Investigation of Novel Thrust Parameters to Variable Geometry Turbojet Engines

Although propulsion systems in commercial aviation rely on high bypass ratio turbofan engines, there is still a niche in which turbojet engines can be utilized. Despite some promising experiments that offer the on-wing measurement of the most important parameter, the thrust of the engine is still not available during flight. Turbofan Power Ratio, which is a compound thermodynamic value of various pressures and temperatures across the engine, is proportional to the thrust output of the turbofan, and the same relationship was proven by the author earlier regarding turbojet engines with fixed geometry exhaust nozzle. This paper has the main objective to gather data that can reveal how variable geometry affects the relationship between Turbofan Power Ratio and thrust output of the turbojet. This has been performed by carrying out measurements on a real turbojet engine test bed. The results show that the correlation is not suitable directly to determine thrust levels as it is influenced by nozzle position. Therefore, the author has developed a novel thrust parameter that is derived from TPR and can provide additional diagnostic capabilities. The outcome of this research can gain additional importance in the future as several engine manufacturers are about to introduce variable geometry nozzles, and the results presented in this paper may pave the way for these succeeding developments.