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HELICOPTER AERODYNAMICS
 CONING CORIOLIS EFFECT DYNAMIC ROLLOVER DISSYMMETRY OF LIFT GROUND EFFECT GROUND RESONANCE GYROSCOPIC PRECESSION MAST BUMPING SETTLING WITH POWER TORQUE TRANSLATING TENDENCY TRANSLATIONAL LIFT TRANSVERSE FLOW EFFECT WASHOUT

The upward bending of the rotor blades caused by the combined forces of lift and centrifugal force.  Coning is more pronounced with heavier loads and high density altitude.  Lift decreases because it now has a horizontal component.  The pilot should increase centrifugal force by lowering the collective and increasing RPM.  Coning results in blade bending in a semirigid rotor; in an articulated rotor, the blades assume an upward angle through movement about the flapping hinges.

The tendency of a rotor blade to increase or decrease velocity in the plane of rotation due to movement of the blades' center of mass.  When the center of mass of a rotor blade moves closer to the axis of rotation, its rotational velocity will increase.  As the advancing blade of a rotor disc flaps up, its center of mass moves closer to the axis of rotation, and its velocity increases.  As it becomes the retreating blade it flaps down and its velocity decreases.  Coriolis effect is also known as the conservation of angular momentum.

A lateral rolling tendency which starts when the helicopter has one skid on the ground and it becomes a pivot point for lateral roll.  Each helicopter has a critical rollover angle beyond which recovery is impossible.  If the critical rollover angle is exceeded, the helicopter will roll over on its side regardless of cyclic corrections by the pilot.  An upslope rolling motion results from excessive application of cyclic into the slope.  A downslope rolling motion results from excessive application of collective.  Application of collective pitch is more effective than lateral cyclic in controlling the rolling motion because it changes main rotor thrust.  A smooth, moderate collective pitch reduction may be the most effective way to stop a rolling motion.  Reducing collective too fast, however, may create a roll in the opposite direction.  If collective reduction causes the downslope skid to hit the ground abruptly, the rate of motion may cause a roll or pivot about the downslope gear.

The unequal lift between the advancing and retreating blades created by forward movement of the helicopter, or wind during hovering flight.  Forward flight @ 100 mph:

• Rotor blade tip speed approximately 400 mph in no-wind hover
• Advancing blade airspeed will be 400 mph PLUS the 100 mph forward airspeed.
• Retreating blade airspeed will be 400 mph MINUS the 100 mph forward airspeed.

Increased efficiency of the rotor disc due to proximity of the ground -- usually occurs less than one rotor diameter above the surface.  It is caused by the rotor downwash field being altered from its free air state by the presence of the surface.  A helicopter will require a smaller angle of attack and less manifold air pressure to hover in ground effect than out of ground effect.

Hard contact with the ground which causes the blades to become out of balance.  Ground resonance generally occurs only with three-bladed, fully articulated rotor systems.  Corrective action could be an immediate takeoff if RPM is in the proper range, or an immediate closing of the throttle and down collective if the RPM is low.

When a force is applied to a rapidly spinning object, the reaction occurs 90 degrees away from the point that the force was applied, in the direction of rotation.  The pitch change links are connected 90 degrees away from the appropriate blade, compensating for gyroscopic precession.

A low-G situation could cause the rotor shaft to violently strike the mast, possibly shearing off the main rotor.

A condition of flight in which the helicopter settles in its own downwash.  The helicopter is descending in turbulent air that has just been accelerated downward by the rotor.  Reaction of this air on rotor blades at high angles of attack stalls the blades at the hub (center of the rotor) and the stall progresses outward along the blade as the rate of   descent increases.  The combination of conditions likely to cause settling with power:

• 10 knots airspeed or less
• 20% or more engine power
• 300 fpm or greater rate of descent
Hovering out of ground effect and a tailwind approach are conducive to a settling-with-power situation.  A proper glidepath will avoid settling with power.  Recovery may be accomplished by increasing forward speed and/or partially lowering the collective pitch.

Newton's third law of motion states, "To every action there is an equal and opposite reaction".  As the main rotor of a helicopter turns in one direction, the fuselage tends to rotate in the opposite direction.  Torque is counteracted by a tail rotor and pedals.  Since torque effect on the fuselage is a direct result of engine power supplied to the main rotor, any change in engine power brings about a corresponding change in torque effect.  Since there is no engine power supplied to the main rotor during autorotation, there is no torque reaction.

The entire helicopter has a tendency to move in the direction of tail rotor thrust while hovering.  Torque is also turning the helicopter to the right.  This movement is sometimes referred to as "drift".  To counteract this drift, the rotor mast in some helicopters is rigged slightly to the left side so that the tip-path plane has a built-in tilt to the left, thus producing a small sideward thrust.