The use of electric and piston-based propulsion systems has been prominent for Group III and smaller unmanned aerial vehicles (UAV) for the last two decades. Recently, small turbojet engines (10-50 1b-thrust class) have appeared on the market making their use on UAVs feasible. This can open the design space for smaller aircraft that potentially need multi-mode propulsion systems (e.g., dash capability). Although these gas turbine systems can provide the needed specific power, the resulting acoustic signature is a concern from both an annoyance and detection perspective. The purpose of this study is to examine the acoustic signature of a small turboprop engine relative to a small turbojet engine. Far field acoustic measurements of the turboprop and turbojet engines were acquired simultaneously with thrust measurements. The turboprop was run with a 2-bladed, 20-inch diameter propeller in both pusher and tractor mode. A comparison of the acoustic data at equivalent thrust levels shows that the overall sound (integrated over 20kHz) of the turbojet is approximately 17 dB higher than the turboprop in pusher configuration. However, narrowband spectral data shows that the strong propeller tones beginning at 180 Hz are comparable to the turbojet noise at that frequency. Since lower frequencies travel farther without attenuation than higher frequencies, this can impact detectability.
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