When we last wrote about the Whisper Sling, a version of the Sling LSA used in the Sling Pilot Training Academy in Torrance, California, one teeny tiny detail was left out of the story by request. It was the next piece of the puzzle, so to speak, to reduce the airplane’s sound signature and one of the places most airframers go to help reduce noise: the prop.
Not just any prop. This special “non-planar” three-blade prop designed by Craig Catto features an unusual bit of blade shape near the tips, all aimed at reducing noise while retaining or, even better, increasing efficiency. As tested on the Whisper Sling, the Catto prop showed strong thrust measurements (as seen in the video) and appears to have reduced noise, both on the ground and in the aircraft.
The key to reducing noise is to control the tip vortices.