Abstract

Electric propulsion is gaining popularity with over 100 electrically propelled aircraft in development worldwide. There is a growing interest in vertical lift vehicles either for package delivery or for urban air taxis. If these vehicles are to operate near population centers, they must be both quiet and efficient. The goal of this research is to develop a propeller that is more efficient and generates less noise than a stock DJI Phantom 2 quadcopter propeller. Since a large contribution of near field noise generation for a propeller comes from the tip vortex, reducing or minimizing this generated tip vortex was the main objective. After studying the literature on aircraft wing tip vortices and techniques proposed to minimize the wing tip vortex, seven promising tip treatments were selected and applied to a stock DJI Phantom 2 propeller in an attempt to reduce the tip vortex, and, thus, the generated noise. These tip treatments were: 1. Leading Edge Notch, 2. Trailing Edge Notch, 3. Hole, 4. Vortex Generators, 5. Tip Thread, 6. Trailing Edge Sawtooth, and 7. Reverse Half-Delta.

An optimum design would be one that reduces near field noise while at the same time minimizes any additional required power. Several different configurations were tested for each tip treatment to determine the RPM and required power to hold 0.7 lbf thrust, which simulated a static hover condition. For each test, a radial traverse one inch behind the propeller permitted the measurement of the Sound Pressure Level (SPL) to find the maximum SPL and its radial location. Several configurations tested resulted in 8–10 dBA reductions in SPL when compared to the stock propeller, however, these configurations also resulted in an unacceptable increase in the power required to achieve the desired thrust. Thus, in these cases a decrease in SPL comes at the expense of power, a tradeoff that must be considered for any propeller modification.

The most promising tip treatment tested was the Trailing Edge Notch at a radial location of 0.95 r/R with a Double Slot width and a Double Depth (DSDD). The DSDD configuration as tested reduced the SPL 7.2 dBA with an increase in power required of only 3.96% over the stock propeller. This tradeoff, while not the largest reduction in noise generation measured, seems to be an acceptable power increase for the decrease in SPL achieved. Smoke visualization confirms that the tip vortex is minimized for this configuration.

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