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Contra-rotating propellers are used on torpedoes due to the natural torque compensation and are also used in some motor boats. The cost of boring out the outer shafts and problems of mounting the inner shaft bearings are not worth pursuing in case of normal ships .
The torque on the aircraft from a pair of contra-rotating propellers effectively cancels out. Contra-rotating propellers have been found to be between 6% and 16% more efficient than normal propellers. [4] However they can be very noisy, with increases in noise in the axial (forward and aft) direction of up to 30 dB, and tangentially 10 dB. [4]
The configuration can also be used in helicopter designs termed coaxial rotors, where similar issues and principles of torque apply. Contra-rotating propellers should not be confused with counter-rotating propellers, a term which describes propellers rotating in opposite directions but sitting apart from each other on separate shafts instead of ...
Counter-rotating propellers are sometimes used on twin-engine and multi-engine aircraft with wing-mounted engines. These propellers turn in opposite directions from their counterpart on the other wing to balance out the torque and p-factor effects. They are sometimes referred to as "handed" propellers since there are left hand and right hand ...
These "pneumatic" propellers were fitted on the de Havilland DH.88 Comet aircraft, winner of the famed long-distance 1934 MacRobertson Air Race and in the Caudron C.460 winner of the 1936 National Air Races, flown by Michel Détroyat . Use of these pneumatic propellers required presetting the propeller to fine pitch prior to take-off.
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Turboprops have bypass ratios of 50–100, [9] [10] although the propulsion airflow is less clearly defined for propellers than for fans. [11] [12] The propeller is coupled to the turbine through a reduction gear that converts the high RPM/low torque output to low RPM/high torque. This can be of two primary designs, free-turbine and fixed.
Propellers can have a single blade, but in practice there is nearly always more than one so as to balance the forces involved. Archimedes' screw The origin of the screw propeller starts at least as early as Archimedes (c. 287 – c. 212 BC), who used a screw to lift water for irrigation and bailing boats, so famously that it became known as ...