ROTARY-WING FLIGHT CONTROL SYSTEMS
Chapter Objective: Upon completion of this chapter, you &ill have a working
knowledge of the theory of operation and the maintenance requirements for
rotary-wing (helicopter) aircraft.
The helicopter has become a vital part of naval
aviation. The helicopter, known also as a rotary-wing
aircraft, has many military
antisubmarine warfare (ASW) and search and rescue
functions, as well as minesweeping and amphibious
warfare functions. The advantages of the helicopter
over conventional aircraft are that lift and control are
relatively independent of forward speed. A helicopter
can fly forward, backward, sideways, or remain in
above the ground
Helicopters do not require runways for takeoffs or
The decks of small ships or open fields
provide an adequate landing area.
THEORY OF FLIGHT
Recognize the princi-
ples of aerodynamics peculiar to the flight of
The same basic aerodynamic principles apply to
rotary-wing aircraft as fixed-wing aircraft. The main
difference between the two types of aircraft is in the
way lift occurs. The fixed-wing aircraft gets its lift
from a fixed airfoil surface. The helicopter gets lift
from rotating airfoils called rotor blades. The word
helicopter comes from Greek words meaning helical
wing or rotating wing. A helicopter uses one or more
engine-driven rotors, from which it gets lift and
The main rotor of a helicopter consists of two or
more rotor blades. The airfoils of a helicopter are
perfectly symmetrical. This means that the upper and
lower surfaces are alike. This fact is one of the major
differences between a fixed-wing aircrafts airfoil and
the helicopters airfoil.
The airfoil on a fixed-wing
aircraft has a greater camber on the upper surface than
on the lower surface. The helicopters airfoil camber
is the same on both surfaces.
See figure 10-l.
Helicopters have symmetrical airfoils because the
center of pressure across its surface should not move.
On the fixed-wing
airfoil, the center of pressure
moves fore and aft, along the chord line. The center
of pressure changes with changes in the angle of
attack. If this type of airfoil was on a rotary-wing
aircraft, it would cause the rotor blades to jump
With the symmetrical airfoil,
this undesirable effect does not exist.
when rotated, travels smoothly through the air.
Rotor lift can be explained by either of two
theories. The first theory uses Newtons
momentum. Lift results from accelerating a mass of
air downward. This action is similar to jet thrust,
which develops by accelerating a mass of air out the
The second theory is the blade element
The airflow over an airfoil section (blade
element) of the rotor blade acts the same as it does on
a fixed-wing aircraft. The simple momentum theory
determines only the lift characteristic, while the blade
element theory gives both lift and drag characteristics.
This theory gives us a more complete picture of all the
forces acting on a rotor blade.
Lift changes by increasing the angle of attack or
pitch of the rotor blades.
This action produces
enough lift to raise the helicopter off the ground and
keep it in the air. On a helicopter, when the rotor is
turning and the blades are at zero angle of attack, no