V.G.'s and stall fences.

[Rodney]

John - very nice looking airplane.

Could I get you to comment on the - I'm searching for the right word here - I guess stall strips - fences ??? that you placed inboard of each wing tip.

Why did you place them at that location on the wing? Also, have you had a chance to fly the airplane enough to evaluate if they are doing what you intended for them to do.

Given the rest of the care you took in building your airplane, I'm sure you did some significant research on this subject.

Thanks - and looks like you have an airplane that should be a lot of fun to fly for many years.

Regards
Rodney

[War Eagle]

Hi Rodney,

I have done some research when it comes to vg's, stall fences, wing cuffs and other technics used to improve stall performance. I am most interested in vg's as a technic to help keep the laminar air flow attached to the wing as you increase the angle of attack. This benefit can help you lower your stall performance.

So in the future I plan to perform some experiments using vg's (various quantities and locations) to see if I can lower the current stall performance of the plane. I have seen enough KF's with vg's to have high confidence this will prove benficial on my plane.

Another area of interest is the stall fence. If you were to watch wind tunnel tests using tufts to see the behavior of airflow over the wing as the AOA is increased you would see that the laminar flow seperation from the wing usually begins at the root of the wing and then works its way out to the tip of the airplane. When you get enough laminar flow seperation across the surface of the wing then you get the stall. The theory behind the fence is to break up the propagation of the laminar flow seperation so it doesn't so easily spread across the wing. In effect a stall fence creates something similar to a wall. The seperation is disrupted (not necessarily stopped) and then effectively has to restart on the other side of the stall fence but with a lower magnitude. You in effect don't stall the whole wing but it sort of goes in sections. The net affect is that you should be able to increase your AOA even more thus decreasing your stall and providing more contol of your stall behavior.

So my plan is to do some experimenting with the addition of stall fences with the hope that I can see improved stall performance with the combination of vg's.

So prior to covering my wings, I designed several attachment points to my wing ribs so that I could add up to 3 stall fences across the span of the wing (this includes the wing tip fences you see in the picture). When I complete all the experiments I hope to do, I don't know if I'll want to use one, two or three fences across the wing surface. However, it was important to me to provide the capability in case it proved out to be beneficial. If my testing doesn't prove to be benefical then I don't have to redo my wing or try to cover up some external attachment points etc.

Now to your question on the stall fence at the wing tip. I designed this fence to be sandwiched between the wing tip and the end rib because the fence extends above and below the entire surface of the airfoil. This created a more solid attachment for this design. Since I was finish painting the plane I didn't want to come back at a later date and try to remove a portion of the painted wing tip and then reset all my wing tip attachment holes in order to make room for the wing tip fence in between the end rib and the wing tip. Therefore, out of the shoot I installed the wing tip fence and modified the wing tip so that it would mount up close to the fence so that I wouldn't be faced with the wing tip changes a a later date. So if I decided that I was to remove the wing tip fence I would now just fill in that gap with black RTV to cover it up and it would not be seen unless you were looking for it.

In practicality, I had several experienced KF pilots fly the plane with the wing tip fence in place and with unpainted wing tips to help me judge if there were any negative affects from using the fence in this location.

There were none that could be detected so I decided to leave the wing tip stall fence in place and finish paint the wing tip even though I had not completed any formal vg or stall fence testing as I have discussed above.

Since I was planning to take the plane to Arlington soon after the completion I thought it would look cool to leave them installed and not worry above filling a gap between the wing tip and the wing.

So to sum this all up, in the not too distant future I do plan to do experimentation (via flight testing) with vg's and stall fences.

If you are interested is being pointed to some of the research material I have studied then send me a private email and I will try to share some details.

[HighWing]

Just a couple of comments.

The twist built into the wing would cause the progressive effect of wing stall from inboard to outboard. This is designed to eliminate a sudden and likely catastrophic stall of the whole wing. The Lancair IV has a wing design that where the wing tip stalls all at once and going inverted not uncommon in a stall and attempted recovery. The manual says do not practice stalls, for that reason. I wonder what a stall fence would really do. Will its presence prevent the stall beginning beyond the fence, or will it just start independently as a function of angle of attack just as it does at the wing root? I know of a couple of other Kitfoxes that that have them, but have heard of no data supporting their installation. The fact that there was not degradation of flight characteristics, to me, is not the same as enhancing flight characteristics. I am all ears (eyes) regarding the possible reports of your experiment.

Regarding the vortex generators. I recently received an email from an old time participant in the original Kitfox List forum and this is a quote from his email.

(quote)

Did I mention that I took a few worried e-mails from VG sales people after what I wrote? They worried about sales, but I can't support false claims. I have to see it really work and I was very saddened by the lack of stall speed reduction in my tests. I did a lot of position changes fore and aft as well as spacing. Nothing saved me much on stall speed, but I could drop my top speed down a good bit if I wasn't careful.

I want to try some more tests with them later, especially doing the bottom of the horizontal stab to see if that helps. But I really feel I got full stalls from the stab as is, especially hearing the thump of the airflow reattach to the top of the wing sounding like a flag waving, or the snap of a bed sheet when you make the bed, as I recovered from the stall.

With the VG's I felt very safe doing stalls, maybe getting cocky a bit near the end. I did some below 2000 feet AGl.

The power on stall with VG's did get me down another 5 knots, but the stall was wicked and it snapped left on me. I only did one because my VG's were taped on and I threw a bunch off the left wing with that stall. Power on, full stall landings would mean a tail wheel first slow landing, but there would be no return to flight. You couldn't get the same angle of attack once the mains were down, so I feel the ground run would be nice and short.

He had a lot of ideas on drag reduction as well, but that will be for a later day.

Lowell

[War Eagle]

HighWing,

I too have seen and heard from those that have tried them and found no benefit as well as those who have tried them and were able to get an improvement and now won't fly without them. I can't begin to tell you why there is such a varied affect but I am guessing there are lots of variables that could come into play. But that's what experimentaion is all about.

I agree with your comment on "no flight degradation" is not the same as enhancing flight characteristics. But as I stated, I had other motives for trying to leave those wing tip fences in place and since I hadn't done any experimenting to prove any enhancements I wanted to be sure there was no degredation.

In the video shots of wind tunnel tests that I have seen the stall appears to re-start independantly at the fence (as a function of AOA) just as it does at the wing root.

[Rodney]

John - thanks for taking the time for such a detailed response. I had a feeling that you had done some significant research into the subject.

Been really busy at work these last few days, but I hope to send you a private email shortly to discuss some other ideas I want to explore.

To me, this is one of the major aspects of building your own airplane that is so attractive. What you are doing is so very interesting and really makes having an airplane you can experiment with so much fun.

Regards
Rodney Wren

[HighWing]

John,

I agree that there are a lot of variables and I have read of the great improvement, but my understanding remains that the improvement has to do with handling at slow speeds with minimal to no reduction in stall speed. One guy very experienced in Kitfox mentioned that they sure make washing the wing difficult. That comment spoke volumns to me.

Lowell

[DBVZ]

Here are some VG's offered from Aircraft Spruce. I would like to hear from anyone who used any of these products and how it worked for you. One mentioned "all you need is the glue". So what glue did you use? First 2 are sets of plastic, the third is metal.

6. STOLSPEED VORTEX GENERATORS
Vortex generators (VGs) are well-proven for better low-speed handling, softening the stall characteristics, and lowering the stall speed, which makes it easier to do slower, gentler landings. Stolspeed VGs are streamlined for less drag and better
http://www.aircraftspruce.com/catalo.../stolspeed.php - 2.7kb

05-04458 STOLSPEED VORTEX GENERATORS 80 $69.95

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7. AIR WAVE VORTEX GENERATOR
know for some time now that vortex generators reduce stall speeds and improve the aircraft’s handling performance. vortex generators allow the wing to develop more lift and lower airspeeds. This reduces takeoff speed and improves the rate
http://www.aircraftspruce.com/catalo...evortexgen.php - 2.5kb

05-04662 AIRWAVE MICRO VORTEX GENERATOR $55.00

8. VORTEX GENERATOR FOR EXPERIMENTAL AIRCRAFT
These Vortex Generators were designed and tested in a wind tunnel as well as on various experimental aircraft. They are formed in pairs so that the shape and angles are built right in.
http://www.aircraftspruce.com/catalo...xgenerator.php - 1.9kb

05-01480 HALL VORTEX GEN KIT $184.50