To: Vfrpilotpb
I don't know if this completely answers you post but, here goes.
Leading and lagging are the result of physical forces called conservation of angular momentum and Coriolis forces which are very often intertwined when an explanation of both are given. The advancing blade leads and the retreating blade lags. I don’t know if the aerodynamic forces acting on the leading and lagging blades can be measured but it can be seen that those forces are not effecting the leading and lagging. The advancing blade has the increased relative wind which would be in opposition to its’ leading and the retreating blade has that same relative wind assisting it. Yet, the advancing blade leads and the retreating blade lags.
It should be noted that all of the leading (and lagging) takes place behind the pure radial position of the blade. That is, if you could diagram a four-blade system the radial position would have all four blade disposed 90-degrees to each other. However there is another mechanical force in play and that is inertia. Inertia causes the blades to move back in relation to the radial position and all leading and lagging takes place around that position. The only time the blades move ahead of the radial position in flight is during autorotation when the blades are driving the power train.
Another point to consider is spanwise bending of the blades, which you brought up in your post, as well as leading and lagging on a two-blade system. To counter the leading and lagging, most two-blade systems have their rotorhead underslung which decreases the angular difference between the driving axis and the driven axis when cyclic is applied. Leading and lagging is only present when cyclic input is made in the control system whether the pilot does it or it is built into the control system.
Spanwise bending is a result of the imperfect design of the rotor blade in that it is difficult to get the chordwise CG and the pitch axis of the blade to be coincident with each other. One of these elements will try to get in line with the other by bending the blade forward on the spanwise axis.
There are several ways of combating spanwise bending. Some blades have stiffeners built into the trailing edge of the blades, while others have drag links, which allow spanwise adjustment while at the same time rigidizing the blade. The most common design device used on articulated rotors is to move the pitch axis forward of the rotational axis.
If you could diagram this you would note that the pitch axis when compared to the rotating axis of the rotorhead is approximately ¼” ahead of the rotating axis. This mechanically brings the chordwise CG into the position it would move to if you had spanwise bending. It is not perfect, but it minimizes spanwise bending.
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The Cat
[This message has been edited by Lu Zuckerman (edited 21 March 2001).]
[This message has been edited by Lu Zuckerman (edited 21 March 2001).]