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filercaliforniaMechanics

Nov 14, 2013 (3 years and 4 months ago)

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INTRODUCTION









A

helicopter

is

an

aircraft

that

is

lifted

and

propelled

by

one

or

more

horizontal

rotors,

each

rotor

consisting

of

two

or

more

rotor

blades
.



A

helicopter

works

by

having

its

wings

move

through

air

while

the

body

stays

still
.

The

helicopter

blades

are

called

main

rotor

blades
.

During

flight

there

are

four

forces

on

the

helicopter

and

those

forces

are

lift,

drag,

thrust

and

weight
.



Technical Terms



Bernoulli'

principle

:
This

principle

states

that

as

the

air

velocity

increases,

the

pressure

decreases
;

and

as

the

velocity

decreases,

the

pressure

increases

.



Airfoil

:

is

technically

defined

as

any

surface,

such

as

an


elevator,

rudder,

wing,

main

rotor

blades,

or

tail

rotor

blades

designed

to

obtain

reaction

from

the

air

through

which

it

moves






Angle

of

Attack

:
is

the

acute

angle

measured

between

the

chord

of

an

airfoil

and

the

relative

wind
.







Angle

of

Incidence

:
is

the

acute

angle

between

the

wing's

chord

line

and

the

longitudinal

axis

of

the

airplane
.

(usually

manufacturer

had

built

the

aircraft

with

the

wing

has

some

degrees

to

the

horizontal

plane

or

airplane

longitudinal

axis




Blades

:

The

blades

of

the

helicopter

are

airfoils

with

a

very

high

aspect

ratio

(

length

to

chord

)
.

The

angle

of

incidence

is

adjusted

by

means

of

the

control

from

pilots
.











Swash

Plate

Assembly

:

The

swash

plate

assembly

consists

of

two

primary

elements

through

which

the

rotor

mast

passes
.

One

element

is

a

disc,

linked

to

the

cyclic

pitch

control
.

This

disc

is

capable

of

tilting

in

any

direction

but

does

not

rotate

as

the

rotor

rotates
.












Transmission

:

The

transmission

system

transmits

engine

power

to

the

main

rotor,

tail

rotor,

generator

and

other

accessories


Description of lift on an airfoil




In

a

helicopter,

the

structure

making

flight

possible

is

the

airfoil

-

a

surfaced

body

that

responds

to

relative

motion

between

itself

and

the

air

with

a

useful,

dynamic

reaction

known

as

lift
.

The

term

airfoil,

refers

to

the

rotary

wing,

and

more

specifically

means

the

curvature,

or

camber,

of

the

blade
.





As

the

diagram

indicates,

the

thick

end

of

the

section

is

known

as

the

leading

edge
.

The

small

tapering

end

is

the

trailing

edge
.

The

distance

between

the

leading

edge

and

the

trailing

edge

is

known

as

the

chord

of

the

airfoil
.














The

rotary

wing

blade

in

a

helicopter

is

asymmetrical,

that

is

it

has

a

curvature

that

changes

along

the

entire

length

of

the

chord
.

If

the

blade

were

symmetrical,

then

the

chord

line

would

be

a

straight

line

from

the

leading

to

the

trailing

edges
.

Since

the

curvature

changes

constantly

in

rotor

blade,

the

result

is

that

the

chord

of

the

blade

also

changes
.

When

computing

the

chord

line

of

this

type

of

blade,

an

average

or

mean

aerodynamic

chord

(MAC)

becomes

apparent

.











When

a

blade

(airfoil)

is

moved

through

the

air,

a

stream

of

air

flows

over

and

under

it
.

The

blade

is

designed

so

that

the

flow

of

air

will

be

smooth

and

will

conform

to

the

shape

of

the

moving

blade
.

If

the

blade

is

set

at

the

proper

angle

and

made

to

move

fast

enough,

the

airflow

will

support

the

weight

of

the

blade
.

This

is

the

nature

of

the

action

that

enables

rotary

wings

to

furnish

enough

lift

to

sustain

the

helicopter

in

flight
.











Hence

by

applying

bernoulli’s

theorem

we

can

observe

that

lift

is

produced

by

a

lower

pressure

created

on

the

upper

surface

of

the

helicopter’s

wings

compared

to

the

pressure

on

the

wing's

lower

surfaces
,

causing

the

wing

to

be

lifted

upward
.

The

special

shape

of

the

rotor

(airfoil)

is

designed

so

that

air

flowing

over

it

will

have

to

travel

a

greater

distance

and

faster

resulting

in

a

lower

pressure

area


thus

lifting

the

wing

upward
.




Lift equation



Lift

depends

upon
:



(
1
)

shape

of

the

airfoil




(
2
)

the

angle

of

attack




(
3
)

the

area

of

the

surface

exposed

to

the

airstream




(
4
)

the

square

of

the

air

speed




(
5
)

the

air

density
.





Where,


L

is

lift

force,


ρ

is

air

density,


v

is

air

speed

over

the

airfoil,


A

is

wing

area,

and


C
L

is

the

lift

coefficient

at

the

desired

angle

of

attack








Lift

in

an

established

flow



Established

flow

may

be

considered

as

steady,

laminar

&

incompressible

flow
.


As

fluid

never

crosses

a

streamline

in

a

steady

flow
;

hence

mass

is

conserved

within

each

streamtube
.



One

streamtube

travels

over

the

upper

surface,

while

the

other

travels

over

the

lower

surface
;

dividing

these

two

tubes

is

a

dividing

line

that

intersects

the

airfoil

on

the

lower

surface,

typically

near

to

the

leading

edge
.



The

streamline

leaves

the

airfoil

at

the

sharp

trailing

edge,

a

feature

of

the

flow

known

as

the

Kutta

condition
.





This

image

shows

the

streamlines

over

a

NACA

0012

airfoil

of

the

real

flow
.

The

flow

approaching

an

airfoil

can

be

divided

into

two

streamtubes
,

which

are

defined

based

on

the

area

between

two

streamlines
.






The

upper

stream

tube

constricts

as

it

flows

up

and

around

the

airfoil,

a

part

of

the

so
-
called

upwash
.

From

the

conservation

of

mass,

the

flow

speed

must

increase

as

the

stream

tube

area

decreases
.

The

area

of

the

lower

stream

tube

increases,

causing

the

flow

inside

the

tube

to

slow

down
.

It

is

typically

the

case

that

the

air

parcels

traveling

over

the

upper

surface

will

reach

the

trailing

edge

before

those

traveling

over

the

bottom
.






From

Bernoulli's

principle,

the

pressure

on

the

upper

surface

where

the

flow

is

moving

faster

is

lower

than

the

pressure

on

the

lower

surface
.

The

pressure

difference

thus

creates

a

net

aerodynamic

force,

pointing

upward

and

downstream

to

the

flow

direction
.





The

component

of

the

force

normal

to

the

free

stream

is

considered

to

be

lift
;

the

component

parallel

to

the

free

stream

is

drag
.

In

conjunction

with

this

force

by

the

air

on

the

airfoil,

by

Newton's

third

law,

the

airfoil

imparts

an

equal
-
and
-
opposite

force

on

the

surrounding

air

that

creates

the

downwash
.




Principle
of Helicopter Flight











Helicopter

Lift

is

obtained

by

means

of

one

or

more

power

driven

horizontal

propellers

which

called

Main

Rotor
.








When

the

main

rotor

of

helicopter

turns,

it

produces

lift

and

reaction

torque
.

Reaction

torque

tends

to

make

helicopter

spin
.

On

most

helicopters,

a

small

rotor

near

the

tail

which

called

tail

rotor

compensates

for

this

torque
.












On

twin

rotor

helicopter

the

rotors

rotate

in

opposite

directions,

their

reactions

cancel

each

other
.












MAIN ROTOR


The lifting force is produced by the main rotor . As
they spin in the air and produced the lift. Each blade
produces an equal share of the lifting force. The
weight of a helicopter is divided evenly between the
rotor blades on the main rotor system. If a helicopter
weight 4000 lbs and it has two blades, then each blade
must be able to support 2000 lbs. In addition to the
static weight of helicopter ,each blade must be accept
dynamic load as well . For example, if a helicopter pull
up in a 1.5 time the gravity force, then the effective
weight of helicopter will be 1.5 time of static
helicopter weight or 6000 lbs. due to gravitational
pull.



The

tail

rotor

in

normally

linked

to

the

main

rotor

via

a

system

of

drive

shafts

and

gearboxes

.
Most

helicopter

have

a

ratio

of

3
:
1

to

6
:
1

.

In

most

helicopter

the

engine

turns

a

shaft

that

connected

to

an

input

quill

in

the

transmission

gearbox
.


Torque Reaction








If

you

spin

a

rotor

with

an

engine,

the

rotor

will

rotate,but

the

engine

and

helicopter

body

will

tend

to

rotate

in

opposite

direction

to

the

rotor
.

This

is

called

Torque

reaction
.

Newton's

third

law

of

motion

states

,

"

to

every

action

there

is

an

equal

and

opposite

reaction"

.

The

tail

rotor

is

used

to

compensates

for

this

torque

and

hold

the

helicopter

straight
.

Dissymmetry of Lift


All

rotor

systems

are

subject

to

Dissymmetry

of

Lift

in

forward

flight

.

At

a

hover

,

the

lift

is

equal

across

the

entire

rotor

disk

.

As

the

helicopter

gain

air

speed

,

the

advancing

blade

develops

greater

lift

because

of

the

increased

airspeed

and

the

retreating

blade

will

produce

less

lift

,

this

will

cause

the

helicopter

to

roll

.






In

order

to

overcome

this

problem

blade

flapping

is

done
.

Dissymmetry

of

lift

in

helicopter

aerodynamics

refers

to

an

uneven

amount

of

lift

on

opposite

sides

of

the

rotor

disc
.

The

dissymmetry

is

caused

by

differences

in

relative

airspeed

between

the

advancing

blade

and

the

retreating

blade
.

Blade
Flapping


Dissymmetry

of

lift

is

compensated

by

blade

flapping

,because

of

the

increased

airspeed

and

lift

on

the

advancing

blade

will

cause

the

blade

to

flap

up

and

decreasing

the

angle

of

attack

.

The

decreased

lift

on

the

retreating

blade

will

cause

the

blade

to

flap

down

and

increasing

the

angle

of

attack

.

The

combination

of

decreased

angle

of

attack

on

the

advancing

blade

and

increased

angle

of

attack

on

the

retreating

blade

through

blade

flapping

action

tends

to

equalize

the

lift

over

the

two

halves

of

the

rotor

disc
.


Flight Control


Swash

plate

assembly

:


Its

primary

component

is

the

swash

plate
,

located

below

the

rotor

head
.

This

swash

plate

consists

of

one

non
-
revolving

disc

and

one

revolving

disc

mounted

directly

on

top
.

The

swash

plate

is

connected

to

the

cockpit

control

sticks

and

can

be

made

to

tilt

in

any

direction,

according

to

the

cyclic

stick

movement

made

by

the

pilot,

or

moved

up

and

down

according

to

the

collective

lever

movement
.






The Collective Control :
.



The collective control is made by moving a lever
that rises up from the cockpit floor to the left of
the pilot's seat, which in turn
raises

or
lowers

the swash plate on the main rotor shaft, without
tilting it. This lever only moves up and down and
corresponds directly to the desired movement of
the helicopter; lifting the lever will result in the
helicopter rising while lowering it will cause the
helicopter to sink






The Cyclic Control :


The

cyclic

control

works

by

tilting

the

swash

plate

and

changing

the

pitch

angle

of

a

rotor

blade

at

a

given

point

in

the

rotation
.

As

the

pitch

angle

changes,

so

the

lift

generated

by

each

blade

changes

and

as

a

result

the

helicopter

becomes

'unbalanced',

and

so

tips

towards

whichever

side

is

experiencing

the

lesser

amount

of

lift
.

Thus

with

its

help,

helicopter

can

move

right

or

left

,

backward

or

forward
.


Thank
you