Propulsion by Propellers

Table of Contents

Energy, Power and Force
An Engine
The Propeller
How a Propeller Works
Aerodynamic Characteristics of Propellers
Measuring the Geometry of Propellers
Equivalent Multiblade Propellers
Static Thrust of Propellers

Energy, Power and Force

Moving a vehicle, may it be on the ground, on the water, or in the air, requires a force to overcome the friction and inertia forces and to lift the vehicle to a different elevation. A force can be created from any kind of energy, like the energy contained in liquid fuel for an internal combustion engine, the electric energy stored in a battery or like solar energy being transformed by solar cells into electric power. To actually move an object, the force must be must be translated by some sort of engine into power, pushing the vehicle forward. Burning fuel in an open pan does not create a force and having a bottle of compressed air lying around creates a force on the bottles walls, but does not create any power output. Energy may be static (fuel) or dynamic (flywheel), force is static and power is always dynamic. Power equals force times distance per time.

Process of converting stored energy into power which can be used to move a craft.

An Engine

The lack of powerful, lightweight engines was one of the reasons for the failures of early attempts to fly. Using natural sources of energy like thermals requires quite sophisticated aircraft, which were of course not available in the beginning. When steam engines became available, they proved to be too heavy for flight, compressed air engines or twisted rubber bands could be used for smaller demonstrations, but were not practical or to heavy for man carrying airplanes. Finally the piston engine, burning fuel of high energy content internally, was to become the first successful power plant for aviation. From the early days of the Wright brothers until the 1950s this type of engine was the main source of aerial propulsion.



The Propeller

Having a power plant available is only one part of aircraft propulsion, we also need means to transform the power output of the engine into something useful. Piston engines usually drive a rotating shaft, which must be connected to a device which, literally "in turn", creates a force, finally advancing our vehicle.

The idea of placing an "inclined plane" on a rotating shaft is astonishingly old. It was probably first used to pump water and later in windmills.
Propellers for the propulsion of vehicles have been developed and successfully used on ships first (see timetable).

Propeller or Airscrew?

Propellers have also been called airscrew in the past, but this term may be misleading, because a propeller does not move like a mechanical screw through a rigid medium. You don't call a wing knife or slicer because it also does not slice through the air in the direction of its inclined mean line. Each section of a propeller (or of a wing) has a certain angle of incidence and is moving through the air at its unique angle of attack - both are independent. On the other hand, a mechanical screw or a knife moves through a rigid material exactly in the direction which is given by its pitch or angle of incidence - a screw with different pitches along its radius would get stuck, a propeller does not.


Where do you want to go now?
      Design a Prop      Optimum Propellers      How a Propeller works
      Equivalent Multiblade Propellers

Last update of this page: November 9th 1996

Last modification of this page: 08.09.03

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