Tuft testing an ultralight sail

[Bob Furr]

I have long wondered about how much spanwise flow is disrupting lift on my Starflight UL. It has a very deep (about 50% greater than a quicksilver) single surface airfoil that is open at the tips. That provides lots of lift and makes it a good UL for me (I weigh 250 pounds). So... I have decided to tuft test the bottom surface of the wing and use a video camera to capture what really is going on. Should be an interesting experiment. Anyone have any experience with this?

[jedi]

I expect you will find a reverse flow in cruise flight. The forward half of the wing undersurface will flow from mid cord to the leading edge. If this is the case please report at what speed, relative to stall, the flow becomes LE to Trailing edge. I would expect a relatively small span wise flow except at the tips (one to 1.5 cord width in). Main flow would be within 20 degrees of cordwise. The reverse flow area should get smaller as the AoA increases.

[Ron Blum]

I agree with Jedi, but I think that you will get reverse flow all the time. I'd be very interested to see your videos! Way cool project.