CONTROLLABLE AND FIXED PITCH PROPRLLER

BY,AJAY G623 B

Consists of two or more blades and a central hub to which the blades are attached.

Propeller blades produce forces that create thrust to pull or push the airplane through the air.

Propeller Principles

CONTROLLABLE PITCH PROPELLER FIXED PITCH PROPELLER SCREW PROPELLER SELF PITCHING PROPELLER SURFACE PIERCING PROPELLER SHROUDED OR DUCTED PROPELLER PUMP JETS CONTRA-ROTATING PROPELLER AZIMUTHING PROPELLER

TYPES OF PROPELLER

VERTICAL AXIS PROPELLER WATER JET PROPULSION PADDLE WHEELS WIND

A controllable pitch propeller (CPP) or variable pitch propeller is a special type of propeller with blades that can be rotated around their long axis to change their pitch. If the pitch can be set to negative values, the reversible propeller can also create reverse thrust for braking or going backwards without the need of changing the direction of shaft revolutions.

CONTROLLABLE PITCH PROPELLER

Machinery must develop enough torque to turn the propeller at the revolutions appropriate to the power being developed or the machinery will lock up.

The matching is not possible with fixed blades and some ships are fitted with propellers in which the blades can be rotated about axis normal to drive shaft its termed as controllable pitch propellers(CPP’s)

CONTROLLABLE PITCH PROPELLER

The pitch can be altered to satisfy a range of operating condition which is useful in tugs and trawlers

For such ships there is a great difference in the propeller loading when towing or trawling and when running free

The machinery can be run at constant speed so that full power can be developed over range of operating condition

The pitch of blades is changed by gear fitted in the hub and controlled by linkages passing down the shaft. Thus the CPP has a larger boss than usual which limits the blade ratio to about 0.8 which affects cavitations performance.

It also mechanically fairly complex which limits the total power that can be transmitted.

By reversing the pitch an astern thrust can be produced thus eliminating the need for a reversing gear box. variation in thrust for manoeuvring can be more rapid as it involves changing blade angle rather than shaft revolutions but for maximum acceleration or deceleration there will be an optimum rate of change of blade angle.

The Fixed-Pitch PropellerIn this type of propeller the blades are mounted directly onto the hub in a position determined and "fixed" by the manufacturer There are two types of fixed-pitch propellers; the climb propeller and the cruise propeller. 

The Climb Propeller -This propeller has a lower pitch, therefore less drag.  This results in the capability of higher rpm and more horsepower being developed by the engine.  it increases performance

FIXED PITCH PROPELLER

The Cruise Propeller -This propeller features a higher pitch, therefore more drag than that of the climb propeller.  This results in lower rpm and less horsepower capability.  This decreases performance but increases efficiency .

In FPP the power generated by the engine and the propulsive forces produced by the propeller cannot be controlled. This leads to high amount of power wastage and increased stresses on the propeller. But in a CPP all these can be prevented by just changing the pitch of the propeller. This function of CPP makes it an integral part of the propulsion system of a ship.

A striking aspect of a CPP is that the propeller rotates in only one direction, unlike FPP. Thus there is no need of a reverse clutch, which is an integral part of FPP for producing reverse thrust, in case the ship needs braking or reversing.

A FPP consists of fixed blades. This means that the position of blades cannot be changed. On the Contrary, CPP can move its blade in the desired position by changing the pitch of the blades.

The main disadvantage of CPP is that it is a highly complex system

It is vulnerable due to numerous hydraulic components and sealing rings. As the sealing rings are outside the ship, damage of a single sealing ring can result in oil pollution.

As the system is complex, repairs and maintenance is difficult.

Disadvantages of CPP

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