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  • Now the efficiency of a propeller is the ratio of the useful work is given out by the propeller to the work put into it by the engine.

  • Mechanical work done is measured by the force multiplied by the distance moved, and so when either the force or the distance is zero.
  • the useful work done is zero, and the efficiency nil. Thus when the propeller moves forward in each revolution a distance equal to the experimental pitch.
  •  the fact that there is no thrust means that there is no efficiency. Also, when there is no forward speed, there is no distance moved, no work done, and therefore no efficiency. Between these two extremes are the normal conditions of flight.
  • It might be thought that the object of the propeller is to give the maximum thrust (T) with the minimum torque (Q), i.e. to give the maximum T/Q ratio.
  • However, two things are required – a high value of L/D and the optimum helix angle, which is theoretically around 45 degrees.
  • The high value of L/D is fairly easy, and is an old problem; what is needed is a good aerofoil section, set at near the correct small angle to the relative airflow, and this means twisting the blade, as already explained.
  • Fixed pitch propeller (By courtesy of what was the Fairey Aviation Co Ltd) Two-blader with very large pitch angle, as used in the Schneider Trophy contest, 1931.
  • The provision of the optimum helix angle is more difficult, as this would require matching the rotational speed to the forward speed. In practice, this is impractical, and the propeller is normally run at near-constant speed, as described later.
  • In any case, the optimum helix angle can only be obtained at one position along the blade, since the blade is twisted.
  • However, the tip of the propeller is moving faster than the inboard sections and thus tends to produce a high proportion of the thrust, so it is the angle of the tip that is most important.
  • With fixed pitch propellers, a compromise on the pitch angle has to be made between high-efficiency cruising, and high thrust for take-off.
  • Under conditions of maximum efficiency, the advance per revolution is usually considerably less than the experimental pitch.
  •  The experimental pitch is sometimes called the ideal pitch, while the advance per revolution is the actual practical pitch. The difference between the two is called the slip, and is usually expressed as a percentage.

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