Lesson13 Helicopter

7/29/2019 Lesson13 Helicopter

1/12

Lesson 13

Propel me


2/12

In a helicopter, you can move in any direction or you can rotate 360 degrees


3/12

How do helicopters work?

The wings of a normal airplane obtain upward lift forces from the air as the airplane movesforward through the air.

That's because the shape and angle of the wings is such that air flows faster over the top

surface of each wing than under the bottom surface.

A helicopter spins its wings around in a circle so that they move through the air even when

the helicopter itself is stationary.

Normally, these rotating wings are called blades. Again, the air flows faster over each blade

than beneath it and there is a net upward pressure force on each blade.

These upward forces support the helicopter and they also allow it to tilt itself--by adjusting

the angle of each blade as the blades turn, the helicopter can obtain twists from the air so

accelerates forward, backward, or toward the side.


4/12


5/12

The Main RotorA helicopter's main rotor is the most important part of the vehicle. It providesthe lift that allows the helicopter to fly, as well as the control that allows the

helicopter to move laterally, make turns and change altitude.To handle all of these tasks, the rotor must first be incredibly strong. It must

also be able to adjust the angle of the rotor blades with each revolution of the

hub. The adjustability is provided by a device called the swash plateassembly.


6/12

The swash plate assembly consists of two plates -- the fixed and the rotating swash platesshown above in blue and red, respectively.

The rotating swash plate rotates with the drive shaft (green) and the rotor's blades (grey)because of the links (purple) that connect the rotating plate to the drive shaft.

The pitch control rods (orange) allow the rotating swash plate to change the pitch of the

rotor blades.The angle of the fixed swash plate is changed by the control rods (yellow) attached to thefixed swash plate.

The fixed plate's control rods are affected by the pilot's input to the cyclic and collectivecontrols.The fixed and rotating swash plates are connected with a set ofbearings between the twoplates. These bearings allow the rotating swash plate to spin on top of the fixed swash plate.


7/12

The cyclic changes the angle of attackof the main rotor's wings unevenly by tiltingthe swash plate assembly.On one side of the helicopter,the angle of attack (and therefore the lift) is greater.

The collective lets you change theangle of attack of the main rotorsimultaneously on both blades.


8/12

In order to spin the shaft with enough force to lift a human being and the vehicle,

you need an engine. This arrangement works really well until the moment the

vehicle leaves the ground. At that moment, there is nothing to keep the engine

(and therefore the body of the vehicle) from spinning just like the main rotor

does. So, in the absence of anything to stop it, the body will spin in an oppositedirection to the main rotor. To keep the body from spinning, you need to apply a

force to it.

The usual way to provide a force to the body of the vehicle is to attach another

set of rotating wings to a long boom. These wings are known as the tail rotor.The tail rotor produces thrust just like an airplane's propeller does. By producing

thrust in a sideways direction, counteracting the engine's desire to spin the body,the tail rotor keeps the body of the helicopter from spinning.

The Forces At Work

There are many forces at work when a helicopter flies, and many are specific to

helicopter flight. We all have learnt about: lift, drag, gravity, and thrust

The tail rotor has a 6 to 1 rotational ratio to the main rotor system.


9/12

Most people think that a helicopter will fall like a rock and the rotor system will stop once the engine

fails. This is a totally false assumption. A helicopter can continue to fly without any power from the

engine. "Autorotation" is the term used for "Gliding" a helicopter down after the engine fails or the

throttle is retarded to the idle position.

If you look at a rotor blade from the tip of the blade toward the root, you will see it will twist laterally.

At the tip of the blade, the leading edge may point down while at the root of the blade, the leading edge

may point up. This allows different regions of the blade to perform different tasks, one of which isAutorotation. The outer portion of the blade, when the collective is lowered all the way to what is called

"Flat Pitch", will drive the rotor system as the aircraft glides downward, increasing or maintaining the

speed of the rotor system. The rotor system is driven normally by a centrifugal clutch which is

positively engaged while the engine supplies power, but disengages when power is removed. The rotor

system "Free Wheels", and continues to spin. The air travelling upward through the rotor system

continues to drive the system and maintain rotor RPM.

Autorotation


10/12

Notes


11/12

A rotary motion is the easiest way to keep a wing in continuous

motion. So you can mount two or more wings on a central shaft and

spin the shaft, much like the blades on a ceiling fan. The rotating wings

of a helicopter are shaped just like the airfoils of an airplane wing, but

generally the wings on a helicopter's rotor are narrow and thin becausethey must spin so quickly. The helicopter's rotating wing assembly is

normally called the main rotor. If you give the main rotor wings a

slight angle of attack on the shaft and spin the shaft, the wings start to

develop lift.

In order to spin the shaft with enough force to lift a human being and

the vehicle, you need an engine of some sort. Reciprocating gasoline

engines and gas turbine engines are the most common types. The

engine's driveshaft can connect through a transmission to the main

rotor shaft. This arrangement works really well until the moment the

vehicle leaves the ground. At that moment, there is nothing to keep the

engine (and therefore the body of the vehicle) from spinning just like

the main rotor does. So, in the absence of anything to stop it, the body

will spin in an opposite direction to the main rotor. To keep the body

from spinning, you need to apply a force to it.

The usual way to provide a force to the body of the vehicle is to attach

another set of rotating wings to a long boom. These wings are known

as the tail rotor. The tail rotor produces thrust just like an airplane'spropeller does. By producing thrust in a sideways direction,

counteracting the engine's desire to spin the body, the tail rotor keeps

the body of the helicopter from spinning. Normally, the tail rotor is

driven by a long drive shaft that runs from the main rotor's transmission

back through the tail boom to a small transmission at the tail rotor.

What you end up with is a vehicle that looks something like this:
http://travel.howstuffworks.com/category-engines.htmhttp://travel.howstuffworks.com/engine.htmhttp://travel.howstuffworks.com/engine.htmhttp://travel.howstuffworks.com/turbine.htmhttp://travel.howstuffworks.com/fpte.htmhttp://travel.howstuffworks.com/fpte.htmhttp://travel.howstuffworks.com/turbine.htmhttp://travel.howstuffworks.com/engine.htmhttp://travel.howstuffworks.com/engine.htmhttp://travel.howstuffworks.com/category-engines.htm


12/12

It takes both hands and both feet to fly a helicopter!

One hand grasps a control called the cyclic, which controls thelateral direction of the helicopter (including forward, backward, left

and right). The other hand grasps a control called the collective,which controls the up and down motion of the helicopter (and also

controls engine speed). The pilot's feet rest on pedals that control the

tail rotor, which allows the helicopter to rotate in either direction on itsaxis.

Documents

Lesson13 Helicopter