Propeller Effects And Designs Flashcards
What do we assume about propellers movement
Moves clockwise direction
Methods to increase power absorption of propeller
Increase propeller diameter
Increase propeller solidity (surface area)
Increase blade chord
Increase number of blades
Use contra rotating propellers
What is propeller solidity
Propeller blade frontal area
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Propeller disk area
The ratio of propeller blade frontal area to propeller disc area. Greater the solidity the greater potential for power absorption.
Torque Reaction what is it
Propeller rotates clockwise of a small mass but large angular velocity
Aircraft rotate anti clockwise opposing the clockwise momentum
Rolling tendency to the left or left yaw on ground
What increases torque reaction/How can it be reduced
Greater power setting/faster RPM more roll left
Contra rotating propellers reduce the effect
Slipstream Effect What is it
Propeller creates slipstream under fueselage hit tail from left to create AOA and creates sideway force around CG
Moves tail right creating a left yaw
What factors increase slipstream effect
Slow airspeed and high power setting
Asymmetric Blade Effect What Is it
At high AOA the right hand propeller produces more than left half resulting in thrust line being offset to the right.
Causes ac to yaw left
Known as P factor
Why does asymmetric blade effect occur
Down going blade travel a further distance in one rotation therefore faster RAF and more lift
Gyroscopic Effect - pitch up/down what happens
Force acts 90 degrees to force applied
Occurs in pitch or yaw
Pitch Up = Yaw to the right
Pitch Down = Yaw to the left
Yaw to the right = Pitch down
Yaw to the left = Pitch Up
Why does the gyroscopic effect happen
Mass has rigidity and therefore will process and move as though force displace by 90 degrees
What are the limits to propeller tip speeds
Above Mach 0.85 drag increases rapidly as shockwaves begin
Propeller effect T/O
Torque effect - yaw left
Slipstream effect - Yaw left
Gyro - yaw left
Propeller effect - Rotation
Gyro effect - Yaw right
Torque reaction - roll left
Propeller effect - Initial Climb
Asymmetric Blade Effect (slow/high AOA) - Yaw left
Propeller effect - Go Around
Flaps reduce alphas - reduction in p factor (less left yaw)
Gyroscopic yaw to right (pitch up)
Slipstream - yaw to left at high power setting
Torque reaction from applied thrust (left yaw)
All propeller effects can be expected during go around!
What is the negative effect of increasing blade chord
Reduce in aspect ratio of blade more wing tip vortices less efficient
Variable pitch propeller during takeoff (blade angle)
Increases as forward velocity increases
RPM up - AOA up
Velocity up - AOA Down - therefore governor will coarsen the pitch to maintain optimum AOA
Most favourable situation in failure of a twin engine aeroplane
Failure of the right engine if both engines are counter rotating inwards
Inwards = both arms closer to CG
