90 degrees from the applied force. Applying a
FACTORS AFFECTING
downward force to the right of the disk area will
ROTOR BLADE LIFT
cause the rotor to tilt down in front. This
Factors that affect rotor blade lift are the rotor
downward tilt is true only for a right-to-left
area, pitch of rotor blades, smoothness of rotor
(counterclockwise) rotor rotation.
blades, and density altitude.
The cyclic control applies force to the
main rotor through the swashplate. To simplify
Rotor Area
directional control, helicopters use a mechanical
One assumption in figuring the lift of a rotor
linkage, which places cyclic pitch change 90
is that lift is dependent upon the entire area of
degrees ahead of the applied force. Moving the
the rotor disk. The rotor disk area is the area of
cyclic control forward (right-to-left turning rotor)
the circle. The radius of the rotor disk is equal
places high pitch on the blades to the pilot's left.
to the length of the rotor blade. The lift of a rotor
Low pitch is then found on the blades to the
increases not in direct proportion to the length
pilot's right. Since every pitch change causes a
of the rotor, but in proportion to the square of
flap, reaching its maximum at 90 degrees, this
the length of the rotor. The use of large rotor disk
flapping causes the disk area to tilt forward.
areas is readily apparent. The greater the rotor
If offset linkage were not used, the pilot would
disk area, the greater the drag created. This drag
have to move the cyclic stick 90 degrees out of
results in the need for greater power requirements.
phase. The pilot would have to move the stick to
the right to tilt the disk forward, and forward to
Pitch of Rotor Blades
tilt the disk area to the left, and so on.
If the rotor operates at zero angle of attack
Ground Effect
or zero pitch, no lift would result. When the pitch
increases, the lifting force increases until the angle
When a helicopter hovers close to the ground,
of attack reaches the stalling angle. To even out
the rotor directs air downward faster then it can
the lift distribution along the length of the rotor
escape. This builds up a cushion of dense air
blade, it is common practice to twist the blade.
beneath the helicopter known as ground cushion
Twisting the blade causes a smaller angle of
or ground effect. It is effective to a height of one-
attack at the tip than at the hub.
half the rotor diameter. Ground cushion effect
does not occur at airspeeds greater then 10 mph.
Smoothness of Rotor Blades
Autorotation
Tests have shown that the lift of a helicopter
increases by polishing the rotor blades to a mirror-
Autorotation occurs when the main rotor
like surface. By making the rotor blades as smooth
turns by air passing up through the rotor system
as possible, the parasite drag reduces. Any dirt,
instead of by the engine. The rotor disengages
grease, or abrasions on the rotor blades may be
automatically from the engine during engine
a source of increased drag, which will decrease
failure or shutdown. During autorotation, the
the lifting power of the helicopter.
rotor blades turn in the same direction as when
engine driven. Air passes up through the rotor
Density Altitude
system instead of down. This causes a slightly
In formulas for lift and drag, the density of the
greater upward flex or coning of the blades.
air is an important factor. The mass or density of
Power Settling
the air reacting in a downward direction causes the
upward force or lift that supports the helicopter.
Stalling, as applied to fixed-wing aircraft, will
Density is dependent on two variables. One
not occur in helicopters. However, power settling
variable is the altitude, since density varies from
may occur in low-speed flight. Power settling is
a maximum at sea level to a minimum at high
the uncontrollable loss of altitude. Heavy gross
altitude. The other variable is atmospheric
weights, poor density conditions, and low forward
changes. The density of the air may be different,
speed all contribute to power settling. During low
even at the same altitude, because of changes in
forward speed and high rates of descent, the
temperature, pressure, or humidity.
downwash from the rotor begins to recirculate.
The downwind flows up, around, and back down
through the effective outer disk area. The recir-
FLIGHT CONTROL SYSTEM
culating air velocity may become so high that
The mechanical flight control system consists
full collective pitch cannot control the rate of
of mechanical linkage and controls. See figure 7-4.
descent.
7-4
