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Innovative Propeller Multi-Point Multi-Disciplinary Optimization


OBJECTIVE: Develop a process for multi-point propeller optimization for a range of vehicles, particularly mobility transports. Include method for analyzing propulsion system, airframe/propulsion integration, acoustics, cruise and takeoff performance. DESCRIPTION: For subsonic aircraft, increasing turbine bypass ratio is conducive to lower thrust-specific fuel consumption. Studies have shown that an increase in turbine bypass ratios is a primary driving factor in achieving overall efficiency improvement for subsonic transport aircraft. The limiting case for high-bypass ratio is a turboprop, then for higher performance applications, the open-rotor propulsion system. The latter is the higher-speed and higher-solidity analogue of the former, and its application is for larger, higher speed subsonic transports. But with large increases in turbine power comes more complicated open-rotor staging, blade operation, and gearing integration. This and its airframe integration may negate the benefits to overall system efficiency. Modern designs of open-rotors suggest capacity for high subsonic cruise together with good takeoff/landing performance, but modern open-rotor designs are subject to ambitious air-transportation noise constraints. Methods of propeller or open-rotor analysis tend to either be very low-order, or proprietary. Research engineers cannot render rational systems decisions without access to accurate flexible design methods. Such methods must be multi-disciplinary if they are to be useful in conceptual design of complex and innovative aircraft, with subject-disciplines including aerodynamics, fuel burn rate, weights, acoustics, structural dynamics, and gear coupling between the propellers, gear chain and turbine engine. The relatively efficient fixed propeller designs tend to be single-point performance, at least at the level of conceptual design. Thus, there is a need to develop the design framework for multi-point optimization of efficient propellers for example, for cruise, climb and loiter, takeoff and landing. Propeller aerodynamics and orientation flexibility therefore need development in two senses: first, to improve efficiency per se; and second, to raise efficiency across the expected operating envelope. Also, in a propeller-driven or open-rotor-driven aircraft, an important practical performance consideration is vehicle acoustic footprint, which depends for example on blade tip Mach number, blade airfoil design, and placement of the propeller disk relative to the aircraft fuselage and other components. As with propeller selection itself, many of these considerations have been approached empirically. The lack of unified engineering understanding has led to suboptimal designs. A comprehensive, non-proprietary tool is necessary for propeller and open-rotor designs at the conceptual level, relevant to future advanced subsonic aircraft needs. PHASE I: Develop the feasibility of a multi-point design optimization tool. Develop the preliminary architecture for a computational/analytical propeller and open-rotor design tool, to include blade sizing, design and orientation flexibility; hence, first-order aerodynamic propulsive performance prediction. Assess opportunity for multi-point blade efficiency improvement with suitable shaping. PHASE II: Fully develop and demonstrate the design system tool to include 1) acoustic signature for the propeller in isolation, and 2) propellers installed in pusher and tractor configurations. Using simulated flight test, wind tunnel test and/or numerical assessment, demonstrate improvements in takeoff and landing, cruise, loiter and dash efficiency, and show robustness of efficiency improvements across a range of scales and applications. PHASE III: Applications include USAF legacy aircraft and commercial airliners currently using turbofans, which conceivably could be replaced with open-rotors. REFERENCES: 1. Gur, O., and Rosen, A.,"Optimization of Propeller Based Propulsion System,"J. Aircraft, Vol. 46, No. 1, January-February 2009. 2. Hill, P., and Peterson, C.,"Mechanics and Thermodynamics of Propulsion,"Addison-Wesley Publishing Company, November 1970.
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