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

[War Eagle]

Good Evening HighWing,

I share the same understanding with you that vg benefits (when they are there) such as control improvements and stall improvements (again when they are there) are seen at the slower speeds. I know of no vg installation that has done anything to improve high speed performance or control.

When vg's are installed one of the things you try to accomplish is little or no degradation of the high speed performance due to increased drag.
Placement on the wing and design of the vg can influence this issue. As in many things there usually is a trade off between functionality and performance. But that's what experimentation helps to determine.

From my reading I see stall improvements reported in small increments of 1-2 mph and some up to 5-8mph. And as we have discussed earlier some don't see any decernable improvements in stall at all.

I also do think that vg's will provide a challenge when it's time to wash the wing. Probably good to use a soft long bristle brush when washing the wing to try and avoid injuries to oneself or the wing. Oh those dang tradeoffs!

[HighWing]

I am all ears (eyes) as you proceed with your testing. You mention VG design and location. One thing I tend to think of is wing design. Our scalloped wing leading edge - ribs and false ribs with shrunk fabric between - is much different than a completely flat leading edge (the exact area where the VGs are located). In a conversation with a friend once who had spoken to Harry Riblet, I was told that Riblet felt that the most important area of his design was the first three inches of the airfoil and he considered the Kitfox design a compromise of the design. My thinking and it is only that, is that with the different air pathways over our leading edge, we are already getting some burbling with the interaction of the neighboring air flows, giving the effect of a poor man's VG system. I am hoping that the Rotec SS will experiment with VGs for some answers there, as they do have the Laker fiberglass leading edge and a true Riblet airfoil. If they find marked improvement, it could open a definite market for the new leading edge and VGs. With the traditional Kitfox wing, in all the reading I have done on three forums, I can recall maybe one poster, that noticed significant stall speed reduction. Uniformely, they noted better handling at slow speeds. As I mentioned, I am anxious for the reports.

Lowell

[BigJohn]

I have been slow on posting anything on VG's due to the fact that I wanted to have some personal facts before I did post my findings. I fly a Kitfox S-5 and have done extensive flight testing over the past 5 months prior to putting on VG's with consistant results at various Gross Weights and Significant improvements with VG's as listed below, Most of the flight tests were conducted with two other aircraft in the air at the same time confirming like conditions and there consistant results in those conditions to doubly make sure that various air conditions were not makeing any significant differences.

Prior to VG's
Gross Weight at 1100lb GW
No Flapperons Stall 46 Indicated. Landing Touchdown 42 Indicated
Full Flapperons Stall 42 Indicated. Landing Touchdonw 39 Indicated

Gross Weight at 1300lb GW
No Flapperons Stall 49 Indicated. Landing Touchdown 45 Indicated
Full Flapperons Stall 45 Indicated. Landing Touchdown 42 Indicated

After VG's Installed on Wings Only
Gross Weight 1100lb GW
No Flapperons Stall 41 Indicated. Landing Touchdown 36 Indicated
Full Flapperons Stall 37 Indicated. Landing Touchdown 32 Indicated
Full Flapperons slow flight control in turns at 40)

Gross Weight at 1300lb (Pending) should be completed this week.

I will also be installing the VG's on the underside of the Horizontal Stab and continue flight tests to see if there is any improvements.


Cruise tests showed no loss of airspeed and this was again also done comparing cruise speeds and RPM's prior to VG's and After VG's along with two other aircraft. I have not yet done a max speed comparrison but will try to complete those tests later this week.

The VG's are from Pacific NW Aero LLC in Washington State and I used there recommended placement. All of the VG Flight test results listed above were done after the VG's were perminently attached and painted.

[War Eagle]

Hi BigJohn,

Thanks for sharing your test information.

I believe the vg kit you used sends a test form for the buyer to utilize in testing the performance of the plane with the vg's installed. Can you add a few more details regarding the test process you used? Are you planning on using any vg's on your vertical tail or any stall fences on the wings.

Is it possible to add some pictures? What is the configuration of your plane and have you made any other mods on the fuselage, wings, struts etc..

The results (lower stall speed) you have reported so far are really good and seem to be in the range of other reports where they seem to get 4-8 mph reduced stall speed.

The interest in this subject is generally high but comes with reports of mixed results. Reports are somewhere between "no stall speed reduction" and to reductions of 4-8 mph.

Will be looking forward to your additional posts as you complete your installation and tests.