Not Bent — Whisper Sling Special Prop Breaks Cover

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. According to CFD analysis done by Catto, the non-planar tips split the vortex in two, which helps clear the path for the next blade as it comes around. Catto says testing showed that lift at the tip was zero, “which is what you want.”

Another interesting aspect of the Catto ground-adjustable prop is its use of shims to set pitch. There’s an internal pitch-setting pin that allows the shanks to rotate through the desired pitch range by shimming a central pitch block that contacts pins on the blade shanks. All pins ride on the same block, which is a clever way to reduce the chances of blade-to-blade differences. It also means there’s no need to use a digital protractor to set pitch as is common with other ground-adjustable props. (Sensenich uses a different kind of pitch pin to set blade angle on its two-blade ground-adjustable props.)

Sling is continuing to test the unusual prop, but early results are promising.

Craig Catto explains how the tip vortices react with the non-planar tip.