Delta-3 & Phase Lag

[Dave_Jackson]

Spaced,

The incorporation of delta-3 into a main rotor may be a good thing. It is one of many, many arraignments that have been tried. Delta-3 is used in the Robinson and the K-Max, and both are still being built, whereas most of the other ideas have come and gone.

A delta-3 angle of 45º on a tail rotor means that all flap will be opposed by an equal and opposite pitch, so as to maintain the blade in its 'home' position. On a main rotor the same thing will take place. In other words, the cyclic stick will be useless because a delta-3 angle of 45º not allow the blade to leave its 'home' position. On a tail rotor, of course, there is no cyclic control and there is no desire to tilt the disk.


Nick,

You and Lu have mentioned some of the nuances of delta-3 and phase lag where I haven't, but we appear to be pretty well in agreement on the major points.

One area might do with some clarification. You said; "I think you tend to explain things backwards - not wrong, but rather mixing cause and effect. " Here we differ. Your emphasis seems to be on gust alleviation, and that is what delta-3 in the tail rotor is all about. The technical discussions about delta-3 and the main rotor, that I have read, tend to put the emphasis on the cyclic control input and the mention of gust alleviation is sort of a by-product.

Consider the following two distinctly different activities;

Delta-3:
A tail rotor does not have cyclic control input (from the pilot). It would therefore be meaningless to discuss phase angle when discussing the tail rotor because there is no phase lag. There is only a specific angle of delta-3, which was selected by the designer to give a specific speed of response to gusts etc.

Phase Angle:
Exclude delta-3 from the consideration. A phase lag can be decreased by rotating the pitch links to the blade's pitch horn by 20º CW or by rotating the swashplates inputs from the mixed box by 20º CCW.

You said that; "The blade actually rises earlier with positive delta three, so it needs less phase angle." On this point, I disagree. I think that this explanation is backwards. It is the Phase Angle that determines the timing of the 'blade rise'. Delta-3's responsibility is only to reduce the amount of the blade rise. In other words, if the pitch link was not vertical, the delta-3 would still pull pitch, but it would be uncoordinated with the phasing.

This is why I think that Lu is correct when he questions where did 17º of the 180º go. In an articulated rotor, the blades will flap to position in less than 90º and thus the control plane and the tip path plane will be coplanar in less than 90º. In a rotor with delta-3, the location of maximum flap will be 90º after the azimuth where the control plane asked for maximum pitch.

Edited to lighten an erroneous statement.