Propellers Flashcards
ATM and CTM
ATM - Increasing the blade angle
CTM - fining force - try to align the mass of the blade around the pitch point - uses centrifugal force
Blade angle definition
Angle between the plane of rotation and the chord line
Why is the spinner orientated slightly right on a right hand tractor
To control asymmetric, p factor and torque factor
Washout/ aerodynamic washout
Reducing the AoA and the thickness to reduce the amount of vortices at the tip
Where is most of the thrust produced on a prop
70% of the blade
Efficiency in the cruise - prop
85%
Sole purpose of a spinner
Reduce drag
The hub is connected to
The drive shaft or crankshaft of the engine either directly or through a series of gears
Reduction gear box purpose
Reduces the prop RPM to maintain the optimum engine RPM - while trying to maintain optimum prop RPM
On a fixed pitch prop - neither the prop or the engine are operating at optimum rpm
Common prop construction
Simplest are made from wood
Most are made from metal alloys
Some are made from steel composite
Torque drag
Aerodynamic force acting against rotation motion
2 types of pitch control units
Single acting
Double acting
Single acting PCU - SEP
Uses BFO - big spring and oil pressure moves a piston which then moves the prop
Safest place to fail is in the fine posn - so oil is fed to the piston for coarsening - if it is lost or fails - CTM will cause it to fine - assisted by a spring for low RPMs
Single acting PCU - light twin
Opposite to the SEP - want the prop to go to coarse when the engine fails - spring makes it coarse and oil makes it goes fine
Double acting PCU
Oil in both sides of the piston - hydraulic lock is formed
Directly related to rpm - prop going too slow - bob weight will relax inwards - push the landed valve downwards - oil pressure is sent to fining the blade - when rpm stabilises - landed valve shuts - hydraulic lock forms on the pistons to keep them in place
Overspeed - opposite - bob-weights flyout
Prop lever
Sets the base for which the rpm stabilises - varies the speeder ring - dictates engine rpm
Fully fwd = fully fine
In order to increase power (rev up)
RPM goes up first then throttle - MAP also goes up
In order to decrease power (throttle back)
Throttle goes down first then RPM
Prevents “over gearing”
CSU checks
After engine start - operate the rpm lever to exercise the CSU and encourage oil to flow - check RPM rise and fall
Alpha range - prop
The flight range
Beta range
Range of pitch angles below flight idle
Controlled by weight on wheels
Torque drag increases rapidly the further the blade moves into the beta range - req more reverse thrust req more engine power - only turboprops have the engine power to do true beta ranges
Underspeed governor
In the beta range - power lever directly controls blade pitch angle - prop rpm is maintained at safe levels by an underspeed governor
Synchrophasing unit
Adjusts the prop blade phase - allows for a more constant/calmer noise through the fuselage
Props can also be synchronised at the same rpm
Methods of indicating power - fixed pitch
Amount of thrust is proportional to engine rpm - req power is set as an rpm
Set by a function of mass flow through the engine - MAP
Methods of indicating power - turboprops
Measure of power = torque
Measures the twisting of the shaft - how much mechanical work the engine is doing - can be done electronically or by measuring oil pressure
