Effect of nozzle configuration on thrust of a novel turbojet-turboprop engine for small unmanned aircraft

Abstract

This paper presents the results of testing the effects of nozzle area ratio , nozzle downstream location propeller shape, and propeller size to determine the feasibility of a variable-cycle turboprop/turbojet powerplant for small unmanned applications. The motivation of this project is to inform design decisions for a single aircraft with both high-speed dash and high-endurance loiter capabilities, creating a package that can perform both long range and long-duration surveillance . In this study, thrust was evaluated through static testing of a modified 6.7 horsepower KingTech K45TP engine with an array of turbojet nozzle exit diameters from 1.57in (40mm) to 0.94in (24mm), with offset from the turboprop exhaust exit between 0.75 inches and 1.75 inches. Additional testing was accomplished to measure static thrust with the different nozzle exit diameters and locations in turboprop mode with 2- and 3-bladed, 20-inch diameter propellers. Turbojet mode was achieved by locking the propeller to prevent rotation. Based on initial results, it appears that while the turboprop configuration generates excellent thrust across many nozzle diameters and offsets, the turboprop has difficulty generating usable thrust with the propeller's turbine locked in the flow. Comparison of the propellers indicates that the 3-bladed Biela produced more thrust with no decrement to operability. Future research should consider rocket-assisted take-off for high-speed flight with a folding propeller, rather than attempting to achieve high-speed flight in a turbojet mode. Follow-on developments should include design and implementation of a mechanism for releasing the folding propeller after high-speed flight.