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“for every action there is an equal and opposite reaction,”
the air that is deflected downward also produces
an upward (lifting) reaction.
Since air is much like water, the explanation for this
source of lift may be compared to the planing effect of
skis on water. The lift which supports the water skis
(and the skier) is the force caused by the impact pressure
and the deflection of water from the lower surfaces
of the skis.
Under most flying conditions, the impact pressure and
the deflection of air from the lower surface of the rotor
blade provides a comparatively small percentage of the
total lift. The majority of lift is the result of decreased
pressure above the blade, rather than the increased
pressure below it.
WEIGHT
Normally, weight is thought of as being a known, fixed
value, such as the weight of the helicopter, fuel, and
occupants. To lift the helicopter off the ground vertically,
the rotor system must generate enough lift to
overcome or offset the total weight of the helicopter
and its occupants. This is accomplished by increasing
the pitch angle of the main rotor blades.
The weight of the helicopter can also be influenced by
aerodynamic loads. When you bank a helicopter while
maintaining a constant altitude, the “G” load or load
factor increases. Load factor is the ratio of the load supported
by the main rotor system to the actual weight of
the helicopter and its contents. In steady-state flight,
the helicopter has a load factor of one, which means the
main rotor system is supporting the actual total weight
of the helicopter. If you increase the bank angle to 60°,
while still maintaining a constant altitude, the load factor
increases to two. In this case, the main rotor system
has to support twice the weight of the helicopter and its
contents. [Figure 2-11]
Disc loading of a helicopter is the ratio of weight to the
total main rotor disc area, and is determined by dividing
the total helicopter weight by the rotor disc area,
which is the area swept by the blades of a rotor. Disc
area can be found by using the span of one rotor blade
as the radius of a circle and then determining the area
the blades encompass during a complete rotation. As
the helicopter is maneuvered, disc loading changes.
The higher the loading, the more power you need to
maintain rotor speed.
Leading Edge
Stagnation Point
B
A
Compare the upper surface of an airfoil with the constriction
in a venturi tube that is narrower in the middle
than at the ends. [Figure 2-9]
The upper half of the venturi tube can be replaced by
layers of undisturbed air. Thus, as air flows over the
upper surface of an airfoil, the camber of the airfoil
causes an increase in the speed of the airflow. The
increased speed of airflow results in a decrease in pressure
on the upper surface of the airfoil. At the same
time, air flows along the lower surface of the airfoil,
building up pressure. The combination of decreased
pressure on the upper surface and increased pressure
on the lower surface results in an upward force.
[Figure 2-10]
As angle of attack is increased, the production of lift is
increased. More upwash is created ahead of the airfoil
as the leading edge stagnation point moves under the
leading edge, and more downwash is created aft of the
trailing edge. Total lift now being produced is perpendicular
to relative wind. In summary, the production of
lift is based upon the airfoil creating circulation in the
airstream (Magnus Effect) and creating differential
pressure on the airfoil (Bernoulli’s Principle).
NEWTON’S THIRD LAW OF MOTION
Additional lift is provided by the rotor blade’s lower
surface as air striking the underside is deflected down-
Figure 2-10. Lift is produced when there is decreased pressure
above and increased pressure below an airfoil.
Lift
Decreased Pressure
Increased Pressure
Increased Velocity
Decreased Pressure
Figure 2-9. The upper surface of an airfoil is similar to the
constriction in a venturi tube.
2-5
DRAG
The force that resists the movement of a helicopter
through the air and is produced when lift is developed
is called drag. Drag always acts parallel to the relative
wind. Total drag is composed of three types of drag:
profile, induced, and parasite.
PROFILE DRAG
Profile drag develops from the frictional resistance of
the blades passing through the air. It does not change
significantly with the airfoil’s angle of attack, but
increases moderately when airspeed increases. Profile
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