OPTIMALLY DESIGNED PROPELLERS CONSTRAINED BY NOISE (BLADE, SOUND, AIRCRAFT)

CHRISTOPHER JOHN MILLER, Purdue University

Abstract

A design tool is developed that will produce an optimum geometry for a given trade-off between noise and performance. A vortex lattice model is used for the aerodynamics and a subsonic compact source model for the noise analysis. Optimizing the twist, chord and sweep distributions produced blades with favorable tradeoffs between noise and performance. The techniques used for noise reduction include: an inboard loading shift, diameter reduction, rotational speed reduction, increased blade sweep, and blade number increase. Certain advanced propeller concepts are studied for their benefits. The two blades are (1) propleted, having a tip bent towards the suction side of the blade and similar to a winglet, and (2) a bi-blade, which is two propleted blades (one bent up and one down) joined at the tip. The trade-offs for the optimized blades show that, for the straight blade, the most effective noise reduction method is to increase the blade number which both increases efficiency and decreases the noise. This is followed by blade sweep, which can decrease the noise up to 3 dBA with no efficiency loss, and rotational speed reduction, which reduces noise by 4 dBA per percent in efficiency loss. More favorable trade-offs can be made in conjunction with the advanced concept propleted and bi-blades. Reducing the rotational speed 10% with a propleted blade will drop both the noise level 4.2 dBA and the efficiency 0.1% below that of the baseline straight blade. The bi-blade performance is even better: again using rotational speed reduction, the noise can be 14.8 dBA less at the same efficiency or 1.5% more efficient at the same noise level. The results demonstrate that this design tool is not only effective, but useful in the search for more efficient and lower noise propulsion methods.

Degree

Ph.D.

Subject Area

Aerospace materials

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