Piston Prop. Pt 8

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PISTON ENGINES

Part 8

Propeller Control

How Lift is Generated

The result is LIFT in this direction

Pressure here is constant

Pressure here decreases

The result is LIFT

The difference in direction of travel and aerofoil incline is called ?

Large Pressure

Decrease here

α

Increase here

The Propeller System

On Propellers, LIFT is called THRUST a Helix or HELICAL.

the tips of the propeller blades move in a ‘corkscrew’ path

Three things effect this shape: -

Forward speed.

Propeller rpm.

Propeller diameter.

Exactly how the blade tip travels produces

This path is called a

HELIX

The Helix Angle

The Helix Angle

The optimum Angle of Attack is required to maintain most efficient thrust generation.

If the Helix Angle changes, then we need to change the Angle of Attack.

Line of Rotation

This is the

Blade Angle

Propeller Blade

This is the

Helix Angle

This is the

Angle of Attack

Direction of blade through the air

The Angle of Attack can be changed by altering the rpm or the forward speed.

The Helix Angle

This produces a set

HELIX ANGLE

Rotation - Number of Rotations per Minute

Forward Speed - Distance Travelled over One Minute

At a

Faster

RPM

The angle narrows

The Helix Angle

Changes in FORWARD SPEED a nd/or RPM will change the Helix Angle and the Angle of Attack

At a Faster Forward Speed

The angle widens

Variable Pitch Propellers

Blade Angles

With fixed pitch propellers, changing the rpm or forward speed changes the Angle of Attack, but unfortunately not at the correct angle.

Therefore either increase in drag or a stall results.

Variable Pitch propellers were introduced to alleviate this problem, and provide other advantages.

Variable Pitch Propellers

Blade Angles

The variable pitch propeller is a mechanism by which all the blades on a propeller hub can be rotated about the blade centre axis, whilst the propeller is spinning.

Fine

Pitch

Sliding Piston

Propeller

Blade

Actuating

Lever through to

Coarse

Pitch

Direction of

Rotation

Hard Stops

All propeller blades are actuated by the same mechanical linkage

Direction of Flight

Direction

Of

Rotation

Variable Pitch Propellers

Blade Angles

Fine pitch Coarse pitch or

‘Feathered’

Minimum resistance to rotation

Maximum resistance to forward speed

Maximum resistance to rotation

The blade angle changes through 90 o with piston travel

Minimum resistance to forward speed

At this hard stop the blade is in this position

Piston travels between ‘hard’ stops

Blade angle is relative to piston travel

At this hard stop the blade is in this position

Direction

Of

Rotation

Variable Pitch Propellers

Blade Angles

Fine pitch

Maximum resistance to forward speed

Good for:-

Easier Starting of engine

Running engine with no/minimal thrust

High drag

– braking effect on ground

Minimum resistance to rotation

Bad for:-

In-flight

– loss of control

In-flight engine failure

– loss of control and engine disintegration

Direction of travel

Importance of set blade angle

Variable Pitch Propellers

Blade Angles

Direction

Of

Rotation

Coarse pitch or

‘Feathered’

Maximum resistance to rotation

Good for:-

In-flight

– loss of control

In-flight engine failure

– control maintained engine stops rotating minimizing damage

Minimum resistance to forward speed

Bad for:-

Starting of engine

Could cause engine burn-out if running

Low drag

– NO braking effect on ground

Direction of travel

Importance of set blade angle

Direction

Of

Rotation

Variable Pitch Propellers

Blade Angles

Fine pitch Used for:-

High drag

– high braking effect on ground

speed

REVERSE PITCH

Usually for military aircraft only

Direction of travel

Bad for:-

In-flight

– loss of forward speed, aircraft stalls

In-flight engine failure

– loss of control and reverse rotation increasing engine disintegration

Importance of set blade angle

Direction

Of

Rotation

Variable Pitch Propellers

Blade Angles

Flight

Fine pitch

Used for:-

Low drag on final approach

Flight Fine

&

Cruise Pitch

Both give minimal drag at low power settings

Cruise pitch

Used for:-

In-flight descent

– faster forward speed than final approach

Direction of travel

Importance of set blade angle

Blade Twist

There is a

Twist

’ to all propeller blades

ROOT

MID-SPAN

TIP

Viewed ‘End On’

Blade Twist

The distance the blade travels during rotation is different at various blade sections along its span.

All blades have a ‘coarse’ angle at the root, progressing to a ‘fine’ angle towards the tip.

This ‘blade twist’ maintains an efficient angle of attack along the full length of the propeller blade.

3 Blade Prop Typical Blade

ROOT

MID-SPAN

TIP

COARSE

ANGLE

THICK FOR

STRENGTH

MEDIUM

ANGLE

THINNER FOR

STRENGTH & THRUST

Distance travelled by ROOT, MID-SPAN

&

TIP

FINE

ANGLE

THIN FOR

THRUST

Variable Pitch Control

Variable pitch propeller systems allow the engine to run at a constant speed, irrespective of flight manoeuvres.

This has the advantage of protecting the engine from over-speeding, and possible disintegration, during extreme manoeuvres experienced in combat.

Variable Pitch Control

The rotating hub contains the blade turning mechanism, which is piston driven and hydraulically operated, by a Propeller Control Unit (PCU).

The PCU is the link between pilot demand (power setting), the engine speed, and the aircraft attitude.

Engine Mounted Propeller Hub

Operation Piston

Blade

Turning

Mechanism

Hydraulic

Connections

PCU

Variable Pitch Control

A hydraulic valve directs pressure to either side of the piston in the hub.

The valve is positioned by rotating centrifugal weights

(‘bob’ weights), balanced against spring tension.

Engine Mounted

Spring

Propeller Hub

Counter

Balance

Weights

Operation Piston

Blade

Turning

Mechanism

Hydraulic

Valve

Hydraulic

Connections

PCU

Variable Pitch Control

When the pilot opens the throttle, increasing power, he also compresses the spring to a higher tension.

When the engine accelerates the bob weights spin faster, putting the hydraulic control valve in the balanced position, and steady state rpm is achieved.

Throttle Positions: -

Take Off

Engine Mounted

Spring

Propeller Hub

Cruise

Counter

Balance

Weights

Operation Piston

Start & Idle

Pilot Input

Signal

Blade

Turning

Mechanism

Hydraulic

Return

Hydraulic

Pressure Supply

PCU

Engine RPM Signal

Hydraulic

Valve

Hydraulic

Connections

Variable Pitch Control

The PCU is driven by the engine main rotating shaft, so a s soon as the engine starts to rotate, the internal components of the PCU will rotate as well; ensuring the PCU weights spin to engine speed, sensing rpm.

The mechanical control linkage has to be adjusted so fuel supply at any throttle position is enough to drive the engine to the selected spring tension (rpm) in the PCU.

FMU

Variable Pitch Control

The Sequence of Events

We shall quickly review what happens with the pitch control through a sequence of events from a stationary position, through take-off and level flight, then into a dive, and finally to level flight again.

Straight and Level

Take Off

Dive

Straight and Level

Stationary

Variable Pitch Control

The Sequence of Events

Stage 1

– Engine Stationary

Throttle idle, the PCU spring extended

The hydraulic selector valve to the ‘fine’ port, open, position.

Start & Idle

Variable Pitch Control

The Sequence of Events

Stage 2

– Start Initiated

Rpm starts to increase,

Hydraulic pressure also starts to increase.

Start & Idle

Variable Pitch Control

The Sequence of Events

Stage 3

– Accelerate to Idle

When rpm close to idle, weights start to lift the hydraulic valve.

At idle rpm, the propeller is locked into the fine position.

Start & Idle

Variable Pitch Control

The Sequence of Events

Stage 4

– Idle to Take Off

The PCU loads the spring tension, pushing the hydraulic direction valve down.

Take Off

Variable Pitch Control

The Sequence of Events

Stage 5

– Accelerate to Take Off RPM

Propeller angle lags behind the actual rpm

The hydraulic direction valve is in the fine pitch open position

Take Off

Variable Pitch Control

The Sequence of Events

Stage 6

– at Take Off RPM

The propeller locks in the take off angle.

When brakes release, pitch gradually increases to maintain correct angle of attack.

Take Off

Variable Pitch Control

The Sequence of Events

Stage 7

– Aircraft in Straight and Level Flight

Pitch is hydraulically locked at the cruise angle.

Aircraft is now manoeuvred into a dive attitude, the engine controls are not altered.

Cruise

Variable Pitch Control

The Sequence of Events

Stage 8

– Dive is Initiated

The aircraft gathers speed, relieving drag on the propeller, and allowing it to be driven faster by the engine.

Cruise

Variable Pitch Control

The Sequence of Events

Stage 9

– Dive is Begun

As the engine over-speeds slightly the propeller moves to a coarser pitch.

Cruise

Variable Pitch Control

The Sequence of Events

Stage 10

– Dive Attitude

Pitch coarsened off to maintain the correct angle of attack

Blade pitch is hydraulically locked at the cruise angle.

Cruise

Variable Pitch Control

The Sequence of Events

Stage 11

– Level Out Initiated

Rpm reducing due to the increase drag of the blades at the dive blade angle.

Cruise

Variable Pitch Control

The Sequence of Events

Stage 12

– Levelling Out

The propeller pitch is fined off to increase the rpm

Cruise

Variable Pitch Control

The Sequence of Events

Stage 13

– Aircraft in Straight and Level Flight

Pitch is hydraulically locked at the cruise angle.

Rpm is restored

Cruise

Straight and Level

Variable Pitch Control

The Dive Sequence

The PCU changes propeller pitch and maintains constant engine speeds during the Dive commencement and again at

Level Out

In all of these manoeuvres, all the pilot is doing is flying (redirecting) the aircraft, the throttle is not touched.

Straight and Level

Check of Understanding

The Helix Angle is the angle between what?

The direction of the blade and the angle of attack

The line of rotation and the direction of the blade

The line of rotation and the direction of flight

The line of rotation and the angle of attack

Check of Understanding

As an aircraft pulls forward, at what rate does the propeller spin?

Around 100 rpm

Around 1000 rpm

Around 2000 rpm

Around 4000 rpm

Check of Understanding

The blade angle on a propeller is varied from the root to the tip.

What is this called?

Blade twist

Variable pitch

Blade transition

Adjustable pitch

Check of Understanding

Which of these statements applies to a propeller that has been feathered?

Its leading edge faced 90 o to the direction of flight

It operates at maximum speed

Its leading edge faces forward to the direction of flight

It produces maximum power

Check of Understanding

On a variable pitch propeller, what is the largest obtainable pitch angle called?

Cruise pitch

Fine pitch

Reverse pitch

Coarse pitch

Line of

Rotation

A

Check of Understanding

In the diagram, what is angle ‘A’ known as?

Propeller

Blade

The Blade Angle

The Pitch Angle

The Prop Angle

The Fine Angle

Check of Understanding

Which pitch of propeller gives the maximum resistance to forward speed?

Cruise Pitch

Reverse Pitch

Fine Pitch

Coarse Pitch

PISTON ENGINES

End of Presentation

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