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The helicopter’s main rotor advance angle is...

  • A
    The azimuthal angular difference between the pitch link attachment on the control orbit and the blade axis to which it relates.
  • B
    The angular difference between the movement of the stick and the tilt of the control orbit.
  • C
    The angular difference between the tilt of the control orbit and the subsequent tilt of the rotor.
  • D
    Always 90°.

Refer to figures.
PHASE LAG: The blades reach their highest and lowest positions 90° later than the point where they experience the maximum increase and decrease in their blade pitch angles. This variation between the point of maximum blade pitch angle increase/decrease and subsequent tilt of the rotor is known as PHASE LAG and is ALWAYS NINETY DEGREES (90°).

ADVANCE ANGLE: The angular distance that the pitch-operating arm is positioned on the control orbit in advance of the blade to which it relates is known as the ADVANCE ANGLE.

If the control orbit tilts in the same direction as the cyclic stick is being moved and, as a result of these changes in cyclic pitch, the rotor disc tilts 90° out of phase with the control orbit, then the disc will also be tilting 90° out of phase with the cyclic stick. Thus, unless the control system is compensated in some way, moving the stick forward would cause the helicopter to move sideways.

One way to overcome this undesirable feature is to arrange for the blade to receive the maximum alteration in cyclic pitch change 90° before the blade is over the highest and lowest points on the control orbit; that is, to construct the system with a 90° ADVANCE ANGLE, to compensate for the 90° of phase lag.

Another way is to make the control orbit tilt so that it is out of phase with the cyclic stick by the required angle. This could be 90°, but is usually 45°, with 45° advance angle to compensate fully for phase lag.

In some systems the control orbit is tilted in phase with the cyclic stick but only, for example, with seventy four degrees (74°) of advance angle. This system is often used to compensate for INFLOW ROLL.

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