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It is emphasized that, if noise reductions are to be obtained or if present noise levels are to be maintained for higher power ratings, future propeller designs should operate at lower tip Mach numbers are currently being used. Single- rather than dual-rotating propellers were found to generate the lowest over-all noise levels for a given number of blades.
Theoretical calculations of the sound-pressure levels by the method of NACA TN 2968 predict accurately, for the 10-foot propeller investigated, the location of and the maximum levels to be expected for the overall noise and the first two propeller harmonics. The calculations do not predict accurately the location of the maximum sound-pressure levels ad the maximum calculated levels are 10 and 13 decibels lower than the msximm messured levels for the third and fourth harmonics, respectively.
The effects of primary and runback icing and frost formations on the drag of an 8-foot-chord NACA 651-212 airfoil section were investigated over a range of angles of attack from 2 degrees to 8 degrees and airspeeds up to 260 miles per hour for icing conditions with liquid-water contents ranging from 0.25 to 1.4 grams per cubic meter and datum air temperatures of -30 to 30 degrees F.