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 is also referred to as the azimuthal angle, which means the angle between the control input and and corresponding value of a rotor blade in the plane of movement. 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°).

The blade does not react instantly, however, to the lift peaking at 9 o'clock and the lift troughing at 3 o'clock due to inertia, mass and aerodynamic damping. In fact the blade will continue to flap up after 9 o'clock, reaching its peak at approximately 12 o'clock. This phenomenon is called phase lag. The exact value of the phase lag angle depends on several design features and, to a tiny extent, the air density, but as a generalisation it can be said to usually be about 90.

Mass Influence: The mass (weight) of the rotor blades affects how quickly they can respond to the forces acting on them:

• Heavier Blades: Slower to react, so the delay (phase lag) is longer.
• Lighter Blades: Quicker to react, so the delay is shorter.