CANNOCK CHASE U3A

SCIENCE & TECHNOLOGY GROUP

How Things Work Series:

The HELICOPTER and its CONTROLS

Feb 2017

BarryKJames: HowHelicoptersWorkRevD.ppt

LIFT

The upper air streams constrict

as they flow up and around the

aerofoil.

Conservation of mass says that

the flow speed must increase as

the air streams constrict.

Similarly, the lower air

streams expand and the flow

slows down.

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

where it is moving slower.

This pressure difference

creates a net aerodynamic

force, pointing upward (Lift).

Streamlines and stream tubes around

an airfoil at moderate angle of attack.

“An aerofoil is a streamlined shape that is capable of

generating significantly more lift than drag.”

BERNOULLI’S PRINCIPLE (1738)

AIR

FLOW

Air flow speeds up

in constriction

Liquid from tray is

sucked up the tube

due to the lower

pressure in the

constriction

A Train

• When a fast moving train rushes past the train

platform, the velocity of the air particles next to

the train increases. This leads to a decrease in

the air pressure around the train.

• If standing very close to the edge of the platform,

a person can get sucked into to the low pressure

area created in front of them, pushing them into

the train.

Balloons!

WHAT ELSE RELIES ON BERNOULLI FORCES?

Propellers

Kites

Sails and keels on sailing boats

Hydrofoils

Wings on racing cars

Wind turbines.

Bunsen burner

Carburetter on an engine

Garden sprayer & Paint sprayer

The Helicopter spinning blades generate the

air flow and lift instead of forward speed.

AIR FLOW AROUND AN AEROFOIL

The black dots are on timelines,

which split into two – an upper

and lower part – at the leading

edge.

The colour dots move with the

flow.

The air speeds are much higher

at the upper surface than at the

lower surface.

See the speed difference upper

and lower.

The air columns do not rejoin as

they were before separation.

Colours of the dots

indicate streamlines.

The angle of attack is 8°.

HOW LIFT IS VARIED

ON A HELICOPTER

Lift is created from the air

flow over rotating blades at

around 500rpm.

Lift is varied

by twisting

the blades.

Varies angle

of attack.

Blade pitch

control is by

tie rods.

Blades twist is forced by a “swash plate” and tie rods

HELICOPTER PILOT CONTROLS:

The “Collective” control.

Varies the blade pitch.

Pulling up increases pitch

and therefore lift.

Increased lift needs more

engine power.

Turning increases throttle.

1. LIFT

1. COLLECTIVE – LIFT AND FALL

The blades have short vertical rods (pitch links)

attached to them that are connected to a rotating metal

disc called a swash plate.

This swash plate slides on bearings around a second,

similar plate directly underneath that doesn't rotate.

When the pilot moves the collective one way, both swash

plates move upward, pushing up on the pitch links that

tilt all the rotor blades to a steeper angle.

STOPPING TWIST 2. TAIL ROTOR

More lift means more twist.

Tail rotor spins to

counteract twist.

Pilot can adjust tail pitch

as “Collective” lift is

increased.

Pilot’s

pedal

control of

tail rotor

pitch.

Produces

force to

oppose

torque.

HELICOPTER CONTROLS

3. DIRECTION

The main rotors also provide

steering by making more lift

on one side than the other.

The pilot steers using a second

lever called the cyclic pitch,

similar to a joystick.

This makes the blades swivel

as they cycle around,

producing lift on one side.

CYCLIC CONTROL

3. DIRECTION

The swash plate mechanism is used for lift

to right or left, front & back, as well as

straight up lift.

The swash plate can tilt in any direction.

This forces a higher angle on one side.

So there is a steeper angle when say on the

left side of the craft than when the blade is

on the right.

That means more lift is generated on the

left, tilting the craft over to the right and

steering it to the right.

The blades swivel back and forth as they

rotate at 500 rpm (seesaw).

THE INGENIOUS “SWASH PLATE”

Greater angle on one side means

More lift generated on that side

As blades rotate,

they change angle

MODEL HELICOPTER SWASH PLATE

CONTROL COORDINATION

(& THE WIND)

All these four controls need to be carefully

coordinated to make steady flight possible.

1. Moving the cyclic forward causes the helicopter to

move forward and speed up. But it will also start to

descend. So, at the same time, the pilot needs to raise

the collective to prevent the descent, and open the

throttle, if flying a helicopter without a governor.

2. Because raising the collective and opening the

throttle will cause the helicopter to twist to the right

due to the increased power, the left pedal is needed too.

3. Lowering the collective to start a descent will cause

the nose of the helicopter to drop (due to air pushing

upwards on the tail), so aft cyclic will be needed, and

also right pedal to counteract the reduced twist.

Collective (lift),

Cyclic (direction)

Pedals (twist)

Engine Throttle

TWIST - COUNTER-ROTATION

CONTRA-ROTATION ALTERNATIVES

Anyone for Tea?

PART 2

AutoGyros

FLYBARS – WHY?

FLYBAR – THE REASON

The flybar (or stabilizer bar) acts to stabilize the main

rotor by automatically changing the cyclic pitch,

reducing the effect of wind and turbulence and making

the helicopter more controllable.

The flybar has a weight or paddle, or both, for added

stability on smaller helicopters. The flybar is able to

tilt separately from the collective control.

Through mechanical linkages, the stable rotation of the

bar mixes with the swashplate movement to damp

external wind forces on the rotor.

But it does require extra power to rotate it.

RC MODELS HAVE FLYBARS