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.
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.
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.
|DISSYMMETRY OF LIFT|
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:
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.
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.
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.
situation could cause the rotor shaft to violently strike the mast, possibly
shearing off the main rotor.
|SETTLING WITH POWER|
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:
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.
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.
lift obtained when the airspeed over the rotor disk is 15 mph or more.
The rotor system becomes more efficient and produces more lift because
the higher inflow velocity supplies the rotor disc with a greater mass
of air per unit of time. The helicopter is also entering clean (undisturbed)
air. Effective translational lift will be achieved when all of the
air entering the rotor disk is new, and when the rotor-tip vortices are
separated from the rotor (when they trail behind).
|TRANSVERSE FLOW EFFECT|
efficiency of the rear half of the rotor disc due to effects of air flow
across the disc. In forward flight, air passing through the rear
portion of the rotor disc has a higher downwash velocity than air passing
through the forward portion. This is because the air passing through
the rear portion has been accelerated for a longer period of time than
the air passing through the forward portion. This increased downwash
velocity at the rear of the disc decreases the angle of attack and blade
lift, hence in combination with gyroscopic precession, causes the rotor
disc to tilt to the right (the advancing side). The lift on the forward
part of the rotor disc is greater than on the rearward part. Due
to gyroscopic precession, the rotor blades will reach maximum upward deflection
on the left side and maximum downward deflection on the right side.
The pilot compensates for transverse flow effect with left cyclic.
The rotational velocity of the rotor blade is greater at the tip than at the root. Since lift increases as the velocity of the airflow over the airfoil increases, there is an unequal distribution of lift over the rotor blade. This is compensated for by manufacturing the blade with a built-in twist, called washout. Blade twist produces a higher pitch angle at the root where speed is low, and a lower pitch angle at the tip where the speed is high. Washout helps distribute the lift more evenly along the rotor blade.
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