PTC B1.1 Notes - Sub Module 17.3 (Propeller Pitch Control)

PIATrainingCentre(PTC) Module17PROPELLERCategory A/B1 Sub Module 17.3 Propeller Pitch Control

ISO 9001 - 2008 Certified For Training Purpose Only PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.3 Mar 2014

MODULE17

SubModule17.3

PROPELLERPITCHCONTROL


ISO 9001 - 2008 Certified For Training Purpose Only PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.3 - i Mar 2014

ContentsPROPELLERPITCHES1

SPEEDCONTROLANDPITCHCHANGEMETHODS9

FEATHERINGANDREVERSEPITCH21

OVERSPEEDPROTECTION28


ISO 9001 - 2008 Certified For Training Purpose Only PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.3 - ii Mar 2014

Page Intentionally Left Blank


ISO 9001 - 2008 Certified For Training Purpose Only

PTC/CM/B1.1 Basic/M17/01 Rev. 00 17.3 - 1 Mar 2014

PROPELLER PITCHES The roots of propeller blade can be rotated about the pitch change axis by a mechanism in the hub to vary the blade angle by approximately 110. Movement of the blade is controlled by a Propeller Control Unit (PCU) that directs hydraulic pressure to turn the blade. Ground Fine or Superfine Pitch During starting and taxiing in fixed shaft engines, when power available from the turbines is insufficient to drive the propeller efficiently, this position is used to off-load the engine. When the propeller is in the ground fine pitch position, it also acts as an effective brake, being propeller discs producing drag in the airflow. This mode of blade position is only available when the aircraft is on ground. Flight Fine Pitch At this position the angle of attack is small; so accelerates a smaller mass of air per revolution this position is the minimum blade angle allowed in flight. It allows the engine to turn at higher speed, like take off RPM. Although the mass airflow is smaller for high RPM, the slip stream velocity is high and with low forward aircraft speed the thrust is also high.

Coarse Pitch During flight, PCU controls the angle of blades for fine pitch and coarse pitch. At coarse pitch, greater mass of air is accelerated for lower engine RPM. Resulting saving fuel and engine wear during cruising of flight. Reverse Pitch To provide an effective aerodynamic brake on landing and to reverse the aircraft during ground maneuvers, reverse pitch is used to obtain a negative thrust. Thrust reverse is only available when the aircraft is on the ground because of mechanical locking gate on the thrust levers. Feathering In case of engine failure during flight, the airflow will try to rotate (windmill) the propeller and create increased drag that causes a multi-engine aircraft yaw. Feathering position allows leading and trailing edges of the propeller blades to be aligned with the airflow, thus reducing drag. To prevent more than one engine feathering at a time, Protection devices are integrated.




Figure 17.3.01




Alpha and Beta Modes of Operation Alpha mode and Beta modes are the two basic operating modes. Alpha is the flight mode which includes all operations from take off to landing throughout. Whereas Beta is the ground operation mode consisting of engine start, taxi and reverse operations. Controls other than the normal flight range of any turboprop will be under beta range, especially in the thrust reverse range. Usually a mechanical lock or gate on the thrust lever is the transition point between normal (alpha) control and beta control. Different safety devices by means of air / ground sensors ensure that thrust reverse cannot be chosen except the thrust lever is at idle and the aircraft is on the ground.

We will study now how the torque, thrust, total reaction, relative airflow and angle of attack arrange themselves in three conditions. These are normal flight, windmill brake and power on brake.




Normal Flight If you examine the illustration below you will see that the blade angle and the angle of attack are both positive giving positive thrust and positive torque. The engine is driving the propeller. In normal flight:

Blade angle is positive Angle of attack is positive Thrust is positive Torque is positive

Figure 17.3.02




Power on Brake Condition In this condition, the blade angle has been deliberately selected to be negative, creating a large negative angle of attack. This produces negative thrust but because the engine is still driving the propeller, the torque remains positive. This is the reverse pitch facility and it is provided to enable aircraft to use the propeller to provide a braking thrust force to shorten the landing run. The condition is only selected on the ground, never in the air. One drawback of this condition is that the propellers can throw up debris from the runway in front of the aircraft and then ingest it back through the propellers.

During power-on-brake condition:

Blade angle is negative Angle of attack is negative Thrust is negative Torque is positive




Figure 17.3.03




Windmill Brake Condition In this condition the blade angle is still positive but at a very low value. It will be in the ground range. The angle of attack is negative giving negative thrust and negative torque. The propeller will then be driving the engine. It is a dangerous condition if it occurs in flight as it creates massive drag. However, the condition may be deliberately selected when an aircraft that does not possess a reverse pitch facility requires an additional braking force after landing. By selecting minimum or ground fine pitch and then throttling the engine back after landing the propeller can be induced to windmill and thus provide an air brake. It is important to note that there has to be aircraft forward movement for windmill to occur. It will not happen on a stationary aircraft. A strong head-wind will windmill a shut down engine on a parked aircraft, but that is a different matter. In windmill brake condition:

Blade angle is positive Angle of attack is negative Thrust is negative Torque is negative

Figure 17.3.04




Windmill Condition This is similar to the windmill brake condition and is an undesirable and hazardous condition that follows an engine failure. As the rotational speed of the failed engine reduces a constant speed propeller will reduce or fine off its blade angle in an attempt to maintain RPM. This will see the blade angle reduce to the point where, providing there is forward movement, a negative angle of attack will occur and the propeller will then drive the engine. Torque and thrust will have become negative and the propeller will have become a huge spinning airbrake. The condition can also occur if the propeller blade angle inadvertently enters the ground range in flight. Note that there must be forward aircraft movement for windmill to occur. To avoid this unpleasant effect variable pitch propellers are equipped with a feathering facility where the blade angle can be driven to the fully coarse position following an engine failure. The blade chord-line will then be aligned with the oncoming airflow. Once feathered a propeller cannot windmill.




SPEED CONTROL AND PITCH CHANGE METHODS Variable-pitch propellers comprise of a number of separate blades mounted in a central hub along with a mechanism to change the blade angle according to aircraft requirements. The blades and hub are often aluminium alloy forgings, but the hub on a large propeller may be constructed from steel forgings because of the high centrifugal forces that it has to sustain. The blades are mounted in the hub in ball or tapered roller bearings, and the pitch change mechanism is attached to the hub and connected to each blade through rods, yokes or bevel gears. Operation and control of the pitch-change mechanism varies significantly, and the major types are described in the following sections. Single-Acting Propeller System A single acting propeller is shown in the figure 17.3.05. This is a constant-speed, feathering type system, typical for the propellers fitted to light and medium sized twin-engine aircraft. A cylinder is bolted to the front of the hub, and contains a piston and piston rod that move axially to adjust the blade angle. In some propellers, oil under pressure is fed through the hollow piston rod to the front of the piston, moving the piston to the rear to turn the blades to a finer pitch. On other propellers the reverse applies.

When oil pressure is relieved, the counterweights and feathering spring advances the piston to turn the blades to a coarser pitch. Although Counterweights produce a CTF, they are positioned at 90 to the chord line trying to move the blades to a coarser pitch. Counterweights must therefore be located far enough from the blade axis, and must be heavy enough to overcome the natural twisting moment of the blade. But since weight and space are limiting factors, they are generally only used with blades of narrow chord.




Figure 17.3.05




Double-Acting Propeller System This type of propeller is generally fitted on larger engines. Because of engine requirements, it is more complex than the propellers fitted to smaller engines. Construction is similar to that of a single-acting propeller, the hub supporting the blades and the cylinder housing the operating piston. In this case however, the cylinder is closed at both ends and the piston is moved in both directions by means of oil pressure. In the mechanism shown in Figure 17.3.06, links from the annular piston pass through seals in the rear end of the cylinder, and are connected to a pin at the base of each blade. In another type of mechanism, the piston is connected by means of pins and rollers to a cam track and bevel gear, the bevel gear meshing with a bevel gear segment at the base of each blade. Axial motion of the piston causes rotation of the bevel gear and adjustment of the blade angle. Operating oil is supplied to the propeller mechanism through concentric tubes in the bore of the engine reduction gear shaft.

In double acting propeller system, pitch change mechanism can be achieved by either of the mechanisms indicated below.

Moving piston Moving cylinder Geared or Hydromatic




Figure 17.3.06




Moving Piston A moving piston hydraulic pitch change mechanism for a double acting propeller system is illustrated in Figure 17.3.07. Linear movement of the piston inside the cylinder is transmitted to the base of each blade by linkages, and is converted into rotary movement of the blades.

Figure 17.3.07

Moving Cylinder The illustration in Figure 17.3.08 shows a moving cylinder hydraulic pitch change mechanism for a double acting propeller system. Linear movement of the cylinder is transmitted to the base of each blade by linkages, and converted to rotary movement of the blades.

Figure 17.3.08




Geared or Hydromatic The geared or hydromatic pitch change mechanism, shown in figures 17.3.09(a) and 17.3.09(b), uses a piston inside a stationary cylinder. The piston is connected to a pair of co-axial cylindrical cams. The cylindrical cams convert linear motion into rotary motion. It carries a bevel gear meshing with bevel gear segments on the blade roots.

Figure 17.3.09(a)

Figure 17.3.09(b)




Speed Control The great advantage of being able to alter pitch in flight opened new possibilities for better efficiency. Replacing the two-position valve with a flyweight-controlled valve in a governor allows the blade pitch angle to be continuously and automatically adjusted in flight to keep a constant and efficient engine speed. The beginning of an engine-driven centrifugal governor, allowed the blade angle to be changed automatically (within a pre-determined choice), in order to maintain any engine speed selected by the pilot, regardless of aircraft speed or altitude. Propeller Governor A flyweight-type governor (shown in Figure 17.3.10) senses the engine speed and compares it with the speed selected by the pilot. If an air load on the propeller causes it to slow down, the governor senses this RPM decrease and directs oil into or out of the propeller to decrease the blade pitch. The lowered pitch decreases the load, and the engine returns to the desired speed. If the air load decreases, the RPM increases; the governor senses the raise and directs the oil in the right direction to increase the pitch causing the engine to slow down. As the flight conditions are continually varying during a usual flight profile, the engine RPM will vary in response to the changing propeller torque. This is detrimental for a turboprop aircraft, and to manually maintain a constant RPM would be a full time activity for the pilot.

The purpose of the propeller governor is to maintain the RPM of the engine at the shape selected by the pilot, i.e. it is a range speed governor. It is also used to limit the maximum RPM of the engine, i.e. it is a maximum speed governor. This is accomplished by controlling the pitch of the propeller blades and hence the load on the engine. Propeller governors are sometimes known as Constant Speed Units (CSUs) and Propeller Control Units (PCUs). Propeller governors use a pair of L-shaped flyweights, mounted on a flyweight head and driven by the engine, to control the position of the pilot valve in the oil passage between the engine and the propeller. A gear-type pump within the governor boosts engine oil pressure high as much as necessary for it to move the propeller piston against the effect of the counterweights or the low pitch spring. The governor pump and the flyweight head are driven by an accessory gear in the engine. The speeder spring presses down on the toes of the flyweights and, in turn, on the pilot valve plunger. The governor control lever rotates the adjusting worm, which varies the compression of the speeder spring.




Figure 17.3.10




On-Speed Condition When the propeller has fully absorbed the engine power, the governor flyweight force equals that of the spring force. In this "on speed" condition the governor piston valve blanks off the oil ports to the propeller pitch change piston, and high pressure oil from the governor pump is by-passed through the main relief valve to the inlet side of the pump (Figure 17.3.11).

Figure 17.3.11




Overspeed Condition If the RPM rises above the selected speed, the governor flyweight force, being larger than the spring force, lifts the governor piston valve. The valve is raised to a position where operating oil is directed to the front of the pitch change piston, moving it rearwards to increase the pitch angle of the blades. This increases the load on the engine. At the same time, displaced oil from the rear of the piston is directed by the governor piston valve, via drain, to the inlet side of the governor pump. The increased blade pitch angle causes the RPM to fall until equilibrium is reached and the governor piston valve returns to the on speed state (Figure 17.3.12).

Figure 17.3.12

Under speed Condition If the RPM falls below the chosen speed, the spring force, being in surplus of the governor flyweight force, causes a downward movement of the governor piston valve. In this position operating oil is directed to the rear of the propeller pitch change piston, moving it forward and lessening the pitch angle of the blades (i.e. decreasing the load on the engine). At the same time, the oil displaced from the front of the piston is returned, via drain, to the governor pump. This condition will apply until the selected RPM is brought back (Figure 17.3.13).

Figure 17.3.13




Flight Deck Controls A typical engine / aircraft combination uses two propeller control levers that are mounted on the flight deck quadrant. These levers are referred to as the power lever and condition (speed) lever as shown in the figures 17.3.14 and 17.3.15. The power lever relates to the throttle, but it also gives the pilot control over the propeller during ground operation. It affects the fuel flow, torque and exhaust gas temperature (EGT), and has five positions:

Reverse Ground idle Flight idle Take off Maximum power

The LP and HP Shaft Speeds are referred to as NL and NH respectively and the free turbine shaft speed is designated as NP. Power in the reverse mode is controlled on NP and in the forward mode, by NH. The condition / speed lever principally controls the propeller RPM, and also acts as a manual feather and fuel shut off lever. The condition lever has four positions:

Fuel shut off On feather Low RPM (min NP) High RPM (max NP)




Figure 17.3.14

Figure 17.3.15




FEATHERING AND REVERSE PITCH Propeller Feathering Since a double-acting propeller operates by directing oil pressure to either side of the piston in the pitch change mechanism, oil pressure is required in order to feather. Fitting an electrical oil pump in the system that takes oil from the bottom of the oil tank below a stack pipe achieves this. Figure 10.3.16 shows a simplified typical feathering circuit. Pushing in the feathering button (normally illuminated) energizes a holding coil. This activates the electrical pump to supply oil pressure. It also energizes a valve lift solenoid, allowing the pump oil pressure to lift the control valve, allowing pump oil pressure into the pitch change mechanism to feather the propeller. Once reaching the full feather position, a pressure cut out switch turns off the feathering pump. On a manual system, moving the high pressure fuel cock to the feather position, mechanically lifting the control valve, lifts the control valve in the PCU. If insufficient oil pressure is available from the engine-driven PCU pump to move the propeller to feather, then operation of the electrical feather pump becomes necessary.

Propeller Un-feathering In a double-acting propeller, the electrical feathering pump oil pressure directed to the pitch change mechanism achieves un-feathering, with the power levers closed and the high pressure fuel cock open. The rpm lever moves in normal operating range and the control valve lowers under the action of the governor spring. The electrical feathering pump switches on and oil pressure discharges to the PCU, turning the propeller from feather toward coarse position. The propeller then windmills and rotates the engine. Once the engine starts and is on speed, the oil pressure from the feathering pump rises and a cut out switch turns the pump off. Operation of the pump occurs either via manual selection of a switch or automatically via a micro switch mounted on the high pressure fuel cock lever.




Figure 17.3.16




Pitch Stops These are fitted to control the propeller angle for ground and flight operations. The types of pitch stops are: Ground Fine Pitch Stop: This stop type ensures fine pitch

on the ground during engine start and ground running. Flight Fine Pitch Stop: This stop type limits the minimum

pitch in flight to prevent over-speed and resulting high drag. It must be removed to allow selection of ground fine pitch for ground operation.

Beta Range Some gas turbine engines use a form of control known as Beta Control. Beta is blade angle, and during ground operations only, direct control of the propeller pitch by the power levers is achieved in the ground idle and reverse pitch range. To operate in the beta range, the aircraft must be on the ground and have the flight fine pitch stops removed. This gives better control for ground maneuvering.

Reverse Pitch In ground fine pitch, the blade position is 0 and provides high wind-milling drag to aid aircraft retardation on the ground to a low forward speed. To improve this on slippery or short runways, some engine installations are fitted with reverse pitch propellers. This system includes installation of removable ground fine pitch stops. With the ground fine pitch stop removed and reverse selected, moving the power levers rearward beyond ground idle causes the blades to move to a negative pitch, applying the correct amount of engine power to produce reverse thrust. Pitch Locks Pitch locks lock the blades at whatever angle they are currently at should there be a propeller mechanism or PCU failure, which would cause the propeller to run to fine due to CTM. There are various types of lock, two of which appear above: Hydraulic Lock: This responds to fine pitch oil pressure

failure to create a hydraulic lock.

Mechanical Lock: Again, this responds to fine pitch oil pressure failure and mechanically locks the blade.




Figure 17.3.17




Automatic Feathering An automatic feathering system is sometimes provided to automatically feather the propeller in the event that engine power and hence indicated torque pressure falls to a pre-determined value. In this instance, a low torque switch operates, completing the circuit to the piston lift solenoid on the PCU and feathering pump. The relevant feathering button pulls in and a red light illuminates. The control valve rises hydraulically, thus enabling the feathering of the propeller. A switch on the flight deck arms the system, indicated by amber light. The throttles must advance to approximately 45 to 75% of lever movement to close the throttle micro switch. Normally this system is only used during take-off and landing. To prevent the system operating as a result of momentary loss of torque pressure, a time delay unit prevents completion of the circuit until a predetermined time has elapsed, typically one or two seconds. To prevent more than one engine from auto-feathering, a blocking relay is usually fitted either between the master switch and the throttle switch, or incorporated in the feathering button circuit.

Sometimes it can be reset to re-arm the auto-feather system in the event of another engine failure. By activating the feather button, regardless of whether or not the propeller has been auto-feathered, any engine can be feathered at any time. Some engines incorporate an automatic drag limiting (ADL) system or negative torque sensing (NTS) system that do not feather the propeller in the event of engine failure but turn the blades to coarse pitch to limit wind-milling.




Figure 17.3.18




Figure 17.3.19




OVERSPEED PROTECTION Light aircraft propeller speed control is accomplished by the governor, and the actual turbo-prop equipped aircraft are provided with back-up propeller overspeed protection. Mechanical Controlled Propellers An overspeed governor is a back-up for the propeller governor and is mounted on the reduction gearbox. It has its own flyweights and pilot valve, and it releases oil from the propeller whenever the propeller RPM exceed a preset limit. When the propeller speed reaches this limit the flyweights lift the pilot valve and bleed off propeller servo pressure oil into the reduction gearbox sump, causing the blade angle to increase. A greater pitch puts more loads on the engine and slows down the propeller.

Figure 17.3.20

Documents

PTC B1.1 Notes - Sub Module 17.3 (Propeller Pitch Control)