Aerodynamic challenges are discussed in the design of high-efficiency proprotors for application to convertible rotor aircraft. To match the ambitious performance goals being set for new concepts, it is emphasized why peak levels of proprotor efficiency must be maintained over un-precedentedly broad ranges of operating conditions. For hovering flight efficiency, the challenge is to achieve both low disk loadings and high values of figure of merit. In forward flight, the best propulsive efficiency is obtained by minimizing both profile drag and compressibility losses on the blades, which demand reduced tip speeds, smaller disk areas, and ultimately, perhaps less conventional blade designs. These challenges must also be met by maintaining sufficient stall margins for normal flight maneuvers, gusts, and control of the aircraft, as well as accounting for uncertainties in design. It is shown that proprotor blade shapes more suited for forward flight may have significantly reduced stall margins in hovering flight. Furthermore, it is shown that blade stall and compressibility effects can tightly bracket the attainable propulsive efficiency in forward flight at higher airspeeds. Depending on the blade design and the operational tip speeds, the range of conditions between these two boundaries may become increasingly constrained. Proprotor operations at significantly reduced tip speeds are shown to be fundamental for maintaining propulsive efficiency at high airspeeds. While variable tip speeds can be obtained by adjusting rotational (shaft) speeds or by using a variable diameter concept, the best propulsive efficiencies may be obtained by using optimal combinations of both. However, tradable design factors must also include the consideration of drive system weight and specific fuel consumption of the powerplant. A major challenge for the future, is in the development of proprotor analysis methods that have verified predictive capabilities over very broad ranges of operating conditions.
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