Not a good picture but these look like large vortex generators stuck on the pylon of the Icon. They are about 4-5 inches long and stick out about 3 inches. I’m just wondering how they improve the aerodynamics. Do they somehow contribute to yaw stability? Thoughts?
I don't have inside knowledge, but I assumed they were vortex generators.
HBA member Rotax618 added vanes to his Boorabee aircraft design that, similarly, had a fuselage constricting into a pusher prop intake.
looks like those vortex generators smooth airflow around the aft half of the pylon/cabin.
Many small amphibians - with pusher propellers - suffer similar problems. Consider that most taper down from 4 feet (rear seat) to zero thickness in a short space (maybe 8 feet). That short space is needed for balance ... to avoid mounting the prop and engine too far back.
For example, the Glass Goose suffered yaw problems because airflow would briefly attach to one side of the pylon and yaw the airplane to that side. Then the boundary layer would stall and it would yaw in the other direction, rinse and repeat.
VGs help keep flow attached on both sides of the pylon.
A friend built a Mermaid amphibian and found that putting VG’s on the aft fuselage just ahead of the major taper did a lot to improve airflow through the prop and increase the effectiveness of the rudder.
Thanks for all the replies and links. I see this has been discussed in various threads.
I’m still wondering what dictates the size of these things. I guess for this application something that doesn’t stick up far enough wouldn’t be effective. What is the process for determining the needed height? Is it just multiple tuft tests until one finds the right size and placement?