Propellers Flashcards
Propeller Manufacturers
Mcauley
Hartsell
MTU
Components of the Total Reaction
Propellor thrust
Propellor torque
Composite Propeller
Preferable as much lighter than metal or wood
Difficult to repair from foreign object damage (FOD)
Angle of Attack
The angle between its chord and the relative airflow
4 degrees is most efficient
Blade Angle
The angle between the chord line of the propeller and the plane of rotation
Blade angle = helix angle + angle of attack
Helix Angle
The angle between the plane of rotation and the relative airflow
Relationship Between RPM, TAS and AoA
Fixed pitch (blade angle remains constant):
For a given RPM, increased TAS will reduce the angle of attach and vice versa
For a given TAS, increased RPM increases the angle of attack and vice versa
Any angle of attack can be achieved with combinations of TAS and RPM
One RPM value will only have one TAS to achieve a certain AoA
As AoA decreases, thrust decreases as TAS increases
Thrust/Toque Ratio
Changes with the AoA
Best Thrust/Torque Ratio
Achieved using the most efficient AoA
Gives the greatest amount of propeller thrust for the smallest amount of propeller torque
Best value for money
Engine Torque
Acts to overcome propeller torque and enables the blades to rotate
Engine Torque Vs Propeller Torque
ET = PT: the blades rotate at a constant RPM (RPM constant)
ET > PT: RPM increases
ET < PT: RPM reduces
Fixed Pitch Propellers
Fixed blade angle
Efficient only at one TAS
Variable Pitch Propellers
Varies the blade angle to maintain an efficient angle of attack over a wide range of RPM and TAS
Also known as constant speed units (CSU)
Increased TAS on a Variable Pitch Propeller
Blade angle must be increased to maintain the AoA
Fine Pitch
Small blade angle
Good for acceleration (high RPM)
Suitable for takeoff and slow flight
Coarse Pitch
Large blade angle
Suitable for high speeds (cruise at low RPM)
Efficiency of Propellers
Convert BHP to THP
80 - 90% efficient due to working in the fluid air (slip)
Forces Acting on a Propeller
Centrifugal Forces: work to pull blades apart from the hub
Tangential Component (centrifugal twisting): Which wants to turn the blades to fine the pitch
Aerodynamic Twisting: Tries to turn the blades in the opposite direction
Propeller Thrust
Acting perpendicular to the plane of rotation
No thrust with brakes on
Propeller Torque
Acting in the plane of rotation Opposes rotation Opposed by engine torque (turning force) Can overrev when the propeller begins to turn the engine (eg. in a steep dive) Increases with RPM
Feathering
Propeller must feather (become extremely coarse) in an engine failure to ensure it does not create drag and twist the aeroplane
A position of 0 torque
Reverse Pitch
Acts as a braking mechanism and enables aircraft to be reversed
Creates a relatively large negative angle of attack by allowing the blades to turn past the fine pitch limit
Total Flow
Total flow = induced flow + TAS
Slip
The difference between geometric pitch and effective pitch
Reduction due to the propeller working in air
Geometric Pitch
= effective pitch + slip
Constant Speed Unit (CSU) in a Single Engine
When speed is increased, fly waits are flung outwards, lifting the oil valve and pushing oil to coarsen the prop to maintain RPM
When speed decreases, fly waits go in, lowering the valve and pushing away to fine the prop to maintain RPM
Aerodynamics of the Blades on the Prop
Twisted to keep the AoA constant (may stall otherwise)
Tip of prop moves faster than at the hub
Constant Speed Unit (CSU) in a Twin Engine
When speed is increased, fly waits are flung outwards, lifting the oil valve and pushing oil away to coarsen the prop to maintain RPM
When speed decreases, fly waits go in, lowering the valve and pushing oil in to fine the prop to maintain RPM
If oil pressure is lost the propeller feathers
Constant Speed Unit (CSU)
Maintains a constant RPM
As RPM is decreased, the prop coarsens increasing TAS in the cruise
Counterweights
Only in multi-engine
Used to coarsen the pitch
Oil pressure reduces pitch
Overspeed
Engine torque > Prop torque
Prop tries to speed up
CSU coarsens pitch
PQ increases to match EQ
Governor Action When Overspeeding
Prop RPM starts to increase Flyweights spin faster and are flung outwards Pilot valve rises Oil Flows: - To the hub (single) - Away from the hub (multi) Blades move to a coarser pitch Extra prop torque stops the RPM from increasing
Underspeed
Engine torque < prop torque
Prop tries to slow down
CSU drives prop to fine pitch
Governor Actions When Underspeeding
RPM starts to reduce Flyweights slow down and are flung inwards Speeder spring wins Pilot valve moves down Oil flows: - From the hub (single) - To the hub (multi) Blades move to a finer pitch Less prop torque, stops the RPM from reducing
Controls For Power and RPM
Throttle controls MAP (The primary power gauge)
Pitch lever controls RPM
- Increased RPM creates a fine pitch
Coarse Pitch Stop
Max possible blade angle
Eg. A dive with power on
Ground Operation
Prop in fine pitch
Changing Power
Avoid high MAP with low RPM die to the risk of detonation
High MAP: lots of mixture entering cylinders
Low RPM: valves open for longer, allowing more mixture to enter the cylinders
