1. PISTON ENGINES
Part 8
Propeller Control

2. How Lift is Generated α The Angle of Attack
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 ?
in this direction
Large Pressure
Decrease here α
Small Pressure
Increase here
The Angle of Attack

3. The Propeller System The Helix Angle
On Propellers, LIFT is called THRUST
and propeller Blades work the same way as aircraft wings.
As an aircraft pulls forward, the propeller spins at high speed,
this can be around 1000 rpm.
The path the blade tip cuts through the air is called
a Helix or HELICAL.
Three things effect this shape: -
Forward speed.
Propeller rpm.
Propeller diameter.
When a propeller spins and the aircraft moves forward,
the tips of the propeller blades move
in a ‘corkscrew’ path
Exactly how the blade tip travels produces
This path is called a HELIX
The Helix Angle

4. The Helix Angle The Angle of Attack can be changed
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.
Propeller Blade
Line of Rotation
Direction of blade through the air
This is the
Blade Angle
This is the Angle of Attack
This is the
Helix Angle
The Angle of Attack can be changed
by altering the rpm or the forward speed.

5. The Helix Angle This produces a set HELIX ANGLE
Rotation - Number of Rotations per Minute
Forward Speed - Distance Travelled over One Minute

6. The Helix Angle Changes in FORWARD SPEED and/or RPM will change
the Helix Angle and the Angle of Attack
At a Faster Forward Speed
At a
Faster
RPM
The angle
narrows
The angle widens

7. 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.

8. 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.
Sliding Piston
Propeller Blade
Fine Pitch
Actuating Lever
through to
Coarse Pitch
Direction of Rotation
Direction of Flight
Hard Stops
All propeller blades are actuated
by the same mechanical linkage

9. Variable Pitch Propellers
Blade Angles
Direction Of
Rotation
Fine pitch
Coarse pitch or
‘Feathered’
Maximum resistance to forward speed
Minimum resistance to forward speed
Maximum resistance to rotation
Minimum resistance to rotation
The blade angle changes
through 90o with piston travel
At this hard stop the blade is in this position
At this hard stop the blade is in this position
Piston travels between ‘hard’ stops
Blade angle is relative
to piston travel

10. Variable Pitch Propellers
Blade Angles
Direction Of
Rotation
Fine pitch
Good for:-
Maximum resistance to forward speed
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

11. Variable Pitch Propellers
Blade Angles
Direction Of
Rotation
Good for:-
In-flight – loss of control
Coarse pitch or
‘Feathered’
In-flight engine failure – control maintained
engine stops rotating minimizing damage
Minimum resistance to forward speed
Maximum resistance to rotation
Bad for:-
Starting of engine
Could cause engine burn-out if running
Direction of travel
Low drag – NO braking effect on ground
Importance of set blade angle

12. Variable Pitch Propellers
Blade Angles
Direction Of
Rotation
Fine pitch
Used for:-
Maximum resistance to forward speed
High drag – high braking effect on ground
Air pushed forward giving reverse thrust
REVERSE PITCH
Usually for military aircraft only
Minimal resistance to rotation
Minimum resistance to rotation
Bad for:-
In-flight – loss of forward speed,
aircraft stalls
In-flight engine failure – loss of control
and reverse rotation increasing
engine disintegration
Direction of travel
Importance of set blade angle

13. Variable Pitch Propellers
Blade Angles
Direction Of
Rotation
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

14. There is a ‘Twist’ to all propeller blades
Blade Twist
There is a ‘Twist’ to all propeller blades
MID-SPAN
ROOT
TIP
Viewed ‘End On’

15. Blade Twist Distance travelled by ROOT, MID-SPAN & TIP
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
TIP
MID-SPAN
COARSE ANGLE
MEDIUM ANGLE
FINE ANGLE
THICK FOR STRENGTH
THINNER FOR STRENGTH & THRUST
THIN FOR THRUST
Distance travelled by ROOT, MID-SPAN & TIP

16. 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.

17. 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.
Propeller Hub
Engine Mounted
Operation Piston
Blade
Turning Mechanism
Hydraulic Connections
PCU

18. 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.
Propeller Hub
Engine Mounted
Spring
Counter Balance Weights
Operation Piston
Blade
Turning Mechanism
Hydraulic Valve
Hydraulic Connections
PCU

19. 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: -
Propeller Hub
Engine Mounted
Take Off
Spring
Cruise
Counter Balance Weights
Start & Idle
Operation Piston
Pilot Input Signal
Blade
Turning Mechanism
Hydraulic Return
Hydraulic Valve
Hydraulic Pressure Supply
Hydraulic Connections
PCU
Engine RPM Signal

20. Variable Pitch Control
The PCU is driven by the engine main rotating shaft,
so as 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

21. 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

22. 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

23. Variable Pitch Control
The Sequence of Events
Stage 2 – Start Initiated
Rpm starts to increase,
Hydraulic pressure also starts to increase.
Start & Idle

24. 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

25. 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

26. 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

27. 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

28. 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

29. 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

30. 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

31. 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

32. 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

33. Variable Pitch Control
The Sequence of Events
Stage 12 – Levelling Out
The propeller pitch is fined off to increase the rpm
Cruise

34. 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

35. Variable Pitch Control
The Dive Sequence
The PCU changes propeller pitch
and maintains constant engine speeds
during the Dive commencement
Straight and Level
and again at
Level Out
Dive
Straight and Level
In all of these manoeuvres,
all the pilot is doing
is flying (redirecting) the aircraft,
the throttle is not touched.

36. 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

37. 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

38. 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

39. Check of Understanding
Which of these statements applies
to a propeller that has been feathered?
Its leading edge faced 90o
to the direction of flight
It operates at maximum speed
Its leading edge faces forward
to the direction of flight
It produces maximum power

40. Check of Understanding
On a variable pitch propeller,
what is the largest obtainable pitch angle called?
Cruise pitch
Fine pitch
Reverse pitch
Coarse pitch

41. Check of Understanding
In the diagram,
what is angle ‘A’ known as?
The Blade Angle
Line of Rotation
Propeller Blade A
The Pitch Angle
The Prop Angle
The Fine Angle

42. Check of Understanding
Which pitch of propeller gives
the maximum resistance to forward speed?
Cruise Pitch
Reverse Pitch
Fine Pitch
Coarse Pitch

43. PISTON ENGINES
End of Presentation