Transcript Piston Prop. Pt 8

PISTON ENGINES
Part 8
Propeller Control
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
in this direction
and aerofoil
incline is called ?
Large Pressure
Decrease here
α
The Angle of Attack
Small Pressure
Increase here
The Propeller System
On Propellers,
LIFT
called THRUST
As an aircraft
pulls forward,
theispropeller
spins at high speed,
this can
bethe
around
1000
rpm.
and propeller Blades
work
same
way
as aircraft wings.
path the blade
cuts
air is called
WhenThe
a propeller
spins tip
and
thethrough
aircraftthe
moves
forward,
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.
Line of Rotation
If the Helix Angle changes,
then we need to change
the Angle of Attack.
Propeller Blade
This is the
Blade Angle
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
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
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.
Sliding Piston
Fine
Pitch
Propeller
Blade
Actuating
Lever
through to
Coarse
Pitch
Hard Stops
Direction
of
Rotation
All propeller blades are actuated
by the same mechanical linkage
Direction
of Flight
Variable Pitch Propellers
Blade Angles
Direction
Of
Rotation
Fine pitch
Coarse pitch
or
‘Feathered’
Maximum
resistance
to forward
speed
Minimum
resistance to
rotation
At this hard stop
the blade is in
this position
Maximum resistance
to rotation
Minimum
resistance
to forward
speed
The blade angle changes
through 90o with piston travel
Piston travels between ‘hard’ stops
Blade angle is relative
to piston travel
At this hard stop
the blade is in
this position
Variable Pitch Propellers
Blade Angles
Direction
Of
Rotation
Fine pitch
Good for:Maximum
resistance
to forward
speed
Minimum
resistance to
rotation
Direction of travel
Easier Starting of engine
Running engine with no/minimal thrust
High drag – braking effect on ground
Bad for:In-flight – loss of control
In-flight engine failure – loss of control
and engine disintegration
Importance of set blade angle
Variable Pitch Propellers
Blade Angles
Direction
Of
Rotation
Good for:Coarse pitch
or
‘Feathered’
Maximum resistance
to rotation
In-flight – loss of control
In-flight engine failure – control maintained
engine stops rotating minimizing damage
Minimum
resistance
to forward
speed
Bad for:-
Direction of travel
Starting of engine
Could cause engine burn-out if running
Low drag – NO braking effect on ground
Importance of set blade angle
Variable Pitch Propellers
Blade Angles
Direction
Of
Rotation
Fine pitch
Used for:High drag – high braking effect on ground
Air pushed
Maximum
forward
giving
resistance
reverse
thrust
to forward
speed
REVERSE PITCH
Usually for military aircraft only
Minimum
Minimal
resistance
to
resistance
to
rotation
rotation
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
Variable Pitch Propellers
Blade Angles
Direction
Of
Rotation
Used for:Flight
Fine
pitch
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
There is a ‘Twist’ to all propeller blades
MID-SPAN
ROOT
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
COARSE
ANGLE
THICK FOR
STRENGTH
MID-SPAN
MEDIUM
ANGLE
THINNER FOR
STRENGTH & THRUST
Distance travelled by ROOT, MID-SPAN & TIP
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
PCU
Hydraulic
Connections
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
PCU
Hydraulic
Connections
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
Blade
Turning
Mechanism
Pilot Input
Signal
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 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
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
Variable Pitch Control
The Dive Sequence
The PCU changes propeller pitch
and maintains constant engine speeds
during the Dive commencement
Straight and Level
In all of these manoeuvres,
all the pilot is doing
is flying (redirecting) the aircraft,
the throttle is not touched.
and again at
Level Out
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 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
Check of Understanding
On a variable pitch propeller,
what is the largest obtainable pitch angle called?
Cruise pitch
Fine pitch
Reverse pitch
Coarse pitch
Check of Understanding
In the diagram,
what is angle ‘A’ known as?
The Blade Angle
Line of
Rotation
Propeller
Blade
The Pitch Angle
A
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
PISTON ENGINES
End of Presentation