Experimental Evaluation of Open Propeller Aerodynamic Performance and Aero-acoustic Behavior

Abstract

A topic of increasing importance in the Unmanned Aerial System (UAS) community is the design and performance of open propellers used in hand launched, small UASs. The performance of these small propellers directly influences the operational capabilities of the UAS. As such, the design and testing of these propellers is necessary to accurately predict UAS performance. This experimental investigation examined the relationship between diameter, pitch, and number of blades to aerodynamic efficiency and aero-acoustic sound pressure levels. Thrust, torque, propeller rotational speed, and sound pressure level were measured for twelve aero-nautCAMcarbon (ACC) folding propeller configurations currently being used on an operational UAS with diameters ranging from 12 to 15 inches, pitches from 6 to 13 inches and increasing from two to three propeller blades. Each configuration was tested at 44 ft/s tunnel velocity, the typical cruising velocity of a small UAS, while the propeller rotational speed was varied to determine the rotational speed needed to produce 2.5 lbf of thrust, a typical cruise thrust required for a small UAS. As expected, the rotational speed required to achieve the desired thrust decreased approximately 7.7% per inch increase in the propeller diameter. At the same time, the noise signature decreased by approximately 0.8 dB per inch increase and overall efficiency rose by 2.9% per inch increase. Similar results were found for increasing both the number of propeller blades and also increasing the pitch of the propeller. Increasing from two to three blades decreased the rotational speed by 9.1% with a 2.1 dB drop in sound pressure level and an increase in overall efficiency of 3.2%. Increasing the pitch generally decreased rotational speed by 4.6% per inch of pitch increase and decreased noise level by 0.7dB per inch of pitch increase. Overall efficiency slightly increased by 0.6% for an inch increase in pitch. For a given diameter propeller there seems to be an optimum pitch for minimum sound pressure level. For design, this indicates there is an optimum angle of attack for the propeller, which translates to an optimum beta twist angle to achieve minimum sound pressure level. Noise generation was found to be a strong function of propeller rotational speed. Lower rotational speed generally produced less noise.