Ch 26 - Propeller Design And Propeller Effects Flashcards
Engine power
A propeller must be able to absorb the shaft power produced by the engine otherwise it is pointless. There are various designs which help the absorption of power
Blade Length
Increasing the blade length will increase the maximum possible amount of power
Longer blades lead to increase tip vortices which will reduce efficiency and greatly increase noise created
Also need a higher undercarriage and further away from fuselage to let them fit
Propeller Solidity
The ratio of propeller frontal blade area to the propeller disk.
The greater the solidity, the greater the potential power for absorption
Increase number of blade or chord of blade to increase
Propeller Solidiity = frontal area / disk area
Increasing Blade Chord
Allows the propeller to absorb more power because each blade section gene erases more aerodynamic force.
Reduced aspect ratio increase tip vortices making them less efficient
Increasing Number of Blades
Increases solidity
Practical limit (5 for EASA)
Efficiency starts to reduce as each blade is operating in dirty air from last blade
Contra-Rotating Propellers
Two hubs instead of one where the blades rotate in opposite direction to decrease the impact of inflow effects. Eliminates gyroscopic effects
Torque Reaction Effect
On an aircraft where the blade is rotating clockwise, the aircraft will attempt to spin anti-clockwise with equal but opposite angular momentum.
The amount depends on the mass of rotating propeller and the speed at which it is rotating and the thrust it is producing.
Particular noticeable on SE aircraft during TO roll - resulting in yaw.
Slipstream Effect
A rotating propeller rotates the slipstream it generates. As well as flowing rearwards the slipstream rotates in the same direction as the propeller. As a result the slipstream hits the fin at an angle which gives the fin an angle of attack. The fin then produces a sideways aerodynamic force, creating a yawning motion.
Slipstream tightens out slightly at higher TAS so as speed increases, yawning motion decreases.
Greatest at high power setting and slow speed
Asymmetric Blade Effect - P Factor
At high angles of attack (slow flight) the right half of the propeller disc produces more thrust than the left half. This results in the thrust line being offset to the right rather than through the shaft axis of the propeller. The aircraft yaws left around its CG.
Increased AoA on the downgoing blade and increased RAF
Greatest at high power setting and high Aircraft AoA - Climbing and slow speed after take - off
Gyroscopic Effect
Nazi Symbol
Control input: Pitch Up
Reaction: Yaw Right
Ways to Reduce Propeller Effects
By using counter or Contra rotating propellers
