the area of the circle, the radius of which is equal to the
The usual method of counteracting torque in a
length of the rotor blade. Engineers determined that
single main rotor is by a tail (antitorque) rotor. This
the lift of a rotor is in proportion to the square of the
auxiliary rotor mounts vertically, or near vertical, on
length of the rotor blades. The desirability of large
the outer portion of the tail boom. The tail rotor and its
rotor disc areas is readily apparent. However, the
controls serve as a means to counteract torque, and it
greater the rotor disc area, the greater the drag, which
provides a means to control directional heading. See
results in the need for greater power requirements.
figure 17-2.
PITCH OF ROTOR BLADES
DISSYMMETRY OF LIFT
If the rotor is operated at zero pitch (flat pitch), no
Dissymmetry of lift is the difference in lift existing
lift will develop. When the pitch increases, the lifting
between the advancing blade half of the disc and the
force increases until the angle of attack reaches the
retreating blade half. The disc area is the area swept by
stalling angle. To even out the lift distribution along
the rotating blades. Dissymmetry is created by
the length of the rotor blade, it is common practice to
horizontal flight or by the wind when the helicopter is
twist the blade. With the twist, a smaller angle of
hovering. When hovering in a no-wind condition, the
attack results at the tip than at the hub.
speed of the relative wind in relation to the rotor is the
same. However, the speed reduces at points closer to
SMOOTHNESS OF ROTOR BLADES
the rotor hub, as shown in figure 17-3. When the
helicopter moves into forward flight, the relative wind
Tests have shown that the lift of a helicopter
moving over each blade becomes a combination of the
increases by polishing the rotor blades to a mirrorlike
rotor speed and the forward movement. The advancing
surface. By making the rotor blades as smooth as
blade is then the combined speed of the blade speed
possible, the parasite drag reduces. Dirt, grease, or
and helicopter speed. While on the opposite side, the
abrasions on the rotor blades cause increased drag,
retreating blade speed is the blade speed minus the
which decreases the lifting power of the helicopter.
speed of the helicopter. For example, figure 17-4
shows a helicopter moving forward at 100 mph. The
DENSITY ALTITUDE
advancing blade has a tip speed of 350 mph plus the
helicopter speed of 100 mph, or 450 mph. The
In formulas for lift and drag, the density of the air is
an important factor. The mass or density of the air
reacting in a downward direction causes the lift that
supports the helicopter.
Density is dependent on two factors. One factor is
altitude, since density varies from a maximum at sea
level to a minimum at high altitude. The other factor is
atmospheric changes. Because of the atmospheric
changes in temperature, pressure, or humidity, density
of the air may be different, even at the same altitude.
TORQUE
Although torque is not unique to helicopters, it
does present some special problems. As the rotor turns
in one direction, the fuselage rotates in the opposite
direction. Newton's third law of motion (every action
has an equal and opposite reaction) applies. This
tendency for the fuselage to rotate is known as the
torque effect. Since the torque effect on the fuselage is
a direct result of engine power, any change in power
changes the torque. The greater the engine power, the
greater the torque. There is no torque when the
rotary-wing head is not engaged or when the engine is
Figure 17-2.--Torque reaction.
not operating.
17-2
