I took a more pragmatic approach when I was designing a wing. What I wanted was as much lift as a wing could give and I remembered reading once that as the airflow over the wing got closer to the tip it began angling towards the tip instead of heading in a basically straight line from leading edge to trailing edge. I figured why not try to keep as much of that angling across a wing surface? So I used swept back tips. A tip fence could probably have done better but they look ugly .

I'm 100% with Randy on this one.  The pix in #2 illustrate it perfectly.  When the wing is producing a lot of lift the longest trailing edge span(swept tips) produces the most lift  by pushing the tip vortices further apart and minimizes the tip.  You really have to do the experiments if you really want to be convinced.  Like Randy I've tried many different tip shapes both for stunt and combat.  What works the best is a tip where the trailing edge has a longer span than the leading edge.  The exact shape of the leading edge seems to be relatively unimportant.  I had very good luck in combat with a simple square cut tip angled about 15 deg.  It cost a bit of speed in level flight, but performed better in maneuvers.  When Steve Hill invented the rounded tip I found it really works.  Just cutting the leading edge at the tip on  circle with a radius of about 50% of the chord reduces drag and handles better in turns.  For combat an added advantage is that the tip is pre-crashed.  A square tip will always get broken if the plane cartwheels.

You can experiment with making the leading edge an ellipitical shape as shown in the pix, but it takes a lot of care to shape the tip exactly symmetrical.  The slightest variation in the airfoil section in the tip gives you a built in roll and a tendency to stall one tip or the other in sharp turns.  A simple radius is much easier to get right.

The absolute worst tip is one that is sharply swept with a sharp break at the leading edge.  You can actually hear the air roaring over the tip above the sound of the engine as it slows the plane down.  Old George was a good designer, he used a rounded transition from the straight leading edge to tip.

I think the reason that an elliptical wing generally works so well is that it has an even longer trailing edge for a given span.  And, as the air tends to flow towards the tip, it still exits the wing more or less at a right angle to the trailing edge and pushes the tip vortex even further out than a swept tip.

Serge, it wasn't really obvious, but I was comparing the standard ellipse to a tapered wing and or one with swept tips.  I have no analytical data for the thought, but it just seemed to fit the idea of the longest trailing edge having the best lift.  With the outward flow towards the tips, the ellipse would let it hit the trailing edge more straight on.

I can't imagine building a wing with split tips.  The fancy modified ellipses may be a few more percent efficient, but besides being hard to build, using a swept back wing on a stunter introduces trimming problems.  The slightly swept forward quarter chord, angled stab, and other tricks on F2D combat ships really seem to give the most forgiving, best handling of any wing shape for CL maneuvering.

Howard,

The T-rex tip is kinda simi-eliptical is that what you are advocating as the best shaped wing tip???
I don't really know, but my T-Bird did fly exceptionally well back in '69

Jim Pollock   

While on the subject of wingtips, what difference does it make if the wing was eliptical, rather than having the conventional leading edge with a raked wing tip?

If I am doing the appearance judging, you might score higher.  You might talk your math teacher into giving you extra credit for calculating the rib shapes for the elliptical wing, particularly if you use zigzag ribs and flaps.  Other than that, not much.

Howard,
Could you say a bit more about what these consist of: "Discontinuities at the end of control surfaces are bad." ?

Thanks.

I'm inclined to think that this may be true of more massive, highly powered ships with good thrust to weight ratios and line tension, but I'm not so sure I'd be happy with a lighter, lower powered plane losing speed in it's maneuvers and then having to maintain line tension and control efficiency in climb at reduced speed. I've always thought that parasite drag was valuable in preventing windup and excessive speed buildup in descents, but induced drag doesn't seem so desirable to me.

I guess you could increase the aspect ratio to what you think it ought to be.  I don't see little airplanes (or classic airplanes, which were powered by wimpy engines) with appreciably more aspect ratio than modern big ones, but the little or old ones often don't have as much speed regulation from engines, so a high aspect ratio would tend to make them more prone to windup.  I would reckon that the optimal aspect ratio of an electric stunter is greater than that of an Otto engine powered stunter, both because the electric can regulate speed better and because its fuel weighs more.  

I just thought of another aspect of aspect ratio: to wit, higher aspect ratios are more sensitive to turbulence because they have higher lift curve slopes (hence my enthusiasm for super-low aspect ratio canard surfaces, but that's another story). Maybe your fancy wing tips can give you the requisite efficiency with less turbulence sensitivity.  Was David Fitz onto something?

I would avoid stuff like a piece of fixed flap adjacent to a piece of movable flap.  Not only are you opening and closing a gap while you are trying to fly level accurately, but you are changing the sign of swirly air at the TE.   These discontinuities are common on stunt planes, but I've only seen them cause a problem for sure once. 

   I agree - and it's a heck of a lot more important than the wingtip shape!  That's the one intentional change I made from the 98 version of the Infinity to the 2006 version. The airplane flew almost exactly the same, including the trim settings, but it had greatly improved intersections, particularly on the vertical 8. The old airplane had a hitch right at the intersection that I could never get rid of. 2006 airplane, nothing, it was a non-event.

     The other part of the issue is that people are tempted to use the "fixed" part as a trim tab. If you tilt it up or down to adjust the roll angle, it lines up with the flap differently in one direction than the other, and can cause all sorts of weird trim problems. Uncle Jimby flew David's "China" airplane for a few years, and it really never flew right. But it had a warp somewhere and a "fixed flap" trim tab deflected at a pretty good angle. We could never get it to fly just right. After about 10 years of watching it, I figured the problem out, we tweaked the entire flap and moved the tab until the flap lined up with the "fixed" part at neutral on both sides. Immediately better and after removing all the other weird settings it had to try to fix the problem, we were able to get it flying pretty consistently.

    Brett

I guess you could increase the aspect ratio to what you think it ought to be.  I don't see little airplanes (or classic airplanes, which were powered by wimpy engines) with appreciably more aspect ratio than modern big ones, but the little or old ones often don't have as much speed regulation from engines, so a high aspect ratio would tend to make them more prone to windup.  I would reckon that the optimal aspect ratio of an electric stunter is greater than that of an Otto engine powered stunter, both because the electric can regulate speed better and because its fuel weighs more.  

I just thought of another aspect of aspect ratio: to wit, higher aspect ratios are more sensitive to turbulence because they have higher lift curve slopes (hence my enthusiasm for super-low aspect ratio canard surfaces, but that's another story). Maybe your fancy wing tips can give you the requisite efficiency with less turbulence sensitivity.  Was David Fitz onto something?

I suppose I should have been clearer, but I've just been checking in and out of this conversation amid a lot of other distractions. I certainly agree about aspect-ratio effects and, to address an earlier comment, I wouldn't take these considerations out of the context of the entire design in their application. My posting of wing configuration effects was not meant to ask anyone to ignore good design practices. I was just isolating some specific trends.

So my last post was directed only at the statement about induced drag. All else being equal, I think lowering induced drag can be beneficial, while lowering parasite drag is generally not helpful at all in stunt models. I never meant to indicate that tip efficiency should dictate overall design. What you say about t.e. discontinuities - flap, tabs, etc. - also seems quite correct, and this issue sometimes even favors the ellipticasl wings, which also have the furthest inboard a.c of all commonly proportioned (i.e. not pointed) wings. Overall, it seems that the swept tips on tapered wings may have the lift efficiency advantage, but that's obviously not everything either. As you said, not only are high-aspect-ratio wings more sensitive to lateral disturbances, but their efficiency in lift (higher lift-slope curve) causes them to jump more when encountering an airspeed change; that fits my limited experience. I did once post though how within some limits one can diminish lateral high-aspect-ratio effects with increased taper; there's probably a little room to play with there, at least on larger models. My 56" , high-A/R, tapered model with swept tips flew a bit better than I had expected.

As for canards, I know you were addressing only aspect ratio, and I see why lower aspect ratio there would probably move the total a.c. back  and reduce their loading, while decreasing the spanwise wake effects. FWIW though, I see them as advatageous only in 3-surface aircraft and not as a foreplane "tail" for "monoplanes" without aft tails too. This is because of wake interference and inability to use the main wing's total lift, due to c.g. placement relative to the aircraft's total aero center and static margin requirements. There's some good quantitative stuff out there on this topic, including some research and theory by Stanford's Illan Kroo. I know I've posted it more than once, but the biggest thing I wrote on that here on stunthangar was lost with several others in a crash a year or so ago. Anyway, I think that a model with large main wing (or bipe), having fore and aft tails, like the Adamison design, is a reasonable stunt idea, but the aft area is the stabilizing area.

I'm sorry if I addressed too many topics - away from wing tips - especially since I have to be elsewhere much of the time during the next couple weeks. So, if there's need for further participation here, please be patient. I'll try to check in.

P.S. I got that red "warning" that another post has appeared, but time says I need to just post this now and get out.

SK

So my last post was directed only at the statement about induced drag. All else being equal, I think lowering induced drag can be beneficial, while lowering parasite drag is generally not helpful at all in stunt models.

I reckon the relative importance varies from model to model.  I was thinking of my 67-oz. VF-powered airplane on semifinals day at the 2003 Nats and a couple other such experiences.  I found myself wishing for a little more speed on the straightaways and a whole lot less in round loops.  I wouldn't have minded a little more induced drag in the square corners, too.  Those square eights sure went fast. 

Electric people are reducing the "noninduced" drag of their airplanes.  I guess this makes sense, but I sorta doubt if anorexic airfoils are indicated.  One cool thing is that we can now record pertinent flight data.