Aircraft Propellers


Introduction :

A machine is something which takes sufficient and specific input and gives the required output to reduce human labours and problems. Aircraft propellers are also a type of machine which take rotary motion (rotational energy as an input to give a sufficient thrust as an Output). This rotational energy comes from Engine power of aircraft, so we can also say that propeller converts Engine power into powerful required thrust. Aircraft propellers have atleat two blades. Propellers can have any number of blades greater than two. Only one blade in revolving position doesn't let the centre of mass fixed at a point ie center of mass of propellers should belong to the axis of rotation in revolving position hence Aircraft propellers should have atleast Two blades.

History :

wooden Propelers

Archimedes Screw
The idea of Aircraft propellers came from Archmedes screw. In earlier days this screw was used to draw water from lakes and rivers, it is said that afterwards in 1903 Write Brothers constructed first Aircraft propeller and successfully flew the aeromodel, but it is also true that An Indian Scholar Shivkar Bapuji Talpade from Maharashtra, India constructed the first wooden propeller in 1895 and successfully flew the aeroplane prototype. The name of his plane was Marutsakha and it was Unmanned. Now in modern era we use highly advanced Aircraft Propellers. 

Types Of Aircraft Propellers :

On the basis of their adjustment Feature Propellers are classified into three types :
  1.  Fixed Pitch Propellers :
  2.  Ground Adjustible Propellers :
  3. Controllable Pitch Propellers :

Fixed Pitch Propellers : 

Fix pitch Propellers

Fixed pitch propellers are the propellers in which blade angle
 cannot be changed, Blade angle is the angle made by blades
 of aircraft propellers with X Axis. We will come to know 
more about this in construction and working section. 
In ancient times only wooden Fix pitch propeller were in use, 
Fix pitch propellers are Less efficient and have some
disadvantages. Generally Fix pitch propellers were of 
wood but nowadays their prototypes of metal plastic 
fibre etc are available in the market.



Ground Adjustable Propellers :
 Ground adjustible propellers are the propellers in which blade angle of blades can be changed but only when the airplane is on ground and at rest hence the name is given GROUND ADJUSTIBLE propellers. By passing time from fix pitch to ground adjustable made a huge revolution in propellers history, as due to the consideration that change in the blade angle the Efficiency of the propellers can be changed. But with passing time it is also proved that at a certain height certain blade angle can give max efficiency. So Not upto the mark but definitely more than fix pitch propellers Ground adjustible propellers were preferable and efficient.

Controllable Pitch Propellers : 
Controllable Pitch Propellers
It is the kind of propeller in which Blade angle of blades can be c It is the kind of propeller in which Blade angle of blades can be  hanged at any time at any height. This type of aircraft propellers deliver maximum efficiency. In the modern aircrafts Controllable pitch propellers are used. Atmospheric pressure affects the thrust produced by the propellers and rpm of propellers and atmospheric pressure varies with height so feature of changing blade angle at anytime at any position makes this type most efficient.


Construction and Working

There are some parameters which will be considered while studying construction and basic principle to understand :

   
fig A
  • Chord line : It is the imaginary line passing through the blade from its leading edge to its trailing edge. (refer to fig A)
  • Blade/Pitch Angle : It is the angle made by chord line with the plane of rotation. 
  • Angle of Attack : It is the angle between relative direction of wind and chord line.


Aerofoil Cross Section
  •       Airfoil : It is the cross-sectional shape of a wing, blade or propeller. It is such a cross section which produces aerodynamic force when respective propeller rotates. Airfoil shape is shown in figure


Geometric and actual pitch and sleep distance
  •  Geometric pitch : It is the distance which is to be travelled by the aircraft in per revolution of propellers. Due to slippery friction it is not possible to cover distance equal to geometric pitch.
  • Actual pitch : it is the which is practically travelled by aircraft propeller in one revolution
  • Slip Distance : it is the distance between the Geometric pitch and Actual pitch. Slip distance is the result of slippery friction of Propeller blades in air.

Working Of Propellers :
When the aircraft is at rest on the ground with the engine operating, or moving slowly at the beginning of takeoff, the propeller efficiency is very low because the propeller is restrained from advancing with sufficient speed to permit its fixed-pitch blades to reach their full efficiency. In this situation, each propeller blade is turning through the air at an AOA that produces relatively little thrust for the amount of power required to turn it. The air deflection produced by this angle of attack causes the dynamic pressure at the engine side of the propeller blade to be greater than atmospheric pressure, thus creating thrust. To maintain the maximum efficiency when the aircraft is at ground level the blade angle is kept lower which sets AOA is low at ground level so that aircraft will take minimum amount of energy and power to set minimum rpm speed required to create minimum amount of thrust which will lift the aircraft upward and forward. When the aircraft reaches at the cruising altitude aircraft doesn’t require high power and high rpm to produce more thrust and due to its state it gets additional torque so to keep higher efficiency again pilot reduces the blade angle which results in and reduction in rpm and hence engine in aircraft consumes lower power. When blade angle is reduced at cruising height AOA remains still low as it depends upon speed of aircraft. When propeller is in the Rotational motion it is having maximum velocity at its tips due to high radius and hence maximum centrifugal force at tips. Refer following figure :
speed of propellers
For the efficient working the lower AOA is needed which varies from 20 to 40 . In constant speed propellers to adjust the lower AOA blade angle varies with respect to the speed of propeller. Basically a modern controllable pitch propeller gives efficiency from 50% to 80% depending upon how much the propeller slips due to slippery friction. Slip of the propeller is measured in distance which can also called slip distance\


What We Learnt :
after going through this post we have become aware with :
  • What are Aircraft Propellers?
  •  Aim /need of Aircraft propellers
  • Classifications 
  • Basic construction
  • Working principle
  • Actual Working 
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published by -
Shreyas Sanjay Chaudhari.