1979-02-01

Wind Tunnel Performance of Four Energy Efficient Propellers Designed for Mach 0.8 Cruise 790573

The increased emphasis on fuel conservation has stimulated a renewed interest in turboprop powered aircraft. Recent studies by NASA and industry indicate that fuel savings from 15 to 30 percent at Mach 0.8 may be realized by the use of an advanced high-speed turboprop, and over 20% at Mach 0.7 typical of business jet aircraft. This aircraft must be capable of high efficiency at Mach 0.8 cruise above 9.144 km (30 000 ft) altitude if it is to compete with turbofan powered aircraft. In the past, propellers were highly efficient at cruise speeds up to approximately Mach 0.6. However, above this speed, large compressibility losses on the propeller blading caused the efficiency to fall rapidly. If these losses are to be overcome, new design concepts will have to be developed for advanced high-speed propellers. Several advanced aerodynamic and acoustic concepts were investigated in recent wind tunnel tests performed in the NASA-Lewis Research Center 8x6 foot wind tunnel. These concepts included aerodynamically integrated propeller/nacelles, area-ruling, blade sweep, reduced blade thickness, and power (disk) loadings several times higher than conventional designs. Four eight-bladed propeller models were tested to determine aerodynamic performance. Relative noise measurements were made on three of the models at cruise conditions. Three of the models were designed with swept blades and one with straight blades. At the design Mach number of 0.8, power coefficient of 1.7, and advance ratio of 3.06, the straight bladed model had the lowest net efficiency of 75.8%. Increasing the sweep to 30° improved the performance to near 77%. Installation of an area-ruled spinner on a 30° sweep model further improved the efficiency to about 78%. The model with the highest blade sweep (45°) and an area-ruled spinner had the highest net efficiency of 78.7%, and at lower power loadings the efficiency exceeded 80%. At lower Mach numbers the 30° swept model had the highest efficiency. Values near 81% were obtained for the design loading at speeds to Mach 0.7. Relative noise measurements indicated that the acoustically designed 45° sweep model reduced the near field cruise noise by between 5 and 6 dB.

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