Advanced Propeller Flow Control for Increased Propulsive Efficiency

An important mission for NASA is the development of revolutionary flight concepts and technology. The development of unmanned air vehicles (UAVs), the resurgence of general aviation, and growing interest in environmentally conscious, all-electric, emissionless aircraft have brought about a renewed interest in propeller design. Overall, since the propeller's golden age during the WWII era, very little has changed in propeller design. Computers have automated the design processes, but the basic design methodology, from an aerodynamic point of view has changed very little. Strides have been made in acoustics and multidisciplinary optimization (MDO), but the basic aerodynamic design and performance of the subsonic propeller has basically remained unchanged. The explosion of UAVs and a need for more efficient designs allowing greater payload, range, and loiter times have taken UAVs from simple cut-and-try designs to sophisticated, aerodynamically efficient systems. An area as of yet not fully exploited by this class of aircraft, is that of propeller efficiency. Most smaller UAVs and micro-UAVs simply use off-the-shelf radio control propellers, while moderate size UAVs rely on propellers designed using classical blade element theory or those derived for general aviation aircraft. While these propellers provide industry acceptable levels of thrust for a given torque, the majority of propellers suffer some form of flow separation. The extent of flow separation can range from small areas in cruise regions of the flight envelope, to large areas during climb and wind milling. Significant propeller performance gains in the form of increased thrust and reduced torque can be obtained by eliminating these separated regions across the flight envelope. A simple, efficient, and robust flow control technique is proposed to eliminate these separated regions and provide a marked increase in propeller performance and vehicle propulsive performance.

The propeller flow control technology will provide enhanced performance for propeller-based aircraft across a wide range of flight regimes and missions. Considering the recent proliferation of small and moderate size UAV designs, the commercialization potential for the technology is excellent. This fact, coupled with the recent rise in the price of petroleum based fuels and the push for greener, more efficient vehicles, will make the improved performance and efficiency of the propeller flow control system very attractive to both the military and commercial sectors. Additionally, the applicability of the technology to the retrofit market is extremely high. Finally, the propeller flow control technology can also be easily licensed to existing airframers and propeller design and manufacturing companies.

Propeller flow control technology will provide NASA with a means to develop new vehicles, or retrofit existing aircraft that can take advantage of an improvement in propeller propulsion efficiency. In the case of electric powered vehicles, the increased efficiency can be used to extend battery life and range, or to allow a larger payload to be carried to altitude. Exploratory aircraft, whether used for terrestrial missions or on other planets, will benefit from the ability to eliminate separated flow on the propeller, which will improve flight efficiency in off-design flight conditions or particularly challenging flight envelopes, such as those found in very low Reynolds number flight. High-altitude long-endurance (HALE) aircraft, used for such tasks as atmospheric sampling, ground mapping, or communications relaying operate in a very low Reynolds number environment, where laminar separation bubbles can dominate the flow field, and could benefit from improvements in propeller efficiency.

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