NATIONAL ADVISORY COMMITTEE FOR AERONAUTICS LANGLEY FIELD VA LANGLEY AERONAUTICAL LABORATORY
Appreciable sound-pressure reduction may be realized by changing from two blades to seven blades for comparable operating conditions. Small reduction in intensity may be realized by increasing propeller diameter for same tip speed and power absorbed. AT fixed pitch setting, sound-intensity levels expressed in decibels are approximately a linear function of tip speed for test Mach number range for all propellers tested. For same tip speed and power absorbed, seven blades are only slightly less loud than two blades at distances greater than 400 ft even though difference in sound pressure is large.