Wingtip Fence design ?

I've built a set of slatted (not yet) 17' wings, with the flap-aileron tunnel straight to the tip:



I want to built a wing tip fence to finish the wing tip. Something like this wing:


But I don't like the idea having something that ''hangs'' under the wing.
Would it be as efficient if the fence was only on top of the wing?? To force the airflow to stay on top of wing??

Any more design of fences??

I did my try with my paintbrush skills...
From small to big fence only on top.













Thanks

PM me. Jerry Burr

Thanks for the info Jerry.

But I want to built something more like a straight wing tip fence, not a 3D wing tip.
I don't mind having something lower the wing if it really worths it.

Anyone know where to find more pictures or design of wing tip fences like pictured in top of page??

Thanks again

Doesn't Wayne Mackey's wings have a flat end with a fence above the wing on the top?

Hi Jerry, what´s your thought on the idea?

Oli, have you seen this? there is a small program at the end that could help you with your design validation.


http://www.diva-portal.org/smash/get/diva2:547954/FULLTEXT01

I don't think you're going to get much benefit going above the wing out at the tip. I think the biggest goal here would be to trap the high pressure air under the wing thats trying to escape span-wise. I would carry it back at least part way into the aileron area.

I don't pretend to fully understand exactly how it works, but at a recent EAA meeting here in Sandpoint a representative of Tamarack Aerospace did a presentation of how their active winglet saved significant fuel on the Citation wing. It moves to different positions at different speeds and altitudes to optimize lift and reduce drag and it works.

I asked what they had in the works for us "low and slow" folks and in a nut shell it sounded like the low altitude/low speed benefits from wing tip treatment is negligible and the best way to improve performance would be to reduce drag inducing areas on the planes like "dirty" wings, landing gear not retracted and so on....didn't seem to me there was much help from them for us cub folks. The wing issue is the main reason I went with a flush riveted alum skin wing to help with airflow....we'll see if it helps.

Crosswind's mid wing fence at the junction of the flaps and aileron and the wing tip fence aft of chord. We discussed the mid-wing fence and consensus was it didn't really help at slow speeds and I'm guessing the wing tip fence is there to enhance aileron effectiveness at slow speed and not to help with lift....right??

Apart from trying to improve low speed performance, Do you think any of these wing tip designs would improve eficiencies at cruise speed on a cub?

I believe that there was a study done by an engineering team for Cessna for a wing fence for STOL performance, and the outcome was very similar to what Lowrider was implying. Basically it's one of those things that works great in a laminar flow, but any slight deviation from an idealisation of laminar wind flow will make it useless or minimal to the point that they can't quantified.

Lowrider. I'll admit that a consensus of opinion is a lot easier to come by then facts. Or even good test results. Tell you what. You test Crosswinds mid-wing fence and the wingtip fence and we will compare the results of your tests and mine. We may not arrive at the absolute truth but it will be a lot closer than a consensus of opinion and a guess. right?? After all the consensus of opinion was that man will never fly. Jerry B.

Jerry,

I'm not real sure what you are saying but I'm only repeating what I've been told here by Dave Calkins primarily. I had a "stall fence" on a 172 and I thought it was helpful and was told it wasn't worth doing. Are you saying they are effective and should be used or not? I'm sorry, but I have no clue what you have on your plane or what you are suggesting.

A favorite quote of mine: "One good test is worth a thousand expert opinions."

I've wondered about the effectiveness of the mid-wing fence on the Crosswinds leading edge STC, but have not done any with/without comparisons. Jerry, can you share some results in this regard?

...what did I say?

I have been out of the loop for a couple weeks......in Hawaii.......and NOT going home soon

..and folks.....When Jerry Burr speaks...you should listen.

Don't quote me but it was something like...don't bother with mid wing fences. How's Hawaii...taking care of the AK whine?

Lowrider What I suggested was that instead of having a consensus of opinions that ( X-Winds midwing fences ) don't work that, who ever "we" are should test them on a Cub and come up with some useable test results. I would be glad to compare my results with yours to see if they agree or not. If they agree then we have some good information to share on the chat. My results are only good for one airplane. I greatly respect Dave's knowledge and would happily accept any test data he has to that effect. Jerry B.

Jerry,

I would be happy to play but I'm just building an LSA cub like plane and looking for advice on what to include on the build. I don't think using my C-170 as part of the "test" would help much. I also have a lot of confidence in Dave's opinions as well as yours. Opinions from informed folks based upon experience are worth a lot to me whether there is consensus or not. I'm all ears!!

Lowrider said:

Crosswind's mid wing fence at the junction of the flaps and aileron and the wing tip fence aft of chord. We discussed the mid-wing fence and consensus was it didn't really help at slow speeds and I'm guessing the wing tip fence is there to enhance aileron effectiveness at slow speed and not to help with lift....right??

Click to expand...

As I have mentioned before, I have tested a fence placed between the aileron and flap on one wing of a Cessna 185. It made absolutely no noticeable difference. This is because there is very little, if any, span wise flow in the middle of the wing.

cgoldy said:

Apart from trying to improve low speed performance, Do you think any of these wing tip designs would improve efficiencies at cruise speed on a cub?

Click to expand...

I have also modified and tested various wing tip shapes and extensions on several different types of airplanes including Cessna 185, Champion 7GCB, Republic SeaBee, PA-18, Colonial C-1, Cessna 206, Grumman G-44 and Twin Bee. There may have been others, I do not recall. In all cases, in addition to improved low speed, take off, landing and climb, performance, there was also improved cruise and altitude performance. In some cases it was not a lot, in others it was very noticeable. The improved cruise was generated by a reduction of induced drag which came as a function of the increased aspect ratio, or effective aspect ratio. The result of the effective increased span helped the fuselage to fly level (picked up the tail) generating less drag. Thus more cruise speed.

The higher pressure, under the outboard section of the wing, tends to flow outward towards the lower pressure above the wing. This generates a swirling flow (vortex) at the wing tip. When this flow from below curls around to the top, it "pressurizes" the lower pressure air on the top of the tip. This renders a portion of the outboard section of the wing less effective. If you can shape the wing tip area to prevent or reduce this span-wise flow and vortex, the wing tip area will be more efficient. What ever you place on the tip, which will reduce this swirling vortex, will help. The secret will be to design this "fence" to produce the minimum amount of extra drag. An ugly flat plate screwed to a full sized tip rib will work. There are many sizes and shapes which will accomplish the mission. The trick is to find the most optimum shape which is also pleasing to the owners eye.

Also, by reducing this vortex, if the aileron goes to, or nearly to, the tip, the outboard section of the aileron will be more effective. This was particularly noticeable to me on the 7GCB. A 7GCB or most of the Champion/Citabrias have sloppy ailerons. I installed Ferguson drooped tips on the 7GCB with the result of this sloppiness having disappeared. The ailerons became very much more effective.

Thank you skywagon8a.
I will share the results of my mid-span fence testing. Keep in mind that these results are for only one airplane. As a lot of you know I am a weight freak. And the fences looked like a good way to save some, if they were not paying their way. So I removed them, and this is what I observed. Approaching the stall there was a circular (horizontal) flow developed mid chord at the junction of the flap and aileron. This increased with associated turbulent flow until the break, when a fast rush of air from the wingtip moved span wise towards the fuse. Wiping out all semblance of uniform flow. The droop tip was also tufted and the air did not originate from below the tip (as per a standard wing tip vortice) as the only tufts to point around the tip were within 12” of the leading edge where there was no droop. The rest of the tufts stayed in trail under the tip throughout the stall break. The air seemed to come from space beyond the tip. It was a sharp break as this took place very quickly when it went. Then I replaced the fences. The fence had a tendency to impede the circular flow and definitely delayed the span wise flow. Which created a much more docile break. I decided to leave them on if for no other reason than increased low and slow safety. As I stated this may or may not apply to your airplane. You just have to try it and find out.

Very interesting Jerry,
I assume that the airplane is the one shown in your avitar? Are the ailerons and flaps of the original dimensions or have they been altered? Were you able to video the tufts on the top of the wing? It would be interesting to be able to see the picture.

I have not used any tufts nor any specialized instrumentation in any of my tests, only basic instruments and seat of the pants observation. As you stated, the results on different airplanes will likely be a bit different. There are so many different combinations of shape, dimension, weight, aspect ratio, etc, etc.

Skywagon8a Yes, same plane. 42 years now. The flaps are stock, ails longer. Yes I use video and tufts for my observations of the top and bottom of the wing or whatever I'm testing. I use the same test criteria each time so it is easier to repeat a base line. I don't trust seat of the pants. Take care. Jerry B.

here is what i did, VG's, fences, it has droop tips but i also installed a small fence on the top of the wing tip to reduce vorticies. the plane has great control

Not pointing at anyone in particular. but some **** LOOKS cool.. and some stuff WORKS ! Thanks Jerry for the true data.

This may seem like a dumb question but since the airflow on the bottom of the wing flows out to the wing tip and spills over to the low pressure top side causing the vortices would a fence on the BOTTOM of the wing just to the outside of the aileron (cub12 above has a small one on top) stop that flow and improve performance?

It would certainly be worth trying.

Sky,

Just so I'm clear on this...the wing tip fence below the wing is there hoping to improve aileron effectiveness or what? It seems the wingtip vortex occurs after the airflow leaves the wing so I'm not sure what the gain would be in using one...turbulence behind the wing doesn't affect anything does it....or am I missing something?

Lowrider,

The Vortex generate greater airflow over the surface, but since your airspeed is constant you don't gain Cl, but rather drag, its what's called parasitic drag. This also cause interruption in laminar flow as it escape the tip of the wing with some reduction in Cl. Wing tips are designed to create a vortex in the outer surface of the wing area, thus removing the vortices that happen aft of the wing surface. Whitcomb's papers are the top authority on the fencing subject, unfortunately I don't have them to review some of the principals, but in essence you need a certain Re to gain those benefits, something that will occur probably above Vne.

If someone has an inside contact with Glassair, pls check with them, there was a guy with them that talked in another forum that their substantial testing, about 3 years and their findings was that fencing provided minimal to negligible results for their Sportsman. (I am just saying what others have claimed, this is not meant to undermine the findings of other testing and configuration on a cub type airplane, so please don't shoot the messenger.)

OpenFOAM (CFD) testing of other wings show that at about 30% of root there is Cl_max sustained at greater AoA with Re between 4 to 6 x 10^6. Chord of that wing was tapered and when from root of 75" to tip of 32" from my notes. I am hopping to get to this level of efficiency in OpenFoam, but I am still struggling with some of the basics at the moment .

Thanks Spain!!

Your explanations always leave me researching further which is a good thing. It seems we are finding more examples of testing that resulted in limited effectiveness regarding wing fences. It would be interesting to see the Glassair Sportsman test results.

here is a design that seen interesting, but I can´t find much about it



Here is another angle of the Tip/Fence...


There is a Black one that I took several pictures when I saw the airplane in Texas, but I can't seem to find them. They seem to have an interesting shape with a built in fence.