Piston Engines: Propellers
-
Upload
jesscar -
Category
Engineering
-
view
189 -
download
8
Transcript of 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
LIFT
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
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
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
RPM
How the blade tip travel produces the HELIX ANGLE
PROPELLER SYSTEM
PROPELLER SYSTEM
Forward Speed
RPM
How the blade tip travel produces the HELIX ANGLE
Forward Speed
RPM
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
Forward Speed
RPM
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
Forward Speed
RPM
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
Forward Speed
RPM
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
Forward Speed
RPM
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
• 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
Fine
Pitch
Coarse
Pitch
Direction
of
Rotation
Direction
of Flight
Propeller
Blade
Actuating
Lever
Actuating
Link
Note: - blade angle is relative to piston travel
Fine pitch
Coarse pitch
Or
‘Feathered’
Piston travels between ‘hard’ stops
Direction
Of
Rotation
Maximum resistance
to rotation
Minimum
resistance
to forward
speed
Minimum
resistance to
rotation
Maximum
resistance
to forward
speed
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
speed
Maximum resistance
to rotation
Minimum
resistance
to forward
speed
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
Importance of set blade angle
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
Importance of set blade angle
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
Fine
pitch
Cruise
pitch
Both give minimal drag at
low power settings
PROPELLER SYSTEM
Importance of set blade angle
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
ANGLE
MEDIUM
ANGLE
FINE
ANGLE
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
Start
& Idle
Cruise
Take
Off
Throttle
Positions: -
Pilot Input
Signal
Hydraulic
Pressure
Supply
Hydraulic
Return
Operation Piston
Spinner
Connecting Linkage
Hydraulic
Connections
PCU
Sliding
Collar
Engine RPM Signal
(Mechanical Drive)
Spring
Hydraulic
Valve
Counter
Balance
Weights
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
FMU PCU
Piston Engine driven propeller
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
FMU PCU
Jet Engine driven propeller – Turbo-Prop
Propeller HubEngine Mounted
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Start
& Idle
Cruise
Take
Off
Throttle
Positions
Pilot Input
Signal
Engine Stationary
Propeller HubEngine Mounted
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Start
& Idle
Cruise
Take
Off
Throttle
Positions
Pilot Input
Signal
Start Initiated – Engine Begins to Rotate
Propeller HubEngine Mounted
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Start
& Idle
Cruise
Take
Off
Throttle
Positions
Engine At Idle
Propeller HubEngine Mounted
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Start
& Idle
Cruise
Take
Off
Throttle
Positions
Take Off Selected
Propeller HubEngine Mounted
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Start
& Idle
Cruise
Take
Off
Throttle
Positions
Engine Accelerates to Take Off Speed
Propeller HubEngine Mounted
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Start
& Idle
Cruise
Take
Off
Throttle
Positions
Engine Running at Take Off Speed
Flying straight and level
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.
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
And then at Level Out
Propeller HubEngine Mounted
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Throttle
Positions
Start
& Idle
Cruise
Take
Off
Aircraft Starts Dive
Throttle (and Engine) at Cruise –
Propeller HubEngine Mounted
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Throttle
Positions
Start
& Idle
Cruise
Take
Off
RPM Increases
Throttle at Cruise –
Propeller HubEngine Mounted
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Throttle
Positions
Start
& Idle
Cruise
Take
Off
RPM Increases - Prop Goes to Coarser Pitch
Throttle at Cruise –
Propeller HubEngine Mounted
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Throttle
Positions
Throttle at Cruise –
Start
& Idle
Cruise
Take
Off
Aircraft in Dive – RPM Restored
Flying straight and level
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.
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Now at Level Out
Propeller HubEngine Mounted
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Throttle
Positions
Throttle at Cruise –
Start
& Idle
Cruise
Take
Off
Aircraft Levels Out
Propeller HubEngine Mounted
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Throttle
Positions
Start
& Idle
Cruise
Take
Off
RPM Reduces
Throttle at Cruise –
Propeller HubEngine Mounted
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Throttle
Positions
Start
& Idle
Cruise
Take
Off
Prop Pitch Reduces
Throttle at Cruise –
Propeller HubEngine Mounted
PROPELLER SYSTEM – VARIABLE PITCH CONTROL
Throttle
Positions
Start
& Idle
Cruise
Take
Off
RPM Restored
Throttle (and Engine) at Cruise –