Thread: Discussion washout vs inverted tip section
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Old May 21, 2012, 06:26 PM
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Originally Posted by kcaldwel View Post
Despite the 6 degrees of aero washout from the inverted tip airfoil, I still had to add 1 degree more washout to get it to trim with a stable CG position. This doesn't agree with the original chart very well, but I'm not sure how they derived that.
That appears to be a page from an article in the "On The 'Wing" column of Radio Controlled Soaring Digest. The data is derived from the Panknin twist formula. As I understand it that formula is empirically derived. I've always suspected that it doesn't fully account for sweep effect which actually decreases the lift slope on the center section and increases the slope near the tip. Taper also has that effect so combining sweep and taper adds a bit of -Cm regardless of the airfoils chosen. An Eiffel tower planform with wash-in on the center section and wash-out on the outboard panel can fix that.

The lift distribution with the inverted airfoil ends up a bit bell shaped, and the the tip separates on the top surface and stalls before the glider really slows down.
Yep. The mean line at the leading edge slopes up and the upper surface is too flat. You could fix that by sloping the mean line down on the front 15 or 20% of the airfoil but then you'd have a reflexed section.

Unless the tips were lifting down with a substantial negative Cl at all flight speeds, using an inverted cambered airfoil at the tips will be a bad idea. If the tips have that much down load on them, then the overall performance will be poor in any case. I can't see how it would ever be a good idea.
That's probably why the full size designers stopped experimenting with inverted tip sections around 1930

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