Propellers Flashcards
How do propellers generate thrust
Imparting small velocity changes to large masses of air
Converting torque produced by engine into linear force on vehicle (thrust)
How do the propeller blades produce force
Act as rotating aerofoils and generate pressure difference from forward to rearward surfaces (Bernoulli’s principle)
How is the efficiency of a propeller determined
Thrust * axial speed / Resistance torque * rotational speed
How does number of blades affect performance
More blades perform slightly better as power and thrust distributed evenly in wake
However usually means blades are narrower so reduced chord length introduces practical structural considerations
What flow properties determine the pitch distribution
Velocity of incoming fluid
Velocity of rotation
What are the typical regions of efficiency for propellers
Good propulsive efficiency at low flight speeds
Inefficient and noisy above Mach 0.5
How does propeller diameter affect efficiency
Typically larger diameter means higher efficiency
Catches more incoming fluid and distributes power and thrust on larger volumes
What dangers need to be considered with propeller blades
Cavitation on propeller blades under water (vaporisation of water on propeller blades due to low pressure)
Supersonic pockets on tips of high speed blades
Should be designed to have lift coefficients below 0.5
Why are large gearboxes needed for propellers
To transmit high torques produced
What is the aim of blade element analysis
To find interrelationship between lift, drag, thrust and torque
To know difference of pitch angle, advance angle, angle of attack
To calculate propulsive efficiency
What similar characteristics do propeller blades and wings have
Bladed generates thrust through aerodynamic lift component
Demands an engine torque to overcome aerodynamic drag
Will stall if local blade angle > stall angle
Affected by trailing vortex generation, tip losses and compressibility
What assumptions are involved in blade element theory
Local flow is 2D at any given radius
Aerodynamic performance of each element determined entirely by geometry of local section
What is the pitch (blade) angle
The angle between the chord line and propeller’s plane of rotation when a blade twists
Function of radius
Defines geometric pitch of blade element
Sum of the angle of attack and advance angle
What is the airflow velocity seen by propeller
Vector sum of velocity entering propeller and tangential velocity of element
What is geometric pitch
Geometric value of axial advance of blade during one revolution of the propeller
Value is practically same for all blade elements
Depends only on geometry of propeller
What is advance per revolution
Measure of how far propeller (and aircraft) moves forward every revolution
Dependent only on forward speed of aircraft
When is the thrust of a low speed fixed-pitch propeller usually greatest
When there is no forward speed
Static thrust gives aircraft good acceleration for take-off
Why do high speed fixed-pitch propeller aircraft struggle with take off
Propeller is designed for maximum pitch meaning large pitch at static speed
Results in separation on blade when there is no forward speed meaning lack of thrust
What are constant speed propellers
Propellers with continuously variable pitch to maintain a constant engine speed at maximum efficiency point under varying load
Qualitatively explain how are thrust and torque calculated
Lift and drag components on the blade are combined with the advance angle
Lift and drag found from dynamic pressure of fluid, blade geometric properties and local coefficients
Integrate differential equations to get the thrust and torque per blade
What design considerations are there in propellers
Blade tip speed
Number of blades
Slipstream
What differences are there in the analysis of helicopter rotor’s compared to airplane propellers
Orientation of the rotor disk is different with respect to incoming airflow
Main rotor produces lift and thrust
Tail rotor produces thrust to compensate for main rotor torque
What motion do rotor blades undergo during flight
Advance into air flow in forward flight and retreat during the other half of rotation
What implication do rotor aerodynamics have
Separation can happen at both the leading edge and trailing edge
Dynamic stall triggered weakly by blade vortex interaction
Dynamic stall triggered strongly by blade kinematics
What is vortex ring state
Airflow upward on inner blade section produces a secondary vortex in addition to normal wingtip vortices
Turbulent airflow results in loss of efficiency
Uncommanded pitch and roll oscillations can occur with large descent rate
What impact does ground effect have
Reduced pitch angle and reduced induced flow
What is collective pitch control
All blades angled equally and simultaneously, allows aircraft to rise vertically
What is cyclic pitch control
Each blade angled individually, allows aircraft to move forward or backward, nose upward or downward, roll from side to side
What is differential pitch control
Allows pitch of one rotor to be increased over the collective pitch of the other rotor, controls the yaw of aircraft (rotation around vertical axis)
What controls the amount of lift generated by a main rotor
Engine throttle setting for desired level of main rotor rotational speed
Collective pitch setting which sets angle of incidence of main rotor blades collectively for uniform lifting force
What is the purpose of a tail rotor
Controls yaw forces and moments (due to main-rotor-induced torque)
Not used if the helicopter has two contra-rotating main rotors
What is a ducted tail fan
Alternative to tail rotor
How is range performance of a helicopter improved
Tilt-wing and tilt-rotor design for V/STOL applications
What is the velocity into the propeller
U_in = U_a * (1+a)
What is the tangential velocity of propeller blade
r * w
What is the advance angle of the propeller blade
tan(phi) = U_in/rw
What is the geometric pitch of the blade
p = 2 * pi * r * tan(beta)
What is the thrust element on a blade
dT = (dL * cos(phi)) - (dD * sin(phi))
What is the torque element on a blade
dF = (dL * sin(phi)) + (dD * cos(phi))
What is the propulsive efficiency in blade element theory
np = (dT * U_a)/(dF * rw)
np = tan(phi) * dT/dF
