Dissymmetry of Lift & Blade Flapping

In the hover every blade sees the same airflow. Each one is travelling at its own rotational speed and nothing is added or subtracted, so lift is the same all the way round the disc and there is no tendency to roll.

Forward flight breaks that symmetry. On the advancing side the aircraft's forward speed adds to the blade's rotational speed; on the retreating side it subtracts. Since lift goes with the square of speed, even a modest forward speed produces a large difference in lift between the two sides. Left alone, that difference is a rolling moment towards the retreating side, and it was what destroyed the earliest attempts at helicopter flight.

Flapping hinges are the fix. A blade free to flap responds to extra lift by rising, and as it rises the induced flow through the disc changes its angle of attack downwards, which sheds the extra lift. The retreating blade does the reverse: it falls, its angle of attack increases, and it recovers the lift it was missing. The blades trade angle of attack for flapping angle and the disc stays in balance, which is why they must also be free to lead and lag, and why the Coriolis effect matters. Note the consequence: as forward speed rises, the retreating blade has less and less airspeed to work with and needs an ever larger angle of attack, until it stalls. Retreating blade stall is what ultimately limits the forward speed of a conventional helicopter.

• Try this: Lock the hinges and add forward speed. Which way does the rolling moment act, and why does it grow so quickly?
• Try this: Free the hinges at the same speed. The lift curve flattens out, but look at what the angle of attack trace had to do to achieve it.
• Try this: Keep raising airspeed with the hinges free until the retreating blade stalls. What could a pilot do to delay it, and what is the cost?