Description and Operation of Helicopter
Helicopter designs date back to the late
1400’s with Leonardo da Vinci’s rotating helix. In the modern flight era, Igor
Sikorsky began designing helicopters for Russia in 1910. He also made the first
successful helicopter flight in 1939, when he lifted off in the VS-300 in the
United States.
Start from Lesson 1: Principle of Airframe
Go to Lesson 2 : PRINCIPLES OF AERODYNAMICS
Summary
DESCRIPTION
of Helicopter Description and Operation
Drive
train of Helicopter.
Rotor
Systems of Helicopter.
Main
Rotor of Helicopter.
Main
Rotor Head of Helicopter.
Tail
Rotor of Helicopter
OPERATION
of Helicopter.
Cyclic
Pitch Lever of Helicopter.
Collective
Pitch Lever of Helicopter.
Anti-Torque
Pedals of Helicopter.
Swash
plate of Helicopter.
Horizontal
Stabilizer or Stabilator of Helicopter.
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DESCRIPTION of Helicopter, Operation of Helicopter, Drive train, Rotor Systems, Main Rotor, Main Rotor, Tail Rotor, OPERATION of Helicopter, Cyclic Pitch Lever, Collective Pitch Lever, Anti-Torque Pedals, Swash plate, Horizontal Stabilizer.
Helicopters
have served in every conflict beginning with World War II. They have been used
as observation platforms, liaison and casualty evacuation vehicles, and later for
search and rescue, armed escort, anti-submarine, and attack missions. The Air Force
presently has helicopter units throughout the world performing special
operations, rescue, and training missions.
So far, we have covered aerodynamics as they
apply to airplanes. After covering so much information, you might be wondering
why there is a separate section devoted to helicopters. Helicopters seem to
defy the traditional forces of aerodynamics. As you will see, essentially the
same forces apply but helicopters overcome these forces by different means.
Rather than lift being created by a static wing being forced through the air by
thrust, the wing rotates around the vertical axis. This relative motion of the rotating
blades creates lift.
DESCRIPTION of Helicopter Description
and Operation
The
structure of most helicopters consists of a fuselage, tail boom and pylon, main
and tail rotor systems, and landing gear or systems, see figure 1- 23. Landing
gear types vary somewhat. There are skids (UH-1/Huey), fixed conventional gear
(HH-60), retractable tricycle (H-53), or quadra-cycle gear (CH 47/Chinook).
Some helicopters are equipped with floats to allow for water landings (H
3/Jolly Green Giant).
Figure
1-23, Helicopter Structure (HH-60)
Drive train of Helicopter.
The
Drive train consists of the transmission, the main rotor, and the tail rotor.
Power is supplied for Air Force helicopters by turbo-shaft jet engines and most
have twin engines. In emergency conditions they can fly on one engine. Each engine
feeds through a series of drive shafts into the helicopter transmission system.
The transmission consists of the engines, gearboxes, and rotor drive shafts.
Power from the engines turns the main gearbox through the input drive shafts.
In turn, the main gearbox drives the accessories (hydraulics/electrics) and
outputs to the tail driveshaft system. The tail driveshaft turns the tail
gearbox and the tail rotor system.
Rotor Systems of Helicopter.
The
majority of rotary-wing aircraft use one main rotor system to generate lift and
thrust, and a tail rotor system to counteract the torque of the main rotor
system and to provide yaw control. Figure 1-24. Alternatively, counter-rotating
and tandem main rotor configurations can be used.
Figure
1-24, Main Rotor Configurations
Main Rotor of Helicopter.
There
are currently two types of main rotor systems: Semi-rigid (Huey) and fully
articulated (Sikorsky – H-60/3/53). Fully articulated rotor systems allow for
main rotor movement in all directions (like your hand on your wrist). To
provide forward movement, AOA is increased on the rotor blades towards the rear
of the aircraft. The main rotor on most helicopters is allowed to tilt forward
such as the 3- degree tilt on an H-60. The primary components of the main rotor
system are the main gearbox, lower and upper (non-rotating and rotating) swash
plates pitch control rods, main rotor head, scissors, dampers, droop
restrainers, and main rotor blades. The
main gearbox/transmission, figure 1-25, drives the main and tail rotor while reducing
the speed from the engine inputs. Swash plates are mounted on a ball and socket
assembly which keeps them parallel with one another at all times but allows them
to be tilted, raised, or lowered simultaneously. Servos connected to the main gearbox
also connect the lower (non-rotating) swash plate and directional changes are transmitted
to the upper (rotating) swash plate and on to the main rotor blades through the
pitch change rods or pitch links and scissors.
Figure
1-25, Breakdown of Typical Helicopter
Main Rotor Head of Helicopter.
The main rotor head, is actually
an assembly consisting of everything from the swash plates to the rotor head
cap. It includes a hub assembly, which has the main blade attachment points and
allows the blade to flap drag and pitch. These attachment points have dampers
that cushion the movement of the main rotor blades while starting, stopping and
in flight. Droop restrainers prevent the blades from flapping during start-up
and shutdown.
Tail Rotor of Helicopter
.
The tail rotor system, figure 1-27, is used to counteract the torque produced
by the movement of the main rotor blades, and will be used to control the helicopter’s
yaw. This system consists of the tail gearbox, tail rotor head, pitch change links,
and tail rotor blades. Basic operation is much the same as the main rotor system
except that the tail rotor is not fully articulated.
Figure
1-27, Tail Rotor Assembly
OPERATION of Helicopter.
A
pilot maneuvers a helicopter by changing the pitch, or angle, of the rotor
blades as they rotate through the air. As the blades rotate, they create lift. When
the pitch (AOA) of a blade is increased, more lift is produced. By directing
the lift, the helicopter can be propelled in different directions. Pilots use
three different controls to maneuver helicopters: anti-torque pedals, a cyclic
pitch stick, and a collective pitch stick.
Figure
1-28, Relative Blade Angle and Speed in Forward Flight
Cyclic Pitch Lever of Helicopter.
The cyclic pitch lever, item number 1 in
figure 1-29, moves a helicopter in a longitudinal/lateral direction by
controlling the direction of the main rotor’s thrust. This lever affects the
pitch of the rotor blades as they cycle through a rotation. Increasing the
pitch of a blade at a particular point during its rotation increases the amount
of lift at that point. By selecting where along the rotor’s path lift is
increased, the pilot can tilt the helicopter forward, backward, or to either
side.
Collective Pitch Lever of Helicopter.
The
collective pitch lever, item number 2 in figure 1-29, allows the helicopter to
climb and descend vertically (along the vertical axis). It changes the pitch of
all the main rotor blades equally, and performs much the same function as the
pedals perform on the tail rotor. Pulling or pushing on the lever increases or decreases
the thrust produced, varying the lift. Most collective pitch levers also have a
twist grip that changes the speed of the engine, in much the same way as the
throttle of a motorcycle. Increasing rotor speed is another way to increase
lift, but this is not normally done.
Figure
1-29, Cyclic and Collective Pitch Levers
Anti-Torque Pedals of Helicopter.
The pilot’s feet control two anti-torque
pedals, which are used to control yaw. Unlike with fixed wing aircraft,
helicopters will turn or point their nose to the left or right as a part of
normal flight operations. The pedals control the pitch of the tail rotor
blades, increasing or decreasing the aerodynamic thrust produced by that rotor.
The tail rotor provides the sideways thrust needed to counteract the torque
produced by the main rotor. When the thrust from the tail rotor balances the torque
on the main rotor’s shaft, the helicopter points forward. However, when the
right pedal is pushed, the pitch of the tail rotor blades decreases and the
thrust is reduced. The torque from the main rotor shaft then turns the nose of
the helicopter to the right. When the left pedal is pushed, the tail rotor
thrust increases, and the nose turns to the left. Tandem-rotor helicopters,
which use two main rotors instead of a main rotor and a tail rotor, turn by
tilting the rotors in different directions.
Swash plate of Helicopter.
The
cyclic pitch lever and anti-torque pedals change rotor pitch through a device
called a swash plate. This device consists of two circular plates that surround
the rotor shaft. The upper plate rotates with the rotor blades and rests on the
lower plate, which is controlled by the lever/pedals. Moving the cyclic pitch
lever forward, for example, tilts the lower plate, which in turn tilts the
upper plate controlling the rotor blades. The swash plate lowers the pitch of
the blades as they pass the right side of the helicopter, momentarily
decreasing lift and causing the blades to flap downward. The swash plate at the
same time increases the pitch of the blades as they pass the left side of the
helicopter, increasing lift and causing the blades to flap upward. The front of
the helicopter then points lower than the rear, and so the helicopter moves forward.
Pushing the cyclic pitch lever in any direction will tip the rotor blades accordingly,
allowing the helicopter to travel in any direction. When the lever is centered,
the helicopter hovers in midair.
Horizontal Stabilizer or Stabilator of Helicopter.
See
Figure 1-30. Some helicopters have a horizontal tail surface synchronized with
the cyclic pitch control to create a downward force that increases when the
cyclic control tilts the main rotor forward. Without this provision, the
fuselage will assume an excessive nose-down pitch at high airspeeds. Helicopters
without a moveable stabilator may have a fixed stabilizer mounted on the tail
boom.
Figure
1-30, Stabilizer on an MH-53J and Stabilator on an HH-60