Propellers Flashcards
Blade angle
Angle between plane or rotation (vertical axis) and chord line.
Is the sum of helix angle + AOA
Helix angle
Angle between plane of rotation (vertical axis) and RAF.
AOA
Between RAF and chord line
Geometric pitch
Distance travelled towards in one rotation if the prop was travelled in the direction of its chord line.
Effective pitch
Actual distance travelled forward in one rotation as the prop travels in the direction of the RAF.
Shorter than geometric pitch.
Propeller slip
Difference between effective and geometric pitch.
Forces in normal flight.
Thrust: acts parallel to direction of flight and perpendicular to direction of rotation.
Torque (Aerodynamic): acts parallel to direction of rotation in the opposite direction. Perpendicular to direction of flight.
Torque
2 type: engine and aerodynamic.
When engine > aerodynamic, rpm increases
Opposite, rpm decrease
Same, rpm constant
RPM and TAS vs AOA
As TAS increases, AOA decreases and will eventually become negative.
Ad RPM increases, AOA increases.
Propeller thrust
Highest amount of thrust occurs at 0 TAS
Thrust distribution
Low near root.
Zero at tip.
Highest in the middle.
Propeller twist
Blade angle is reduced along the span from root to tip to maintain a similar AOA along its span.
Types of propeller
Fixed pitch: fixed blade angle, usually a large angle suitable for cruise.
Two-pitch prop: 2 fixed blade angles, fine (small), coarse (large).
Variable pitch/ constant speed prop: blade angle can be varied. Allows an optimum AOA to be maintained resulting in higher efficiency.
Operating a variable pitch prop.
Most efficient at 4 degree AOA.
To keep 4 degrees, at a faster TAS we must coarse the blade.
An increased RPM, requires a fine angle
Controls
Throttle (black) : controls amount of air, shows as MAP.
Pitch (blue) : controls prop pitch, affects seen as rpm gauge.
Mixture (red) : controls amount of fuel.
