Challenges in the Aerodynamic Optimization of High-Efficiency Proprotors

摘要

Aerodynamic challenges are discussed in the design of high-efficiency proprotors for application to convertible-rotor (CR) aircraft such as tiltrotors and tiltwings. Maintaining balanced CR aircraft performance margins over the entire flight envelope, as well as accounting for numerous design constraints and uncertainties in aerodynamic prediction, poses unique challenges in the design of advanced proprotors. For hovering flight efficiency of a CR aircraft, the challenge in proprotor design is to achieve relatively low disk loadings as well as high values of figure of merit. In forward flight, the best propulsive efficiencies are obtained by minimizing profile drag and compressibility losses on the blades; this goal demands reduced tip speeds, smaller disk areas, less blade area, perhaps swept tip blades, and generally geometrically different blade shapes compared to those needed for hovering flight. It is shown that blade stall and compressibility effects can tightly bracket the attainable propulsive efficiency of a proprotor in forward flight at higher airspeeds. For some highly optimized proprotor designs, their efficient operation may become increasingly constrained, especially tor flight at higher altitudes. Proprotor designs that limit the growth of helical Mach numbers and delay the onset of compressibility losses will be fundamental for maintaining propulsive efficiencies at high forward airspeeds, although such designs will not just be limited to better airfoil selection and the use of blade sweepback. It is also shown that blade shapes more suited for forward flight may also have significantly reduced performance and lower stall margins in hovering flight. Variable rotational (shaft) speed or variable diameter proprotor concepts are both shown to be useful for maintaining good propulsive efficiencies; however, the best overall levels of performance and widest operational margins may be obtained by using combinations of both concepts. Tradable design factors must also include the consideration of drive system weight and the specific fuel consumption of the powerplant. Another future challenge for new CR designs will be in the development of analyses for proprotor design that have verified predictive capabilities over very broad ranges of operating conditions, thereby requiring wind tunnel testing at the conditions much closer to those of eventual flight.