Propellers Flashcards
What are the 2 most important forces acting on a propeller?
Thrust and PTQ.
What is PTQ and ETQ?
Propeller torque is the resistance the propeller experiences due to its motion.
Engine torque is torque produced by the engine directly.
What are the TR positions for normal operation, windmilling and reverse thrust?
2, 8 and 10 clock code.
What initially happens to RPM of a fixed pitch when forward speed is increased?
When forward velocity is increased, the advance per revolution of the propeller blade increases. This will reduce the AOA the blade experiences, thus producing a smaller TR and PTQ.
As PTQ is now smaller than ETQ, the RPM will increase.
What initially happens to RPM of a fixed pitch propeller when forward speed is decreased?
When forward velocity is decreased, the advance per revolution of the propeller blade will also decrease, thus increasing the AOA. This will produce a greater TR and therefore more thrust and PTQ.
PTQ>ETQ, RPM will decrease accordingly.
Why does propeller blade twist exist?
Because the propeller blade tip is travelling through a greater circumference for a given RPM than a point closer to the centre of the propeller, it will be producing more thrust.
To help compensate this, a twisted blade propeller will have a more fine edge near the tip and and a coarser blade near the centre of the propeller hub.
Explain what asymmetric blade effect is.
When tilted upwards. The down-going blade of the propeller travels through a greater forward distance than the up-going blade. It also experiences a higher AOA than the downing blade. In a clockwise rotating propeller, this shifts the thrust line to the right of the propeller hub, causing the aircraft to swing to the left.
What happens when an aircraft propeller is windmilling.
During an engine failure, the ETQ will decrease to zero. As a result, the CSU will attempt to fine the blade off to try and compensate for the imbalance of ETQ<PTQ. As a result, the propeller blade will pass into a slightly negative AOA, where the TR acts in the opposite direction to the forward velocity. The PTQ will now act in the direction of the plane of rotation and ETQ.
Because of this, there is a tendency to overspeed the propeller as PTQ is now acting in the same direction as where ETQ would have acted from.
What happens when an aircraft propeller is set to reverse thrust.
During reverse thrust, the aircraft’s propeller is fined to a point beyond windmilling at a significant negative AOA (-20). This produces thrust in the opposite direction to normal, although not as efficiently.
What is CTM?
CTM is centrifugal twisting moment. This occurs due to the perpendicular component of the centrifugal reaction to the pitch axis, acting on the leading and trailing edge of the propeller blade which will act to fine the blade. (smaller blade angle).
What is ATM?
Aerodynamic Twisting Moment. This is caused by the TR acting in front of the pitch change axis. This will act to coarsen the blade (larger blade angle), however the ATM is weaker than the CTM).
When windmilling the ATM will act together with the CTM to fine the blade.
What is experimental mean pitch?
experimental mean pitch is the maximum advance per revolution the propeller blade will travel while no thrust is being produced. EMP will show the zero thrust AOA.
What is propeller slip?
Propeller slip is the difference between the actual advance of the propeller and the experimental mean pitch. It is a measure of efficiency, the most efficiency occurring at around 30% slip.
How do you calculate propeller solidity?
(# propeller blades x blade chord at r)/ propeller disk circumference at r.
What is the most efficiency you can get out of a propeller in ideal conditions? What about the real world?
90% ideal. 50 - 85% real world.
