The problem of minimizing blade profile power for a conceptual high-altitude UAV compound helicopter was addressed through airfoil design. Some example performance calculations were presented for a generic configuration at altitudes ranging from 5;000 ft to 60;000 ft. These performance estimates helped to define the design requirements for high-altitude rotor blade airfoils. Results showed that the combination of natural laminar flow with improved transonic characteristics could enable cruise altitudes up to 50,000 ft. The results also showed that for optimum endurance the rotor tip Mach number had to be increased with altitude. The objective of the design effort was the development of transonic, natural laminar flow airfoils for cruise at 50,000 ft. The aerodynamic challenges involved the combination of high-subsonic Mach numbers with low Reynolds numbers. This combination of parameters resulted in the appearance of both shock waves and laminar separation bubbles in the relevant flow physics. The design of a highly reflexed inboard airfoil, AFDD 50K30, enabled the use of more camber over outboard sections. This allowed the working section airfoil, AFDD 50K60, to be designed for significant runs of laminar flow. Finally the tip airfoil, AFDD 50K80 was designed for low drag at zero to slightly negative lift found on the advancing blade at high speeds. Results were compared to existing rotorcraft airfoils over the high-altitude design space.
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