Adapting a Small Ground Turbine to Very Light Airborne Applications: a “Lean” Approach

There is a need of practical and real alternatives to the more common gasoline internal combustion engines burning the increasingly difficult to find “Avgas” fuel, in particular for the Very Light Airborne applications, and this is expected to increase even further in the next future. This paper describes the authors efforts in research and experimental activity to verify the feasibility of the adaptation of an existing turbine for ground based, power generation use to Very Light Airborne applications, primarily Very Light Rotorcrafts (VLR), UAV and Very Light Aircrafts (VLA). The selected starting point was an existing SOLAR T-62T-32 ground based, power generation gas turbine. The SOLAR T-62T-32 was originally used as prime mover for ground based, generator sets. It is of the single shaft design, single stage centrifugal compressor, reverse annular combustor, single stage radial in-flow turbine, so it is of very simple and rugged design even if particular care has to be paid to its assembly and tuning to achieve acceptable performances, efficiency and controllability in the new airborne environment. The team initial work involved CFD simulation of the compressor-diffuser assembly to identify critical performance parameters and subsequent rig (“Dyno”) testing to correlate analytical predictions with actual data to fine tune the analytical models and to establish a performance baseline to be used later on to verify and assess the possible improvements. Then, a complete review of all the external and internal mechanical parts was performed. The objective was to achieve an output shaft power of about 130 HP at S.L. ISA at 6000 RPM (corresponding to the striking speed of about 61,091 RPM at the turbine mainshaft!) but at lower weight and with increased reliability. The weight reduction was achieved by detail work on turbine inlet case, gear cases, gears and accessories substantiated by stress analysis, gear and bearing calculations, rotor dynamic studies and the definition of specific operational limitations to avoid exceedance of unacceptable stresses. In addition, a dedicated Electronic Engine Control (EEC) has been designed, manufactured and tested to allow the representative operation of the modified engine. Extensive bench testing (in excess of 300 running hours on the lead modified turbine) confirmed the consistent achievement of the above mentioned objectives and lead to the experimental installation of a modified turbine on a Aviotecnica RAVEN prototype (AUW about 450 Kg, two seats VLR) where it has been flight tested.