Propellers Flashcards
What does a propeller do?
- Moves a relatively large mass of air, relatively slowly rearward creating thrust.
- Converts torque to thrust - has an optimum angle of attack at which it is most efficient, also known as the best hurst to torque ratio.
Fixed pitch propeller.
- Two bladed
- Aluminium or wood
Variable pitch propeller
- Pitch can be changed by the pilot
Constant speed propeller
- Two or three blades
- Pilot sets a desired RPM and the “constant speed unit” (CSU) automatically adjusts the pitch to maintain this RPM.
Disadvantages of fixed pitch prop
- Inability to maintain the best thrust to torque ratio with a change to TAS or RPM.
- Operates only under one set of RPM and airspeed conditions.
- With an increase in airspeed while maintaining the throttle setting, the angle of attack will reduce, reducing the propeller torque, resulting in an increase in RPM.
Could led to overspeed if not monitored.
The CSU Governor
CSU - maintains a constant rotational speed of the aircraft engine and propeller. Automatically adjusts the propeller’s blade pitch to ensure optimal engine performance.
- Core of governor has:
Speeder spring and a set of fly weights mounted of a rotating part connected to the engine - As the engine speed increases, the centrifugal force on the flyweights increases, causing them to move outward.
- If the engine RPM increases - it force the flyweights apart and vice versa
CSU cockpit control
- Within the constant speed range, the throttle controls the intake manifold pressure and therefore the engine power.
- CSU automatically varies the pitch of the blade, using a governor, to maintain the set RPM.
- Reduction of power - CSU fines the blade
Changes is power setting
- Opening the throttle, RPM will want to increase due to the increase in engine torque. Governor inside the CSU sense this and increases the blade angle.
Opposite happens when closing the throttle.
Changes in airspeed
- Airspeed will increase if aircraft nose is lowered without changed the throttle setting.
- Decreases the blade’s angle of attack, leading to a reduction in propeller torque, which in turn will increase the RPM.
- CSU will sense this change and coarsen the blade to maintain propeller RPM.
What is coarse-pitch stop?
The point to which the CSU can maintain the RPM by coarsening the blade.
Reverse thrust
Thrust produced in the opposite direction
How is reverse thrust created?
Certain propellers have the capacity to decrease their blade angles, pass the fine-pitch stop, to a negative blade angle.
What is reverse thrust used for?
Reducing landing distance.
Centrifugal Twisting Moment CTM
Resultant of the centrifugal force felt by the blade while spinning.
Pitch changing mechanisms: Simple piston type
- Blade is attached via linkage to a piston inside a cylinder.
- CSU introduces engine oil inside the piston to increase the blade angle.
To reduce the blade angle the CSU will lower the oil pressure, a spring and the CTM forces will act to fine the blade.
Pitch changing mechanisms: Moveable dome type
- Similar to simple piston type
- Increase in oil pressure, the dome moves forward resulting in an increase in blade angle
- A reduction in oil pressure, together with CTM and a spring, will fine the blade
- Can work by using counterweights to the blade. As prop spins, the counterweight will try to get in line with the rotation, acting to coarsen the blade
Pitch changing mechanism: Electrical mechanism
- Electrical pitch controller
- Lighter and less complex than a hydraulic oil system
- Electric motor changes the blade’s angle via a driveshaft and bevel gears.
- Likely to be controlled via a knob on the dash rather than by a RPM lever.
Normal control handling with a CSU when the aircraft is on the ground.
- Throttle lever: Set to idle or a low power setting.
- Propeller control lever: Set to high RPM
Normal control handling with a CSU for excercising the CSU.
- Throttle lever: Increase power to a specific engine RPM as specified in the aircraft’s operating manual. 1700-2000RPM to create a load on the propeller.
- Propeller control lever: Rapidly cycle the propeller control lever between low RPM and high RPM a few times then return to high RPM position.
Ensures proper lubrication of the propeller pitch change mechanism.
Normal control handling with a CSU prior to take-off and landing.
Throttle lever:
Takeoff - advance the throttle lever smoothly to full power.
Landing - reduce power to the recommended approach setting as specified in the aircraft’s operating manual.
Propeller control lever: Set propeller lever to the high RPM position for both takeoff and landing.
Normal control handling with a CSU when increasing or decreasing power in the air.
- For increase: Rich (mixture full rich), Pitch (set the desire RPM), Power (slowly increase power)
- For decrease: Power (set desired power), Pitch (reduce rpm), Mixture (lean if needed)
Failure in a CSU
- If the governor fails, the blade angle will remain fixed in its current position.
- If an oil system fails, oil pressure will decrease. Internal spring will either force the propeller into a fully fine or fully coarse position.
Full coarse scenario
- Propeller may become inoperable or exert excessive stress on the engine
- Necessary to shut down engine and feather the propeller
Full fine scenario
- AoA decreases
- Reduces propeller torque
- Increase in engine RPM
- ROM can exceed maximum limit and potentially damage both engine and propeller = propeller runaway.
Actions:
- Decrease throttle
- Lower airspeed
- Adjust pitch control to its maximum coarse position
Engine failure with a CSU
- Propeller stats windmilling
- Generates a lot of drag
- Some increase the blade angle passed the coarse pitch stop to feather their blades.
Feathered blade creates no net propeller torque.
- Reduces overall drag
