Aircraft Propeller

7/30/2019 Aircraft Propeller

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Background

Aerodynamics (25%)

Helicopter Aerodynamics

a.Aerodynamic Forces, Moments, Torque

b.Flight Performance, Stability and Control


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Outline

Introduction

Definitions

Classification Principles

Systems and Operations

Sample Questions


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History

In the latter part of the 1830s the Swedish-

American engineer John Ericsson and the

English inventor Sir Francis P. Smith

independently patented screw propellers.


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Definitions

Propeller

is a device for propelling an aircraft through the air

having blades mounted on a power-driven shaft which

when rotated produces by its action on the air a thrustapproximately parallel to the longitudinal axis of the

aircraft and shall also include a system of rotating

airfoils which serve either to counteract the effect of

the main rotor torque of a rotorcraft or to maneuver a

rotorcraft about one or more of its three principalaxes.


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Definitions

Airfoil

Any surface designed to produce lift or thrust

when air passes over it

Chordline

Imaginary straight line from the leading edge

to the trailing edge.


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Definitions


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Definitions

Blade Face / Flat side

is the surface of the propeller blade that

corresponds to the lower surface of an airfoil.

Thrust Face / Cambered side

is the curved surface of the airfoil.


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Definitions


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Definitions

Leading Edge is the cutting edge that slices into the air. As the

leading edge cuts the air, air flows over the blade face

and the camber side. Blade Shank (Root)

is the section of the blade nearest the hub.

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


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Definitions

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 andthe plane of rotation. The blade anglethroughout the length of the blade is not thesame.


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Definitions

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.


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Definitions

Geometric Pitch is the theoretical 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 theair is a fluid and always slip.


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Definitions


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Definitions


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Definitions

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 isobtained at an angle of attack around 2 to 4 degrees.

Blade Path

is the path of the direction of the blade elementmoves.


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Definitions


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Definitions


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Definitions


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Definitions

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 theplane of rotation.


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Classification

According to installation

Tractor type and Pusher type

According to pitch

Fixed pitch, ground adjustable, controllable

pitch and automatic


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Classification

Tractor Type

Tractor propellers are those mounted on the

upstream end of a drive shaft in front of the

supporting structure. Most aircraft areequipped with this type of propeller. A major

advantage of the tractor propeller is that lower

stresses are included in the propeller as it

rotates in relatively undisturbed air.


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Classification

Pusher Type

Pusher propellers are those mounted on the

downstream end of a drive shaft behind the

supporting structure. Pusher propellers areconstructed as fixed- or variable-pitch

propellers. Seaplanes and amphibious aircraft

have used a greater percentage of pusher

propellers than other kinds of aircraft.


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Classification

Fixed-pitch Type

a fixed-pitch propeller has the blade pitch, or

blade angle, built into the propeller. The blade

angle cannot be changed after the propeller isbuilt.


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Classification

Ground-adjustable Type The ground-adjustable propeller operates as a fixed-

pitch propeller. The pitch or blade angle can bechanged only when the propeller is not turning. This is

done by loosening the clamping mechanism whichholds the blades in place. After the clampingmechanism has been tightened, the pitch of theblades cannot be changed in flight to meet variableflight requirements. Like the fixed-pitch propeller, the

ground-adjustable propeller is used on airplanes oflow power, speed, range, or altitude.


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Classification

Ground-adjustable Type


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Classification

Controllable-pitch Type

The controllable-pitch propeller permits a change of

blade pitch, or angle, while the propeller is rotating.

This permits the propeller to assume a blade angle

that will give the best performance for particular flight

conditions. The number of pitch position may be

limited, as with a two-position controllable propeller;

or the pitch may be adjusted to any angle between

the minimum and maximum pitch settings of givenpropeller.


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Classification

Controllable-pitch Type


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Classification


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Classification

Automatic Type In automatic propeller system, the Control system

adjusts pitch, without attention by the operator, tomaintain a specific preset engine rpm. For example, if

engine speed increases, the controls automaticallyincrease the blade angle until desired rpm has beenre-established. A good automatic control system willrespond to such small variations of rpm that, for allpractical purposes, a constant rpm will be maintained.

Automatic propellers are often termed constantspeed propellers.


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Principles

Dryewiecki Theory

The first satisfactory theory for the design of

aircraft propellers was known as the blade-

element theory. A Polish scientist namedDryewiecki developed this theory in 1909;

hence, it is sometimes referred to as the

Dryewiecki Theory.


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Principles

This theory assumes that the propeller blade

from the end of the hub barrel to the tip of the

propeller blade is divided into various small,

rudimentary airfoil sections. For example, if a propeller 10 ft in diameter

has a hub 12 in. in diameter, then each blade

is 54 in. long and can be divided into fifty-four

1-in. airfoil sections


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Principles

According to the blade-element theory, the

many airfoil sections, or elements, being

joined together side by side, unite to form an

airfoil, (the blade) that can create thrust whenrevolving in a plane about a central axis. 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 itsbest design speed.


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Principles


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Principles

The thrust developed by a propeller is in accordance withNewtons third law of motion: For every action, there is an equaland opposite reaction, and the two are directed along the samestraight line. In the case of a propeller, the first action is theacceleration of a mass of air to the rear of the airplane. This

means that, if the propeller is exerting a force of 200 lb. toaccelerate a given mass of air, it is, at the same time exerting aforce of 200 lb. tending to pull the airplane in the directionopposite that in which the air is accelerated. That is, when the airis accelerated rearward, the airplane is pulled forward. Thequantitative relationships among mass, acceleration, and force

can be determined by the use of the formula for NewtonsSecond Law: F = ma, or force is equal to the product of massand acceleration.


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Forces

Lift

Differential pressure on the upper camber and

lower camber

Drag

Force that resist movement of the airfoil

through the air


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Forces

Thrust

is the air force on the propeller which is

parallel to the direction of advance and induce

bending stress in the propeller.

Centrifugal force

is caused by rotation of the propeller and

tends to throw the blade out from the center.


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Forces

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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Forces


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Stresses Bending stresses are induced by the thrust

forces. These stresses tend to bend the bladeforward as the airplane is moved through the air

by the propeller.

Tensile stresses are caused by centrifugal

force.

Torsion stresses are produced in rotating

propeller blades by two twisting moments. One

of these stresses is caused by the air reactionon the blades and is called the aerodynamic

twisting moment. Another stress is caused by

centrifugal force and is called the centrifugal

twisting moment


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Stresses


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Operation To understand the action of a propeller, consider

first its motion, which is both rotational andforward. Thus, shown by the vectors of propellerforces in figure 1H, a section of a propeller blademoves downward and forward. As far as the

forces are concerned, the result was the sameas if the blade were stationary and the air werecoming at it from a direction opposite its path.The angle at which this air (relative wind) strikesthe propeller blade is called angle of attack. Theair deflection produced by this angle causes thedynamic pressure at the engine side of thepropeller blade to be greater than atmospheric,thus creating thrust.


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Operation

Propeller Feathering The term feathering refers to the operation of

rotating the blades of a propeller to an edge-

to-the-wind position for the purpose ofstopping the rotation of the propeller whoseblades are thus feathered and to reduce drag.Therefore, a feathered blade is in an

approximate in-line-of-flight position,streamlined with the line of flight. Some, butnot all, propellers can be feathered.


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Operation


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Operation

Reverse Thrust When propellers are reversed, their blades

are rotated below their positive angle, that is,

through flat pitch, until a negative bladeangle is obtained in order to produce a thrust

acting in the opposite direction to the forward

thrust normally given by the propeller.


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Operation

Propeller Governor The propeller governor is geared to the engine in order to sense

the rpm of the engine at all times. The speed sensing isaccomplished by means of rotating flyweights in the upper partof the governor body. As shown in the following diagrams, the

flyweights are L-shaped and hinged at the outside where theyattach to the flyweight head. The speed adjustment control leveris attached to the propeller control in the cockpit. As the speedadjusting lever is moved, it rotates the adjusting worm andincreases or decreases the compression of the speeder spring.This affects the amount of flyweight force necessary to move the

pilot valve plunger.


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Operation


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Operation

To increase the rpm of the engine, the

speed adjusting control lever is rotated in

the proper direction in the cockpit so as to

increase speeder spring compression. Itis therefore necessary that the engine rpm

increase in order to apply the additional

flyweight force to raise the pilot valveplunger to an on speed condition.


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Operation


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Operation


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Operation

When the governor is in the overspeed

condition, the engine rpm is greater than that

selected by the control, and the flyweights are

pressing outward. The toes of the flyweightshave raised the pilot valve plunger to a position,

which permits oil pressure from the propeller to

return to the engine. The propeller

counterweights and feathering spring can thenrotate the propeller blades to a higher angle,

thus causing the engine rpm to decrease.


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Operation


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Operation


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Operation

When the governor is in an underspeed

condition, that is, with engine rpm below the

selected value, the governor flyweights are held

inward by the speeder spring and the pilot valveplunger is in the down position. This position of

the valve directs governor oil pressure from the

governor gear pump to the propeller cylinder

and causes the propeller blades to rotate to alower pitch angle. The lower pitch angle allows

the engine rpm to increase.


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Operation


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Operation


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Operation


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Helicopters

Main Parts

Classification

Systems

Technical Terms


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Main Parts

Cabin

Airframe

Landing Gears

Powerplant

Transmission

Main rotor system Tail rotor system


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Main Parts of a Helicopter


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Classification

Single main rotor system

Dual rotor system


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Classification


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Rotor Systems

Fully articulated rotor system

Semi rigid rotor system

Rigid rotor system


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Antitorque systems

Antitorque systems

Tail rotor system

Fenestron system

Notar system


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Antitorque systems


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Antitorque systems


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Flight controls system


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Technical Terms

Autorotation

The condition of flight during which the main

rotor is driven only by aerodynamic forces

with no power from the engine.


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Technical Terms


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Technical Terms

Blade Flap

The ability of the rotor blade to move in a

vertical direction. Blades may flap

independently or in unison.


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Technical Terms

Blade lead or lag

The fore and aft movement of the blade in the

plane of rotation. It is sometimes called

hunting or dragging.


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Technical Terms

Blade loading

The load imposed on rotor blades, determined

by dividing the total weight of the helicopter by

the combined area of all the rotor blades.


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Technical Terms

Centrifugal force

The apparent force that an object moving

along a circular path exerts on the body

constraining the object and that actsoutwardly away from the center of rotation


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Technical Terms

Centripetal force

The force that attracts a body toward its axis

of rotation. It is opposite centrifugal force.


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Technical Terms

Collective pitch control

The control for changing the pitch of all the

rotor blades in the main rotor system equally

and simultaneously and, consequently, theamount of lift or thrust being generated.


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Technical Terms


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Technical Terms

Coriolis effect

The tendency of a rotor blade to increase or

decrease its velocity in its plane of rotation

when the center of mass moves closer orfurther from the axis of rotation


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Technical Terms

T h i l T


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Technical Terms

Cyclic pitch control

The control for changing the pitch of each

rotor blade individually as it rotates through

one cycle to govern the tilt of the rotor discand, consequently, the direction and velocity

of horizontal movement.

T h i l T


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Technical Terms

T h i l T


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Technical Terms

Dissymmetry of lift

The unequal lift across the rotor disc resulting

from the difference in the velocity of air over

the advancing blade half and retreating bladehalf of the rotor disc area

T h i l T


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Technical Terms

T h i l T


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Technical Terms

Feathering

The action that changes the pitch angle of the

rotor blades by rotating them around their

feathering (spanwise) axis.

T h i l T


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Technical Terms

Flare

A maneuver accomplished prior to landing to

slow down a rotorcraft.

T h i l T


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Technical Terms

Ground effect

A usually beneficial influence on rotorcraft

performance that occurs while flying close to

the ground. It results from a reduction in upwash,

downwash, and blade tip vortices, which

provide a corresponding decrease in induced

drag.

T h i l T


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Technical Terms

T h i l T


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Technical Terms

Gyroscopic precession

An inherent quality of rotating bodies, which

causes an applied force to be manifested 90

degrees in the direction of rotation from thepoint where the force is applied.

T h i l T


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Technical Terms

T h i l T


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Technical Terms

Load factor

The ratio of a specified load to the total weight

of the aircraft.

T h i l T


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Technical Terms

Pendular action

The lateral or longitudinal oscillation of the

fuselage due to it being suspended from the

rotor system.

Technical Terms


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Technical Terms

Technical Terms


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Technical Terms

Rotor

A complete system of rotating airfoils creating

lift for a helicopter or gyroplane.

Technical Terms


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Technical Terms

Torque In helicopters with a single, main rotor

system, the tendency of the helicopter to turn

in the opposite direction of the main rotorrotation.

Technical Terms


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Technical Terms

Translating tendency The tendency of the single-rotor helicopter to

move laterally during hovering flight. Also

called tail rotor drift.

Technical Terms


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Technical Terms

Technical Terms


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Technical Terms

Translational lift The additional lift obtained when entering

forward flight, due to the increased efficiency

of the rotor system.

Technical Terms


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Technical Terms

Technical Terms


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Technical Terms

Transverse-flow effectA condition of increased drag and decreased

lift in the aft portion of the rotor disc caused by

the air having a greater induced velocity andangle in the aft portion of the disc.


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Sample Questions

Sample Questions


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Sample Questions

1. When the propeller is moved rapidlythrough the air, a mass of air will be

accelerated rearwards. The propeller is

exerting a force which is equal to themass of the air times the acceleration.

This is according to

a. Newton's 2nd Lawb. Dryewiecki Theory

c. Newton's 3rd Law

Sample Questions


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Sample Questions

2. As a result of the action of the propeller,there will be an equal but opposite

reaction called thrust which will tend to pull

the airplane forward. This is according to

a. Newton's 2nd Law

b. Dryewiecki Theoryc. Newton's 3rd Law

Sample Questions


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Sample Questions

3. The working theory behind the design ofan efficient propeller

a. Aerodynamics

b. Dryewiecki Theory

c. Otto Cycle

Sample Questions


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Sample Questions

4. The angle formed between the plane ofrotation and the chord of the blade

a. angle of attack

b. blade angle

c. angle of incidence

Sample Questions


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Sample Questions

5. The power supplied by the engine (RE) tothe propeller

a. Brake Hp

b. Indicated Hp

c. Thrust Hp

Sample Questions


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Sample Questions

6. The symbol for propeller efficiency is theGreek letter (eta). It is an expression of

the ratio between...

a. propeller and airplane speedsb. airplane speed and engine rpm

c. thrust hp and brake hp

Sample Questions


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Sample Questions

7. Part of the propeller system to which thepropeller blades are attached.

a. blade shank

b. blade buttc. hub

Sample Questions


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Sample Questions

8. For a fixed-pitch propeller and a givenrotational speed, as the airplane velocity

increases, the angle of attack of the

relative wind...a. increases

b. does not change

c. decreases

Sample Questions


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Sample Questions

9. For a variable-pitch propeller and a givenrotational speed, with no forward aircraft

movement, the angle of attack of the

relative wind...a. is less than the blade angle

b. is equal to the blade angle.

c. is greater than the blade angle.

Sample Questions


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Sample Questions

10. In an efficiently designed propeller, theblade angle of the propeller section

___________, as the distance from the

axis of rotation decreases.a. Increases

b. is constant

c. decreases

Sample Questions


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Sample Questions

11. The distance that a propeller bladeelement would advance in one revolution

if it were moving through a solid medium,

that is, no slip.a. Pitch

b. geometric pitch

c. effective pitch

Sample Questions


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Sample Questions

12. The actual distance the airplane movesforward in flight in one revolution of the

propeller.

a. pitchb. geometric pitch

c. effective pitch

Sample Questions


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Sample Questions

13. The force that tends to pull the bladesaway from the hub.

a. aerodynamic force

b. thrust

c. centrifugal force

Sample Questions


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Sample Questions

14. Type of propeller which can only beadjusted when the aircraft is on the ground

and engine is not running.

a. fixed-pitch propellerb. controllable-pitch propeller

c. ground-adjustable propeller

Sample Questions


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Sample Questions

15. Type of propeller mounted forward of thesupporting structure.

a. fixed pitch

b. pusher typec. tractor type

Sample Questions


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Sample Questions

16. During take-off, controllable-pitchpropellers are set at _________ to enable

the engine to run at maximum rpm.

a. high blade angleb. low blade angle

c. no specific setting

Sample Questions


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Sample Questions

17. In the constant-speed propeller, whenthe governor is in "on-speed" condition,

the flyweight force is ____________ the

speeder spring force.a. greater than

b. equal to

c. lesser than

Sample Questions


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Sample Questions

18. In a constant-speed-propeller-equippedairplane, moving the throttle lever from low

power to high power setting will

momentarily cause an...a. overspeed condition.

b. underspeed condition.

c. on-speed condition.

Sample Questions


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Sample Questions

19. The operation wherein the propellerblade is turned to an "edge to the wind"

position.

a. featheringb. throttling

c. thrust reversing

Sample Questions


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Sample Questions

20. During landing, in order to help theaircraft come to a stop sooner, reversethrust is employed by...

a. turning the propeller blade to a"negative angle

b. turning the propeller blade to a "highangle

c. propellers are not capable of reversethrust


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