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The helicopter rotor blades are designed such that their Centre of Pressure...

  • A
    Has a large degree of movement for stability at high and low speeds.
  • B
    Moves forward and backward according to the rotor speed.
  • C
    Can move forward rapidly to counteract forward Centre of Gravity.
  • D
    Is positioned close to the feathering axis.

Refer to figure.
Centre of Pressure (CP) is the point on the chord line where the resultant of all the aerodynamic forces acts.

To prevent pitching (twisting) moments and extra loads on the control links, the CP should be as close to the feathering axis as possible.

Most rotor blades sections are designed to produce the least variation in CP with angle of attack and to achieve good torsional stiffness, with the blade balanced, so that the Centre of Gravity is coincident on the chord line with the CP, Aerodynamic Centre and feathering axis of the blade.

Torsional stiffness can be more easily achieved when using symmetrical airfoils, where the position of the CP remains relatively unchanged at about 25% of the chord line with the change in Angle of Attack.

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