to Compensate for Torque
Torque
Torque
Blade Rotation
Figure 1-3. The antitorque rotor produces thrust to oppose
torque and helps prevent the helicopter from turning in the
opposite direction of the main rotor.
Figure 1-4. Compared to an unprotected tail rotor, the fenestron
antitorque system provides an improved margin of
safety during ground operations.
1-3
nose or tail gear is free to swivel as the helicopter is
taxied on the ground.
POWERPLANT
A typical small helicopter has a reciprocating engine,
which is mounted on the airframe. The engine can be
mounted horizontally or vertically with the transmission
supplying the power to the vertical main rotor
shaft. [Figure 1-6]
Another engine type is the gas turbine. This engine is
used in most medium to heavy lift helicopters due to its
large horsepower output. The engine drives the main
transmission, which then transfers power directly to the
main rotor system, as well as the tail rotor.
FLIGHT CONTROLS
When you begin flying a helicopter, you will use four
basic flight controls. They are the cyclic pitch control;
the collective pitch control; the throttle, which is
usually a twist grip control located on the end of the
collective lever; and the antitorque pedals. The collective
and cyclic controls the pitch of the main rotor
blades. The function of these controls will be explained
in detail in Chapter 4—Flight Controls. [Figure 1-7]
Figure 1-5. While in a hover, Coanda Effect supplies approximately
two-thirds of the lift necessary to maintain directional
control. The rest is created by directing the thrust from the
controllable rotating nozzle.
Main Rotor
Wake
Rotating
Nozzle
Downwash
Air
Jet
Lift
Air Intake
Main
Rotor
Main
Transmission
Antitorque
Rotor
Engine
Figure 1-6. Typically, the engine drives the main rotor through
a transmission and belt drive or centrifugal clutch system.
The antitorque rotor is driven from the transmission.
Cyclic
Throttle
Collective
Antitorque
Pedals
Figure 1-7. Location of flight controls.
1-4
2-1
There are four forces acting on a helicopter in flight.
They are lift, weight, thrust, and drag. [Figure 2-1] Lift
is the upward force created by the effect of airflow as it
passes around an airfoil. Weight opposes lift and is
caused by the downward pull of gravity. Thrust is the
force that propels the helicopter through the air.
Opposing lift and thrust is drag, which is the retarding
force created by development of lift and the movement
of an object through the air.
AIRFOIL
Before beginning the discussion of lift, you need to be
aware of certain aerodynamic terms that describe an
airfoil and the interaction of the airflow around it.
An airfoil is any surface, such as an airplane wing or a
helicopter rotor blade, which provides aerodynamic
force when it interacts with a moving stream of air.
Although there are many different rotor blade airfoil
designs, in most helicopter flight conditions, all airfoils
perform in the same manner.
Engineers of the first helicopters designed relatively
thick airfoils for their structural characteristics.
Because the rotor blades were very long and slender, it
was necessary to incorporate more structural rigidity
into them. This prevented excessive blade droop when
the rotor system was idle, and minimized blade twisting
while in flight. The airfoils were also designed to
be symmetrical, which means they had the same camber
(curvature) on both the upper and lower surfaces.
Symmetrical blades are very stable, which helps keep
blade twisting and flight control loads to a minimum.
[Figure 2-2] This stability is achieved by keeping the
center of pressure virtually unchanged as the angle of
attack changes. Center of pressure is the imaginary
point on the chord line where the resultant of all aerodynamic
forces are considered to be concentrated.
Today, designers use thinner airfoils and obtain the
required rigidity by using composite materials. In addition,
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