Aircraft Propeller Control And Operation 1l196f
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AIRCRAFT PROPELLER CONTROL AND OPERATION ( page 1
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) Control and Operation (page 1)
Propeller Control basic requirement: For flight operation, an engine is demanded to deliver power within a relatively narrow band of operating rotation speeds. During flight, the speed-sensitive governor of the propeller automatically controls the blade angle as required to maintain a constant r.p.m. of the engine. Three factors tend to vary the r.p.m. of the engine during operation. These factors are power, airspeed, and air density. If the r.p.m. is to maintain constant, the blade angle must vary directly with power, directly with airspeed, and inversely with air density. The speed-sensitive governor provides the means by which the propeller can adjust itself automatically to varying power and flight conditions while converting the power to thrust.
Fundamental Forces : Three fundamental forces are used to control blade angle . These forces are: 1. Centrifugal twisting moment, centrifugal force acting on a rotating blade which tends at all times to move the blade into low pitch. 2. Oil at engine pressure on the outboard piston side, which supplements the centrifugal twisting moment toward low pitch. 3. Propeller Governor oil on the inboard piston side, which balances the first two forces and move the blades toward high pitch Counterweight assembly (this is only for counterweight propeller) which attached to the blades , the centrifugal forces of the counterweight will move the blades to high pitch setting
Constant Speed, Counterweight Propellers The Counterweight type propeller may be used to operate either as a controllable or constant speed propeller. The hydraulic counterweight propeller consists of a hub assembly, blade assembly, cylinder assembly, and counterweight assembly. The counterweight assembly on the propeller is attached to the blades and moves with them. The centrifugal forces obtained from rotating counterweights move the blades to high angle setting. The centrifugal force of the counterweight assembly is depended on the rotational speed of the propellers r.p.m. The propeller blades have a definite range of angular motion by an adjusting for high and low angle on the counterweight brackets.
Controllable : the operator will select either low blade angle or high blade angle by two-way valve which permits engine oil to flow into or drain from the propeller.
Constant Speed : If an engine driven governor is used, the propeller will operate as a constant speed. The propeller and engine speed will be maintained constant at any r.p.m. setting within the operating range of the propeller.
Governor Operation (Constant speed with counterweight ) the Governor supplies and controls the flow of oil to and from the propeller. The engine driven governor receives oil from the engine lubricating system and boost its pressure to that required to operate the pitch-changing mechanism. It consists essentially of : 1. A gear pump to increase the pressure of the engine oil to the pressure required for propeller operation. 2. A relief valve system which regulates the operating pressure in the governor. 3. A pilot valve actuated by flyweights which control the flow of oil through the governor 4. The speeder spring provides a mean by which the initial load on the pilot
valve can be changed through the rack and pulley arrangement which controlled by pilot. The governor maintains the required balance between all three control forces by metering to, or drain from, the inboard side of the propeller piston to maintain the propeller blade angle for constant speed operation. The governor operates by means of flyweights which control the position of a pilot valve. When the propeller r.p.m. is below that for which the governor is set through the speeder spring by pilot , the governor flyweight move inward due to less centrifugal force act on flyweight than compression of speeder spring. If the propeller r.p.m. is higher than setting , the flyweight will move outward due to flyweight has more centrifugal force than compression of speeder spring . During the flyweight moving inward or outward , the pilot valve will move and directs engine oil pressure to the propeller cylinder through the engine propeller shaft.
Principles of Operation (Constant Speed with Counterweight Propellers) The changes in the blades angle of a typical constant speed with counterweight propellers are accomplished by the action of two forces, one is hydraulic and the other is mechanical. 1. The cylinder is moved by oil flowing into it and opposed by centrifugal force of counterweight. This action moves the counterweight and the blades to rotate toward the low angle positon. 2. When the oil allowed to drain from the cylinder , the centrifugal force of counterweights take effect and the blades are turned toward the high angle
position. 3. The constant speed control of the propeller is an engine driven governor of the flyweight type.
Continue on Propeller Control and Operation Page 2
Control and Operation (page 2)
Governor Operation Condition
On-Speed Condition The on-speed condition exists when the propeller operation speed are constant . In this condition, the force of the flyweight (5) at the governor just balances the speeder spring (3) force on the pilot valve (10) and shutoff completely the line (13) connecting to the propeller , thus preventing the flow of oil to or from the propeller.
The pressure oil from the pump is relieved through the relief valve (6). Because the propeller counterweight (15) force toward high pitch is balanced by the oil force from cylinder (14) is prevented from moving, and the propeller does not chang pitch Under-Speed Condition The under-speed condition is the result of change in engine r.p.m. or propeller r.p.m.which the r.p.m. is tend to lower than setting or governor control movement toward a high r.p.m. Since the force of the flyweight (5) is less than the speeder spring (3) force , the pilot valve (10) is forced down. Oil from the booster pump flows through the line (13) to the propeller. This forces the cylinder (14) move outward , and the blades (16) turn to lower pitch, less power is required to turn the propeller which inturn increase the engine r.p.m. As the speed is increased, the flyweight force is increased also and becomes equal to the speeder spring force. The pilot valve is move up, and the governor resumes its on-speed condition which keep the engine r.p.m. constant.
Over-Speed Condition The over-speed condition which occurs when the aircraft altitude change or engine power is increased or engine r.p.m. is tend to increase and the governor control is moved towards a lower r.p.m. In this condition, the force of the flyweight (5) overcomes the speeder spring (3) force and raise the pilot valve (10) open the
propeller line (13) to drain the oil from the cylinder (14). The counterweight (15) force in the propeller to turn the blades towards a higher pitch. With a higher pitch, more power is required to turn the propeller which inturn slow down the engine r.p.m. As the speed is reduced, the flyweight force is reduced also and becomes equal to the speeder spring force. The pilot valve is lowered, and the governor resumes its on-speed condition which keep the engine r.p.m. constant.
Flight Operation This is just only guide line for understanding . The engine or aircraft manufacturers' operating manual should be consulted for each particular aircrat.
Takeoff : Placing the governor control in the full forward position . This position is setting the propeller blades to low pitch angle Engine r.p.m. will increase until it reaches the takeoff r.p.m. for which the governor has been set. From this setting , the r.p.m. will be held constant by the governor, which means that full power is available during takeoff and climb. Cruising : Once the crusing r.p.m. has been set , it will be held constant by the governor. All changes in attitude of the aircraft, altitude, and the engine power can be made without affecting the r.p.m. as long as the blades do not the pitch
limit stop. Power Descent : As the airspeed increase during descent, the governor will move the propeller blades to a higher pitch inorder to hold the r.p.m. at the desired value. Approach and Landing : Set the governor to its maximum cruising r.p.m. position during approach. During landing, the governor control should be set in the high r.p.m. position and this move the blades to full low pitch angle. Control and Operation (page 3)
Hydromatic Propellers
Basic Operation Principles : The pitch changing mechanism of hydromatic propeller is a mechanical-hydraulic system in which hydraulic forces acting upon a piston are transformed into mechanical forces acting upon the blades.
Piston movement causes rotation of cam which incorporates a bevel gear (Hamilton Standard Propeller) . The oil forces which act upon the piston are controled by the governor
Single Acting Propeller: The governor directs its pump output against the inboard side of piston only, A single acting propeller uses a single acting governor. This type of propeller makes use of three forces during constant speed operation , the blades centrifugal twisting moment and this force tends at all times to move the blades toward low pitch , oil at engine pressure applied against the outboard side of the propeller piston and this force to supplement the centrifugal twisting moment toward the low pitch during constant speed operation., and oil from governor
pressure applied against the inboard side of the piston . The oil pressure from governor was boosted from the engine oil supply by governor pump and the force is controlled by metering the high pressure oil to or draining it from the inboard side of the propeller piston which balances centrifugal twisting moment and oil at the engine pressure.
Double Acting Propeller: The governor directs its output either side of the piston as the operating condition required. Double acting propeller uses double acting governor. This type of propeller , the governor pump output oil is directed by the governor to either side of the propeller piston.
Principle Operation of Double Acting : Overspeed Condition : When the engine speed increases above the r.p.m. for which the governor is set . Oil supply is boosted in pressure by thr engine driven propeller governor , is directed against the inboard side of the propeller piston. The piston and the attached rollers move outboard. As the piston moves outboard , cam and rollers move the propeller blades toward a higher angle , which inturn, decreases the engine r.p.m. Underspeed Condition : When the engine speed drops below the r.p.m. for which the governor is set. Force at flyweight is decrease and permit speeder spring to lower pilot valve, thereby open the oil age allow the oil from inboard side of piston to drain through the governor. As the oil from inboard side is drained , engine oil from engine flows through the propeller shaft into the outboard piston end. With the aid of blade centrifugal twisting moment, The engine oil from outboard moves the piston inboard. The piston motion is transmitted through the cam and rollers . Thus, the blades move to lower angle
The Feathering System
Feathering : For some basic model consists of a feathering pump, reservoir, a feathering time-delay switch, and a propeller feathering light. The propeller is feathered by moving the control in the cockpit against the low speed stop. This causes the pilot vave lift rod in the governor to hold the pilot valve in the decrease r.p.m. position regardless of the action of the governor flyweights. This causes the propeller blades to rotate through high pitch to the feathering position.
Some model is initiated by depressing the feathering button. This action, auxiliary pump, feather solinoid, which positions the feathering valve to tranfer oil to feathering the propeller. When the propeller has been fully feathered, oil pressure will buildup and operate a pressure cutout switch which will cause the auxiliary pump stop. Feathering may be also be accomplished by pulling the engine emergency shutdown handle or switch to the shutdown position.
Unfeathering : Some model is accomblished by holding the feathering buttn switch in the out position for about 2 second . This creates an artificial underspeed condition at the governor and causes high-pressure oil from the feathering pump to be directed to the rear of the propeller piston. As soon as the piston has moved inward a short distance, the blades will have sufficient angle to start rotation of the engine. When this occurs , the un-feathering switch can be released and the governor will resume control of the propeller.
Propeller Aerodynamics, I A propeller is an airfoil and like a wing it will generate an aerodynamic force much the same way. It has a leading and trailing edge, camber and a chord line. The cambered side is called blade back and the flatter side the blade face. The angle which the chord makes to the plane of rotation is the blade angle. The propeller is rotated by the engine and this creates thrust and moves the aircraft forward. Previous pages discussed the type of propellers and their controls, in this section we will try to describe some of the aerodynamic principles.
Propeller Aerodynamics As the propeller moves through the air the static pressure is reduced ahead of each blade and at the same time at the blade face the flow is retarded resulting in an increase of static pressure. The changes in pressure around the rotating blades causes air to be drawn into the propeller disc and this results in a rearward movement of a column of air. The result being a forward thrust pulling the aircraft. All of this in accordance with Newton's Third Law. The amount of thrust generated by a propeller depends on the mass of the air and its acceleration toward the rear.
Blade twist and pitch The need for a changing blade angle from the hub to the tips stems from the fact that angular speed varies also and is greatest at the tips. Combined with any forward speed the propeller may have, the relative airflow is also different from the hub to the tips. To keep thrust equal along the blade, they have a build in twist. The design is such that the blade is thick at the hub with a large blade angle and thin at the tip with a low blade angle. During rotation and forward movement the propeller describes a rotational path, called a helix. If the propeller would move forward without giving thrust the distance of one revolution is called experimental mean pitch (angle between plane of rotation and the zero thrust angle of attack). The actual advance is the difference between experimental mean pitch and slip. Slip is the angle between the zero thrust and the actual angle of attack. If the propeller rotates in a solid medium it would advance according to its pitch (angle between plane of rotation and the blade face, chord line,) also called geometric pitch.
Propeller performance
If this was an ideal world the propeller would convert all power to thrust. But as this is not the case, losses occur in the slipstream, aerodynamic drag. Under normal conditions the propeller is able to convert 85% of the brake horse power from the engine into thrust. Thus propeller efficiency is the ratio between thrust horsepower and brake horsepower. that Power = Force x Distance / Time (rate of doing work), we can equate propeller efficiency as Thrust x TAS / Brake Horse Power. It follows that propeller efficiency is zero (0) under two conditions: when there is no forward speed (TAS) or when there is no thrust generated. With the aircraft at standstill (beginning of takeoff roll or taxi) the propeller has zero efficiency until it reaches its optimum forward speed for the propeller where maximum thrust is generated (max efficiency), increasing forward speed beyond that point will decrease efficiency (propeller with fixed blade angle). You could say that there is a relation between RPM and airspeed for fixed pitch propellers. It is obvious that a controllable propeller has a wider range of airspeeds where efficiency is at its maximum, until the governor reaches a position where the blades can no longer be adjusted, which is at the full fine and coarse pitch stops.
Blade forces During rotation the blades generate lift and drag. But with propellers we talk about thrust (lift) and propeller torque (drag). Another one: with a wing, drag must be overcome to provide lift but with a propeller it is propeller torque that must be overcome by engine torque. Increasing power with the throttle increases engine torque, resulting in a higher RPM until propeller torque is equal to engine torque and RPM stabilizes. If you would place an aircraft with a fixed pitch propeller into a shallow dive, as forward speed increases the relative airflow changes and the angle of attack is reduced. Resulting in a reduced thrust and propeller torque and as engine torque remained the same (there was no change in throttle setting) engine/propeller RPM will increase.
Blade effectiveness A propeller blade is the most effective between station 60% and 90% with a peak at 75%. It is this point (station) where blade angle is usually reported.
Asymmetric blade effect This is most noticeable during high angles of attack of the airplane, take off rotation, tail wheel aircraft on take-off run. The cause is that the propeller disc (normally at a right angle with the airflow) is tilted backwards and the relative airflow between the up going and down going blades are different. The down going blades have a larger angle of attack (the distance travelled seems greater due to the forward movement of the aircraft) and produce more thrust, pulling the aircraft with a higher force than the up going blades. With an engine rotating clockwise (as seen from the cockpit) the aircraft wants to turn to the left.
Twin engine aircraft On twin engined propeller driven aircraft and with both propellers rotating in the same direction the arm (or distance) of the downward going blades towards the center of gravity are not the same. The engine where the distance is the smallest is called the critical engine. If that one fails the other engine will yaw the aircraft with more force (thrust x arm) than if the critical engine had failed.
Propeller Aerodynamics, II A propeller is an airfoil and like a wing it will generate an aerodynamic force much the same way. It has a leading and trailing edge, camber and a chord line. The cambered side is called blade back and the flatter side the blade face. The angle which the chord makes to the plane of rotation is the blade angle. The propeller is rotated by the engine and this creates thrust and moves the aircraft forward. Previous page discussed the propeller blade twist and performance, in this section we will discuss blade twisting forces and length and tip speeds.
Propeller Twisting Forces Large stresses are involved when the propeller rotates. Think of centrifugal force and twisting and aerodynamic twisting forces acting on the blades, hub and pitch changing mechanism. Centrifugal force is opposed by centripetal force acting toward the hub (if this was not the case the blade would be thrown away), this force is proportional to the mass of the blade times the square of the rotational velocity.
Centrifugal twisting During rotation centrifugal force acts from the hub spanwise along the blades outward (pulling on the blade) and also on the leading and trailing edges along the blade. As the blade has a twist (blade angle) compared to the plane of rotation, the centrifugal force will try to fine the propeller along the pitch change axis toward the plane of rotation. This twisting force places a great demand on the pitch mechanism, counterweights are used to balance out this twisting toward the fine pitch. The force will be greater on wide blade propellers compared to narrow blade types.
Aerodynamic twisting This force develops when the total reaction force acts on a point ahead of the pitch axis of the blade. It tries to increase the blade angle, coarsen the propeller. This only partially offsets the centrifugal twisting force. With a windmilling propeller the total reaction force is opposite and acts in the same direction as the centrifugal twisting force both trying to fine the propeller. During a steep dive these forces might become stronger than the pitch changing mechanism, effectively locking the propeller into fine pitch and possibly overspeeding the engine. Despite the fact of having a constant speed propeller.
Blade length As the propeller rotates in a circle it becomes clear that with increasing engine/propeller RPM the propeller tip speed rises too. When the tips come close or reach the sound barrier (Mach 1.0) they will become very noisy and propeller effectiveness (thrust) is reduced. But reducing the tip speed by reducing the RPM has one big disadvantage, propeller thrust will reduce also. Some propellers solve this noise problem by curving leading edges to the rear near the tips. The trick is to keep the RPM high enough to generate useful thrust but low enough to keep the propeller quiet. This maximum tip speed is between Mach .88 and .92 and the RPM where that is reached depends on the propeller diameter.
Speed of sound
In the ISA atmosphere (15 °C) the speed of sound is 340,3 m/s (1225 km/h or 661,4 kts) and it varies with temperature. The formula to calculate this is: 331,3 √ (1 + T / 273,15), where T is the outside air temperature in Celsius.
Calculating tip speed The formula VTIP = π d n, were π is a constant (3,14159), d is diameter in meters and n is angular velocity (RPM) in revolutions per minute. VTIP then results in meters/minute. For example: a propeller of 72" diameter (1m83) with a RPM of 2600 gives a tip speed of: π * 1,83 * 2600 = 14947,7 meters/minute or 896,9 km/h.
Propeller tip mach speed Divide the propeller tip speed by the actual speed of sound at standard ISA (15 °C) and our 72" propeller at 2600 RPM will then have a tip mach number of 896,9 / 1225 = .73 Mach. Which is below mach .88 so we can increase RPM a bit or use longer blades to increase thrust without generating a lot of extra noise.
Aircraft Propellers Without a method of converting the power produced by the engine into useful thrust an aircraft would just be sitting still and creating a lot of noise and not get anywhere. In the past, all kinds of different propeller design were used: sometimes flat wooden blades, from one bladed to multiple blade models rotated at various speeds. Until it was realised that a propeller is just like a wing but instead of carrying the aircraft it displaces a volume of air backwards. Albeit slowly, compared to a pure JET engine, which displaces a small volume of air at high speeds. The same aerodynamic laws that apply to wings are valid for propellers too only not horizontal but in a vertical motion and so thrust (lift) is created and the aircraft is able to move forward, after reaching liftoff speed it will get airborne. These pages shine a small light on propeller design, some aerodynamics and operation of controllable propellers during preflight, cruise and the influence on fuel consumption.
Basic Propeller Design Propellers are usually constructed of wood, aluminum alloy or a combination of composites and wood. The leading edge is sometimes reinforced with nickel for protection against rain, sand and stones. They come in two, three or more blades depending of the application. Some high end turbo prop aircraft even have six blades to absorb the power. But, more blades means more weight and more propeller torque, the engine must be able to handle this. The function of the propeller is to convert brake horse power from the engine into thrust. To do this there are two types of propellers in use: fixed pitch (or ground adjustable, which remains a fixed pitch during flight) and the constant speed propeller with some variations possible.
Blade angle Looking at a propeller, its blade angle varies from the root to the tip. This is because angular speed of the blade is at it highest at the tip (reaching the speed of sound) and lowest at the root. If the blade angle would be constant then the angle of attack (and thrust) of the relative airflow (result of forward speed and
RPM) would vary across the propeller disc, and the blade would probably be stalled. To make sure that the generated thrust is equal from root to tip, the blade angle is high at the root (low angular speed) and low at the tip (high angular speed).
Propeller thrust During propeller rotation the airflow past the propeller blade produces an aerodynamic reaction with can be resolved into thrust and propeller torque (called lift with wings). To rotate the propeller the engine needs to create a torque. The resistance to this rotation is called propeller torque and when these two forces are in balance or stabilized, engine/propeller RPM is constant. Thus power created by the engine is absorbed by the propeller and as a result thrust is generated.
Fixed pitch With this propeller the blades are fixed and cannot move (except rotate), thus propeller blade angle is fixed (but the blade angle changes from the root to the tip as explained above). It can be seen that there is also one RPM / airspeed combination where the propeller is operating at its optimum angle of attack and produces its maximum thrust. On all other airspeeds (or conditions of flight) there is less thrust available from the propeller. In other words, its effective angle of attack depends on RPM and airspeed. To compensate, manufacturers sometimes have a climb and a cruise propeller available for the same model or type. Although convenient and simple to operate, a fixed pitch propeller will always be a compromise between a number of factors as RPM, airspeed, relative airflow, angle of attack, two or three blades, blade chord and length, emitted sound level etc...
Constant speed To overcome all these disadvantages the constant speed (or variable pitch) propeller is able to adjust its blade angle. Thereby operating closer to or at its optimum angle of attack over a wide range of RPM and airspeed combinations. The result is that the maximum amount of thrust is obtained from standstill to maximum speed (VNE) or from minimum to maximum propeller pitch angle. A shorter takeoff roll and much better climb performance may be expected and a lower specific fuel consumption is also one of the advantages.
Propeller control With a fixed pitch propeller the pilot has only one control, throttle, to control power and RPM. With a constant speed propeller there are two controls: power (throttle, black knob) and RPM (propeller, blue knob). In addition there should be a MAP indicator (manifold pressure) which relates to engine power. Moving the propeller control (pitch) changes the RPM at which the engine and propeller will rotate and moving the throttle changes the amount of power delivered (MAP) to the propeller at the preset RPM held constant by the propeller governor. An aircraft with a constant speed propeller is bit more complex and is not usually used for basic (ab initio) flight training. Those training for higher licences (L, ATPL) will need to understand and properly operate these more complex aircraft.
Propeller Controls, I Fixed pitch propellers are operated by changing the throttle only, thereby changing thrust and RPM at the same time. Constant speed propellers are more complicated because the pitch can be set separately from the throttle and has a range from full fine to coarse during flight and into feather and reverse for certain operations as emergency and ground maneuvers. A full understanding of operating propellers is required of the pilot as overspeeding the engine or setting reverse in flight or on finals would be nothing less than a real problem. This page focuses on preflight checks applicable for any propeller and the in-flight checks for constant speed propellers as the fixed pitch types usually do not have controls in the cockpit to manipulate them during flight.
Propeller Operation A substantial amount of aircraft have variable pitch propellers of some sort in use today, it is important that the pilot is very familiar with the operation of the propeller in normal and emergency situations. Even the type of engine (gasoline, diesel or turboprop) will change operating procedures for the propeller. Limitations on certain combinations of RPM and manifold pressures are not uncommon due to frequency resonations with the engine/propeller combination.
Preflight checks Check the propeller blades for damage, nicks and any oil leaks under the spinner. This would indicate an unserviceable aircraft as engine oil is used to change the pitch of the blades. For optimum propeller performance do clean the blades after every flight, this gives you the chance to check the blades properly.
Engine startup Set the propeller control in the forward (full fine) position for piston engines. Free turbine turboprop aircraft usually leave the propeller feathered until the engine is fired up and at idle N1 speed.
Pre-takeoff checks During the runup, hydraulic operated propellers are are checked by changing the control from full fine to coarse (or a set RPM drop) two or three times. This makes sure that any cold oil left in the propeller is run through the engine for warmup so that propeller operation will not be sluggish.
Cruise checks During cruise operate the engine RPM and manifold pressure according to the aircraft flight manual (read it thoroughly). Avoid overboosting by more than the recommended values. Any change in power settings should be done per procedure described below. To increase power at a higher RPM: •
Increase RPM with the PROPELLER control
•
Increase MAP with the THROTTLE control
To decrease power for a lower RPM: •
Reduce MAP with the THROTTLE control
•
Reduce RPM with the PROPELLER control
The procedure shown above discusses the correct way of changing power with a constant speed propeller without the possibility of overloading the engine.
Oversquare It is perfectly fine to operate the engine in a so called oversquare condition where the RPM divided by 100 is lower than MAP, for example: 2300 RPM and 25" MAP, read here about running oversquare and in the aircraft/engine manuals. This will lower fuel consumption and increase the range of the aircraft.
Emergencies In case of engine failure place the constant speed propeller in the full coarse pitch (blue lever fully backward) as to reduce the drag from it and increase glide range.
Pre-landing checks During landing and go-around the propeller should be in the full fine position to assure the maximum power available should this be needed. Short final is the place where this check can be done together with setting carb heat cold.
Propeller Controls, II Fixed pitch propellers are operated by changing the throttle only, thereby changing thrust and RPM at the same time. Constant speed propellers are more complicated because the pitch can be set separately from the throttle and range from full fine to coarse during flight and into feather and reverse for certain operations as emergency and ground maneuvers. A full understanding of operating propellers is required of the pilot as overspeeding the engine or setting reverse in flight or on finals would be nothing less than a disaster. Constant speed propellers are driven by oil pressure or electric power and need a control unit (governor) to maintain the preset RPM by varying the blade angle. Both types will be discussed.
Propeller governors To sense the engine RPM a mechanical or electrical device is used. In todays aviation this usually an oil or electrical operated governor. Some models are able to set the propeller blades into feather or even in a reverse trust position (not during flight).
Hydraulic propeller governors
Also called constant speed unit, it consists of a governor which regulates engine oil pressure to a hydraulic operated piston (in the propeller) and changes the propeller blade angle in order to keep RPM constant. The governor contains flyweights which senses engine RPM and change position in relation to a change in RPM, thus changing oil pressure to the propeller and regulating the blade pitch. The blades themselves contain flyweights compensating for aerodynamic forces so that the oil pressure from the engine only needs a little bit of effort to change the blade angle. This means that oil pressure remains almost constant and engine lubrication is guaranteed.
Electric propeller governors Detection of propeller RPM can be done in several ways: by optical (light), magnetic (flux) or sensing the engine RPM by connecting to the ignition or RPM indicator (Rotax). This signal is then fed to a controller which calculates any change in RPM and drives an electric motor inside the propeller to keep the preset RPM constant.
Microprocessor Thus with the flick of a switch the propeller can be set to any RPM, feather or even operate as a fixed pitch with sophisticated, sometimes even microprocessor controlled electronics (Airmaster Propellor, NZ).
Responsive Most modern constant speed propellers (hydraulically or electrical) have an accurate and responsive operation. Any change or tendency to change the RPM, by the pilot, turbulence or change in aircraft attitude, is sensed quickly so blade angle is changed accordingly to keep the preset RPM constant and this occurs almost unnoticeable by the pilot or engers.
Preflight With an hydraulic constant speed propeller you must that no oil leaks exist either at the propeller or near the governor. Any leak is reason not to fly as engine lubrication is impaired and failure or seizure of the engine could result due to loss of oil.
Propeller Controls, III Fixed pitch propellers are operated by changing the throttle only, thereby changing thrust and RPM at the same time. Constant speed propellers are more complicated because the pitch can be set separately from the throttle and range from full fine to coarse during flight and into feather and reverse for certain operations as emergency and ground maneuvers. A full understanding of operating propellers is required of the pilot as overspeeding the engine or setting reverse in flight or on finals would be nothing less than a disaster. In case of an engine failure the propeller should be positioned in the low drag setting. Thus with the blades perpendicular on the airflow thus full coarse, this is called feathering the prop.
Special operations Some types of variable pitch propellers have the option to select a feather or reverse thrust position. Its obvious that it must be impossible to select reverse thrust during flight.
Feather
This position is used when an engine has failed. The drag from a windmilling propeller/engine is very high and will reduce aircraft performance. Feathering the propeller will stop the engine from rotating and this reduces drag. Some motor gliders use this feature when, after climbing to altitude, they shutdown their engine and feather the propeller for much better glide performance of their aircraft. Feathering can be done manually of automatically. Large turboprop aircraft will have the latter. On aircraft without a feather position it is important that the propeller be set in the full coarse position after engine failure and thus reducing drag and increasing the glide distance.
Reverse Thrust This is used to reduce the landing roll. The blades are first set to a negative pitch angle (20°) and then power is applied. Adding power while the pitch angle is around 0° could result in an engine overspeed, mechanical devices are used to prevent this. It is even possible to use this to back the aircraft out from a parking stand, although caution is required. The aid from a marshaller is recommended as the pilot usually is unable to look backwards.
Overspeed condition A constant speed propeller has two mechanical stops: one at the full fine pitch and one at the full coarse pitch. With each settings there is a maximum airspeed the aircraft can reach. If the propeller overspeed fails then the propeller blades end up at the full fine pitch setting due to aerodynamic forces and with an airspeed probably higher than is normally attainable with that setting. With the application of power the engine will then overspeed and produces no thrust until the airspeed is reduced below the maximum of the propeller at the fine pitch stops.
Special Airworthiness Information Bulletin The FAA has issued a Special Airworthiness Information Bulletin CE-10-21 (SAIB) to alert operators, pilots, and aircraft manufacturers of concerns for an optimum response to a propeller overspeed in piston engine aircraft with variable pitch propellers. At this time, the FAA has concluded that the airworthiness concern is not an unsafe condition that would warrant airworthiness directive (AD) action. The appropriate emergency procedures should be followed to mitigate the emergency situation in the event of a propeller overspeed; however, pilots should be aware that some reduction in airspeed may result in the ability for continued safe flight and landing. The determination of an airspeed that is more suitable than engine out best glide speed should only be conducted at a safe altitude when the pilot has time to determine an alternative course of action other than landing immediately.
Section 6.3 - PROPELLERS We can say that the propeller is the action end of an aircraft's reciprocating engine, because it converts the useful energy of the engine into thrust as it spins around and around. The propeller has the general shape of a wing, but the camber and chord (curvature and cross-sectional length) of each section of the propeller are different, as shown in Figure 6-4 . The wing provides lift upward, while the propeller provides lift forward. The wing has only one motion which is forward, while the propeller has forward and rotary motion. The path of these two motions is like a corkscrew as the propeller goes through the air (see figure 6-5 ).
Like a wing, a propeller blade has a thick leading edge and a thin trailing edge. The blade back is the curved portion and is like the top of a wing. The blade face is comparatively flat and corresponds to the underside of a wing (see figure 6-6 for definitions of blade back and blade face). The blade shank is thick for strength and fits into a hub which is attached to the crankshaft directly or indirectly. The outer end of the blade is called the tip.
Blade pitch is loosely defined as the angle made by the chord of the blade and its plane of rotation, as shown in Figure 66 . When the angle is great, the propeller is said to have high pitch. A high-pitch propeller will take a bigger bit of air and move the aircraft farther forward in one rotation than will a low-pitch propeller. Propellers may be classified as to whether the blade pitch is fixed or variable. The demands on the propeller differ according to circumstances. For example, in takeoffs and climbs more power is needed, and this can best be provided by low pitch. For speed at cruising altitude, high pitch will do the best job. A fixed-pitch propeller is a compromise.
There are two types of variable-pitch propellers adjustable and controllable. The adjustable propeller's pitch can be changed only by a mechanic to serve a particular purpose-speed or power. The controllable-pitch propeller permits pilots to change pitch to more ideally fit their requirements at the moment. In different aircraft, this is done by electrical or hydraulic means. In modern aircraft, it is done automatically, and the propellers are referred to as constant-speed propellers. As power requirements vary, the pitch automatically changes, keeping the engine and the propeller operating at a constant rpm. If the rpm rate increases, as in a dive, a governor on the hydraulic system changes the blade pitch to a higher angle. This acts as a brake on the crankshaft. If the rpm rate decreases, as in a climb, the blade pitch is lowered and the crankshaft rpm can increase. The constant-speed propeller thus ensures that the pitch is always set at the most efficient angle so that the engine can run at a desired constant rpm regardless of altitude or forward speed. Click here to see examples of early aviation propellers. The constant-speed propellers have a full-feathering capability. Feathering means to turn the blade approximately parallel with the line of flight, thus equalizing the pressure on the face and back of the blade and stopping the propeller. Feathering is necessary if for some reason the propeller is not being driven by the engine and is windmilling, a situation that can damage the engine and increase drag on the aircraft. Most controllable-pitch and constant-speed propellers also are capable of being reversed. This is done by rotating the blades to a negative or reverse pitch. Reversible propellers push air forward, reducing the required landing distance as well as reducing wear on tires and brakes.
Propellers
Flying Handbook Menu > Transition to Multiengine Airplanes > Operation of Systems > Propellers The propellers of the multiengine airplane may outwardly appear to be identical in operation to the constant-speed propellers of many single-engine airplanes, but this is not the case. The propellers of multiengine airplanes are featherable, to minimize drag in the event of an engine failure. Depending upon single-engine performance, this feature often permits continued flight to a suitable airport following an engine failure. To feather a propeller is to stop engine rotation with the propeller blades streamlined with the airplane’s relative wind, thus to minimize drag. [figure12-2]
figure12-2. Feathered propeller. Feathering is necessary because of the change in parasite drag with propeller blade angle. [figure12-3] When the propeller blade angle is in the feathered position, the change in parasite drag is at a minimum and, in the case of a typical multiengine airplane, the added parasite drag from a single feathered propeller is a relatively small contribution to the airplane total drag.
figure12-3. Propeller drag contribution. At the smaller blade angles near the flat pitch position, the drag added by the propeller is very large. At these small blade angles, the propeller windmilling at high r.p.m. can create such a tremendous amount of drag that the airplane may be uncontrollable. The propeller windmilling at high speed in the low range of blade angles can produce an increase in parasite drag which may be as great as the parasite drag of the basic airplane. As a review, the constant-speed propellers on almost all single-engine airplanes are of the nonfeathering, oil-pressure-to-increase-pitch design. In this design, increased oil pressure from the
propeller governor drives the blade angle towards high pitch, low r.p.m. In contrast, the constant-speed propellers installed on most multiengine airplanes are full feathering, counterweighted, oil-pressure-to-decrease-pitch designs. In this design, increased oil pressure from the propeller governor drives the blade angle towards low pitch, high r.p.m.— away from the feather blade angle. In effect, the only thing that keeps these propellers from feathering is a constant supply of high pressure engine oil. This is a necessity to enable propeller feathering in the event of a loss of oil pressure or a propeller governor failure. The aerodynamic forces alone acting upon a windmilling propeller tend to drive the blades to low pitch, high r.p.m. Counterweights attached to the shank of each blade tend to drive the blades to high pitch, low r.p.m. Inertia, or apparent force called centrifugal force acting through the counterweights is generally slightly greater than the aerodynamic forces. Oil pressure from the propeller governor is used to counteract the counterweights and drives the blade angles to low pitch, high r.p.m. Areduction in oil pressure causes the r.p.m. to be reduced from the influence of the counterweights. [figure12-4] To feather the propeller, the propeller control is brought fully aft. All oil pressure is dumped from the governor, and the counterweights drive the propeller blades towards feather. As centrifugal force acting on the counterweights decays from decreasing r.p.m., additional forces are needed to completely feather the blades. This additional force comes from either a spring or high pressure air stored in the propeller dome, which forces the blades into the feathered position. The entire process may take up to 10 seconds. Feathering a propeller only alters blade angle and stops engine rotation. To completely secure the engine, the pilot must still turn off the fuel (mixture, electric boost pump, and fuel selector), ignition, alternator/generator, and close the cowl flaps. If the airplane is pressurized,
figure12-4. Pitch change forces. there may also be an air bleed to close for the failed engine. Some airplanes are equipped with
firewall shutoff valves that secure several of these systems with a single switch. Completely securing a failed engine may not be necessary or even desirable depending upon the failure mode, altitude, and time available. The position of the fuel controls, ignition, and alternator/generator switches of the failed engine has no effect on aircraft performance. There is always the distinct possibility of manipulating the incorrect switch under conditions of haste or pressure. To unfeather a propeller, the engine must be rotated so that oil pressure can be generated to move the propeller blades from the feathered position. The ignition is turned on prior to engine rotation with the throttle at low idle and the mixture rich. With the propeller control in a high r.p.m. position, the starter is engaged. The engine will begin to windmill, start, and run as oil pressure moves the blades out of feather. As the engine starts, the propeller r.p.m. should be immediately reduced until the engine has had several minutes to warm up; the pilot should monitor cylinder head and oil temperatures. Should the r.p.m. obtained with the starter be insufficient to unfeather the propeller, an increase in airspeed from a shallow dive will usually help. In any event, the AFM/POH procedures should be followed for the exact unfeathering procedure. Both feathering and starting a feathered reciprocating engine on the ground are strongly discouraged by manufacturers due to the excessive stress and vibrations generated. As just described, a loss of oil pressure from the propeller governor allows the counterweights, spring and/or dome charge to drive the blades to feather. Logically then, the propeller blades should feather every time an engine is shut down as oil pressure falls to zero. Yet, this does not occur. Preventing this is a small pin in the pitch changing mechanism of the propeller hub that will not allow the propeller blades to feather once r.p.m. drops below approximately 800. The pin senses a lack of centrifugal force from propeller rotation and falls into place, preventing the blades from feathering. Therefore, if a propeller is to be feathered, it must be done before engine r.p.m. decays below approximately 800. On one popular model of turboprop engine, the propeller blades do, in fact, feather with each shutdown. This propeller is not equipped with such centrifugally-operated pins,due to a unique engine design. An unfeathering accumulator is an optional device that permits starting a feathered engine in flight without the use of the electric starter. An accumulator is any device that stores a reserve of high pressure. On multiengine airplanes, the unfeathering accumulator stores a small reserve of engine oil under pressure from compressed air or nitrogen. To start a feathered engine in flight, the pilot moves the propeller control out of the feather position to release the accumulator pressure. The oil flows under pressure to the propeller hub and drives the blades toward the high r.p.m., low pitch position, whereupon the propeller will usually begin to windmill. (On some airplanes, an assist from the electric starter may be necessary to initiate rotation and completely unfeather the propeller.) If fuel and ignition are present, the engine will start and run. For airplanes used in training, this saves much electric starter and battery wear. High oil pressure from the propeller governor recharges the accumulator just moments after engine rotation begins.
Propeller Synchronization
Flying Handbook Menu > Transition to Multiengine Airplanes > Operation of Systems > Propeller Synchronization Many multiengine airplanes have a propeller synchronizer (prop sync) installed to eliminate the annoying “drumming” or “beat” of propellers whose r.p.m. are close, but not precisely the same. To use prop sync, the propeller r.p.m. are coarsely matched by the pilot and the system is engaged. The prop sync adjusts the r.p.m. of the “slave” engine to precisely match the r.p.m. of the “master” engine, and then maintains that relationship. The prop sync should be disengaged when the pilot selects a new propeller r.p.m., then re-engaged after the new r.p.m. is set. The prop sync should always be off for takeoff, landing, and single-engine operation. The AFM/POH should be consulted for system description and limitations. A variation on the propeller synchronizer is the propeller synchrophaser. Prop sychrophase acts much like a synchronizer to precisely match r.p.m., but the synchrophaser goes one step further. It not only matches r.p.m. but actually compares and adjusts the positions of the individual blades of the propellers in their arcs. There can be significant propeller noise and vibration reductions with a propeller synchrophaser. From the pilot’s perspective, operation of a propeller synchronizer and a propeller syncrophaser are very similar. A synchrophaser is also commonly referred to as prop sync, although that is not entirely correct nomenclature from a technical standpoint. As a pilot aid to manually synchronizing the propellers, some twins have a small gauge mounted in or by the tachometer(s) with a propeller symbol on a disk that spins. The pilot manually fine tunes the engine r.p.m. so as to stop disk rotation, thereby synchronizing the propellers. This is a useful backup to synchronizing engine r.p.m. using the audible propeller beat. This gauge is also found installed with most propeller synchronizer and synchrophase systems. Some synchrophase systems use a knob for the pilot to control the phase angle.
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) Control and Operation (page 1)
Propeller Control basic requirement: For flight operation, an engine is demanded to deliver power within a relatively narrow band of operating rotation speeds. During flight, the speed-sensitive governor of the propeller automatically controls the blade angle as required to maintain a constant r.p.m. of the engine. Three factors tend to vary the r.p.m. of the engine during operation. These factors are power, airspeed, and air density. If the r.p.m. is to maintain constant, the blade angle must vary directly with power, directly with airspeed, and inversely with air density. The speed-sensitive governor provides the means by which the propeller can adjust itself automatically to varying power and flight conditions while converting the power to thrust.
Fundamental Forces : Three fundamental forces are used to control blade angle . These forces are: 1. Centrifugal twisting moment, centrifugal force acting on a rotating blade which tends at all times to move the blade into low pitch. 2. Oil at engine pressure on the outboard piston side, which supplements the centrifugal twisting moment toward low pitch. 3. Propeller Governor oil on the inboard piston side, which balances the first two forces and move the blades toward high pitch Counterweight assembly (this is only for counterweight propeller) which attached to the blades , the centrifugal forces of the counterweight will move the blades to high pitch setting
Constant Speed, Counterweight Propellers The Counterweight type propeller may be used to operate either as a controllable or constant speed propeller. The hydraulic counterweight propeller consists of a hub assembly, blade assembly, cylinder assembly, and counterweight assembly. The counterweight assembly on the propeller is attached to the blades and moves with them. The centrifugal forces obtained from rotating counterweights move the blades to high angle setting. The centrifugal force of the counterweight assembly is depended on the rotational speed of the propellers r.p.m. The propeller blades have a definite range of angular motion by an adjusting for high and low angle on the counterweight brackets.
Controllable : the operator will select either low blade angle or high blade angle by two-way valve which permits engine oil to flow into or drain from the propeller.
Constant Speed : If an engine driven governor is used, the propeller will operate as a constant speed. The propeller and engine speed will be maintained constant at any r.p.m. setting within the operating range of the propeller.
Governor Operation (Constant speed with counterweight ) the Governor supplies and controls the flow of oil to and from the propeller. The engine driven governor receives oil from the engine lubricating system and boost its pressure to that required to operate the pitch-changing mechanism. It consists essentially of : 1. A gear pump to increase the pressure of the engine oil to the pressure required for propeller operation. 2. A relief valve system which regulates the operating pressure in the governor. 3. A pilot valve actuated by flyweights which control the flow of oil through the governor 4. The speeder spring provides a mean by which the initial load on the pilot
valve can be changed through the rack and pulley arrangement which controlled by pilot. The governor maintains the required balance between all three control forces by metering to, or drain from, the inboard side of the propeller piston to maintain the propeller blade angle for constant speed operation. The governor operates by means of flyweights which control the position of a pilot valve. When the propeller r.p.m. is below that for which the governor is set through the speeder spring by pilot , the governor flyweight move inward due to less centrifugal force act on flyweight than compression of speeder spring. If the propeller r.p.m. is higher than setting , the flyweight will move outward due to flyweight has more centrifugal force than compression of speeder spring . During the flyweight moving inward or outward , the pilot valve will move and directs engine oil pressure to the propeller cylinder through the engine propeller shaft.
Principles of Operation (Constant Speed with Counterweight Propellers) The changes in the blades angle of a typical constant speed with counterweight propellers are accomplished by the action of two forces, one is hydraulic and the other is mechanical. 1. The cylinder is moved by oil flowing into it and opposed by centrifugal force of counterweight. This action moves the counterweight and the blades to rotate toward the low angle positon. 2. When the oil allowed to drain from the cylinder , the centrifugal force of counterweights take effect and the blades are turned toward the high angle
position. 3. The constant speed control of the propeller is an engine driven governor of the flyweight type.
Continue on Propeller Control and Operation Page 2
Control and Operation (page 2)
Governor Operation Condition
On-Speed Condition The on-speed condition exists when the propeller operation speed are constant . In this condition, the force of the flyweight (5) at the governor just balances the speeder spring (3) force on the pilot valve (10) and shutoff completely the line (13) connecting to the propeller , thus preventing the flow of oil to or from the propeller.
The pressure oil from the pump is relieved through the relief valve (6). Because the propeller counterweight (15) force toward high pitch is balanced by the oil force from cylinder (14) is prevented from moving, and the propeller does not chang pitch Under-Speed Condition The under-speed condition is the result of change in engine r.p.m. or propeller r.p.m.which the r.p.m. is tend to lower than setting or governor control movement toward a high r.p.m. Since the force of the flyweight (5) is less than the speeder spring (3) force , the pilot valve (10) is forced down. Oil from the booster pump flows through the line (13) to the propeller. This forces the cylinder (14) move outward , and the blades (16) turn to lower pitch, less power is required to turn the propeller which inturn increase the engine r.p.m. As the speed is increased, the flyweight force is increased also and becomes equal to the speeder spring force. The pilot valve is move up, and the governor resumes its on-speed condition which keep the engine r.p.m. constant.
Over-Speed Condition The over-speed condition which occurs when the aircraft altitude change or engine power is increased or engine r.p.m. is tend to increase and the governor control is moved towards a lower r.p.m. In this condition, the force of the flyweight (5) overcomes the speeder spring (3) force and raise the pilot valve (10) open the
propeller line (13) to drain the oil from the cylinder (14). The counterweight (15) force in the propeller to turn the blades towards a higher pitch. With a higher pitch, more power is required to turn the propeller which inturn slow down the engine r.p.m. As the speed is reduced, the flyweight force is reduced also and becomes equal to the speeder spring force. The pilot valve is lowered, and the governor resumes its on-speed condition which keep the engine r.p.m. constant.
Flight Operation This is just only guide line for understanding . The engine or aircraft manufacturers' operating manual should be consulted for each particular aircrat.
Takeoff : Placing the governor control in the full forward position . This position is setting the propeller blades to low pitch angle Engine r.p.m. will increase until it reaches the takeoff r.p.m. for which the governor has been set. From this setting , the r.p.m. will be held constant by the governor, which means that full power is available during takeoff and climb. Cruising : Once the crusing r.p.m. has been set , it will be held constant by the governor. All changes in attitude of the aircraft, altitude, and the engine power can be made without affecting the r.p.m. as long as the blades do not the pitch
limit stop. Power Descent : As the airspeed increase during descent, the governor will move the propeller blades to a higher pitch inorder to hold the r.p.m. at the desired value. Approach and Landing : Set the governor to its maximum cruising r.p.m. position during approach. During landing, the governor control should be set in the high r.p.m. position and this move the blades to full low pitch angle. Control and Operation (page 3)
Hydromatic Propellers
Basic Operation Principles : The pitch changing mechanism of hydromatic propeller is a mechanical-hydraulic system in which hydraulic forces acting upon a piston are transformed into mechanical forces acting upon the blades.
Piston movement causes rotation of cam which incorporates a bevel gear (Hamilton Standard Propeller) . The oil forces which act upon the piston are controled by the governor
Single Acting Propeller: The governor directs its pump output against the inboard side of piston only, A single acting propeller uses a single acting governor. This type of propeller makes use of three forces during constant speed operation , the blades centrifugal twisting moment and this force tends at all times to move the blades toward low pitch , oil at engine pressure applied against the outboard side of the propeller piston and this force to supplement the centrifugal twisting moment toward the low pitch during constant speed operation., and oil from governor
pressure applied against the inboard side of the piston . The oil pressure from governor was boosted from the engine oil supply by governor pump and the force is controlled by metering the high pressure oil to or draining it from the inboard side of the propeller piston which balances centrifugal twisting moment and oil at the engine pressure.
Double Acting Propeller: The governor directs its output either side of the piston as the operating condition required. Double acting propeller uses double acting governor. This type of propeller , the governor pump output oil is directed by the governor to either side of the propeller piston.
Principle Operation of Double Acting : Overspeed Condition : When the engine speed increases above the r.p.m. for which the governor is set . Oil supply is boosted in pressure by thr engine driven propeller governor , is directed against the inboard side of the propeller piston. The piston and the attached rollers move outboard. As the piston moves outboard , cam and rollers move the propeller blades toward a higher angle , which inturn, decreases the engine r.p.m. Underspeed Condition : When the engine speed drops below the r.p.m. for which the governor is set. Force at flyweight is decrease and permit speeder spring to lower pilot valve, thereby open the oil age allow the oil from inboard side of piston to drain through the governor. As the oil from inboard side is drained , engine oil from engine flows through the propeller shaft into the outboard piston end. With the aid of blade centrifugal twisting moment, The engine oil from outboard moves the piston inboard. The piston motion is transmitted through the cam and rollers . Thus, the blades move to lower angle
The Feathering System
Feathering : For some basic model consists of a feathering pump, reservoir, a feathering time-delay switch, and a propeller feathering light. The propeller is feathered by moving the control in the cockpit against the low speed stop. This causes the pilot vave lift rod in the governor to hold the pilot valve in the decrease r.p.m. position regardless of the action of the governor flyweights. This causes the propeller blades to rotate through high pitch to the feathering position.
Some model is initiated by depressing the feathering button. This action, auxiliary pump, feather solinoid, which positions the feathering valve to tranfer oil to feathering the propeller. When the propeller has been fully feathered, oil pressure will buildup and operate a pressure cutout switch which will cause the auxiliary pump stop. Feathering may be also be accomplished by pulling the engine emergency shutdown handle or switch to the shutdown position.
Unfeathering : Some model is accomblished by holding the feathering buttn switch in the out position for about 2 second . This creates an artificial underspeed condition at the governor and causes high-pressure oil from the feathering pump to be directed to the rear of the propeller piston. As soon as the piston has moved inward a short distance, the blades will have sufficient angle to start rotation of the engine. When this occurs , the un-feathering switch can be released and the governor will resume control of the propeller.
Propeller Aerodynamics, I A propeller is an airfoil and like a wing it will generate an aerodynamic force much the same way. It has a leading and trailing edge, camber and a chord line. The cambered side is called blade back and the flatter side the blade face. The angle which the chord makes to the plane of rotation is the blade angle. The propeller is rotated by the engine and this creates thrust and moves the aircraft forward. Previous pages discussed the type of propellers and their controls, in this section we will try to describe some of the aerodynamic principles.
Propeller Aerodynamics As the propeller moves through the air the static pressure is reduced ahead of each blade and at the same time at the blade face the flow is retarded resulting in an increase of static pressure. The changes in pressure around the rotating blades causes air to be drawn into the propeller disc and this results in a rearward movement of a column of air. The result being a forward thrust pulling the aircraft. All of this in accordance with Newton's Third Law. The amount of thrust generated by a propeller depends on the mass of the air and its acceleration toward the rear.
Blade twist and pitch The need for a changing blade angle from the hub to the tips stems from the fact that angular speed varies also and is greatest at the tips. Combined with any forward speed the propeller may have, the relative airflow is also different from the hub to the tips. To keep thrust equal along the blade, they have a build in twist. The design is such that the blade is thick at the hub with a large blade angle and thin at the tip with a low blade angle. During rotation and forward movement the propeller describes a rotational path, called a helix. If the propeller would move forward without giving thrust the distance of one revolution is called experimental mean pitch (angle between plane of rotation and the zero thrust angle of attack). The actual advance is the difference between experimental mean pitch and slip. Slip is the angle between the zero thrust and the actual angle of attack. If the propeller rotates in a solid medium it would advance according to its pitch (angle between plane of rotation and the blade face, chord line,) also called geometric pitch.
Propeller performance
If this was an ideal world the propeller would convert all power to thrust. But as this is not the case, losses occur in the slipstream, aerodynamic drag. Under normal conditions the propeller is able to convert 85% of the brake horse power from the engine into thrust. Thus propeller efficiency is the ratio between thrust horsepower and brake horsepower. that Power = Force x Distance / Time (rate of doing work), we can equate propeller efficiency as Thrust x TAS / Brake Horse Power. It follows that propeller efficiency is zero (0) under two conditions: when there is no forward speed (TAS) or when there is no thrust generated. With the aircraft at standstill (beginning of takeoff roll or taxi) the propeller has zero efficiency until it reaches its optimum forward speed for the propeller where maximum thrust is generated (max efficiency), increasing forward speed beyond that point will decrease efficiency (propeller with fixed blade angle). You could say that there is a relation between RPM and airspeed for fixed pitch propellers. It is obvious that a controllable propeller has a wider range of airspeeds where efficiency is at its maximum, until the governor reaches a position where the blades can no longer be adjusted, which is at the full fine and coarse pitch stops.
Blade forces During rotation the blades generate lift and drag. But with propellers we talk about thrust (lift) and propeller torque (drag). Another one: with a wing, drag must be overcome to provide lift but with a propeller it is propeller torque that must be overcome by engine torque. Increasing power with the throttle increases engine torque, resulting in a higher RPM until propeller torque is equal to engine torque and RPM stabilizes. If you would place an aircraft with a fixed pitch propeller into a shallow dive, as forward speed increases the relative airflow changes and the angle of attack is reduced. Resulting in a reduced thrust and propeller torque and as engine torque remained the same (there was no change in throttle setting) engine/propeller RPM will increase.
Blade effectiveness A propeller blade is the most effective between station 60% and 90% with a peak at 75%. It is this point (station) where blade angle is usually reported.
Asymmetric blade effect This is most noticeable during high angles of attack of the airplane, take off rotation, tail wheel aircraft on take-off run. The cause is that the propeller disc (normally at a right angle with the airflow) is tilted backwards and the relative airflow between the up going and down going blades are different. The down going blades have a larger angle of attack (the distance travelled seems greater due to the forward movement of the aircraft) and produce more thrust, pulling the aircraft with a higher force than the up going blades. With an engine rotating clockwise (as seen from the cockpit) the aircraft wants to turn to the left.
Twin engine aircraft On twin engined propeller driven aircraft and with both propellers rotating in the same direction the arm (or distance) of the downward going blades towards the center of gravity are not the same. The engine where the distance is the smallest is called the critical engine. If that one fails the other engine will yaw the aircraft with more force (thrust x arm) than if the critical engine had failed.
Propeller Aerodynamics, II A propeller is an airfoil and like a wing it will generate an aerodynamic force much the same way. It has a leading and trailing edge, camber and a chord line. The cambered side is called blade back and the flatter side the blade face. The angle which the chord makes to the plane of rotation is the blade angle. The propeller is rotated by the engine and this creates thrust and moves the aircraft forward. Previous page discussed the propeller blade twist and performance, in this section we will discuss blade twisting forces and length and tip speeds.
Propeller Twisting Forces Large stresses are involved when the propeller rotates. Think of centrifugal force and twisting and aerodynamic twisting forces acting on the blades, hub and pitch changing mechanism. Centrifugal force is opposed by centripetal force acting toward the hub (if this was not the case the blade would be thrown away), this force is proportional to the mass of the blade times the square of the rotational velocity.
Centrifugal twisting During rotation centrifugal force acts from the hub spanwise along the blades outward (pulling on the blade) and also on the leading and trailing edges along the blade. As the blade has a twist (blade angle) compared to the plane of rotation, the centrifugal force will try to fine the propeller along the pitch change axis toward the plane of rotation. This twisting force places a great demand on the pitch mechanism, counterweights are used to balance out this twisting toward the fine pitch. The force will be greater on wide blade propellers compared to narrow blade types.
Aerodynamic twisting This force develops when the total reaction force acts on a point ahead of the pitch axis of the blade. It tries to increase the blade angle, coarsen the propeller. This only partially offsets the centrifugal twisting force. With a windmilling propeller the total reaction force is opposite and acts in the same direction as the centrifugal twisting force both trying to fine the propeller. During a steep dive these forces might become stronger than the pitch changing mechanism, effectively locking the propeller into fine pitch and possibly overspeeding the engine. Despite the fact of having a constant speed propeller.
Blade length As the propeller rotates in a circle it becomes clear that with increasing engine/propeller RPM the propeller tip speed rises too. When the tips come close or reach the sound barrier (Mach 1.0) they will become very noisy and propeller effectiveness (thrust) is reduced. But reducing the tip speed by reducing the RPM has one big disadvantage, propeller thrust will reduce also. Some propellers solve this noise problem by curving leading edges to the rear near the tips. The trick is to keep the RPM high enough to generate useful thrust but low enough to keep the propeller quiet. This maximum tip speed is between Mach .88 and .92 and the RPM where that is reached depends on the propeller diameter.
Speed of sound
In the ISA atmosphere (15 °C) the speed of sound is 340,3 m/s (1225 km/h or 661,4 kts) and it varies with temperature. The formula to calculate this is: 331,3 √ (1 + T / 273,15), where T is the outside air temperature in Celsius.
Calculating tip speed The formula VTIP = π d n, were π is a constant (3,14159), d is diameter in meters and n is angular velocity (RPM) in revolutions per minute. VTIP then results in meters/minute. For example: a propeller of 72" diameter (1m83) with a RPM of 2600 gives a tip speed of: π * 1,83 * 2600 = 14947,7 meters/minute or 896,9 km/h.
Propeller tip mach speed Divide the propeller tip speed by the actual speed of sound at standard ISA (15 °C) and our 72" propeller at 2600 RPM will then have a tip mach number of 896,9 / 1225 = .73 Mach. Which is below mach .88 so we can increase RPM a bit or use longer blades to increase thrust without generating a lot of extra noise.
Aircraft Propellers Without a method of converting the power produced by the engine into useful thrust an aircraft would just be sitting still and creating a lot of noise and not get anywhere. In the past, all kinds of different propeller design were used: sometimes flat wooden blades, from one bladed to multiple blade models rotated at various speeds. Until it was realised that a propeller is just like a wing but instead of carrying the aircraft it displaces a volume of air backwards. Albeit slowly, compared to a pure JET engine, which displaces a small volume of air at high speeds. The same aerodynamic laws that apply to wings are valid for propellers too only not horizontal but in a vertical motion and so thrust (lift) is created and the aircraft is able to move forward, after reaching liftoff speed it will get airborne. These pages shine a small light on propeller design, some aerodynamics and operation of controllable propellers during preflight, cruise and the influence on fuel consumption.
Basic Propeller Design Propellers are usually constructed of wood, aluminum alloy or a combination of composites and wood. The leading edge is sometimes reinforced with nickel for protection against rain, sand and stones. They come in two, three or more blades depending of the application. Some high end turbo prop aircraft even have six blades to absorb the power. But, more blades means more weight and more propeller torque, the engine must be able to handle this. The function of the propeller is to convert brake horse power from the engine into thrust. To do this there are two types of propellers in use: fixed pitch (or ground adjustable, which remains a fixed pitch during flight) and the constant speed propeller with some variations possible.
Blade angle Looking at a propeller, its blade angle varies from the root to the tip. This is because angular speed of the blade is at it highest at the tip (reaching the speed of sound) and lowest at the root. If the blade angle would be constant then the angle of attack (and thrust) of the relative airflow (result of forward speed and
RPM) would vary across the propeller disc, and the blade would probably be stalled. To make sure that the generated thrust is equal from root to tip, the blade angle is high at the root (low angular speed) and low at the tip (high angular speed).
Propeller thrust During propeller rotation the airflow past the propeller blade produces an aerodynamic reaction with can be resolved into thrust and propeller torque (called lift with wings). To rotate the propeller the engine needs to create a torque. The resistance to this rotation is called propeller torque and when these two forces are in balance or stabilized, engine/propeller RPM is constant. Thus power created by the engine is absorbed by the propeller and as a result thrust is generated.
Fixed pitch With this propeller the blades are fixed and cannot move (except rotate), thus propeller blade angle is fixed (but the blade angle changes from the root to the tip as explained above). It can be seen that there is also one RPM / airspeed combination where the propeller is operating at its optimum angle of attack and produces its maximum thrust. On all other airspeeds (or conditions of flight) there is less thrust available from the propeller. In other words, its effective angle of attack depends on RPM and airspeed. To compensate, manufacturers sometimes have a climb and a cruise propeller available for the same model or type. Although convenient and simple to operate, a fixed pitch propeller will always be a compromise between a number of factors as RPM, airspeed, relative airflow, angle of attack, two or three blades, blade chord and length, emitted sound level etc...
Constant speed To overcome all these disadvantages the constant speed (or variable pitch) propeller is able to adjust its blade angle. Thereby operating closer to or at its optimum angle of attack over a wide range of RPM and airspeed combinations. The result is that the maximum amount of thrust is obtained from standstill to maximum speed (VNE) or from minimum to maximum propeller pitch angle. A shorter takeoff roll and much better climb performance may be expected and a lower specific fuel consumption is also one of the advantages.
Propeller control With a fixed pitch propeller the pilot has only one control, throttle, to control power and RPM. With a constant speed propeller there are two controls: power (throttle, black knob) and RPM (propeller, blue knob). In addition there should be a MAP indicator (manifold pressure) which relates to engine power. Moving the propeller control (pitch) changes the RPM at which the engine and propeller will rotate and moving the throttle changes the amount of power delivered (MAP) to the propeller at the preset RPM held constant by the propeller governor. An aircraft with a constant speed propeller is bit more complex and is not usually used for basic (ab initio) flight training. Those training for higher licences (L, ATPL) will need to understand and properly operate these more complex aircraft.
Propeller Controls, I Fixed pitch propellers are operated by changing the throttle only, thereby changing thrust and RPM at the same time. Constant speed propellers are more complicated because the pitch can be set separately from the throttle and has a range from full fine to coarse during flight and into feather and reverse for certain operations as emergency and ground maneuvers. A full understanding of operating propellers is required of the pilot as overspeeding the engine or setting reverse in flight or on finals would be nothing less than a real problem. This page focuses on preflight checks applicable for any propeller and the in-flight checks for constant speed propellers as the fixed pitch types usually do not have controls in the cockpit to manipulate them during flight.
Propeller Operation A substantial amount of aircraft have variable pitch propellers of some sort in use today, it is important that the pilot is very familiar with the operation of the propeller in normal and emergency situations. Even the type of engine (gasoline, diesel or turboprop) will change operating procedures for the propeller. Limitations on certain combinations of RPM and manifold pressures are not uncommon due to frequency resonations with the engine/propeller combination.
Preflight checks Check the propeller blades for damage, nicks and any oil leaks under the spinner. This would indicate an unserviceable aircraft as engine oil is used to change the pitch of the blades. For optimum propeller performance do clean the blades after every flight, this gives you the chance to check the blades properly.
Engine startup Set the propeller control in the forward (full fine) position for piston engines. Free turbine turboprop aircraft usually leave the propeller feathered until the engine is fired up and at idle N1 speed.
Pre-takeoff checks During the runup, hydraulic operated propellers are are checked by changing the control from full fine to coarse (or a set RPM drop) two or three times. This makes sure that any cold oil left in the propeller is run through the engine for warmup so that propeller operation will not be sluggish.
Cruise checks During cruise operate the engine RPM and manifold pressure according to the aircraft flight manual (read it thoroughly). Avoid overboosting by more than the recommended values. Any change in power settings should be done per procedure described below. To increase power at a higher RPM: •
Increase RPM with the PROPELLER control
•
Increase MAP with the THROTTLE control
To decrease power for a lower RPM: •
Reduce MAP with the THROTTLE control
•
Reduce RPM with the PROPELLER control
The procedure shown above discusses the correct way of changing power with a constant speed propeller without the possibility of overloading the engine.
Oversquare It is perfectly fine to operate the engine in a so called oversquare condition where the RPM divided by 100 is lower than MAP, for example: 2300 RPM and 25" MAP, read here about running oversquare and in the aircraft/engine manuals. This will lower fuel consumption and increase the range of the aircraft.
Emergencies In case of engine failure place the constant speed propeller in the full coarse pitch (blue lever fully backward) as to reduce the drag from it and increase glide range.
Pre-landing checks During landing and go-around the propeller should be in the full fine position to assure the maximum power available should this be needed. Short final is the place where this check can be done together with setting carb heat cold.
Propeller Controls, II Fixed pitch propellers are operated by changing the throttle only, thereby changing thrust and RPM at the same time. Constant speed propellers are more complicated because the pitch can be set separately from the throttle and range from full fine to coarse during flight and into feather and reverse for certain operations as emergency and ground maneuvers. A full understanding of operating propellers is required of the pilot as overspeeding the engine or setting reverse in flight or on finals would be nothing less than a disaster. Constant speed propellers are driven by oil pressure or electric power and need a control unit (governor) to maintain the preset RPM by varying the blade angle. Both types will be discussed.
Propeller governors To sense the engine RPM a mechanical or electrical device is used. In todays aviation this usually an oil or electrical operated governor. Some models are able to set the propeller blades into feather or even in a reverse trust position (not during flight).
Hydraulic propeller governors
Also called constant speed unit, it consists of a governor which regulates engine oil pressure to a hydraulic operated piston (in the propeller) and changes the propeller blade angle in order to keep RPM constant. The governor contains flyweights which senses engine RPM and change position in relation to a change in RPM, thus changing oil pressure to the propeller and regulating the blade pitch. The blades themselves contain flyweights compensating for aerodynamic forces so that the oil pressure from the engine only needs a little bit of effort to change the blade angle. This means that oil pressure remains almost constant and engine lubrication is guaranteed.
Electric propeller governors Detection of propeller RPM can be done in several ways: by optical (light), magnetic (flux) or sensing the engine RPM by connecting to the ignition or RPM indicator (Rotax). This signal is then fed to a controller which calculates any change in RPM and drives an electric motor inside the propeller to keep the preset RPM constant.
Microprocessor Thus with the flick of a switch the propeller can be set to any RPM, feather or even operate as a fixed pitch with sophisticated, sometimes even microprocessor controlled electronics (Airmaster Propellor, NZ).
Responsive Most modern constant speed propellers (hydraulically or electrical) have an accurate and responsive operation. Any change or tendency to change the RPM, by the pilot, turbulence or change in aircraft attitude, is sensed quickly so blade angle is changed accordingly to keep the preset RPM constant and this occurs almost unnoticeable by the pilot or engers.
Preflight With an hydraulic constant speed propeller you must that no oil leaks exist either at the propeller or near the governor. Any leak is reason not to fly as engine lubrication is impaired and failure or seizure of the engine could result due to loss of oil.
Propeller Controls, III Fixed pitch propellers are operated by changing the throttle only, thereby changing thrust and RPM at the same time. Constant speed propellers are more complicated because the pitch can be set separately from the throttle and range from full fine to coarse during flight and into feather and reverse for certain operations as emergency and ground maneuvers. A full understanding of operating propellers is required of the pilot as overspeeding the engine or setting reverse in flight or on finals would be nothing less than a disaster. In case of an engine failure the propeller should be positioned in the low drag setting. Thus with the blades perpendicular on the airflow thus full coarse, this is called feathering the prop.
Special operations Some types of variable pitch propellers have the option to select a feather or reverse thrust position. Its obvious that it must be impossible to select reverse thrust during flight.
Feather
This position is used when an engine has failed. The drag from a windmilling propeller/engine is very high and will reduce aircraft performance. Feathering the propeller will stop the engine from rotating and this reduces drag. Some motor gliders use this feature when, after climbing to altitude, they shutdown their engine and feather the propeller for much better glide performance of their aircraft. Feathering can be done manually of automatically. Large turboprop aircraft will have the latter. On aircraft without a feather position it is important that the propeller be set in the full coarse position after engine failure and thus reducing drag and increasing the glide distance.
Reverse Thrust This is used to reduce the landing roll. The blades are first set to a negative pitch angle (20°) and then power is applied. Adding power while the pitch angle is around 0° could result in an engine overspeed, mechanical devices are used to prevent this. It is even possible to use this to back the aircraft out from a parking stand, although caution is required. The aid from a marshaller is recommended as the pilot usually is unable to look backwards.
Overspeed condition A constant speed propeller has two mechanical stops: one at the full fine pitch and one at the full coarse pitch. With each settings there is a maximum airspeed the aircraft can reach. If the propeller overspeed fails then the propeller blades end up at the full fine pitch setting due to aerodynamic forces and with an airspeed probably higher than is normally attainable with that setting. With the application of power the engine will then overspeed and produces no thrust until the airspeed is reduced below the maximum of the propeller at the fine pitch stops.
Special Airworthiness Information Bulletin The FAA has issued a Special Airworthiness Information Bulletin CE-10-21 (SAIB) to alert operators, pilots, and aircraft manufacturers of concerns for an optimum response to a propeller overspeed in piston engine aircraft with variable pitch propellers. At this time, the FAA has concluded that the airworthiness concern is not an unsafe condition that would warrant airworthiness directive (AD) action. The appropriate emergency procedures should be followed to mitigate the emergency situation in the event of a propeller overspeed; however, pilots should be aware that some reduction in airspeed may result in the ability for continued safe flight and landing. The determination of an airspeed that is more suitable than engine out best glide speed should only be conducted at a safe altitude when the pilot has time to determine an alternative course of action other than landing immediately.
Section 6.3 - PROPELLERS We can say that the propeller is the action end of an aircraft's reciprocating engine, because it converts the useful energy of the engine into thrust as it spins around and around. The propeller has the general shape of a wing, but the camber and chord (curvature and cross-sectional length) of each section of the propeller are different, as shown in Figure 6-4 . The wing provides lift upward, while the propeller provides lift forward. The wing has only one motion which is forward, while the propeller has forward and rotary motion. The path of these two motions is like a corkscrew as the propeller goes through the air (see figure 6-5 ).
Like a wing, a propeller blade has a thick leading edge and a thin trailing edge. The blade back is the curved portion and is like the top of a wing. The blade face is comparatively flat and corresponds to the underside of a wing (see figure 6-6 for definitions of blade back and blade face). The blade shank is thick for strength and fits into a hub which is attached to the crankshaft directly or indirectly. The outer end of the blade is called the tip.
Blade pitch is loosely defined as the angle made by the chord of the blade and its plane of rotation, as shown in Figure 66 . When the angle is great, the propeller is said to have high pitch. A high-pitch propeller will take a bigger bit of air and move the aircraft farther forward in one rotation than will a low-pitch propeller. Propellers may be classified as to whether the blade pitch is fixed or variable. The demands on the propeller differ according to circumstances. For example, in takeoffs and climbs more power is needed, and this can best be provided by low pitch. For speed at cruising altitude, high pitch will do the best job. A fixed-pitch propeller is a compromise.
There are two types of variable-pitch propellers adjustable and controllable. The adjustable propeller's pitch can be changed only by a mechanic to serve a particular purpose-speed or power. The controllable-pitch propeller permits pilots to change pitch to more ideally fit their requirements at the moment. In different aircraft, this is done by electrical or hydraulic means. In modern aircraft, it is done automatically, and the propellers are referred to as constant-speed propellers. As power requirements vary, the pitch automatically changes, keeping the engine and the propeller operating at a constant rpm. If the rpm rate increases, as in a dive, a governor on the hydraulic system changes the blade pitch to a higher angle. This acts as a brake on the crankshaft. If the rpm rate decreases, as in a climb, the blade pitch is lowered and the crankshaft rpm can increase. The constant-speed propeller thus ensures that the pitch is always set at the most efficient angle so that the engine can run at a desired constant rpm regardless of altitude or forward speed. Click here to see examples of early aviation propellers. The constant-speed propellers have a full-feathering capability. Feathering means to turn the blade approximately parallel with the line of flight, thus equalizing the pressure on the face and back of the blade and stopping the propeller. Feathering is necessary if for some reason the propeller is not being driven by the engine and is windmilling, a situation that can damage the engine and increase drag on the aircraft. Most controllable-pitch and constant-speed propellers also are capable of being reversed. This is done by rotating the blades to a negative or reverse pitch. Reversible propellers push air forward, reducing the required landing distance as well as reducing wear on tires and brakes.
Propellers
Flying Handbook Menu > Transition to Multiengine Airplanes > Operation of Systems > Propellers The propellers of the multiengine airplane may outwardly appear to be identical in operation to the constant-speed propellers of many single-engine airplanes, but this is not the case. The propellers of multiengine airplanes are featherable, to minimize drag in the event of an engine failure. Depending upon single-engine performance, this feature often permits continued flight to a suitable airport following an engine failure. To feather a propeller is to stop engine rotation with the propeller blades streamlined with the airplane’s relative wind, thus to minimize drag. [figure12-2]
figure12-2. Feathered propeller. Feathering is necessary because of the change in parasite drag with propeller blade angle. [figure12-3] When the propeller blade angle is in the feathered position, the change in parasite drag is at a minimum and, in the case of a typical multiengine airplane, the added parasite drag from a single feathered propeller is a relatively small contribution to the airplane total drag.
figure12-3. Propeller drag contribution. At the smaller blade angles near the flat pitch position, the drag added by the propeller is very large. At these small blade angles, the propeller windmilling at high r.p.m. can create such a tremendous amount of drag that the airplane may be uncontrollable. The propeller windmilling at high speed in the low range of blade angles can produce an increase in parasite drag which may be as great as the parasite drag of the basic airplane. As a review, the constant-speed propellers on almost all single-engine airplanes are of the nonfeathering, oil-pressure-to-increase-pitch design. In this design, increased oil pressure from the
propeller governor drives the blade angle towards high pitch, low r.p.m. In contrast, the constant-speed propellers installed on most multiengine airplanes are full feathering, counterweighted, oil-pressure-to-decrease-pitch designs. In this design, increased oil pressure from the propeller governor drives the blade angle towards low pitch, high r.p.m.— away from the feather blade angle. In effect, the only thing that keeps these propellers from feathering is a constant supply of high pressure engine oil. This is a necessity to enable propeller feathering in the event of a loss of oil pressure or a propeller governor failure. The aerodynamic forces alone acting upon a windmilling propeller tend to drive the blades to low pitch, high r.p.m. Counterweights attached to the shank of each blade tend to drive the blades to high pitch, low r.p.m. Inertia, or apparent force called centrifugal force acting through the counterweights is generally slightly greater than the aerodynamic forces. Oil pressure from the propeller governor is used to counteract the counterweights and drives the blade angles to low pitch, high r.p.m. Areduction in oil pressure causes the r.p.m. to be reduced from the influence of the counterweights. [figure12-4] To feather the propeller, the propeller control is brought fully aft. All oil pressure is dumped from the governor, and the counterweights drive the propeller blades towards feather. As centrifugal force acting on the counterweights decays from decreasing r.p.m., additional forces are needed to completely feather the blades. This additional force comes from either a spring or high pressure air stored in the propeller dome, which forces the blades into the feathered position. The entire process may take up to 10 seconds. Feathering a propeller only alters blade angle and stops engine rotation. To completely secure the engine, the pilot must still turn off the fuel (mixture, electric boost pump, and fuel selector), ignition, alternator/generator, and close the cowl flaps. If the airplane is pressurized,
figure12-4. Pitch change forces. there may also be an air bleed to close for the failed engine. Some airplanes are equipped with
firewall shutoff valves that secure several of these systems with a single switch. Completely securing a failed engine may not be necessary or even desirable depending upon the failure mode, altitude, and time available. The position of the fuel controls, ignition, and alternator/generator switches of the failed engine has no effect on aircraft performance. There is always the distinct possibility of manipulating the incorrect switch under conditions of haste or pressure. To unfeather a propeller, the engine must be rotated so that oil pressure can be generated to move the propeller blades from the feathered position. The ignition is turned on prior to engine rotation with the throttle at low idle and the mixture rich. With the propeller control in a high r.p.m. position, the starter is engaged. The engine will begin to windmill, start, and run as oil pressure moves the blades out of feather. As the engine starts, the propeller r.p.m. should be immediately reduced until the engine has had several minutes to warm up; the pilot should monitor cylinder head and oil temperatures. Should the r.p.m. obtained with the starter be insufficient to unfeather the propeller, an increase in airspeed from a shallow dive will usually help. In any event, the AFM/POH procedures should be followed for the exact unfeathering procedure. Both feathering and starting a feathered reciprocating engine on the ground are strongly discouraged by manufacturers due to the excessive stress and vibrations generated. As just described, a loss of oil pressure from the propeller governor allows the counterweights, spring and/or dome charge to drive the blades to feather. Logically then, the propeller blades should feather every time an engine is shut down as oil pressure falls to zero. Yet, this does not occur. Preventing this is a small pin in the pitch changing mechanism of the propeller hub that will not allow the propeller blades to feather once r.p.m. drops below approximately 800. The pin senses a lack of centrifugal force from propeller rotation and falls into place, preventing the blades from feathering. Therefore, if a propeller is to be feathered, it must be done before engine r.p.m. decays below approximately 800. On one popular model of turboprop engine, the propeller blades do, in fact, feather with each shutdown. This propeller is not equipped with such centrifugally-operated pins,due to a unique engine design. An unfeathering accumulator is an optional device that permits starting a feathered engine in flight without the use of the electric starter. An accumulator is any device that stores a reserve of high pressure. On multiengine airplanes, the unfeathering accumulator stores a small reserve of engine oil under pressure from compressed air or nitrogen. To start a feathered engine in flight, the pilot moves the propeller control out of the feather position to release the accumulator pressure. The oil flows under pressure to the propeller hub and drives the blades toward the high r.p.m., low pitch position, whereupon the propeller will usually begin to windmill. (On some airplanes, an assist from the electric starter may be necessary to initiate rotation and completely unfeather the propeller.) If fuel and ignition are present, the engine will start and run. For airplanes used in training, this saves much electric starter and battery wear. High oil pressure from the propeller governor recharges the accumulator just moments after engine rotation begins.
Propeller Synchronization
Flying Handbook Menu > Transition to Multiengine Airplanes > Operation of Systems > Propeller Synchronization Many multiengine airplanes have a propeller synchronizer (prop sync) installed to eliminate the annoying “drumming” or “beat” of propellers whose r.p.m. are close, but not precisely the same. To use prop sync, the propeller r.p.m. are coarsely matched by the pilot and the system is engaged. The prop sync adjusts the r.p.m. of the “slave” engine to precisely match the r.p.m. of the “master” engine, and then maintains that relationship. The prop sync should be disengaged when the pilot selects a new propeller r.p.m., then re-engaged after the new r.p.m. is set. The prop sync should always be off for takeoff, landing, and single-engine operation. The AFM/POH should be consulted for system description and limitations. A variation on the propeller synchronizer is the propeller synchrophaser. Prop sychrophase acts much like a synchronizer to precisely match r.p.m., but the synchrophaser goes one step further. It not only matches r.p.m. but actually compares and adjusts the positions of the individual blades of the propellers in their arcs. There can be significant propeller noise and vibration reductions with a propeller synchrophaser. From the pilot’s perspective, operation of a propeller synchronizer and a propeller syncrophaser are very similar. A synchrophaser is also commonly referred to as prop sync, although that is not entirely correct nomenclature from a technical standpoint. As a pilot aid to manually synchronizing the propellers, some twins have a small gauge mounted in or by the tachometer(s) with a propeller symbol on a disk that spins. The pilot manually fine tunes the engine r.p.m. so as to stop disk rotation, thereby synchronizing the propellers. This is a useful backup to synchronizing engine r.p.m. using the audible propeller beat. This gauge is also found installed with most propeller synchronizer and synchrophase systems. Some synchrophase systems use a knob for the pilot to control the phase angle.
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