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PROPELLERS

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keith mccabe

on 3 January 2014

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Transcript of PROPELLERS

The twisted airfoil (aerofoil) shape of modern aircraft propellers was pioneered by the Wright brothers. While some earlier engineers had attempted to model air propellers on marine propellers, they realized that a propeller is essentially the same as a wing, and were able to use data from their earlier wind tunnel experiments on wings. They also introduced a twist along the length of the blades. This was necessary to ensure the angle of attack of the blades was kept relatively constant along their length.[2] Their original propeller blades were only about 5% less efficient than the modern equivalent, some 100 years later.[3] The understanding of low speed propeller aerodynamics was fairly complete by the 1920s, but later requirements to handle more power in smaller diameter have made the problem more complex.

Alberto Santos Dumont was another early pioneer, having designed propellers before the Wright Brothers (albeit not as efficient[citation needed]) for his airships. He applied the knowledge he gained from experiences with airships to make a propeller with a steel shaft and aluminium blades for his 14 bis biplane. Some of his designs used a bent aluminium sheet for blades, thus creating an airfoil shape. They were heavily undercambered, and this plus the absence of lengthwise twist made them less efficient than the Wright propellers. Even so, this was perhaps the first use of aluminium in the construction of an airscrew

NOMENCLATURE
PROPELLERS
Introduction

T
o
PROPELLERS
PROPELLER THEORY
Forces acting on a propeller[edit]

Five forces act on the blades of an aircraft propeller in motion, they are:[7]
Thrust bending force Thrust loads on the blades act to bend them forward.Centriorfugal twisting force. Acts to twist the blades to a low, fine pitch angle.Aerodynamic twisting force As the centre of pressure of a propeller blade is forward of its centre. line the blade is twisted towards a coarse pitch position.Centrifugal force The force felt by the blades acting to pull them away from the hub when turning.Torque bending forceAir resistance acting against the blades, combined with inertial effects causes propeller blades to bend away from the direction of rotation.

FORCES ACTING ON A PROP
PROPELLER PITCH
The purpose of varying pitch angle with a variable pitch propeller is to maintain an optimal angle of attack (maximum lift to drag ratio) on the propeller blades as aircraft speed varies. Early pitch control settings were pilot operated, either two-position or manually variable. Following World War I, automatic propellers were developed to maintain an optimum angle of attack. This was done by balancing the centripetal twisting moment on the blades and a set of counterweights against a spring and the aerodynamic forces on the blade. Automatic props had the advantage of being simple, lightweight, and requiring no external control, but a particular propeller's performance was difficult to match with that of the aircraft's powerplant. An improvement on the automatic type was the constant-speed propeller. Constant-speed propellers allow the pilot to select a rotational speed for maximum engine power or maximum efficiency, and a propeller governor acts as a closed-loop controller to vary propeller pitch angle as required to maintain the selected engine speed. In most aircraft this system is hydraulic, with engine oil serving as the hydraulic fluid. However, electrically controlled propellers were developed during World War II and saw extensive use on military aircraft, and have recently seen a revival in use on homebuilt aircraft.
PROPELLER CLASSIFICATION

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.


propeller construction
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.
SUMMARY CHECK LIST
SUMMARY CHECK LIST!!!!!


-Propellers convert rotational engine power to thrust in both reciprocating and with turbo shaft engines.
-Propellers pitch is the theoretical distance that a propeller advances forward in a single revolution.
-A tractor propeller is located on the front end of an engine and pulls an aircraft forward.
-A pusher propeller is located on the aft end of an engine and pushes an aircraft forward.
-Fixed-pitch propellers are designed with a particular performance characteristic. Based on desired aircraft operations and performance, a pilot can choose either a climb or a cruise propeller.
-Constant-speed propellers change blade angle to maintain an engine speed selected by the pilot and controlled by a governor.
-Feathering propellers rotate the blades straight into the wind. If the engine stops turning during flight this position reduces drag and enables the aircraft to glide father.
-Reversible-pitch propellers enable an aircraft to reduce its landing roll; in addition, reversible-pitch propellers enable an aircraft to back up during taxi.

QUESTIONS
AND ANSWER
QUESTIONS””

1-The basic function of a propeller on an airplane is to convert engine torque into? Thrust
2-Propeller blade angle is the angle between the chord of the propeller blade and the?plane of rotation
3-The flat surface of a propeller blade is called the ?face of the blade
4-Propeller blade stations are measured in inches from the ? center of the hub
5-A blade cuff is designed primarily to increase the flow of cooling air to the engine nacelle.
6-The gradual twist in the propeller blade from the shank to tip is known as ? pitch distribution.
7-On a given propeller blade the blade angle nearest the hub will be the ? highest.
8-The six forces that act on a rotating propeller are? 1-centrifugal 2-thrust bending 3-torque bending 4-aerodynamic twisting 5-centrifigal twisting 6-vibration
9-Thrust bending force tries to bend the tip of the propeller blade?forward.
10-Torque bending force tries to bend the propeller blades in the ?opposite direction of their rotation.
11-Aerodynamic twisting moment tries to move the propeller blades toward a ? high pitch angle.
12-Centrifugal twisting moment tries to move the propeller blades toward a ? low pitch angle.
13-Normally aerodynamic twisting moment is greater than centrifugal twisting moment? False
14-Because of vibration problems encountered with certain engine propeller combinations the critical range is indicated on the tachometer by a ? red arc.
15-The geometric pitch is the distance in inches that a propeller would move forward in a medium that allowed no slip.
16-The effective pitch is the distance in inches that the aircraft actually moves forward in one revolution of the propeller.
17-The difference between geometric and effective pitch is called? Slip
18-Tractor propellers are those mounted on the upstream end of a drive shaft in front of the supporting structure.
19-Automatic propellers are usally termed?constant speed propellers.
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