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
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
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