Figure 4. Counter-rotating turbine design

Selection of second shaft rotation speed is dictated by overall engine layout, i.e. joined compressor part rotation speed, gearbox presence or absence etc. It’s natural that higher rotation speed allows to obtain more specific work from every stage of turbine.

Search region for the second shaft rotation speed was chosen in ranges 10-15% less than primary shaft rotation speed. To receive exact resulting speed the design of experiment study was performed. In the figure below response surface between total-to-total efficiency vs first and second shaft rotation speeds (X and Y axis correspondingly) is shown. Basing on the highest total-to-total efficiency criterion rotation the speed of 4500 RPM was chosen for the second shaft.

Figure 5. Efficiency chart vs rotation speeds