To: Vfrpilotpb
As I had indicated previously blade design on paper or on a CAD program is precise. However, when the blades are constructed the positioning of the chordwise CG is not very precise. Most blades have movable weights in the tips and these serve two purposes. 1) They establish chordwise balance in that they can be moved on the whirl stand to compensate for climbing or diving and 2) The weights establish the spanwise CG of the blade mass.
On the ideal blade the chordwise CG would be on the pitch change axis but in reality it is slightly behind that axis. During flight the blade bends spanwise so that the chordwise CG is on the pitch change axis. As previously stated most helicopter manufacturers will design the rotorhead so that the pitch axis is slightly ahead of the rotating axis of the spinning rotor mass. This moves the chordwise CG to the point it wants to be. However even this cannot compensate for manufacturing inaccuracies so that the blade is bent spanwise at all times during its’ rotation and it doesn’t matter if the blade is advancing or retreating. The mechanical forces that cause the bending are independent of the forces that cause leading and lagging.
There have been many incidents where a blade came off in flight but it was due to one of several reasons-which include, manufacturing error, design error or, faulty overhaul but I have never heard of a blade failure due to lead/lag or spanwise bending. That is not to say it has never happened.
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The Cat
[This message has been edited by Lu Zuckerman (edited 22 March 2001).]