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Which statement about propeller noise is correct?

I. Propeller noise decreases when the blade tip speed increases.
II. For a given engine and propeller blade shape, an increase in the number of propeller blades allows for a reduction in propeller noise.

  • A
    I is incorrect, II is correct.
  • B
    I is correct, II is incorrect.
  • C
    I is correct, II is correct.
  • D
    I is incorrect, II is incorrect.
The ability of a propeller to convert engine power into forward thrust is its power-absorption capability.
It is affected by:
  • the propeller-blade shape.
  • the number of propeller blades.
  • the propeller solidity.
The blade diameter is an important factor since the greater the diameter the greater the tip speeds at a lower rpm, therefore allowing the tips to reach sonic velocity earlier. At sonic velocity, the compressibility effects reduce thrust and increase drag, therefore reducing the propeller efficiency. This effect also results in unacceptably high noise levels. Therefore, statement 1 is incorrect.
The number of propeller blades is usually limited to six because of the complexity of the pitch control contained in the propeller hub and aerodynamic interference between propeller blades. Having a large number of blades has the advantage that it decreases propeller noise, but its primary purpose is to improve the power absorption properties and efficiency of the propeller. Therefore, statement 2 is correct.

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