reduces the distance you can glide. |
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Term
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Definition
| The ratio of a specified load weight to the total weight of the aircraft. |
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Term
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Definition
| The part of total drag created by the form or shape of helicopter parts. |
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Term
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Definition
| The lateral or longitudinal oscillation of the fuselage due to its suspension from the rotor system. |
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Term
| What is pitch angle (with respect to a rotor blade)? |
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Definition
| The angle between the chord line of the rotor blade and the reference plane of the main rotor hub or the rotor plane of rotation. |
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Term
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Definition
| Drag incurred from frictional or parasitic resistance of the blades passing through the air. It does not change significantly with the angle of attack of the airfoil section, but it increases moderately as airspeed increases. |
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Term
| What is resultant relative wind? |
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Definition
| Airflow from rotation that is modified by induced flow. |
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Term
| What is the retreating blade? |
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Definition
| Any blade, located in a semicircular part of the rotor disk, in which the blade direction is opposite to the direction of flight. |
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Term
| What is the solidity ratio of a rotor system? |
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Definition
| The ratio of the total rotor blade area to total rotor disk area. |
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Term
| What is the span of a rotor blade or airfoil? |
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Definition
| The dimension of a rotor blade or airfoil from root to tip. |
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Term
| What is a symmetrical airfoil? |
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Definition
| An airfoil having the same shape on the top and bottom. |
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Term
| What is a teetering hinge? |
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Definition
| A hinge that permits the rotor blades of a semirigid rotor system to flap as a unit. |
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Term
| What does "unloaded rotor" mean? |
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Definition
| The state of a rotor when rotor force has been removed, or when the rotor is operating under a low or negative G condition. |
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Term
| What is the purpose of a tail rotor? |
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Definition
| The main purpose of the tail rotor is to counteract the torque on the fuselage produced by driving the main rotor. |
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Term
| How do the anti-torque pedals compensate for torque effect? |
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Definition
| The pedals vary the pitch angle on the tail rotor blades, thereby varying the amount of thrust produced by the tail rotor. |
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Term
| How does a twin-rotor system (two main rotors) compensate for torque effect? |
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Definition
| Each main rotor turns in a direction opposite the other, thereby cancelling the torque effect. |
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Term
| How is dissymmetry of lift compensated for in a fully articulated rotor system? |
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Definition
| Each blade is on a hinge that allows it to flap up and down, counteracted by centrifugal force. The advancing blade flaps up and reduces AOA/lift, the retreating blade flaps down and increases AOA/lift. This balances out dissymmetry of lift. |
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Term
| How is dissymmetry of lift compensated for in a semi-rigid rotor system? |
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Definition
| The semi-rigid rotor system uses a teetering hinge which allows the two opposing blades to flap up and down as a unit. |
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Term
| How is dissymmetry of lift compensated for in a fully rigid rotor system? |
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Definition
| The blade roots are rigidly attached to the rotor hub. Behaves like fully articulated system through aerodynamics, but lack flapping or lead/lag hinges. Blades accommodate both by bending. |
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Term
| What is coning as applied to the main rotor system, and what causes it? |
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Definition
| When the main rotor system is placed under load, the lift acting upward acts against the centrifugal force acting outward. As the lift increases, the blades bend or flap up together in response to the increased lifting force.
[image] |
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Term
| What determines the maximum rotor RPM? |
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Definition
| The limit on the centrifugal force generated by rotation. It cannot exceed the structural limitations of the main rotor system. |
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Term
| What is the transverse flow effect? |
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Definition
The condition of increased drag and decreased lift in the aft portion of the rotor disk caused by the air having a greater induced velocity and angle in the aft portion of the disk.
[image] |
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Term
| What is translating tendency? |
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Definition
| The tendency of the single-rotor helicopter to move laterally during hovering flight. Also called tail rotor drift. |
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Term
| Explain gyroscopic precession and its influence on rotor control |
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Definition
| An inherent quality of rotating bodies wherein an applied force is manifested 90° in the direction of rotation from the point of application. In general, then, forces must be applied 90° away from the intended reaction. For example the swashplate would be forced to tilt forward to cause the rotor disk to tilt to the left. |
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Term
| Explain the three aerodynamic regions of the main rotor during autorotation |
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Definition
- Driven region: Outer 30% of radius, net dragging
- Driving region: Middle 45% of radius, supplies driving forces needed to turn the blades
- Stall region: Inner 25% or radius, produces no lift (stalled), all drag.
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Term
| How are the three aerodynamic regions during autorotation changed by forward flight? |
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Definition
| The forward motion causes the relative wind to increase on the advancing side and decrease on the retreating side. The effect is to displace the driving and stall regions to the left. |
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Term
| Over what types of surfaces will ground effect be most pronounced? |
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Definition
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Term
| What is a major factor in determining Vne? |
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Definition
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Term
| Why does OGE hover require more power than IGE hover in the same conditions? |
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Definition
| Induced drag increases due to the increase in blade tip vortices, requires higher pitch angle and thus more engine power |
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Term
| Why is the H/V diagram used only for the takeoff profile and not for the landing profile? |
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Definition
| Most engine failures occur during takeoff and climb when the highest power output is required. |
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Term
| When is translating tendency most significant? |
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Definition
| In a hover, when tail rotor thrust is the greatest. |
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Term
| Does the tail rotor in the H269C experience dissymetry of lift? |
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Definition
| Yes, due to the induced dowward flow and tip vortices of the nearby main rotor |
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Term
| What feature of 269C is used to compensate for tail rotor dissymetry of lift? |
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Definition
| A delta hinge on the tail rotor |
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Term
| What is the purpose of the vertical fin on the H269C |
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Definition
| To provide yaw stability, particularly in case of a tail rotor failure |
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Term
| What is the purpose of the horizontal fin on the H269C |
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Definition
| To provide pitch stability, counteract divergent pitch tendency (tuck). I believe it is angled (dihedral) to provide a bit of yaw stability as well. The original 269/TH-55 had a much smaller vertical fin. |
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Term
| Where is the harmonic damper of the H269C located? |
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Definition
| It is located about midway on the tail rotor drive shaft. |
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Term
| What is the purpose of the harmonic damper on the H269C? |
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Definition
| It helps to suppress harmonic vibrations in the tail rotor driveshaft. |
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Term
| Where are the main rotor dampers located in the H269C? |
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Definition
| Along the trailing edges of each blade, at the root? |
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Term
| What is the function of main rotor dampers in a fully articulated main rotor system? |
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Definition
| They resist lead/lag "hunting" motion caused by Coriolis Effect, thereby helping to prevent ground resonance. |
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Term
| What is the purpose of the droop stops in a fully articulated rotor system? |
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Definition
| Prevent excessive downward flapping of main rotor blades when not in motion. |
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Term
| Where are the droop stops located in a fully articulated rotor system? |
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Definition
| The upper part is on the blade yoke and lower part on the mast. There is one for each blade. |
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Term
| What is droop stop pounding, and where is it most commonly encountered in the H269C |
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Definition
| A condition where the rotor disk and mast are at an angle away from 90 degrees sufficient to cause the droop stops to be hit during flight. It is most commonly encountered during slope operations. |
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Term
| What is the purpose of the vertical hinge in a fully articulated rotor system? |
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Definition
| To allow lead/lag (drag, hunting) motion which compensates for Coriolis Effect. |
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Term
| How does the main rotor in a fully articulated system compensate for Coriolis Effect? |
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Definition
| By allowing the blades to lead/lag. |
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Term
| What is the purpose of the horizontal hinge in a fully articulated rotor system? |
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Definition
| Allows blades to flap up and down and thereby compensate for dissymetry of lift. |
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Term
| How does the main rotor of a fully articulated rotor system compensate for disymmetry of lift? |
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Definition
| By allowing the blades to flap up and down, changing their angle of attack and therefore generated lift. |
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|
Term
| What are the three types of main rotor system? |
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Definition
| Fully articulated, semi-rigid, rigid. |
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|
Term
| What are the three types of tail rotor systems? |
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Definition
| Tail rotor, fenestron, NOTAR. |
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|
Term
| At or below what height does ground effect occur? |
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Definition
| One rotor diameter or less. |
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