Propeller Theory Flashcards
Propeller Torque
Addition of forces to the drag that oppose the plane of rotation.
The propeller will run at constant speed when the torque force equals the torque produced by the engine
How does AoA change on a fixed pitch propeller
For the same RPM if the forward speed increases the AoA reduces.
At times of low RPM and high forward speed, a negative AoA can be produced causing drag. (Windmilling)
Conversely, if the forward speed of the aircraft is constant and the RPM increases AoA will increase due to the change in the lift vector.
As forward speed increases, AoA reduces, less lift is produced therefore less drag is created. Therefore there is less drag and propeller torque reduces and RPM increases which could cause the a/c to exceed its RPM limit.
Gyroscopic Effect
As the prop is spinning, it acts like a gyro. Therefore, any force is felt. 90* to the plane of rotation.
E.g a prop rotating anti clockwise, pitching up will cause a yaw to the left.
Slipstream Effect
As the rotating propeller causes the airflow to spiral around the fuselage, this creates a force on the tail fin.
Roughly increases with RPM.
This effect is reduced on twins as their prop wash is relatively free from hitting the tail fin
Asymmetric Blade Effect
As the blades travel the same distance. When the plane of rotation is inclined, the up/down going blade will be traveling further. Therefore it is subject to a faster relative airflow, allowing it create more lift.
This imbalances the thrust between the 2 blades and the a/c will yaw to the left or right. It offsets the thrust line of the propeller.
Aerodynamic and Centrifugal Twisting Moments
Aerodynamic
Lift acts forward of the centre of rotation (centre of the propeller aerofoil). Therefore this causes a moment which wants to try and coarsen the blade angle. A negative windmilling AoA will cause an opposite effect.
Centrifugal
As the blades rotate, the mass of the leading and trailing edge of the propeller wants to fly out due to the centrifugal forces. Therefore it will try and fine off the balde angle
CSU
Allows the balde to be coarsened or fined.
Once the RPM is set, the fly weights move out/in by centrifugal forces, allowing oil to go into the propeller unit. A single acting actuator moves to either coarsen or fine the blade using engine oil to push it. Fine = oil is brought in. Coarsen = oil is taken out and a spring with compressed air moves the piston to increase blade angle.
Sequence of Use of Levers
When you want to go faster = right to left = Mixture, richen, increase rpm then increases power.
When you want to slow down, left to right = opposite actions
