The difference in direction of travel and aerofoil incline is
called ? How Lift is Generated Pressure here is constant Pressure
here decreases in this direction The result is LIFT Small Pressure
Increase here Large Pressure Decrease here in this direction The
result is LIFT The Angle of Attack
Slide 3
The Propeller System Exactly how the blade tip travels produces
The Helix Angle 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. 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
Slide 4
The Helix Angle Line of Rotation Propeller Blade Direction of
blade through the air This is the Helix Angle This is the Angle of
Attack If the Helix Angle changes, then we need to change the Angle
of Attack. The optimum Angle of Attack is required to maintain most
efficient thrust generation. The Angle of Attack can be changed by
altering the rpm or the forward speed. This is the Blade Angle
Slide 5
The Helix Angle Rotation - Number of Rotations per Minute
Forward Speed - Distance Travelled over One Minute This produces a
set HELIX ANGLE
Slide 6
The Helix Angle Changes in FORWARD SPEED and/or RPM will change
the Helix Angle and the Angle of Attack At a FasterRPM At a Faster
Forward Speed The angle narrows The angle widens
Slide 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.
Slide 8
Variable Pitch Propellers Blade Angles Direction of Rotation
Direction of Flight Propeller Blade Sliding Piston Actuating Lever
Hard Stops Fine Pitch Coarse Pitch All propeller blades are
actuated by the same mechanical linkage 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. through to
Slide 9
Variable Pitch Propellers Blade Angles Blade angle is relative
to piston travel Direction Of Rotation Piston travels between hard
stops Fine pitch At this hard stop the blade is in this position
Minimum resistance to rotation Maximum resistance to forward speed
Coarse pitch orFeathered At this hard stop the blade is in this
position Maximum resistance to rotation Minimum resistance to
forward speed The blade angle changes through 90 o with piston
travel
Slide 10
Variable Pitch Propellers Blade Angles Importance of set blade
angle Direction Of Rotation Minimum resistance to rotation Maximum
resistance to forward speed Direction of travel Easier Starting of
engine Good for:- Running engine with no/minimal thrust Bad for:-
In-flight loss of control High drag braking effect on ground
In-flight engine failure loss of control and engine disintegration
Fine pitch
Slide 11
Variable Pitch Propellers Blade Angles Importance of set blade
angle Direction Of Rotation Direction of travel In-flight loss of
control Good for:- Could cause engine burn-out if running Bad for:-
Starting of engine Low drag NO braking effect on ground In-flight
engine failure control maintained engine stops rotating minimizing
damage Coarse pitch orFeathered Maximum resistance to rotation
Minimum resistance to forward speed
Slide 12
Variable Pitch Propellers Blade Angles Importance of set blade
angle Direction Of Rotation Minimum resistance to rotation Maximum
resistance to forward speed Direction of travel High drag high
braking effect on ground Used for:- Bad for:- In-flight loss of
forward speed, aircraft stalls In-flight engine failure loss of
control and reverse rotation increasing engine disintegration
REVERSE PITCH Minimal resistance to rotation Air pushed forward
giving reverse thrust Usually for military aircraft only Fine
pitch
Slide 13
Variable Pitch Propellers Blade Angles Importance of set blade
angle Direction Of Rotation Direction of travel Low drag on final
approach Used for:- In-flight descent faster forward speed than
final approach Flight Fine & Cruise Pitch Both give minimal
drag at low power settings Flight Fine pitch Cruise pitch
Slide 14
Blade Twist There is a Twist to all propeller blades Viewed End
On ROOT MID-SPAN TIP
Slide 15
Blade Twist Distance travelled by ROOT, MID-SPAN & TIP
Typical Blade 3 Blade Prop ROOT MID-SPAN TIP COARSE ANGLE MEDIUM
ANGLE FINE ANGLE THICK FOR STRENGTH THINNER FOR STRENGTH &
THRUST THIN FOR THRUST 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.
Slide 16
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
Slide 17
Propeller Hub Engine Mounted 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. PCUBlade Turning Mechanism Operation Piston
Hydraulic Connections Variable Pitch Control
Slide 18
Propeller Hub Engine Mounted 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. PCUBlade Turning Mechanism Operation Piston Hydraulic
Connections Hydraulic Valve Spring Counter Balance Weights Variable
Pitch Control
Slide 19
Propeller Hub Engine Mounted 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
PCUBlade Turning Mechanism Operation Piston Hydraulic Connections
Hydraulic Valve Spring Counter Balance Weights Engine RPM Signal
Hydraulic Pressure Supply Hydraulic Return Pilot Input Signal
Cruise Start & Idle Variable Pitch Control
Slide 20
FMU 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.
Slide 21
Variable Pitch Control Stationary Take Off Straight and Level
Dive Straight and Level 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.
Slide 22
The Sequence of Events Start & Idle Variable Pitch Control
Stage 1 Engine Stationary Throttle idle, the PCU spring extended
The hydraulic selector valve to the fine port, open, position.
Slide 23
Start & Idle Stage 2 Start Initiated Rpm starts to
increase, Hydraulic pressure also starts to increase. The Sequence
of Events Variable Pitch Control
Slide 24
Start & Idle The Sequence of Events Variable Pitch Control
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.
Slide 25
Take Off The Sequence of Events Variable Pitch Control Stage 4
Idle to Take Off The PCU loads the spring tension, pushing the
hydraulic direction valve down.
Slide 26
Take Off The Sequence of Events Variable Pitch Control 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
Slide 27
Take Off The Sequence of Events Variable Pitch Control 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.
Slide 28
Cruise The Sequence of Events Variable Pitch Control 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.
Slide 29
Cruise The Sequence of Events Variable Pitch Control Stage 8
Dive is Initiated The aircraft gathers speed, relieving drag on the
propeller, and allowing it to be driven faster by the engine.
Slide 30
Cruise The Sequence of Events Variable Pitch Control Stage 9
Dive is Begun As the engine over-speeds slightly the propeller
moves to a coarser pitch.
Slide 31
Cruise The Sequence of Events Variable Pitch Control Stage 10
Dive Attitude Pitch coarsened off to maintain the correct angle of
attack Blade pitch is hydraulically locked at the cruise
angle.
Slide 32
Cruise The Sequence of Events Variable Pitch Control Stage 11
Level Out Initiated Rpm reducing due to the increase drag of the
blades at the dive blade angle.
Slide 33
Cruise The Sequence of Events Variable Pitch Control Stage 12
Levelling Out The propeller pitch is fined off to increase the
rpm
Slide 34
Cruise The Sequence of Events Variable Pitch Control Stage 13
Aircraft in Straight and Level Flight Pitch is hydraulically locked
at the cruise angle. Rpm is restored
Slide 35
Variable Pitch Control The PCU changes propeller pitch and
maintains constant engine speeds during the Dive commencement and
again at Level Out Straight and Level Dive Straight and Level In
all of these manoeuvres, all the pilot is doing is flying
(redirecting) the aircraft, the throttle is not touched. The Dive
Sequence
Slide 36
Check of Understanding The Helix Angle is the angle between
what? The line of rotation and the angle of attack The direction of
the blade and the angle of attack The line of rotation and the
direction of flight The line of rotation and the direction of the
blade
Slide 37
As an aircraft pulls forward, at what rate does the propeller
spin? Around 4000 rpm Around 100 rpm Around 2000 rpm Around 1000
rpm Check of Understanding
Slide 38
The blade angle on a propeller is varied from the root to the
tip. What is this called? Adjustable pitch Blade twist Blade
transition Variable pitch Check of Understanding
Slide 39
Which of these statements applies to a propeller that has been
feathered? It produces maximum power Its leading edge faced 90 o to
the direction of flight Its leading edge faces forward to the
direction of flight It operates at maximum speed Check of
Understanding
Slide 40
On a variable pitch propeller, what is the largest obtainable
pitch angle called? Coarse pitch Cruise pitch Reverse pitch Fine
pitch Check of Understanding
Slide 41
In the diagram, what is angle A known as? The Fine Angle The
Blade Angle The Prop Angle The Pitch Angle Check of Understanding
Line of Rotation Propeller Blade A
Slide 42
Which pitch of propeller gives the maximum resistance to
forward speed? Coarse Pitch Cruise Pitch Fine Pitch Reverse Pitch
Check of Understanding
