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.

[N82HB]

I am not very happy with the high approach speed of the Kitfox and would love to try and find a solution; it is the only thing less than great about the airplane.
I have a Model 4 w/ a 912s now, but I previously had a 3 with a 582. I did install VG's on the 3. I used the ones from Landshorter.com and located them a 10% of the chord which is known as a common success point on most airfoils. I did not see anything noticable after the installation. But remember that is a totally different airfoil. I also did not put any on the horizontal and I think that could help.
On the 4, I tend to run out of elevator at slow speeds. I did try taping the gap as suggested by many forums about many different airplanes, but the problem persisted. I then removed the tape.
I am very interested in trying a few more things to get this solved. 4 mph would be a great win in my opinion.
I am once again thinking about snow falling and installing my skis. It could be a good winter for that up here.
Just letting you know people are reading and listening,
Kelly

[HighWing]

Kelly,

I just helped a neighbor attach the empenage on his RV10 before his inspection. He had spacers on the forward attach point of the horizontal stabilizer to srt the incidence and I asked him how he determined the thickness of the spacers. He said it was a factory thing. The factory recommends a starting point, then during the test phase, he is to observe the elevator's relative position to the stabilizer in cruise and if it was not nearly parallel, to change the thickness of the shim to correct the relationship.

This is rarely if ever discussed on Kitfox so I wonder how many of us are flying and living with horizontal stabilizer issues. I wonder if dropping the leading edge a bit might help in the flare.

Lowell

[N82HB]

I'll look at that the next time I am standing at the Kitfox. I hadn' thought about that being adjustable. It was on my Tailwind and I took advantage of that.
Thanks,
Kelly

[War Eagle]

I'm not much of a KF historian but I believe that over time, design changes increased the size of the tail feathers (rudder and horizontal) to improve some of the flying characteristics (which included flair behavior etc). I believe that between the model 1 and the model 4 each model saw larger tail feathers. When the model 5 was introduced then it's tail feathers were again modified to help provide improved control. The model 6 has the same set of tail feathers as the 5 but when the model 7 was introduced the motor mounts were lengthened (4 inches I think) to move the cg forward (this allowed higher baggage compartment capability and I guess potentially lighter tail loading) and a larger elavator was added to again improve the flair control. The horizontal on the 7 is the same as on the 5 and 6 (I believe).

So in theory you can add the larger horizontal from the 7 to the model 5 and 6 and get more elavator authority. The elavator from the 7 does not fit the 4 or earlier models.

Vg's added to the underside of the horizontal are supposed to to have the effect of increasing the flair authority. I have several model 7s (in tail dragger configuration)that I fly with that have installed vg's on the underside of their horizontal and they reported an improvement in elevator authority.

I don't personally know anyone that has added vg's to the underside of their model 4 horizontal so I can't say anything about any improvement in their elavator authority but that might be one thing to try and see if it provides more control for you.

[DesertFox4]

Kitfox Aircraft LLC has had an enlarged elevator for sale for some time now that fits the 3,4 and Lite Squared models.
Page 31 of their online parts catalog.
Part #51018.201 (with out electric trim)

Part # 51018.301 for those using electric trim.

Powder coating is optional on both versions.
I run the last version on my Classic 4 and it has all the flair authority i'll ever need.

[BigJohn]

I did more flight testing with the VG's and found that at Gross weight I was unable to fully trim with full Flaperons on, I again listed below along with the Gross Weight #'s. I have since installed the VG's to the underside of the Horizontal Stabalizer as in Pacific NW Aero plans and saw improvement in more rapid recovery at Stall and slightly lower speeds, very solid 1mph with 2mph at times but not consistant, I still have to test at Gross Weight with the Horiz Stab VG's installed. Thus far all the findings are very favorable, I did loose approx 50rpm at max speed that is I am flying at max speed now at what was the same speed at about 50rpm less, not much of a loss at top speed 2-4mph but at normal Cruise speed I lost nothing measurable.

Short Field Takeoff's and Landing tests at 3800 Density Altitude showed consistant improvements of approx 50-70'. This was accomplished by comparing landing and takeoff distances with two other aircraft (Avid B & Remos) prior to VG's and After wing VG's were Installed. Prior to wing VG's both the Avid and Remos were able to takeoff slightly shorter and land as short or shorter. After the Installation of VG's I was able to consistantly out perform both aircraft on Takeoff and Landing and this was with between 100 & 250lbs more gross weight than the other two. We were seeing consistant takeoff and landing sub 200' at 3800' density altitude at 1100lbs and sub 300' at 1300lbs.

Gross Weight 1100lbs Stall (Indicated)
Prior to VG's / After Wing Vg's / Wing & Horiz Vg's
No Flapperons 46 41 40
Full Flapperons 42 37 36
Full flight control with flapperons on at 40mph

Gross Weight at 1300lb Stall (Indicated)
No Flapperons 49 45 Still Testing
Full Flapperons 45 42 With 1st Notch only on, With full flapperons on at this loading full trip could not be achieved.
Still testing with wing and Horiz vg's at 1300lbs and full flapperons.

The 1300lb testing was done with very little loading in the baggage area mostly accomplished by Pilot, Co-Pilot weights, fuel and only about 30lbs of weight in the cargo area.

The 1100 lb testing was done with Pilot, Fuel and about 30lbs in the cargo area.

Aft loading of CG certianly would change things and allow for more successful trimming, i.e. not running out as soon.

I know that all of these findings are some what Subjective based on Pilot skill weather variances etc but this is as accurate as I am able to get and I am very happy with the overall results.

[BigJohn]

Completed the flight testing with Wing and Horiz Stab VG's at 1300lbs with the following results.

No Flapperons 45mph
1st Notch 41mph
Full Flapperons 38mph

[BigJohn]

Forgot to add photo's to the above posts, So here you go.

[N82HB]

Using your photos as a guide I installed a set of VG's on a Kitfox IV. The VG's I received from Landshorter.com had a template, but only a suggestion of the fore/aft placement. I placed these like yours and am very happy. This was on a friend's airplane, but I will be doing exactly the same on mine soon.

I did make one other change. I did not put any VG's on the wing inboard of the fuel caps. This area will typically be boosted by prop blast so the loss should be minimal. My thought was that I did not want to have them in the way while fueling, with cans especially.

I have not done any full on flight test yet, but I will say I was flying final at speeds I had not seen before. I would guess a solid 5 mph change.

Thanks for the quality info!
Kelly

[BigJohn]

Kelly,

Glad to hear that you have innitially found some good reduction in approach speeds, I have seen significant improvement in the STOL performance with the VG's and have been able to get full touch to stop landings on dirt in 150' now at DA of 3400' at 1100lbs. also on the measurement on the VG placement I am curious if you are about 5" from the leading edge to the front of the VG on your placement or where they ended up?

[wildirishtime]

Did any of you ever test stall fences? Recommended locations with any notable success? Are there any that are easy to install to the surface without having planned for them during the build? Thx guys!

[rogerh12]

I thought a stall fense was something you only found on a Russian swept wing jet fighter. Yes?

[Dave F]

Stol fence I do not think would do much for the Kitfox.
as we have full span flapperons. You would normally put them on top of wing at the outboard end of flaps to keep undusturbed air ove the ailerons eg - C 180/185
Tha being said - you could put on at root of wing and at outer end of wing in line with flapperon to try to hold in the low pressure on top of wing

Kitfox placement in those pics of Big John are way back -- I thought I read about 5 " ? His look like 6 to 8 " back

this is about 10% here http://www.cfisher.com/vg.html
which should be about 5.1 " if I recall.

I have done thorough flight testing and still have them on my Kitfox - some have fallen off. I would like to experiment more with then to try to find the claims of some folks but I think alot of the gains are not exact.

I will say the stall is docile to begin on a IV that I have them on and the VG just made it more docile.
as far as shorter take off ? small gain
you can cllimb at a a little steeper AOA
top speed loss ?: None in this case but we only talking a 92 mph cruise now.
Power on stalls the buffet is non exisitant so you have little or no warning of stall. But when it ready to go you gonna have a fun ride with a instant wing drop -- kick in some rudder to get it more fun :)

This is where the VG are not good -- stall forewaring - buffet not there and more. in- experienced pilot could get in trouble in a hurry.

Be safe out there.

[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/catalog/appages/stolspeed.php - 2.7kb

05-04458 STOLSPEED VORTEX GENERATORS 80 $69.95

05-04457 STOLSPEED VORTEX GENERATOR 120 $99.95

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/catalog/appages/airwavevortexgen.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/catalog/appages/vortexgenerator.php - 1.9kb

05-01480 HALL VORTEX GEN KIT $184.50

[MotReklaw]

DBVZ,

I put the stolspeed VGs on a Zenith 701, both the wings and the horizontal stabilizer. I left the slats off. We probably picked up about 3-4 mph and the stall speed seemed to be about the same. But the 701 is almost stall proof anyway. The VGs came with a template to align them with each other and it took about a hour after fixing the self sticking adhesive on each VG. I think the adhesive was a 3M product that came in strips that you trimmed to fit the VGs. I forget the distance from the leading edge that the instructions said to place them, but it was all spelled out in the literature.

Good luck,

Added: I bought mine from the guy who makes them in Australia. He's done a lot of testing and I believe he's on the web at this address: http://www.stolspeed.com/

[DBVZ]

Found them. Part 2420-SP for $289. Not much detail in the catalog, like how many and how attached. Aluminum. Thanks.

[DBVZ]

Comment on this plan please:

I have the short Avid Aerobat wings. I am thinking the 120 piece STOLSPEED VG's will be more than enough for both wings and tail. The other brand is 100 for both. I suspect if I need to cut anywhere on VG's, it should be very near the fuselage where prop wash will have some effect and a partial wing stall has least leverage.

Thanks for your advice.

[MotReklaw]

DBVZ,

I think the spacing for the VGs was different in the area where the prop wash was for the 701. I would assume that that would hold true for the KF.

Funny think about the tail VGs on the 701.... One would think they went on the leading edge of the stabilizer, but no, they were placed on the elevator and in normal flight they were hidden from the slip stream by the horizontal stabilizer. They were not exposed to the slip stream until the flare while landing. That presumably gave you more elevator control at the flare....

(But, we ain't building a 701 are we)! :)

been there done that!

[DBVZ]

These guys post a diagram of the tail showing them all just ahead of the rudder and elevator, and it looks like 3" spacing. On the vertical stab it shows them all 5* off level flight. I assume the diagrams from each mfg will have similar recommended positioning.

http://www.aircraftspruce.com/catalog/graphics/05-04662.jpg

[kitfoxnick]

The vg's that come from Kitfox come with everything you need, right down to a little tape measure. They have vinyl decal material that is a template that you apply to the wing. It makes placement really easy. As far as placement, they're installed further apart near the wing root so that the inboard will stall first and the stall will progress outboard. I installed mine at 8% of cord after talking with the owner of Pacific Northwest Aero who is the owner of the company that the Mcbeans get them from.

Some people don't have much good to say about the vg's, but they made such a dramatic difference in slow flight. The plane is rock solid with very little mush. If you power off stall, with the stick all the way back, the airplane stalls then breaks, and instantly recovers without ever letting the stick forward. Power on the break is more abrupt with little warning. You used to be able to spin mine without much trouble, just stall it uncoordinated. With the vg's, it took three attempts before I could get it to spin. I would stall it uncoordinated and as the wing would drop a little bit it would recover. I've never flown something where you could stall it with the stick back and the ball off to the side and it wouldn't spin, or at least spiral. The plane just kinda slides off to the side, stalling and recovering. I was finally able to get it to spin by stalling it then kicking the rudder abruptly.

I like my vg's:)

Nick W.

[DesertFox4]

kitfoxnick- thanks for the experience based testimonial. Sounds like you got excellent results from the Kitfox Aircraft supplied V.G.'s. They do take the guess work out of installation and hours/days of test flying to learn best location and spacing.

Part 2420-SP for $289 in the Kitfox Aircraft parts catalog.

Kitfox Aircraft LLC. Parts Catalog (http://www.kitfoxaircraft.com/Product_Catalog/KApartscatalog.pdf)