The scope of the paper is the problem of the sensitivity of airfl ows to changes in the resistance of side branches in simple diagonal networks. Sensitivity εir of air output Vi to an increase of side branch resistance Rr is the limit of the differential quotient ΔVi /ΔRr, where ΔRr is approaching zero. Reference is made to the methods of designating sensitivity illustrated by examples of an ancillary network. The ancillary network describes the propagation of a disturbance that appears in a basic network, evoked by changes in the resistance of a selected side branch. In diagonal networks sensitivity signs are not always explicit. This feature distinguishes diagonal networks from normal networks, due to the so called: “relative diagonality” of side branches. In an ancillary network other side branches are diagonal in relation to the source branch that enforces flows.

 

By employing the ancillary network the conditions concerning the sensitivity signs in a simple diagonal network were derived for the couples of side branches belonging to the same cross-section and for the couples belonging to the same route. For some couples in the crosssection the signs were determined as positive or negative. Likewise, for some couples belonging to the same route the sings can also be positive or negative. Accordingly, some dependencies between airflow outputs and resistance may be increasing, very small (insignifi cant) or decreasing, which may be resolved on the grounds of sensitivity. A regularity distinguishing simple diagonal networks from normal networks was indicated. Hence, the problem of side branches diagonality concerns not only airflow  directions but also sensitivity signs. The form of the conditions determining the sensitivity signs is very similar to H. Czeczott’s conditions describing the direction of airfl ows in a diagonal network.