Leading Cadet Training Principles of Flight Leading Cadet Training Propellers Lecture 7

A propeller is essentially an aerofoil Propellers A propeller is essentially an aerofoil Leading Edge Trailing Edge Chord Line

This rotational airflow Propellers At a Zero Airspeed As the blade produces lift, air is projected vertically, known as the Induced Flow. Induced Flow The blade generates Rotational Airflow, parallel to and opposite the blade’s plane of rotation. This rotational airflow generates lift. Relative Airflow (RAF) Chord Line  = AofA Airflow due to Rotational Velocity = Blade Angle 

Propellers Blade Twist Rotational Velocity Approx 4o Angle of Attack Total Inflow

-  Propellers Airspeed Effects At a Forward Airspeed Need larger  True Air Speed + Induced Flow Total Inflow = -  Chord Line Relative Airflow (RAF) Airflow due to Rotational Velocity  = AofA Need larger  for same  = Blade Angle 

Propellers Airspeed Effects Propeller Pitch Course Pitch Feathered 90o Fine Pitch Feathered 90o Ground Pitch 0o Reverse Pitch 20o

Propellers Airspeed Effects Propeller Pitch Propeller Efficiency at Max Power _ 100% Fine Variable Pitch _ 75% Coarse _ 50% _ 25% True Airspeed

Aerodynamic Forces Feathered Propeller Therefore drag at minimum. Although twisted, in aggregate, blade at “Zero Lift α”. Therefore drag at minimum. Note that in Firefly/Tutor prop goes to “Fine Pitch” if engine rotating, “Coarse Pitch” if engine seized

Aerodynamic Forces Total Inflow  (RAF) Airflow due to Relative Airflow (RAF)  Airflow due to Rotational Velocity Chord Line Lift Drag  Total Reaction

Aerodynamic Forces High Slow speed fixed pitch TAS + Induced Flow  Relative Airflow (RAF)  Airflow due to Rotational Velocity Chord Line Thrust Thrust reduced Total Reaction reduced RPM increases, Rotational Drag reduced  Prop Rotational Drag Total Reaction

Aerodynamic Forces High Slow speed variable pitch Faster TAS + Induced Flow Relative Airflow (RAF)  Airflow due to Rotational Velocity Chord Line Thrust (eventually reduces) RPM increases, Rotational Drag increased  (same or greater) Prop Rotational Drag Total Reaction

(Driving the propeller) Aerodynamic Forces Windmilling TAS Chord Line Total Reaction Negative AOA  Negative Rotational Drag (Driving the propeller) Negative Thrust (Drag) Relative Airflow (RAF) This may cause further damage, even Fire. Airflow due to Rotational Velocity

Take Off Swings Yaw due to torque reaction Yaw due to slipstream effect Torque Reaction means more rolling resistance on one wheel Helical slipstream acts on one side of fin more than the other

Take Off Swings Tail wheel aircraft only: Asymmetric blade effect Top picture both propeller sections are equal also distance travelled are equal. Picture exaggerated but downgoing blade has greater AOA more thrust. Distance travelled by downgoing blade greater. Gyroscopic effect When tailwheel raised all effects are zero (until rotate?) But, force applied to top of disc (gyro) then clockwise rotation will cause yaw to the left Obviously anticlockwise all reversed. Affect all aircraft on rotate? All Aircraft: Don’t forget crosswind effect. (if yaw to left because of slipstream then get a crosswind from the right)

Centrifugal Twisting Moment Tries to fine blade off This makes it more difficult for the pitch changing mechanism when increasing pitch.

Aerodynamic Twisting Moment Relative Airflow Total Reaction Tries to coarsen blade up Aerodynamic Twisting Movement (ATM) tries to increase the AOA, this partially offsets Centrifugal Twisting Moment (CTM)

Aerodynamic Twisting Moment Total Reaction Relative Airflow Windmilling Tries to fine blade off When windmilling in a dive the Aerodynamic Twisting Movement (ATM) is reversed and enhances the Centrifugal Twisting Moment CTM. This could prevent the pitch mechanism from working.

Check of Understanding What is the blade angle of attack measured between? The chord line and the total reaction The chord line and the relative airflow The relative airflow and the rotational velocity The rotational velocity and the induced flow

Check of Understanding What are the Resultant Forces that a propeller produces? Total Reaction and Drag Thrust and Propeller Rotational Drag Total Reaction and Lift Lift and Propeller Rotational Drag

Check of Understanding What effect will increasing speed with a fixed pitch propeller have? Reduce efficiency Increase efficiency Reduce the engine speed Increase the engine speed

Check of Understanding What are the two main forces trying to alter the propeller blade Angle of Attack? RAF and ATM CTM and REV ATM and CTM REV and ATM

Check of Understanding Which of the following statements regarding take off swing is true? Equal helical stripstream on the fin Equal rolling resistance on all wheels Less rolling resistance on all wheels More rolling resistance on one wheel

Check of Understanding What is the expression that makes it more difficult for the pitch changing mechanism to work when increasing pitch? Centrifugal twisting moment Aerodynamic twisting motion Windmilling Rotational velocity

Leading Cadet Training Principles of Flight Leading Cadet Training End of Presentation