lecture 1 amt 113

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Lecture 1

INTRODUCTION TO PROPELLERS

En r. Ronald P. Soriano

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What is an Aircraft Propeller?

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What is an Aircraft Propeller?


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Definition and Theory


A propeller is part of the propulsion systemof an airplane.

It consists of two or more blades connectedtogether (by a hub) that convertaerodynamic pressure into propulsive thrustfor the airplane.

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Definition and Theory


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Propeller Theory

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Parts of an Aircraft Propeller


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Leading Edge - ofthe airfoil is thecutting edge thatslices into the air.As the leading

edge cuts the air,air flows over theblade face and thecamber side.


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Blade Face is the surface of the propeller bladethat corresponds to the lower surface of an airfoilor flat side.


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Blade Back / Thrust Face is the curved surfaceof the airfoil.


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Blade Shank (Root) is

the section of the bladenearest the hub.

Blade Tip is the outer endof the blade farthest from

the hub.

Plane of Rotation is animaginary planeperpendicular to the

shaft. It is the plane thatcontains the circle inwhich the blades rotate.


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Blade Angle is formed between

the face of an element and theplane of rotation. The bladeangle throughout the length ofthe blade is not the same. Thereason for placing the blade

element sections at differentangles is because the varioussections of the blade travel atdifferent speed. Each elementmust be designed as part of theblade to operate at its own bestangle of attack to create thrustwhen revolving at its best designspeed.


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Blade Element are theairfoil sections joined sideby side to form the bladeairfoil.

Placed at different anglesin rotation of the plane ofrotation because thevarious sections of theblade travel at different

speeds.


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The inner part of the bladesection travels slower thanthe outer part near the tipof the blade.

If all the elements along a

blade is at the same bladeangle, the relative wind willnot strike the elements atthe same angle of attack.This is because of the

different in velocity of theblade element due todistance from the center ofrotation.


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Thrust is produced by thepropeller attached to theengine driveshaft. Whilethe propeller is rotatingin flight, each section of

the blade has a motionthat combines theforward motion of theaircraft with circular

movement of thepropeller.


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The slower the speed,the steeper the angle ofattack must be togenerate lift. Therefore,the shape of the

propeller's airfoil (crosssection) must changefrom the center to thetips. The changing shape

of the airfoil (crosssection) across the bladeresults in the twistingshape of the propeller.


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Relative Wind is the airthat strikes and pass overthe airfoil as the airfoil isdriven through the air.

Angle of Attackis theangle between the chord ofthe element and the relativewind. The best efficiency ofthe propeller is obtained at

an angle of attack around 2to 4 degrees.


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Blade Path is the path of the direction of the bladeelement moves. Relative Wind is the air that strikesand pass over.


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Pitch refers to the distance a spiral threaded objectmoves forward in one revolution. As a wood screwmoves forward when turned in wood, same with the

propeller move forward when turn in the air.


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Geometric Pitch is the theoretical distance a

propeller would advance in one revolution.

Effective Pitch is the actual distance a propelleradvances in one revolution in the air. The effectivepitch is always shorter than geometric pitch due to

the air is a fluid and always slip.


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Forces Acting on a Propeller in Flight

1. Thrust is the air force on the propeller which isparallel to the direction of advance and inducebending stress in the propeller.

2. Centrifugal force is caused by rotation of the

propeller and tends to throw the blade out fromthe 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.


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Stresses Acting on a Propeller in Flight

1. Bending stresses are induced by the trustforces. These stresses tend to bend the bladeforward as the airplane is moved through the airby the propeller.

2. Tensile stresses are caused by centrifugalforce.

3. Torsion stresses are produced in rotatingpropeller blades by two twisting moments. one of

these stresses is caused by the air reaction on theblades and is called the aerodynamic twisting moment.The another stress is caused by centrifugal force andis called the centrifugal twisting moment.


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Stresses Acting on a Propeller in Flight


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Types of Aircraft Propellers

1. Fixed pitch:The propeller is made in one

piece. Only one pitch setting is possible and isusually two blades propeller and is often madeof wood or metal.

a. Wooden Propellers - were used almostexclusively on personal and business aircraft prior toWorld War II .

-is not cut from a solid block but is built up of a

number of separate layers of carefully selected-most commonly used wood materials are yellowbirch, sugar mable, black cherry, and black walnut


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Wood lamination reduces the tendency for the

propeller to warp. For standard one-piece woodpropellers, from five to nine separate woodlaminations about 3/4 in. thick are used.


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b. Metal Propellers Solid steel

propellers were used for military in the1940s.

- Modern propellers are made fromhigh-strength , heat-treated aluminumalloy by forging a single bar of

aluminum alloy to the required shape.

-Metal propellers are now extensivelyused in the aircraft industry.

-The general appearance of the metal

propeller is similar to the woodpropeller, except that the sections aregenerally thinner.


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2. Ground adjustable pitch- Pitch setting can beadjusted only with tools on the ground before theengine is running.

- has a split hub

- blade angle is specified by the aircraftspecifications


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-permits compensation for the location of the flying

field at various altitudes and for variations in thecharacteristics of airplanes using the same engine

-blade angles are set by loosening the clamps andthe blade is rotated to the desired angle and then

tightening the clamps


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3. Two-position- A propeller which can have its

pitch changed from one position to one other angleby the pilot while in flight.

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4. Controllable pitch -The pilot can change the pitch

of the propeller in flight or while operating the engineby means of a pitch-changing mechanism that may beoperated hydraulically.

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5. Constant speed- utilizes a hydraulically orelectrically operated pitch-changing mechanism which iscontrolled by governor.

-The setting of the governor is adjusted by the pilotwith the rpm lever in the cockpit. During operation, the

constant speed propeller will automatically change itsblade angle to maintain a constant engine speed. Ifengine power is increased, the blade angle is increasedto make the propeller absorb the additional power whilethe rpm remains constant.

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-At the other position, if the engine power isdecreased, the blade angle will decrease to makethe blades take less bite of air to keep engine rpmremain constant. The pilot select the engine speedrequired for any particular type of operation.

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f f

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6.Full Feathering - A constant speed propeller whichhas the ability to turn edge to the wind and therebyeliminate drag and windmilling in the event of enginefailure.

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-The term Feathering refers to the operation of rotating

the blades of the propeller to the wind position for thepurpose of stopping the rotation of the propeller toreduce 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 whenit is desirable to shutoff an engine in flight.

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7. Reversing- A constant speed propeller which has theability to assume a negative blade angle and produce areversing thrust.

-When propellers are reversed, their blades are rotatedbelow their positive angle , that is, through flat pitch,

until a negative blade angle is obtained in order toproduce a thrust acting in the opposite direction to theforward thrust .

-Reverse propeller thrust is used where a large aircraft

is landed, in reducing the length of landing run.

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

lecture 1 amt 113

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