Turboprop performance and acoustic evaluation for a small unmanned aircraft variable-cycle engine
Abstract
Range and endurance of small unmanned aircraft systems (sUAS) driven by gas turbine engines are constrained by high thrust specific fuel consumption (TSFC) resulting from low overall efficiency. This study evaluates the feasibility of a variable-cycle propulsion system to improve the versatility of sUAS by allowing for a dash capability or a lower TSFC when needed. Two variable-cycle concepts are presented, each with the characteristics of a turbojet and a turboprop. These concepts were designed primarily with commercial off-the-shelf components, and parts were tested for operability. The first concept features a turbojet driving a turbine coupled to a propeller. The second concept features a turboprop in its stock configuration with an optional clutch on the propeller shaft. Qualitative results from the variable-cycle concepts show difficulties in engine startup with an aft propeller assembly and higher than nominal temperatures in the stock configuration with a clutched propeller. To evaluate the effectiveness of the variable-cycle concepts, static performance and acoustic contributions were studied from a small commercial off-the-shelf turboprop. A thermodynamic model and resultant mission analysis are also developed to study performance expectations and potential capabilities. The sensitivity on thrust and SFC for parameters is presented to evaluate the impact of the assumptions. Performance data is used to quantify the expected benefits of each variable-cycle engine mode, and the acoustic signatures of the engines are also a concern from both an annoyance and detection perspective. A thrust stand was designed with the capability to mount various propulsors and measure their thrust. Far field acoustic measurements of the turboprop were acquired simultaneously with thrust measurements. Propwash and exhaust properties and their contributions to noise were studied in a wind tunnel with a traverse pitot probe and a thermocouple. Performance results of this study show approximately a 60% decrease in small turboprop TSFC when compared to turbojets. A notional mission analysis reveals a wider operating range with a variable-cycle engine when compared to a turbojet or turboprop alone. Acoustic data shows distinct blade passing frequencies from turbomachinery and propeller noise which could impact detectability.
Collections
- OSU Theses [15752]