Evaluation of a dual-mode, turbojet-turboprop engine for unmanned aircraft

Abstract

This paper provides experimental feedback on the feasibility of a small-scale, dual-mode turbojet to turboprop engine. The engine used for testing was a KingTech K45-TP. Key parameters focused on for this experiment were turbojet and turboprop thrust. A parametric cycle analysis was performed on a pure turbojet configuration and turboprop configuration, then using mass flow parameter various nozzles were sized to be designed and tested. Mission analysis was also performed to get an idea of the required thrust that could be expected for takeoff and flight. The experiment examines the effect of turbojet thrust with respect to varying nozzle exit diameters, as well as nozzle location relative to the exhaust exit. The engine was run in both turbojet mode and turboprop mode with the various nozzle designs with exit diameters measuring from roughly 1.5 inches to 1 inch. During each test, the nozzles started at a backoff distance of two inches and as the engine was running the nozzles were moved forward at quarter inch increments until the backoff distance was half an inch. While running the engine in turbojet mode, there was a highlighted focus on a potential performance increase in turbojet thrust as the nozzles were moved closer to the exhaust. While running the engine in turboprop mode, the highlighted focus was if there was a performance decrease in turboprop thrust as the nozzles were moved closer to the exhaust. Results show that there is an optimal nozzle geometry and placement for turbojet mode, with little effect while running in turboprop mode. Overall, the turbojet thrust recorded during testing is found to be inadequate for high-speed cruise, however the thrust that is produced could be useful for a transition to turboprop mode from a RATO takeoff with a propeller lock.