propeller system 1 st - look at how lift is generated then see how it applies to propellers

PROPELLER SYSTEMPROPELLER SYSTEM

1st - Look at how lift is generated

Then see how it applies to propellers

How lift is generated

PROPELLER SYSTEM

In this example

Pressure Remains Constant here

Pressure Decreases hereIn this direction

The result is LIFT

How lift is generated

PROPELLER SYSTEM

Small Pressure Increase here

Greater Pressure Decrease here

The result is

MORE LIFT

How lift is increased

PROPELLER SYSTEM

Direction of travel

Aerofoil incline

The difference in direction of travel and aerofoil incline is called:-

The ANGLE of ATTACK

How lift is increased

PROPELLER SYSTEM

How does lift apply to PROPELLORS?

On Propellers, LIFT is called THRUST

And propeller Blades work the same way as aircraft wings

When a propeller spins and the aircraft moves forward, the tips of the propeller blades move in a ‘corkscrew’ path

This path is called a HELIX

PROPELLER SYSTEM

How the HELIX ANGLE is generated

How the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEM

PROPELLER SYSTEM



PROPELLER SYSTEM

Forward Speed - Distance Travelled over One Minute

Rotation - Number of Rotations per Minute

Forward Speed

RPM


PROPELLER SYSTEM

PROPELLER SYSTEM

Forward Speed

RPM


Forward Speed

RPM

PROPELLER SYSTEM


How the HELIX ANGLE is changed by engine rpm and forward speed

Forward Speed

RPM

How an increase in RPM changes the Helix Angle

Changes in FORWARD SPEED and RPM will change the Helix Angle

Faster RPM

PROPELLER SYSTEM


Forward Speed

RPMRPM

Faster Forward Speed

Changes in FORWARD SPEED and RPM will change the Helix Angle

How an increase in FORWARD SPEED changes the HELIX ANGLE

PROPELLER SYSTEM

Let’s take a closer look at the blade aerofoil and the Helix Angle and thrust (lift) generation

If the Helix Angle changes, then we need to change the

blade angle.

Remember (from the comparison with the aircraft wing), the optimum Angle of Attack is required to maintain most efficient thrust generation.

This is the Helix Angle

This is the Angle of AttackDirection of

rotation

Direction of blade through the air with forward speed

PROPELLER SYSTEM

Mechanical STOPS and blade angles

All propeller blades are actuated by the same mechanical linkage

PROPELLER SYSTEM

Sliding Piston

Hard Stops

Fine Pitch

Coarse Pitch

Direction of

Rotation

Direction of Flight

Propeller Blade

Actuating Lever

Actuating Link

Note: - blade angle is relative to piston travel

Fine pitchCoarse pitch

Or

‘Feathered’

Piston travels between ‘hard’ stops

Direction

Of

Rotation

Maximum resistance to rotation

Minimum resistance to forward

speed

Minimum resistance to

rotation

Maximum resistance to forward

speed

The blade angle changes through 90deg with piston travel

At this hard stop the blade is in this

position

At this hard stop the blade is in this

position

PROPELLER SYSTEM

Easier Starting of engine

Direction of travel

Direction of Rotation

Good for:-

Running engine with no/minimal thrust

Bad for:-

In-flight – loss of control

High drag – braking effect on ground

Zero pitch – or Ground Fine Pitch

In-flight engine failure – loss of control andengine disintegration

PROPELLER SYSTEMImportance of set blade angle

Fine pitch

Minimum resistance to

rotation

Maximum resistance to forward

speed

Maximum resistance to rotation

Minimum resistance to forward

speed

Starting of engine

Direction of travel


Bad for:-

Could cause engine burn-out if running

Low drag – NO braking effect on ground

Maximum pitch – or Feathered

Good for:-

In-flight – loss of control

In-flight engine failure – control maintained and engine stops rotating minimizing damage


Minimal resistance to

rotation

Air pushed forward giving reverse thrust

Direction of travel


Used for:-

Bad for:-

In-flight – loss of forward speed, aircraft stalls

High drag – high braking effect on ground

Reverse Pitch

In-flight engine failure – loss of control and reverse rotation increasingengine disintegration

Usually for military aircraft only


Direction of travel


Used for:-

Low drag on final approach

Flight Fine and Cruise Pitch

Used for:-

In-flight descent – faster forward speed thanfinal approach

Flight Fine pitch

Cruise pitch

Both give minimal drag at low power settings


Blade Twist

DISTANCE TRAVELLED BY DISTANCE TRAVELLED BY ROOT, MID-SPAN AND TIPROOT, MID-SPAN AND TIP

THICK FOR THICK FOR STRENGTHSTRENGTH

PROPELLER SYSTEMBlade Twist

ROOT MID-SPAN TIP

THINNER FOR THINNER FOR STRENGTH AND STRENGTH AND

THRUSTTHRUST

THIN FOR THIN FOR THRUSTTHRUST

COARSE ANGLE

MEDIUM ANGLE

FINE ANGLE

BLADE ANGLE RELATIVE TO DISTANCE (AND THEREFORE SPEED) BLADE ANGLE RELATIVE TO DISTANCE (AND THEREFORE SPEED) TRAVELLED BY ROOT, MID-SPAN AND TIPTRAVELLED BY ROOT, MID-SPAN AND TIP

Typical Blade

Typical 3 Blade Prop

propeller system 1 st - look at how lift is generated then see how it applies to propellers

Documents

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