Design of an airworthy turboshaft engine quick-start system with compact pressurized air supply for rotorcraft application

Helicopters heavier than light class have two or more turboshaft engines installed, for safety reasons. During helicopter flight missions the maximum installed power is rarely needed. Thus, the engines are mainly running at part load. However, turboshaft engines have the lowest specific fuel consumption (SFC) at high engine loads. This means that the fuel efficiency is poor when the engines are operating at part load conditions. The operational strategy of an intended controlled shutdown of one engine during helicopter flight can be a solution to improve fuel economy. Accordingly, the load of the remaining running engine is increasing and thus, the SFC is shifted to better values and fuel can be saved. But this intended single engine operation (ISEO) strategy is limited to certain areas of the helicopter flight envelope. One constraint is the flight altitude. If the running engine fails during ISEO mode, sufficient altitude margin for autorotation is required. To reduce this margin, the before controlled shutdown engine has to be quick-start capable since regular engine starts last too long until sufficient power can be provided by the engine. A quick-start system (QSS) for turboshaft engines was designed at the Institute for Flight Turbomachinery and Propulsion. This system utilizes pressurized air for gas generator accelerating during engine start-up. At its first development stage for proof-of-concept the system operates with 13 bar(a) air pressure. However, this air is provided by the testbed building. This QSS is currently modified to operate at pressures up to 300 bar(a). Thereby, the QSS gets independent from the shop air and lightweight pressurized air bottles can be used as air reservoir. This paper deals with the preliminary design of a QSS which is lightweight, airworthy, reliable and capable of being integrated into a helicopter fuselage. For this, the already functional QSS with shop air is analyzed and general system parameters are defined for further system design. Numerical simulations of the system's functionality are performed leading to a basic preliminary design. The next design phase comprises a CAD model and fuselage integration aspects. Based on this preliminary design, a rough weight estimation of the QSS is done. After that, a design review is performed. All steps consider aspects for realization of this lightweight QSS on the institute's engine testbed for experimental investigations later one.