That first link doesn't really
explain the phenomenon. I'll have a go and prepare myself for a thousand insults... lol...
The horizontal flow, created by moving forwards, is accelerated for a longer time at the rear of the disk because the airflow has travelled the diameter of the disk (front to back), the molecules of air towards the rear of the disk having had more time under its vertically downward influence. With more flow downwards than sideways (think of that vector diagram now) the angle between the relative airflow and blade chordline gets smaller (reduced angle of attack). The reduced angle of attack here means less rotor thrust (on that vector diagram the total reaction vector would lean further away from from the vertical 'best efficiency' position). So now there is less rotor thrust at the back of the disk, compared to the front and the rear of the disk wants to drop. The result of this is seen 90 degrees later in the blade revolution, giving you the distinctive roll to the right (in a counter-clockwise rotating blade system).
Note: Your text book says this
gives the helicopter a tendency to pitch the nose up
but this is incorrect.
How did I do?
