Naming Coupled Modes
In cases where coupled modes are computed such as in the Campbell diagram analysis the following sections gives details on the naming. A focus is placed on the behaviour when the multi-blade coordinate transform is used.
Support structure modes
For support structure modes, the coupled mode is named after the whole-tower mode that gives the highest contribution. Whole-tower modes are uniquely calculated for the linearisation calculations through a subsequent eigen analysis with fixed-free boundary conditions. This analysis considers the effect of the RNA and any other masses at distal nodes. In case multiple coupled support structure modes share the same whole-tower mode as its prime contributor, then the coupled mode name is made unique by appending letters A,B,C, and so on.
Rotor modes rotating frame
If no MBC transformation is used for the rotor modes, then the following logic applies to naming the coupled rotor modes:
If a single blade mode gives >75% contribution to the coupled rotor mode, then the coupled rotor mode is named after that blade mode. In other words, the mode is called “Blade” instead of “Rotor” mode.
Else, the rotor mode is named after its prime contributor and made unique by appending letters A,B,C, etc. in case multiple coupled rotor modes share the same uncoupled blade mode as prime contributor
Rotor modes non-rotating frame
If an MBC transform is applied then the individual blade modes are transformed to a set of rotor modes. For a three bladed rotor there typically is a collective, cosine-cyclic and sine-cyclic rotor mode. The 1st flapwise modes of all blades will be renamed to rotor 1st flapwise collective, rotor 1st flapwise sine-cyclic and rotor 1st flapwise cosine cyclic. In case the number of blades is even there will be a differential mode as well.
After the transformation and renaming of the individual blade modes the coupled rotor modes are named. The whirling modes are identified following the logic in the table below.
| Coupled mode name | 1st uncoupled mode | 2nd uncoupled mode | Phase angle (\(\phi_2\) – \(\phi_1\)) |
|---|---|---|---|
| Forward whirl | Sine cyclic | Cosine cyclic | > 0.0 |
| Cosine cyclic | Sine cyclic | < 0.0 | |
| Backward whirl | Sine cyclic | Cosine cyclic | < 0.0 |
| Cosine cyclic | Sine cyclic | > 0.0 |
If a coupled mode does not meet the criteria of the whirling modes, then the mode is named after its prime contributor. This is analogous with the naming logic of rotor modes in the rotating frame and support structure modes
Last updated 04-12-2024