Rotor Dynamics ~ Lead-Lag

[Dave_Jackson]

There was a thread about a year ago where the subject of led-lag was discussed. I believe it was Lu who mentioned an interesting phenomena, in a 2-blade or 4-blade rotor, where the advancing blade is leading while the retreating blade is lagging. I.e. the opposing blades are not at 180-degrees to each other.


During an-mail discussion about vibration in a teetering rotors, the subject of coning angle, undersling height and high levels of thrust came about. It is obvious to many that the rotor's CG will not be aligned with the teetering hinge, nor with the centerline of the mast when; 1/ the disk is tipped forward, 2/ the rotor thrust is very high or very low, and, 3/ the opposing blades are at 0 and 180-degrees azimuth.

Under the same conditions, he ALSO felt that the rotor's CG will not be aligned with the teetering hinge, nor with the centerline of the mast, when the blades are at 90 and 270-degrees azimuth. It looks like what he is saying gives cause for the apparent lead of the advancing blade when looking at the mast plane, as Lu mentioned.

What he is saying is that; Under the above conditions the blades are subjected to a higher than normal coning angle. This means the blades will be 'higher' than normal. The blade at 90 azimuth is pitched down, therefore the large upward coning will have a forward component (movement toward the front of the craft), when view in the mast plane. The blade at 270 azimuth is pitched up, therefore the large upward coning will have a rearward component (movement toward the front of the craft), when view in the mast plane.

In other words, when looking at the two blades at 90 and 270 azimuths, from the perspective of the mast plane, these two blades will appear to not be at 180-degrees to each other. They will appear to be pointing slightly forward, just as Lu mentioned.

Any thoughts?

Error in initial post corrected in red. The word 'tip path' has been replaced by the word 'mast'.