FIXED-WING AIRCRAFT
You have learned about the physical laws and
forces that affect flight, the airfoil, and the rotational
axes of an aircraft. Now, let's apply these principles to a
fixed-wing aircraft in flight. First, motion must exist.
Motion is provided by the thrust developed by the
engine of the aircraft. This is accomplished by the force
exerted by the exhaust gases of a jet aircraft or by the
action of the propeller blades on a propeller-driven
aircraft. The thrust overcomes the force of inertia and,
as the fixed-wing aircraft accelerates, the air flows by
the wings. The relative wind striking the leading edge
of the wings is split and flows across the upper and
lower surfaces. The camber of the upper surface acts as
a constriction, which speeds up the airflow and reduces
the pressure of the air. The lower surface, being
relatively flat, doesn't affect the speed or pressure of the
air. There is lower air pressure on the upper surface of
the wing than on the lower surface. The fixed-wing
aircraft is lifted into the air.
Now that the aircraft is safely in the air, rotational
axes come into play. If the nose of the aircraft is raised,
the angle of attack changes. Changing the angle of
attack causes the aircraft to pivot on its lateral or pitch
axis. If you lower the right wing of the aircraft, the left
wing rises. The aircraft moves about its longitudinal or
roll axis. Assume that the aircraft is in a straight and
level flight. There is a strong wind striking the aircraft's
nose on the left side, pushing the nose to the right. This
causes the tail of the aircraft to move to the left, and the
aircraft is pivoting on its vertical or yaw axis. All of
these forces are necessary for flight to begin or be
sustained.
ROTARY-WING AIRCRAFT
(HELICOPTERS)
The same basic aerodynamic principles you read
about earlier in this chapter apply to rotary-wing
aircraft. The main difference between fixed-wing and
rotary-wing aircraft is the way lift is achieved.
Lift
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 the Greek words meaning helical wing or rotating
wing. A helicopter uses two or more engine-driven
rotors from which it gets lift and propulsion.
The helicopter's airfoils are the rotor blades. The
airfoils of a helicopter are perfectly symmetrical. This
means that the upper and lower surfaces are shaped the
same. This fact is one of the major differences between
the fixed-wing aircraft's airfoil and the helicopter's
airfoil. A fixed-wing aircraft's airfoil has a greater
camber on the upper surface than on the lower surface.
The helicopter's airfoil camber is the same on both
surfaces (fig. 3-9). The symmetrical airfoil is used on
the helicopter because the center of pressure across its
surface is fixed. On the fixed-wing airfoil, the center of
pressure moves fore and aft, along the chordline, with
changes in the angle of attack (fig. 3-9). If this type of
airfoil were used on a rotary-wing aircraft, it would
cause the rotor blades to jump around (dive and climb)
uncontrollably. With the symmetrical airfoil, this
undesirable effect is removed. The airfoil, when
rotated, travels smoothly through the air.
The main rotor of a helicopter consists of two or
more rotor blades. Lift is accomplished by rotating the
blades through the air at a high rate of speed. Lift may
be changed by increasing the angle of attack or pitch of
the rotor blades. When the rotor is turning and the
blades are at zero angle (flat pitch), no lift is developed.
This feature provides the pilot with complete control of
the lift developed by the rotor blades.
Directional Control
A pilot controls the direction of flight of the
helicopter by tilting the main rotor. If the rotor is tilted
forward, the force developed by the rotor is directed
downward and aft. Now, apply Newton's third law of
motion (action and reaction). Lift will be developed in
an upward and forward direction, and the helicopter
will tend to rise and move forward. From this example,
3-6
Figure 3-9.—Center of pressure.
