Contrarotating propeller

A contrarotating propeller or propeller with counter-rotation indicates a particular case of Hélice used on certain planes and ships.
It includes/understands two propellers placed one behind the other in the axis, and turning in contrary direction.

Aeronautics

Principle

The contrarotating propellers are by couples or doublets. They are two coaxial propellers, driven by the same engine, or two different engines (Bristol-board " Brabazon"), and one turns right and the other on the left.

They are intended to transmit great powers engine without the tips of blades not reaching the Speed of sound (in the case of planes), which would degrade the output of the blades considerably.

This speed of sound in blade tip can be reached because of a large diameter or an high speed of rotation.

Favors - Disadvantages

  • Possibility of passing more a great power with the same diameter of propeller.
  • Suppression of the Torque reaction of the plane around its axis of rolling.
This couple poses problem, especially on takeoff, for very powerful propeller single-engined aircrafts.
  • Improvement of the propelling output, the helicoid flow of the first propeller being rectified by the second.

  • the major disadvantage of the contrarotating propellers is the complexity of the transmission:

reducing, trees, gear-boxes; the cost, the weight and the brittleness of the unit.

This formula was the song of the swan of the propulsion by piston engine but of the planes with Turbopropulseur S still use them for their excellent output (Tupolev You-95 " Bear" , 4 times 15.000 cv, propellers diameter 5.60 m).

Contrarotating propeller planes

  • Using the driving Griffon 85 of 2 400 ch: Spitfire 22

  • Using an engine (Centaurus Bristol-board) for each propeller: Bristol-board Brabazon

  • Using the turbopropeller Kouznetsov NK-12:

Antonov Year-22
Tupolev You-95
Tupolev You-114

  • Using the turbopropeller Ivtchenko D-27 Progress:

Antonov Year-70

Maritime field

Applications

  • the contrarotating propellers (or CRP for Propeller Countered-Rotating) are mainly used on the Torpille S, although some tests were carried out on Navire S in the real face. The principle is to use two propellers assembled out of two coaxial trees; the propeller before is on the external tree and the back propeller on the interior tree. In general, the two propellers will have neither same the Diamètre nor the same number of revolutions.

  • They are sometimes assembled on powerful outboard motors for fast patrol boats (pleasure).

The advantages of the contrarotating propellers include the possibility of recovering part of energy of rotation lost by the front propeller, from where a better total effectiveness; the possibility of using a lower number of revolutions for the same diameter as with a traditional propeller (or same number of revolutions for a lower diameter); a weaker load by blade, which makes it possible to reduce the Cavitation and the associated noise. This explains the very current application on the torpedes, which need high speed. Moreover, these propellers make it possible to partly compensate for the moment of Roulis which would be induced by only one propeller, very extremely on a torpedo.

On a tradind ship on the other hand, the mechanical complications and the maintenance costs are too high to be able to adopt this system.

Bonds

Source

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