Propellers Flashcards
Blade Angle
Angle between the plane of rotation and the chord line
What happens to the blade angle from the root to the tip
Blade angle decreases towards the tip
As speed increases what happens to the blades AOA
Decreased AOA with increase in speed
As speed decreases what happens to the AOA of the blade
AOA will increase with reduction in forward speed
Component at 90 degree to rotational axis is…
Torque Drag
Component parallel to propellers rotational axis is
Thrust
Fixed Pitch Propeller produces a….
Produces it optimum AOA at a pre determined speed and operates and less efficiently at any other speed
RPM increases what happens to the AOA
AOA will increase
What happens when RPM is reduced but TAS stays the same on propeller
Causes negative AOA and considerable drag for no thrust
Blade Twist is
To preserve same AOA along the whole length of the blade
Geometric Pitch
The distance travelled in one revolution of the propeller
Geometric pitch for a large blade angle
Coarse pitch will have a larger geometric pitch
(Geometric pitch is distance propeller advanced in one revolution)
Variable Pitch Propeller purpose and operations
Designed to maintain optimum blade angle
Can be made to be fine pitches or fully coarse
Blade Pitch - Feathered (90 degrees)
Fully coarse - minimum drag in flight
Used for failed engine
Blade Angle - Fully Fine (0 degrees)
Minimum drag for engine start or max drag for braking on the ground
Blade Angle - Reverse - (-18 degrees)
Generates reverse thrust on the ground
Centrifugal turning moment
Tries to fine off the propeller
Aerodynamic Turning Moment
Tries to coarsen the propeller
ATM vs CTM
CTM (Centrifugal turning moment) is more dominant than aerodynamic turning moment
Hub Attachment on propeller system is attached to what
Drive shaft or crankshaft directly or via reduction gearbox
Purpose of reduction gearbox
Translate large amounts of power into torque
Reduce propeller rpm to optimum
Propeller Construction
Composites/metal alloys
Torque drag is
Force opposing rotation
How does a variable pitch system work/What happens when TAS increases
Propeller coarsens to absorb power
RPM will be constant
Blade angle coarser
Thrust and torque will be higher
AOA will be back to an efficient value
Pitch Change Unit role
Moves propeller blades
Constant Speed Unit
CSU moves propeller blade angle to maintain RPM (picture computer doing work)
Matches torque drag to engine torque
Uses oil to pitch change unit to coarsen blade pitch angle and increase torque drag
Pitch Control Unit
Mechanism that changes the blade angle
Single acting - pressured oil to one side to coarsen pitch and when oil pressure reduces returns to fine pitch via CTM moment
Advantage of PCU single acting system on a single engine AC
Engine output drops so therefore oil pressure in PCU so blade will fine off to reduce torque drag
Single acting PCU on twin A/C
Has fly weights to coarsen off the propeller to feather position
Creates a couple under centrifugal force to overcome CTM force and twist towards feathered position
Double Acting PCU
Has oil pressure supplied to both sides of piston to adjust propeller pitch towards coarse or fine
Used on larger propellers where greater centrifugal force would not be able to cope with just fly weights
Centrifugal Latch
Prevent twin engine from feathering on ground at shutdown
Spring pressure overcomes centrifugal force
Propeller RPM falls - CSU response is
Move blades to a finer pitch
If propeller RPM rises - CSU will
Move blades to coarser pitch
CSU over speed steps
RPM increases
Governors flyweights thrown outwards
Valve lifts against spring
Pressurised oil directed to coarse side of PCU
Propeller torque drag increases and rpm reduces
CSU On speed
Propeller torque and engine output match
Governor flyweight moves inwards to shut off oil supply
Hydraulic lock which holds propeller at pitch angle
Counterweights is exactly counterbalanced by pressure on regulating spring
CSU Underspeed
Governor flyweights experience less centrifugal force so move inwards
Lowering landed valve
Direct oil to fine pitch side of PCU and allows oil to drain from coarse pitch back to pump.
Moves to finer pitch
Torque drag reduces below engine torque allow propeller to accelerate
RPM lever does what
Acts on the governor spring to change compression force on the CSU unit
Forward RPM level will
Finer pitch adjustment
RPM will increase
Prop will accelerate until prop drag matches torque output
Where is the pitch control unit found
Inside the hub
Rearward RPM Lever
Feather the propeller
Unfeathering
Moving from coarse to finer pitch using a accumulator with high pressure oil to move blade
Alpha Flight Range
Anything from flight idle to coarse
Positive angle
Beta Range
Range below flight idle - negative angle
Negative pitch creates large amount of torque drag which increases further into beta range
Turboprop engines usually only have enough power to drive to true beta range
Torque Gauges Role
Measure engine torque which is the turning force exerted by engine on propeller shaft
Power = torque x rpm
Beta range is anywhere between
Reverse and flight fine
More power
Prop lever first when you need more power to climb
Reduce power
Throttle lever first when you need to reduce power in descent
