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Contra-rotating propellers

 Contra-rotating propellers

  • The propeller is capable to absorb the power that given to it by the engine of aircraft and it has resisting torque to balance the engine torque, the both of those propeller and engine will become inefficient.
  •  It’s difficult to get efficiency in high speed because the propeller becomes a compromise and the climbing conditions are particularly difficult to satisfy since high power is being used at a low forward speed of aircraft.
  •  There some things can increase the ability of the propeller to absorb the power, by increasing the number of blades and the blade angle and thus the angle of attack of the blades and increase the camber of the aerofoil section of which the blade is made, and increase the chord of blades and the length of blades and thus the diameter of the propeller.
  •  There are many possibilities on might think this was an easy problem to solve, but in reality it is one that has caused difficulty, the blade angle must be such that the angle of attack is that giving maxi efficiency and we can lose efficiency of trying to absorb more power, the other possibility is to increase the diameter mean increase the blade aspect ratio but quite apart from the bogey of tip speed.
  • The other is high up speed and consequent loss of efficiency and  mean aerofoils would less efficient section and loss efficiency at high speed.
  •  The solidity of the propeller is that part of the propeller disc which when viewed from the front, this is solid part with air, the greater the solidity with the greater the power that can be absorbed, to increase solidity just by the increase of the number of blades and increase of chord,
  •  The latter is more efficient and the former is the easier and the socalled paddle blades are examples of the former method but is make the blades less efficient so an increase the number of blades is the most attractive proposition, five blades it becomes inconvenient to fit all the blades into one hub in effect is necessary to have two propeller we may rotate them is opposite directions and then gain the advantages because it become more apparent.




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