THE FLEXIBLE LFT MODEL
Uncertainties are introduced in the natural frequencies of the 6 flexible modes. A real modal state-space representation of the flexible model is used. If the method of chapter 3 (section 4.) is applied, the flexible LFT model contains 6 twice repeated real scalars. Otherwise, if the method of subsection 2.2 is used, the LFT contains 6 non repeated real scalars.
The complete LFT model у = Fi(H(s),A)u is simply computed by adding at the physical outputs yr and уf the rigid and flexible LFT models (see Figure 4.3): remember indeed that the interconnection of LFTs is an LFT. Д = diag(/S. i, Д2), where Д; is a real model perturbation containing 14 real non repeated scalars (corresponding to uncertainties in the 14 stability derivatives). Д2 is a real model perturbation containing, either 6 real twice repeated scalars, or 6 real non repeated scalars (corresponding to uncertainties in the frequencies of the 6 bending modes).