Wingtip design

I'm working on a new scratch built stunter and am at the wingtip design stage. I own a Pathfinder and a Cardinal. They are both fantastic flyers with opposite style tips. I know wingtips don't fly the plane but I would be interested in opinions as to which style of wingtip people prefer. Would it be only for the sake of appearance or is there an advantage to one style over the other on the same plane?
Thanks guys.

There's good flying airplanes with all sorts of tip shapes. However, I did hear once that someone did an analysis and found the Flite Streak tip complied with "the ideal shape". This makes sense to me. The airflow from front to back of the wing also tends to slide outwards towards the tip. I guess if the general angle of the tip co-incides with the angle of slip, it reduces the tendency for tip vortices to form.

I love elliptical tips

Regards

Mike

I love eliptical tips for the beauty, but they are a drill to construct and cover, particularly if you make the planform eliptical.

I think the best wingtip is the Flight Streak design, to the extent that I modified my Jetco Dolphin to extend the straight trailing edge all the way out to a point where the eliptical arc from the front comes back and meets it. Can't tell you much about the actual effective difference as the airframe is unduly heavy and the airplane not flown yet.

But I believe the Flight Streak style design is the best to give you effective aspect ratio and spin the vortex out ward from the wing tip.

My two sense.

Cy

Make a tip that's easy to get monokote on.

I'll be covering with koverall which is a cloth type of heat shrink covering. So the wingtips won't be subjected to wrinkles, double ironing marks and blue language.

I would just like to point out that the Infinity type tip works just as well as any other kind of tip for model aeroplane purposes and is much easier to construct.

I'm with Jim....
I think that if there was an advantage to be gained in tip sedign, Brett would have utilized whatever shape gave the advantage.

See Ya

Norm,

I saw a film on tip vortex that showed how the air leaves the trailing egde of a wing tip.Yes these were full size planes,but I thought it would be the same for models.That`s why I used the shape on the pathfinder.I think the tip helps in rough air.Have no proof of this other than my opionion.My combat planes had the same shaped tips on them. The turbulence was worse on heavy planes and at high angle of attack.Hope this helps.

Gordy

Hi guys

Somewhere in my collection of old magazine scans is an article on this very issue.

Wind tunnel tests were conducted on team race models to check drag, tip vortex, etc and the conclusion was that the Flight Streak/Pathfinder tip was the best—for whatever reason. I think it had something to do with keeping the vortex or turbulence generated at the LE attached along the length of the tip but I cannot be sure.

I also sesem to recall a comment several years ago on a stunt forum that this tip style produced a wing that punched above its weight so to speak. Even though the effect may be small, if you can reduce the drag created by known turbulence, it's reasonable to expect that the wing should perform above expectation.

As I recall, the article is three pages and, because of pics, not small files. I can look for them over the next couple of days and provide them. If someone has a web site where they might like to give the article a home, please let me know.

Regards, GEoff

Thanks for the help. I was leaning towards a Pathfinder tip.
I can't find a pic of the Infinity. If someone would post one here, I'd appreciate it.
Thanks,
Norm

For many years the "East Coast" style wing i.e. flat plate style was use with great success. That to me means it just doesn't matter what you use. A stunt ship is a complex "stew" of various elements. Use a wing tip shape that gets you all hot and bothered.

"I don't know about this tuned pipe business. I practiced on that thing until my lips turned blue, and I still can't play a stinkin note."
"Homer J. Simpson"

LAST EDITED ON Nov-12-04 AT 05:36 PM (CST)
 
i could be wrong ... not bein an aeronautical engineer .... but i believe wing tips are an appearance thing ... do what ever looks good to ya ... covering w/o wrinkles can be difficult for some designs ... but if ur a scratch builder you can do it .... jim

wingloading and spanloading drown out any small effect the tips may have. Use what looks pretty or is easiest to build/cover.

I have the article somewhere...early 60s I think. Just looked for it, but no success. Can't remember if it was MAN or AM, for starters. Anyone know?

The study was done on Rat Racers, as I recall. Not stunt ships.

Hoener's book on drag basically said that a squared off tip (a la Mooney)is nearly as good as the perfect tip. Being a beginner, and no points for looks, that's pretty much what I use on my short life expetancy planes. Easy to cover. It also make adjustable lead outs and tip weight box easy to install. I've read some NACA reports that indicate the "scimitar" shape tip has the best aerodynamic characteristics. These have become VERY popular in the eletric sailplane competition world. The Flight streak tip is very close to that in shape. Even as far back as WW I there has been much written about negative raked tips vs positive raked tips. One of the considerations in those old reports was keeping the wing tip vortex off the aileron. Bottom line IMHO: use what looks good to you.

Here's what to do:

Go to each percentage of chord point on the root and tip airfoils and define the wing thickness taper by an ellipse passing through each pair of points. (There is only one ellipse for each pair of points.) Where this ellipse intersects with the horizontal centerline of the wing defines a very pleasant (and mathematically correct) tip perimeter shape.

It has no choice but to fly well and be extremely efficient.

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"Few maxims are true from every point of view." -Vauvenargues

Norm, build what looks good, here's my arguement.

In the seventies I flew with a great group of guys in the Minneapolis Model Aero Club. A very competitive, friendly, accomplished group of modelers.

The best of our group, at that time, was an engineer (aeronautical I think) named Herman Fessler. Herman won every high-point award at the end of the year during the years I was a member and ,as far as I know, he's still winning them.

One night, at one of our club meetings, Herman brought the latest of his beautiful new creations for show-and-tell. One of the club members asked "hey Herman, what kind of airfoil you got on that thing?".

Herman's answer?

Florsheim... size 9!

True story.

Bill Ervin

"What crashes in Vegas gets rebuilt in Vegas"

...Florsheim... size 9!

Wow, a chance to plot airfoils from wingtips!

Holy Cow George!, it honestly never occurred to me that the Florsheim old Herman was referring to could be... a...a...a wingtip! I beleive this is what engineers dub "design serindipity", Where dumb luck, BS, and actual aeronuttycal data intersect to produce a beautiful stunt design. I've seen it happen before and, truth be told, I rely on this technique to build and designs my own planes. That is when I'm not actually stealing other guys design ideas outright. Norm, your in good hands, we're gonna have this problem licked within a few more beers, "trust me".

Bill Ervin

"*typewriter* is the longest word you can make on a single row of the keyboard"

Here is an example of a bad tip. Friend built three or four combat planes with this tip, back when Richard Wilkins was popularizing his foam Blasta. The Blasta tips were basically short, straight-tapered tips, more or less cut free hand with one template. Don decided to sweep the tips to copy one of Pat Willcox's planes, a Stealer.

Plane flew fine, but boy could you hear the tips roaring in tight turns, even over a screaming 36. Plane also slowed down excessively. It was a plug-in wing design, so we cut some straight tapered wings with no tip, just a square cut off. Much better performance.

It seemed that the sharp corner in the leading edge was causing the problem. Rounding the tips, either elliptical or just circular, doesn't make the extra noise or drag.

The Flite Streak and Pathfinder tips are probably narrow enough that they don't do much of anything bad. If you're going to sweep the tips I'd make sure the LE corner is rounded, and don't make them very wide. Keep their span less than ~25% of the tip chord.

I agree about rounding the leading edge transition to swept tip. I wasn't going to chime in, since I've written about this before and don't have the time to find documentation, but I think I can summarize reasonably. Here goes...

1) The raked back tip is best, because it prevents some vortex formation in turns (induced drag), where it is generally felt that stunters should not slow.

2) This effect is less in models than on full-scale aircraft, because molecular separation and inter-molecular forces (viscosity) do not scale down.

3) The raked tip on most models - e.g. the Flight Streak - is less than optimum. I found this in an NACA or NASA document, but do not remember the actual quantitative description or source. I designed mine to incorporate slightly less than the suggested ratio of increase in span to chord (delta b/c), and it was considerably more that the Streak's. (See photo)

4) Efficiency in turns can be increased also by using multiple tips to divide the vortex into smaller ones. Stanford shows an 11% increase in span efficiency over elliptical wings, due to split tips, which ARE applicable to symmetrical airfoils. This is the URL:

http://aero.stanford.edu/Reports/Nonplanarwings/SplitTip.html

5) I think that a combination of raked tip with concave upper and lower surfaces, like a double Hoerner tip would work too to reduce final vortex intensity.

7) Spanwise straight trailing edges are most efficient.

8) Tapered wings are probably more important to handling in wind and efficiency, compared to rectangular wings, than the tip shape.

9) The effects at our low Reynolds Numbers are probably small enough as to be obscured by other issues. So my favorite classical shapes with t.e. shorter than l.e. are probably not going to harm a good design.

10) A slightly higher aspect ratio than any given elliptical form can give higher lift/drag for a given wing root bending moment. This is relevant to stunt design, where too many folded wings have been reported recently. If doing so, remember that the consequent position of the spanwise MAC (thus center of lift) closer to the root also helps stability in gusts.

You may be more successful than I in finding my old SSWF post. If so, there's probably more there.

SK

P.S. Oh, and the article on raked tips in rat racers was by John Kilsdonk. I have it somewhere. John once told one of my highschool buddies at a midwestern contest (Indiana) that he felt he had overstated his case in favor of the raked tips.