Back when I built my original wingtips, I was in too much of a hurry to spend a few hours and do the research to see exactly what that means. So I sort of did a Hoerner tip, and then nicely rounded all the edges, which killed the effectiveness completely, I now know.
To create a Hoerner tip, glue a chunk of foam on the end of the wing and contour the top and bottom to match the adjacent airfoil. Glass it or carbon fiber it, and with a square end on it (perpendicular to the ground). This is the first tip I made, using styrofoam insulation. I quickly learned to hate this stuff due to the difficulty in sanding, and it's nasty habit of devloping a static charge and sticking to everything nearby.
So on the next one I used a solid chunk of urethane (I still had some left over from my aft deck, but could have done it the plans way out of 1" panels glued in place. In fact, in 20/20 hindsight, that would have given me a perfectly flat end, which would have turned out a lot better than my "hack" job".
Then take a hacksaw or something similar and starting at either the front or the rear, and using the top edge of the airfoil cut about a 60 degree angle (measured from vertical) inward towards the fuselage using the upper "corner" as a guide. Now you know what I mean by "hack job"!
I then carbon fibered it all with two layers of carbon fiber, since the tips are prone to hangar rash and mishandling at airshows.
The usual micro smoothing layer and some finish sanding.
Good news folks...I finally painted the bottoms of the wings!
This is how it turned out. Not perfect, but not too bad.
Here's the plane the next day, ready to race the 2011 Airventure Cup Race from Dayton to Oshkosh.
The photo above comes pretty close to telling the whole story of my wing extensions and Hoerner tips. That's what eluded me 10 years ago. Viewed from the top you'd never know it was anything but squared off. The theory is that you can radius the bottom a little, but the top has to be a sharp corner so the air can't turn that sharp cornerand turn inward into a vortex, which would be a drag producer. A glide test from 13,000 feet covered 11,050' of altitude in 17.5 minutes (these numbers came off the EIS, so they are exact, not rounded off. The result was a descent rate of 631.4 feet/minute at an average of 87.5 mph, which translates into a 12.1:1 glide ratio. That's better than my previous number of 11.65 feet/minute on the previous wings, done with similar loading. Knowing numbers like these is handy if your engine quits and you need to know which airports you can make it to, and which ones you can't. My test was done with the engine at idle, which I proved several years ago yields the same results as if the engine were switched off. High compression Corvairs with short woodend props don't windmill, at least I've never seen it happen.
Tufting of the new wing versus the old tip showed the flow moving almost parallel to the fuselage, rather than at a 30 degree angle inward like before...a sign that drag is reduced. Wide open throttle tests showed a 1 to 1.5 mph improvement in top speed over the previous wing shape, and this is with a 12% increase in wingspan, so there was a real drag reduction from the Hoerner tips.