PISTON ENGINES Part 8 Propeller Control

How Lift is Generated α The Angle of Attack The result is LIFT in this direction Pressure here is constant Pressure here decreases The result is LIFT The difference in direction of travel and aerofoil incline is called ? in this direction Large Pressure Decrease here α Small Pressure Increase here The Angle of Attack

The Propeller System The Helix Angle On Propellers, LIFT is called THRUST and propeller Blades work the same way as aircraft wings. As an aircraft pulls forward, the propeller spins at high speed, this can be around 1000 rpm. The path the blade tip cuts through the air is called a Helix or HELICAL. Three things effect this shape: - Forward speed. Propeller rpm. Propeller diameter. When a propeller spins and the aircraft moves forward, the tips of the propeller blades move in a ‘corkscrew’ path Exactly how the blade tip travels produces This path is called a HELIX The Helix Angle

The Helix Angle The Angle of Attack can be changed The optimum Angle of Attack is required to maintain most efficient thrust generation. If the Helix Angle changes, then we need to change the Angle of Attack. Propeller Blade Line of Rotation Direction of blade through the air This is the Blade Angle This is the Angle of Attack This is the Helix Angle The Angle of Attack can be changed by altering the rpm or the forward speed.

The Helix Angle This produces a set HELIX ANGLE Rotation - Number of Rotations per Minute Forward Speed - Distance Travelled over One Minute

The Helix Angle Changes in FORWARD SPEED and/or RPM will change the Helix Angle and the Angle of Attack At a Faster Forward Speed At a Faster RPM The angle narrows The angle widens

Variable Pitch Propellers Blade Angles With fixed pitch propellers, changing the rpm or forward speed changes the Angle of Attack, but unfortunately not at the correct angle. Therefore either increase in drag or a stall results. Variable Pitch propellers were introduced to alleviate this problem, and provide other advantages.

Variable Pitch Propellers Blade Angles The variable pitch propeller is a mechanism by which all the blades on a propeller hub can be rotated about the blade centre axis, whilst the propeller is spinning. Sliding Piston Propeller Blade Fine Pitch Actuating Lever through to Coarse Pitch Direction of Rotation Direction of Flight Hard Stops All propeller blades are actuated by the same mechanical linkage

Variable Pitch Propellers Blade Angles Direction Of Rotation Fine pitch Coarse pitch or ‘Feathered’ Maximum resistance to forward speed Minimum resistance to forward speed Maximum resistance to rotation Minimum resistance to rotation The blade angle changes through 90o with piston travel At this hard stop the blade is in this position At this hard stop the blade is in this position Piston travels between ‘hard’ stops Blade angle is relative to piston travel

Variable Pitch Propellers Blade Angles Direction Of Rotation Fine pitch Good for:- Maximum resistance to forward speed Easier Starting of engine Running engine with no/minimal thrust High drag – braking effect on ground Minimum resistance to rotation Bad for:- In-flight – loss of control In-flight engine failure – loss of control and engine disintegration Direction of travel Importance of set blade angle

Variable Pitch Propellers Blade Angles Direction Of Rotation Good for:- In-flight – loss of control Coarse pitch or ‘Feathered’ In-flight engine failure – control maintained engine stops rotating minimizing damage Minimum resistance to forward speed Maximum resistance to rotation Bad for:- Starting of engine Could cause engine burn-out if running Direction of travel Low drag – NO braking effect on ground Importance of set blade angle

Variable Pitch Propellers Blade Angles Direction Of Rotation Fine pitch Used for:- Maximum resistance to forward speed High drag – high braking effect on ground Air pushed forward giving reverse thrust REVERSE PITCH Usually for military aircraft only Minimal resistance to rotation Minimum resistance to rotation Bad for:- In-flight – loss of forward speed, aircraft stalls In-flight engine failure – loss of control and reverse rotation increasing engine disintegration Direction of travel Importance of set blade angle

Variable Pitch Propellers Blade Angles Direction Of Rotation Flight Fine pitch Used for:- Low drag on final approach Flight Fine & Cruise Pitch Both give minimal drag at low power settings Cruise pitch Used for:- In-flight descent – faster forward speed than final approach Direction of travel Importance of set blade angle

There is a ‘Twist’ to all propeller blades Blade Twist There is a ‘Twist’ to all propeller blades MID-SPAN ROOT TIP Viewed ‘End On’

Blade Twist Distance travelled by ROOT, MID-SPAN & TIP The distance the blade travels during rotation is different at various blade sections along its span. All blades have a ‘coarse’ angle at the root, progressing to a ‘fine’ angle towards the tip. This ‘blade twist’ maintains an efficient angle of attack along the full length of the propeller blade. 3 Blade Prop Typical Blade ROOT TIP MID-SPAN COARSE ANGLE MEDIUM ANGLE FINE ANGLE THICK FOR STRENGTH THINNER FOR STRENGTH & THRUST THIN FOR THRUST Distance travelled by ROOT, MID-SPAN & TIP

Variable Pitch Control Variable pitch propeller systems allow the engine to run at a constant speed, irrespective of flight manoeuvres. This has the advantage of protecting the engine from over-speeding, and possible disintegration, during extreme manoeuvres experienced in combat.

Variable Pitch Control The rotating hub contains the blade turning mechanism, which is piston driven and hydraulically operated, by a Propeller Control Unit (PCU). The PCU is the link between pilot demand (power setting), the engine speed, and the aircraft attitude. Propeller Hub Engine Mounted Operation Piston Blade Turning Mechanism Hydraulic Connections PCU

Variable Pitch Control A hydraulic valve directs pressure to either side of the piston in the hub. The valve is positioned by rotating centrifugal weights (‘bob’ weights), balanced against spring tension. Propeller Hub Engine Mounted Spring Counter Balance Weights Operation Piston Blade Turning Mechanism Hydraulic Valve Hydraulic Connections PCU

Variable Pitch Control When the pilot opens the throttle, increasing power, he also compresses the spring to a higher tension. When the engine accelerates the bob weights spin faster, putting the hydraulic control valve in the balanced position, and steady state rpm is achieved. Throttle Positions: - Propeller Hub Engine Mounted Take Off Spring Cruise Counter Balance Weights Start & Idle Operation Piston Pilot Input Signal Blade Turning Mechanism Hydraulic Return Hydraulic Valve Hydraulic Pressure Supply Hydraulic Connections PCU Engine RPM Signal

Variable Pitch Control The PCU is driven by the engine main rotating shaft, so as soon as the engine starts to rotate, the internal components of the PCU will rotate as well; ensuring the PCU weights spin to engine speed, sensing rpm. The mechanical control linkage has to be adjusted so fuel supply at any throttle position is enough to drive the engine to the selected spring tension (rpm) in the PCU. FMU

Variable Pitch Control The Sequence of Events We shall quickly review what happens with the pitch control through a sequence of events from a stationary position, through take-off and level flight, then into a dive, and finally to level flight again. Straight and Level Take Off Dive Straight and Level Stationary

Variable Pitch Control The Sequence of Events Stage 1 – Engine Stationary Throttle idle, the PCU spring extended The hydraulic selector valve to the ‘fine’ port, open, position. Start & Idle

Variable Pitch Control The Sequence of Events Stage 2 – Start Initiated Rpm starts to increase, Hydraulic pressure also starts to increase. Start & Idle

Variable Pitch Control The Sequence of Events Stage 3 – Accelerate to Idle When rpm close to idle, weights start to lift the hydraulic valve. At idle rpm, the propeller is locked into the fine position. Start & Idle

Variable Pitch Control The Sequence of Events Stage 4 – Idle to Take Off The PCU loads the spring tension, pushing the hydraulic direction valve down. Take Off

Variable Pitch Control The Sequence of Events Stage 5 – Accelerate to Take Off RPM Propeller angle lags behind the actual rpm The hydraulic direction valve is in the fine pitch open position Take Off

Variable Pitch Control The Sequence of Events Stage 6 – at Take Off RPM The propeller locks in the take off angle. When brakes release, pitch gradually increases to maintain correct angle of attack. Take Off

Variable Pitch Control The Sequence of Events Stage 7 – Aircraft in Straight and Level Flight Pitch is hydraulically locked at the cruise angle. Aircraft is now manoeuvred into a dive attitude, the engine controls are not altered. Cruise

Variable Pitch Control The Sequence of Events Stage 8 – Dive is Initiated The aircraft gathers speed, relieving drag on the propeller, and allowing it to be driven faster by the engine. Cruise

Variable Pitch Control The Sequence of Events Stage 9 – Dive is Begun As the engine over-speeds slightly the propeller moves to a coarser pitch. Cruise

Variable Pitch Control The Sequence of Events Stage 10 – Dive Attitude Pitch coarsened off to maintain the correct angle of attack Blade pitch is hydraulically locked at the cruise angle. Cruise

Variable Pitch Control The Sequence of Events Stage 11 – Level Out Initiated Rpm reducing due to the increase drag of the blades at the dive blade angle. Cruise

Variable Pitch Control The Sequence of Events Stage 12 – Levelling Out The propeller pitch is fined off to increase the rpm Cruise

Variable Pitch Control The Sequence of Events Stage 13 – Aircraft in Straight and Level Flight Pitch is hydraulically locked at the cruise angle. Rpm is restored Cruise

Variable Pitch Control The Dive Sequence The PCU changes propeller pitch and maintains constant engine speeds during the Dive commencement Straight and Level and again at Level Out Dive Straight and Level In all of these manoeuvres, all the pilot is doing is flying (redirecting) the aircraft, the throttle is not touched.

Check of Understanding The Helix Angle is the angle between what? The direction of the blade and the angle of attack The line of rotation and the direction of the blade The line of rotation and the direction of flight The line of rotation and the angle of attack

Check of Understanding As an aircraft pulls forward, at what rate does the propeller spin? Around 100 rpm Around 1000 rpm Around 2000 rpm Around 4000 rpm

Check of Understanding The blade angle on a propeller is varied from the root to the tip. What is this called? Blade twist Variable pitch Blade transition Adjustable pitch

Check of Understanding Which of these statements applies to a propeller that has been feathered? Its leading edge faced 90o to the direction of flight It operates at maximum speed Its leading edge faces forward to the direction of flight It produces maximum power

Check of Understanding On a variable pitch propeller, what is the largest obtainable pitch angle called? Cruise pitch Fine pitch Reverse pitch Coarse pitch

Check of Understanding In the diagram, what is angle ‘A’ known as? The Blade Angle Line of Rotation Propeller Blade A The Pitch Angle The Prop Angle The Fine Angle

Check of Understanding Which pitch of propeller gives the maximum resistance to forward speed? Cruise Pitch Reverse Pitch Fine Pitch Coarse Pitch

PISTON ENGINES End of Presentation