Piston Engines: Propellers

Piston Engine Propulsion

Propellers

How lift is generated

PROPELLER SYSTEM

In this example

Pressure Remains Constant here

Pressure Decreases hereIn this direction

The result is

How lift is generated

PROPELLER SYSTEM

Small Pressure Increase here

Greater Pressure Decrease

hereThe result is

MORE LIFT

How lift is increased

PROPELLER SYSTEM

Direction of travel

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

The ANGLE of ATTACK

How lift is increased

PROPELLER SYSTEM

• On propellers, lift is called thrust.

• The propeller blades work in the same way as aircraft wings.

• The corkscrew angle which is produced by the tips is called the Helix Angle

Forces Acting on Propeller Blades

• Bending - Due to thrust and torque forces on the blade.

• Centrifugal - Caused by the propeller blade mass rotating at high speeds.

• Torsion - Due to the affects of CTM and ATM and pitch change loads.

• Thrust is the component acting at right anglesto the plane of rotation.

• Torque is the component acting in the plane of rotation opposing engine torque and is the resistance offered by the propeller to rotation.

• Thrust and Torque values developed by the propeller depend on the angle of attack, the R.P.M. and air density.

• As air density increases so will thrust, but as increased resistance is felt by the propeller, torque will also increase.

• Thrust and torque will alter in direct proportion to propeller speed and any increase in the Angle of Attack (below stalling speed) will produce more thrust and torque.

• There is an optimum angle of attack for all propellers, usually about 4.

How the blade tip travel produces the HELIX ANGLE

PROPELLER SYSTEM

Forward Speed - Distance Travelled

over One Minute

Rotation -

Number of

Rotations

per Minute

Forward Speed

PROPELLER SYSTEM

Forward Speed

PROPELLER SYSTEM

Forward Speed

PROPELLER SYSTEM

Forward Speed

PROPELLER SYSTEM

Forward Speed

PROPELLER SYSTEM

Forward Speed

PROPELLER SYSTEM

• The edge of the blade which is at the front as the propeller rotates is termed the Leading edge, and obviously the other is the Trailing edge.

• One of the faces is relatively flat and is called the Pressure or Blade face due to a slight positive pressure being built up here during operation.

• The cambered face where a depression is created is termed the Suction face or Blade Back

• A Fixed Pitch propeller is one where the blade angle only changes along its length and is set to be most efficient at the aircraft’s normal cruise speed.

• Below this speed the blade angle is too coarse resulting in more engine power being required to overcome the Force for Torque being generated by a large angle of attack.

• A variation to Fixed Pitch is the Adjustable Propeller.

• This has the facility to allow adjustment of the blade angle on the ground over a limited range to accommodate different airframe / engine combinations.

• It can also cater for different atmospheric conditions

• There is the Controllable Pitch Propeller. • In this case the pitch setting of the blades can be

changed in flight. • A Pitch Control Lever in the cockpit is used by the

pilot to set an appropriate propeller pitch fixed positions as the flight progresses.

• A Fine position is selected for take off allowing maximum engine rpm and an efficient angle of attack at relatively low forward airspeed.

• As the speed increases, the blade angle can be increased to maintain optimum efficiency.

• The lever is moved in the opposite direction as airspeed is decreased at the end of a flight.

• The ultimate is the Constant Speed Propeller which will allow the engine to run at a constant selected rpm.

• The lever in the cockpit is labelled the R.P.M. Lever and controls a Constant Speed Unit (CSU)

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

All propeller blades are actuated by the same mechanical linkage

PROPELLER SYSTEM

Sliding Piston

Hard Stops

Coarse

Direction

Rotation

Direction

of Flight

Propeller

Actuating

Note: - blade angle is relative to piston travel

Fine pitch

Coarse pitch

‘Feathered’

Piston travels between ‘hard’ stops

Direction

Rotation

Maximum resistance

to rotation

Minimum

resistance

to forward

Minimum

resistance to

rotation

Maximum

resistance

to forward

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 SYSTEM

Importance of set blade angle

Fine pitch

Minimum

resistance to

rotation

Maximum

resistance

to forward

Maximum resistance

to rotation

Minimum

resistance

to forward

Starting of engine

Direction of travel

Direction of

Rotation

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

PROPELLER SYSTEM

Minimal

resistance to

rotation

Air pushed

forward giving

reverse thrust

Direction of travel

Direction of

Rotation

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

increasing

engine disintegration

Usually for military

aircraft only

PROPELLER SYSTEM

Direction of travel

Direction of

Rotation

Used for:-

Low drag on final approach

Flight Fine and Cruise Pitch

Used for:-

In-flight descent – faster forward speed than

final approach

Flight

Cruise

Both give minimal drag at

low power settings

PROPELLER SYSTEM

DISTANCE TRAVELLED BY

ROOT, MID-SPAN AND TIPTHICK FOR

STRENGTH

PROPELLER SYSTEM

Blade Twist

ROOT MID-SPAN TIP

THINNER FOR

STRENGTH AND

THRUST

THIN FOR

THRUST

COARSE

MEDIUM

BLADE ANGLE RELATIVE TO DISTANCE (AND THEREFORE SPEED)

TRAVELLED BY ROOT, MID-SPAN AND TIP

Typical Blade

Typical 3

Blade Prop

Constant Speeding Variable Pitch Propeller System

Propeller Control Unit (PCU) Operation

How the PCU changes propeller pitch and maintains

constant engine speeds

• The Pilot moves the throttle lever which, as well as changing fuel flow and therefore engine power, selects an RPM for the engine to run at.

• The throttle is connected to both the Fuel Control Unit and the PCU.

• The PCU maintains the selected speed by adjusting propeller pitch. This eases pilot workload.

Propeller HubEngine Mounted

Simplified System

PROPELLER SYSTEM – VARIABLE PITCH CONTROL

& Idle

Cruise

Throttle

Positions: -

Pilot Input

Signal

Hydraulic

Pressure

Supply

Hydraulic

Return

Operation Piston

Spinner

Connecting Linkage

Hydraulic

Connections

Sliding

Collar

Engine RPM Signal

(Mechanical Drive)

Spring

Hydraulic

Counter

Balance

Weights

FMU PCU

Piston Engine driven propeller

FMU PCU

Jet Engine driven propeller – Turbo-Prop

& Idle

Cruise

Throttle

Positions

Pilot Input

Signal

Engine Stationary

& Idle

Cruise

Throttle

Positions

Pilot Input

Signal

Start Initiated – Engine Begins to Rotate

& Idle

Cruise

Throttle

Positions

Engine At Idle

& Idle

Cruise

Throttle

Positions

Take Off Selected

& Idle

Cruise

Throttle

Positions

Engine Accelerates to Take Off Speed

& Idle

Cruise

Throttle

Positions

Engine Running at Take Off Speed

Flying straight and level

Look at how the PCU changes propeller pitch

and maintains constant engine speeds during

Dive commencement

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

And then at Level Out

Throttle

Positions

& Idle

Cruise

Aircraft Starts Dive

Throttle (and Engine) at Cruise –

Throttle

Positions

& Idle

Cruise

RPM Increases

Throttle at Cruise –

Throttle

Positions

& Idle

Cruise

RPM Increases - Prop Goes to Coarser Pitch

Throttle

Positions

& Idle

Cruise

Aircraft in Dive – RPM Restored

Flying straight and level

Looked at how the PCU changes propeller pitch

and maintains constant engine speeds during

Dive commencement

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

Now at Level Out

Throttle

Positions

& Idle

Cruise

Aircraft Levels Out

Throttle

Positions

& Idle

Cruise

RPM Reduces

Throttle

Positions

& Idle

Cruise

Prop Pitch Reduces

Throttle

Positions

& Idle

Cruise

RPM Restored

Throttle (and Engine) at Cruise –

Piston Engines: Propellers

Engineering

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