How is disymmetrty of lift compensated for?
I know most text books I've read describe the advancing blade flapping up due to the increase in airspeed. The flapping up then reduces angle of attack because the now upward direction of the blade changes the relative wind. And the retreating blade basically does the opposite. While I argee that by itself this theory of flapping to equality is valid I don't see it being valid in the question of dysimmetry of lift with a helicopter in forward flight.
First of all the book says that the advancing blade flaps up and the retreating blade flaps down. If we simply look at a helicopter in forward flight we instantly see that this cannot be true. If the disc is tilted forward, and, as someone else stated earlier, the tip path plane remains at a constant angle to maintain thrust and lift the same, then how can the advancing blade be moving up? With the disc tilted forward the advancing blade is moving DOWNWARD and the retreating blade is moving UPWARD. This is the opposite of what the book says. Forward cyclic is what takes angle of attack away from the advancing blade and gives more to the retreating.
If you go back to the days of the gyro planes you will see where the theory of flapping to equality compensating for dysymmetry of lift came from. Juan de la cierva's gyros had no cyclic or any type of attitude control in their rotor systems, they also did not have any flapping hinges. So he came up with the idea of putting a flapping hinge on each blade theorizing that the blades would flap to equality thereby reducing an unwanted rolling moment (disymmetry of lift). It seems to work, because there was no cyclic in these designs. I think he did however design the flapping hinge at an angle to produce delta 3 pitch coupling. Think about the disc orientation on a gyro though. In forward flight it is tilted back and you can see that the advancing blade is flapping up and the retreating blade is descending, just like the book says. Maybe whoever wrote the book just figured this must apply to helicopters too. And everyone else who wrote a book just followed suit.
[This message has been edited by helisphere (edited 22 January 2001).]