Simplistic explanation of delta3 and phase lag.
The phase lag in a conventional teetering rotor is 90º. This means that the teetering (flap) angle will match the pitch angle, but this match will be 90º further on. The first three columns in the following table show this relationship.
Delta3 is a flap-pitch coupling. As the flap angle gets greater, the delta3 will remove more and more pitch. In other words, the increasing blade flap removes some of the pitch instruction that is telling the blade to flap, in the first place.
The last 2 columns show the results of delta3. The difference between the 'Pitch' column and the 'Modified pitch' column is due to the delta3. The 'Modified flap' column shows the flap of the blade due to its receiving the 'Modified pitch' instruction.
Lu. The 'Modified pitch' column shows that at approximately 72º azimuth there is no more pitch instruction going to the blade. Therefor the blade will stop flapping at 72º azimuth. To point the helicopter in the desired direction the rotor must be rigged to start the pitch instruction 18º later.
[To put it another way; if delta3 causes 7º of flap to give -2º of pitch, then at 70º azimuth, in the following table, the -2º will cancel the +2º. In other words at 70º azimuth, there is no more pitch instruction to make the blade flap anymore.]
Addition rotations of the rotor will be required to eventually work the maximum flap up the cycle's 9º
Azimuth . Pitch . . . Flap . Modified pitch . Modified flap
00` . ` . `.9` . ` . ` 0` . ` . ` 9.0` . ` . ` 0.0
10` . ` . ` 8` . ` . ` 1` . ` . ` 7.9` . ` . ` 1.0
20` . ` . ` 7` . ` . ` 2` . ` . ` 6.7` . ` . ` 1.9
30` . ` . ` 6` . ` . ` 3` . ` . ` 5.4` . ` . ` 2.8
40` . ` . ` 5` . ` . ` 4` . ` . ` 4.0` . ` . ` 3.7
50` . ` . ` 4` . ` . ` 5` . ` . ` 3.5` . ` . ` 4.6
60` . ` . ` 3` . ` . ` 6` . ` . ` 1.9` . ` . ` 5.5
70` . ` . ` 2` . ` . ` 7` . ` . ` 0.2` . ` . ` 6.4
80` . ` . ` 1` . ` . ` 8` . ` . ` 0.0` . ` . ` 6.3` . ` . `
90` . ` . ` 0` . ` . ` 9
The incorporation of delta3 in a main rotor plus the inclusion of the flapping hinges does move Robinson's rotor head away from the purity of a basic teetering rotor, but IMHO, it will take much more than an interesting discussion to determine the degree of safety, if any, that is lost due to the configuration of the Robinson hub.