momentum. Lift results from accelerating a mass
already entered the picture. In any discussion of
of air downward. This lift is similar to jet thrust
the principles of flight of the helicopter, the
that develops by accelerating a mass of air out
different velocities being considered must be
the exhaust. The second theory is the blade
specified. This also applies to such factors as
element theory. The airflow over the rotor blade
torque, drag, and other forces.
acts the same as it does on the wing of a fixed-
FACTORS AFFECTING HELICOPTER
wing aircraft. The simple momentum theory
FLIGHT
determines only lift characteristic while the
blade element theory gives both lift and drag
Helicopters are subject to several rotary-
characteristics. This comparison gives us a more
wing aerodynamic effects. These forces act
complete picture of all the forces acting on a rotor
independently. Their cumulative sum are factors
blade.
that affect helicopter flight.
The blade element theory divides the blade
into parts (blade elements), as shown in figure 7-1.
Torque
Engineers analyze the forces acting on each blade
Although torque is not unique to helicopters,
element. Then the forces of all elements are added
it does present some special problems. As the main
to give the rotor characteristics. Each rotor blade
rotors turn in one direction, the fuselage may
element has a different velocity, and possibly a
rotate in the opposite direction. Newton's third
different angle of attack. These differences make
law of motion states that "every action has an
analysis a complicated problem.
equal and opposite reaction." This tendency for
If the helicopter hovers in a no-wind condi-
the fuselage to rotate is known as torque effect.
tion, the rotors plane of rotation is parallel to the
Since torque effect on the fuselage is a direct result
level ground. This attitude also makes the relative
of engine power, any change in power changes
wind parallel to the ground. The angle of attack
the torque. The greater the engine power, the
is the same on any blade element throughout the
greater the torque. There is no torque reaction
cycle of rotation. The lifting force is perpendicular
when an engine is not operating. Therefore, there
to the plane of rotation.
is no torque reaction during autorotation.
If the helicopter is rising, there is a component
The usual method of counteracting torque in
of velocity parallel to the axis of the rotor. Then
a single main rotor is by a tail (antitorque) rotor.
the relative wind is the result of the rotational
This auxiliary rotor is mounted vertically on the
velocity and the vertical velocity of the helicopter.
outer portion of the tail boom. The tail rotor and
Lift acts perpendicular to the relative wind. The
its controls serve as a means to counteract torque,
relative wind is no longer parallel to the plane of
and it provides a means to control directional
rotation. Lift is not acting perpendicular to the
heading.
plane of rotation. The vertical thrust then, or the
force acting to overcome gravity, is slightly less
Dissymmetry of Lift
than the lifting force.
So far the discussion has been about the forces
Dissymmetry of lift is the lift difference
in the vertical direction. These forces support the
existing between the advancing blade half of the
helicopter, but do not give it any horizontal
disk and the retreating blade half. The disk area
motion. Rotational and vertical velocities have
is the area swept by the rotating blades. It is
Figure 7-1.-Blade element theory.
7-2
