Design parameter identification of the air supply for a turboshaft engine quick-start system

A crucial safety requirement for intended single engine operation flights of twin-powered helicopters can be quick-start capable turboshaft engines. This quick-start capability of an Allison 250-C20B engine was proven by the Institute of Turbomachinery and Flight Propulsion. For this, a quick-start system based on pressurized air was designed and successfully tested. The pressurized air is used to create a flow-jet with de Laval nozzles. These nozzles are integrated in the radial compressor casing of the engine. The nozzle’s flowjet is impinging the trailing edge of the single radial compressor stage what leads to additional gas generator acceleration during engine start-up. For proof of concept tests, this system operates with shop air at approximately 13 bar(a) air total pressure. Since an airworthy quick-start system based on 13 bar(a) air pressure supply would be too heavy and the nozzles integrated in the radial compressor casing cannot be changed anymore, a new pressurized air supply has to be designed. First, design parameters of the quick-start system are identified. As a result, the nozzle count and the nozzle entry total pressure are main design parameters. Based on these findings, a nozzle test rig was created for nozzle and air supply performance investigations at pressure levels above 13 bar(a). This test setup was validated with the shop air and analytical equations for rough performance estimations are assessed. Simultaneously, numerical simulations with ESPSS are carried out for further validation purposes. Finally, recommendations are given to meet requirements of system weight, complexity, maintainability and safety.