Propeller Thrust

Propeller Thrust


This post is about how a propeller Works, from a Principles of Flight point of view.


What is a propeller?

A propeller is a device that transforms rotational power into linear thrust by acting upon a working fluid.

In principles of flight the propeller is considered an aerofoil, so we can apply all definitions of the wing to the propeller plus:

  • Plane of rotation: 

The Plane of Rotation is an imaginary plane perpendicular to the propeller shaft.



  • Blade chord line.

 is an imaginary straight line joining the centre of curvature of the leading edge of the propeller blade to the blade’s trailing edge.




  • Blade Angle or blade Pitch.

The Blade Angle or Blade Pitch is the angle between the blade chord line and the plane of rotation.


  • Blade Angle of Attack

The Blade Angle of Attack is the angle between the chord line of any given blade element and the relative airflow.





  • Geometric Pitch

The Geometric Pitch is the distance the propeller would travel forward in one complete revolution.

  • Effective pitch

The distance that the propeller actually moves forward with one revolution

  • Propeller Slip

The difference between Geometric Pitch and Effective Pitch is called Propeller Slip.





  • Helix Angle

is the angle between the Plane of Rotation of the propeller and the path of the Effective Pitch.




What is thrust and how is created ?


Thrust is force in the direction of the movement created by the engine in concrete by the propeller.

We must consider the propeller as an aerofoil, so the lift and drag are actuation to it.

The concept of how the lift force is created, we can apply it in this case.

Notice that the incoming air to the propeller is slower that the outcome air resulting in creating a force backwards. Due to the third Newton´s law it results in a forwards movement.

This movement forwards is called Thrust. Depends on the quantity of air we are able to accelerate, and the acceleration of the air.

T= G (Ve- Vo)

  • T: Thrust
  • G: Air mass flow  ( mass/time)
  • Ve: Exit velocity of air
  • Vo: Entry velocity of air

As we have created the “lift”, also appears the “Drag”. This “lift” we have called it Thrust, and this “Drag” we will call it Torque. Which is always opposite to the movement direction.




How does it work ?


While the propeller is working we can encounter a velocity triangle, where the TAS is true air speed, RPM it is a circular speed.



Depending of the angle of attack, the Torque (Drag) will be modified so the efficiency of the propeller will change, as the thrust will change.

The efficiency it is measured with this formula:

Propeller Efficiency= Thrust Power/Shaft Power




Types of propeller:

We have two types of propeller, depending on the blade angle. Fix pitch propeller and variable pitch propeller.


Fix pitch propeller Consideration:

If  the True Air Speed increases, with constant RPM. The angle of attack is reduced, so the torque is reducing as well. It can happen in a descend with a constant RPM



If we increase the RPM maintaining constant the TAS the angle of attack increases so the Torque increases. This situation can happen in a constant TAS climbing.





This problematic it is solved with the variable pitch propeller, it can adjust the angle of attack in each phase of the flight.


Why The Propeller is twisted ?

Every propeller best efficiency is only at one combination of aircraft forward speed, and propeller rotational speed. The RPM of the root of the propeller is higher than the tip of the propeller, that´s why we have to modify the angle of attack for having the best efficiency.




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