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PROPELLER SYSTEMPROPELLER SYSTEM
1st - Look at how lift is generated
Then see how it applies to 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 here
The result is
MORE LIFT
How lift is increased
PROPELLER SYSTEM
Direction of travel
Aerofoil incline
The difference in direction of travel and aerofoil incline is called:-
The ANGLE of ATTACK
How lift is increased
PROPELLER SYSTEM
How does lift apply to PROPELLORS?
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
PROPELLER SYSTEM
How the HELIX ANGLE is generated
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
How the HELIX ANGLE is changed by engine rpm and forward speed
Forward Speed
RPM
How an increase in RPM changes the Helix Angle
Changes in FORWARD SPEED and RPM will change the Helix Angle
Faster RPM
PROPELLER SYSTEM
How the blade tip travel produces the HELIX ANGLE
Forward Speed
RPMRPM
Faster Forward Speed
Changes in FORWARD SPEED and RPM will change the Helix Angle
How an increase in FORWARD SPEED changes the HELIX ANGLE
PROPELLER SYSTEM
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
Mechanical STOPS and blade angles
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 pitchCoarse 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 SYSTEMImportance 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 SYSTEMImportance 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 increasingengine disintegration
Usually for military aircraft only
PROPELLER SYSTEMImportance 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 thanfinal approach
Flight Fine pitch
Cruise pitch
Both give minimal drag at low power settings
PROPELLER SYSTEMImportance of set blade angle
Blade Twist
DISTANCE TRAVELLED BY DISTANCE TRAVELLED BY ROOT, MID-SPAN AND TIPROOT, MID-SPAN AND TIP
THICK FOR THICK FOR STRENGTHSTRENGTH
PROPELLER SYSTEMBlade Twist
ROOT MID-SPAN TIP
THINNER FOR THINNER FOR STRENGTH AND STRENGTH AND
THRUSTTHRUST
THIN FOR THIN FOR THRUSTTHRUST
COARSE ANGLE
MEDIUM ANGLE
FINE ANGLE
BLADE ANGLE RELATIVE TO DISTANCE (AND THEREFORE SPEED) BLADE ANGLE RELATIVE TO DISTANCE (AND THEREFORE SPEED) TRAVELLED BY ROOT, MID-SPAN AND TIPTRAVELLED BY ROOT, MID-SPAN AND TIP
Typical Blade
Typical 3 Blade Prop