I understand some basics about aerodynamics (Turn engine on, put plane on run way, go fast, plane flies because air likes to play with planes).

No really, I was wondering if anyone could explain to me WHY my UCD3D responds and tracks so much differently now that I have squared off the wing tips. I didn't just hack the tips off, I built up an extra wing rib so as not to loose any wing area...Still 65" span.

Any insight would be much appreciated. Thanks.

It could be that the tip shape is causing the differences, but...

Many of us here have done experiments with different wing tip shapes, tip plates, winglets, etc., and they usually don't make a visible performance difference in .46-size and smaller aircraft. (Do a search here in "Aerodynamics" for some of those discussions.)

It's more likely that in the process of changing your wing tips, the weight was changed, causing a CG-related issue.

Casey - be a little more specific in your description. Looking over the U-Can-Do I can't help but echo what Mike said. Granted you have the .65 version but his comments still apply. Most finesse shapes show up in drag which is not easily noticed in something like the U-Can-Do.

Are you sure you arn't suffering from memory problems :-)

I'm hoping my memory isn't too bad, since I flew the plane with the tips the very day before I made the modification then flew it the following day with the modified tips. Or was it a...hmm. No maybe...

Whatever, anyhow... I'm quite certain it made a difference. It wan't what I would describe as night and day but it was noticeable. It seemed to knife edge a bit easier (less elevator input) and flew inverted with less tendancy to pull to the ground. On up lines, it felt less affected by crosswinds (our field has a perpetual cross wind it seems like) and it lands nice and easy like it's nose heavy even though the CG is 6" back. Before I did the tips, flaring just meant less down elevator... I actually had to force it out of the air to land or it would stall. Now it lands like it did at 4 7/8" CG. It looks like there is apx twice as much wing area from the last wing former out (where the tip used to be), so maybe the wing loading lightened up a bit...but with a Saito .91 it's not really heavy. I'm guessing it probably weighs in around 9 lbs with my reinforcements and class A mishap M.A.L.S. repairs.(Marine Aviation Logistics Squadron. They fixed our AV-8B Harriers when we hit deer with them... Semper Fi.)

Or I could just be crazy :stupid: ... My shrink said I was once, but he changed his mind after I threw him out the window.

Actually, I'd say you did add wing area. The area beyond the old last rib wasn't really doing much to create lift. You replaced that with an entire extra rib bay on both ends, making the wing effectivly longer.

I say a better test would be to just chop off the UCD's tips and leave the wing with the resulting length. I can't speak from personal experience, but a couple of guys at my field have made that kind of modification to some of their fun flys, and all reported better handling as a result.

I'm not sure, but I think that some of those wing tips like the UCD has can have a bit of AOA to them, creating what ammounts to a little washin or washout at the tip. But since the "airfoil" of the tip itself is so thin, the effects are really small. Or, it's also possible that the "airfoil" on those tips actually stalls before the rest of the wing. Those tips are so big that I wonder if you can just write them off as "wingtips" and not actually take a look at the airfoil on them, and it's actions at various AOAs. I'm totally guessing here though.

Many years ago, a friend and I made some wing tip tests. He built a "sort of" wind tunnel and we used a stub wing mounted so we could change the angle. We had thread glued to the leading edge and wing tips that screwed on. We were mainly looking for the tip vortex. We did not have any measuring equipment and only observed the path of the thread as the angle increased.

The downward curved, Hoerner, wing tips and end plates were the best with straight back threads. Next best was a squared off tip like a UCD with the tip removed. Block wing tips were next, followed by the curved backward "Stick" or UCD type tipe. Worst were the round, 30's style tips.

Basically, we found that any tip was worse than no tip and the more rounded or curved you got, the more vortex you got causing the threads to wrap around the tip. The Hoerners look nice, but aren't practical for an arco plane that does inverted/outside moaeuvers. End plates were the best, but they can kill stalls, snaps and spins. You will land slower, but you'll need more elevator to get any stalling type maneuver.