Lesson 5: Helicopter Description and Operation Principle

 

Description and Operation of Helicopter 

Helicopter Description and Operation Principale


 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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Click here for the previous lessons, to learn about: Principle of Airframe; Principles of Aerodynamics; Airfoil Characteristics; Primary Flight Control Surfaces; Description and Operation of Helicopter; Miscellaneous Components of an Aircraft…
Helicopter Description and Operation

Tags:

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).


Helicopter Structure (HH-60)


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.


Main Rotor Configurations


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.


Breakdown of Typical Helicopter


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.


Tail Rotor Assembly


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.


Relative Blade Angle and Speed in Forward Flight


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.


Cyclic and Collective Pitch Levers


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.

 

Stabilizer on an MH-53J

Figure 1-30, Stabilizer on an MH-53J and Stabilator on an HH-60