Aircraft Propellers

General Information

     Thrust is the force that move the aircraft through the air.Thrust is generated by the propulsion system of the aircraft. There are different types of propulsion systems develop thrust in different ways, although it usually generated through some application of Newton’s Third Law. Propeller is one of the propulsion system. The purpose of the propeller is to move the aircraft through the air. The propeller consist of two or more blades connected together by a hub. The hub serves to attach the blades to the engine shaft. .

     The blades are made in the shape of an airfoil like wing of an aircraft. When the engine rotates the propeller blades, the blades produce lift. This lift is called thrust and moves the aircraft forward. most aircraft have propellers that pull the aircraft through the air. These are called tractor propellers. Some aircraft have propellers that push the aircraft. These are called pusher propellers.

Description

     Leading Edge of the airfoil is the cutting edge that slices into the air. As the leading edge cuts the air, air flows over the blade face and the cambe side.

Description

     Blade Face is the surface of the propeller blade that corresponds to the lower surface of an airfoil or flat side, we called Blade Face.

     Blade Back / Thrust Face is the curved surface of the airfoil.

     Blade Shank (Root) is the section of the blade nearest the hub.

     Blade Tip is the outer end of the blade fartest from the hub.

     Plane of Rotation is an imaginary plane perpendicular to the shaft. It is the plane that contains the circle in which the blades rotate.

     Blade Angle is formed between the face of an element and the plane of rotation. The blade angle throughout the length of the blade is not the same. The reason for placing the blade element sections at different angles is because the various sections of the blade travel at different speed. Each element must be designed as part of the blade to operate at its own best angle of attack to create thrust when revolving at its best design speed

     Blade Element are the airfoil sections joined side by side to form the blade airfoil. These elements are placed at different angles in rotation of the plane of rotation.

      The reason for placing the blade element sections at different angles is because the various sections of the blade travel at different speeds. The inner part of the blade section travels slower than the outer part near the tip of the blade. If all the elements along a blade is at the same blade angle, the relative wind will not strike the elements at the same angle of attack. This is because of the different in velocity of the blade element due to distance from the center of rotation.
The blade has a small twist (due to different angle in each section) in it for a very important reason. When the propeller is spinning round, each section of the blade travel at different speed, The twist in the peopeller blade means that each section advance forward at the same rate so stopping the propeller from bending.
Thrust is produced by the propeller attached to the engine driveshaft. While the propeller is rotating in flight, each section of the blade has a motion that combines the forward motion of the aircraft with circular movement of the propeller. The slower the speed, the steeper the angle of attack must be to generate lift. Therefore, the shape of the propeller’s airfoil (cross section) must chang from the center to the tips. The changing shape of the airfoil (cross section) across the blade results in the twisting shape of the propeller.

     Relative Wind is the air that strikes and pass over the airfoil as the airfoil is driven through the air.

     Angle of Attack is the angle between the chord of the element and the relative wind. The best efficiency of the propeller is obtained at an angle of attack around 2 to 4 degrees.

     Blade Path is the path of the direction of the blade element moves.

     Pitch refers to the distance a spiral threaded object moves forward in one revolution. As a wood screw moves forward when turned in wood, same with the propeller move forward when turn in the air.

     Geometric Pitch is the theoritical distance a propeller would advance in one revolution.

     Effective Pitch is the actual distance a propeller advances in one revolution in the air. The effective pitch is always shorter than geometric pitch due to the air is a fluid and always slip.

Forces and stresses acting on a propeller in flight

The forces acting on a propeller in flight are :
1. Thrust is the air force on the propeller which is parallel to the directionof advance and induce bending stress in the propeller.
2. Centrifugal force is caused by rotation of the propeller and tends to throw the blade out from the center.
3. Torsion or Twisting forces in the blade itself, caused by the resultant of air forces which tend to twist the blades toward a lower blade angle.

The stress acting on a propeller in flight are :
1. Bending stresses are induced by the trust forces. These stresses tend to bend the blade forward as the airplane is moved through the air by the propeller.
2. Tensile stresses are caused by centrifugal force.
3. Torsion stresses are produced in rotating propeller blades by two twisting moments. one of these stresses is caused by the air reaction on the blades and is called the aerodynamic twisting moment. The another stress is caused by centrifugal force and is called the centrifugal twisting moment.

TYPE OF AIRCRAFT PROPELLERS

Type of propellers

      In designing propellers, the maximum performance of the airplane for all condition of operation from takeoff, climb, cruising, and high speed. The propellers may be classified under eight general types as follows:

      1. Fixed pitch: The propeller is made in one piece. Only one pitch setting is possible and is usually two blades propeller and is often made of wood or metal.
Wooden Propellers : Wooden propellers were used almost exclusively on personal and business aircraft prior to World War II .A wood propeller is not cut from a solid block but is built up of a number of seperate layers of carefully selected .any types of wood have been used in making propellers, but the most satisfactory are yellow birch, sugar mable, black cherry, and black walnut. The use of lamination of wood will reduce the tendency for propeller to warp. For standard one-piece wood propellers, from five to nine seperate wood laminations about 3/4 in. thick are used.


Metal Propellers : During 1940 , solid steel propellers were made for military use. Modern propellers are fabricated from high-strength , heat-treated,aluminum alloy by forging a single bar of aluminum alloy to the required shape. Metal propellers is now extensively used in the construction of propellers for all type of aircraft. The general appearance of the metal propeller is similar to the wood propeller, except that the sections are generally thinner.

      2. Ground adjustable pitch: The pitch setting can be adjusted only with tools on the ground before the engine is running. This type of propellers usually has a split hub. The blade angle is specified by the aircraft specifications. The adjustable – pitch feature permits compensation for the location of the flying field at various altitudes and also for variations in the characteristics of airplanes using the same engine. Setting the blade angles by loosened the clamps and the blade is rotated to the desired angle and then tighten the clamps.

      3. Two-position : A propeller which can have its pitch changed from one position to one other angle by the pilot while in flight.

      4. Controllable pitch: The pilot can change the pitch of the propeller in flight or while operating the engine by mean of a pitch changing mechanism that may be operated by hydraulically.

      5. Constant speed : The constant speed propeller utilizes a hydraulically or electrically operated pitch changing mechanism which is controlled by governor. The setting of the governor is adjusted by the pilot with the rpm lever in the cockpit. During operation, the constant speed propeller will automatically changs its blade angle to maintain a constant engine speed. If engine power is increase, the blade angle is increased to make the propeller absorb the additional power while the rpm remain constant. At the other position, if the engine power is decreased, the blade angle will decrease to make the blades take less bite of air to keep engine rpm remain constant. The pilot select the engine speed required for any particular type of operation.

      6. Full Feathering : A constant speed propeller which has the ability to turn edge to the wind and thereby eliminate drag and windmilling in the event of engine failure. The term Feathering refers to the operation of rotating the blades of the propeller to the wind position for the purpose of stopping the rotation of the propeller to reduce drag. Therefore , a Feathered blade is in an approximate in-line-of-flight position , streamlined with the line of flight (turned the blades to a very high pitch). Feathering is necessary when the engine fails or when it is desirable to shutoff an engine in flight.

      7. Reversing : A constant speed propeller which has the ability to assume a negative blade angle and produce a reversing thrust. When propellers are reversed, their blades are rotated below their positive angle , that is, through flat pitch, until a negative blade angle is obtained in order to produce a thrust acting in the opposite direction to the forward thrust . Reverse propeller thrust is used where a large aircraft is landed, in reducing the length of landing run.

      8. Beta Control : A propeller which allows the manual repositioning of the propeller blade angle beyond the normal low pitch stop. Used most often in taxiing, where thrust is manually controlled by adjusting blade angle with the power lever.


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.

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 contact 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.

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 passage 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.

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