KF7 VG positioning

[wadeg]

I was treated today by my Fedex man delivering my pnwaero.com VG kit today. Their very-well-constructed kit has the proposed position but I was interested in opinions on the placement so I will toss it out there for debate:

How far back is the best spot for VGs on a KF7?

I will post the results of my testing after installation based upon their recommendations.

My current 'numbers':
Clean: 47
1/2 flap: 44
full flap: 40 (well maybe 40.5)

[War Eagle]

We used the same kit for the VG installation on 5 of the Series 7's we made in our group build.

All the technical papers I have read on the subject seem to suggest that 10-12% of the wing cord is the best spot to install them.

As I remember the PNW kit had directions that corresponded to that same area for the best install.

We counted the flaperon into the wing cord and came up with about 60" cord. Therefore the vg's were placed between 6" and 7" back from the tangent line off the leading edge. Remember the wing cord is the straight line that goes through the widest part of the wing (tip to tail so to speak) and not a distant along the surface of the wing (tip to tail).

We did a before and after test and found no drop off of cruise speed when the vg's were placed in this area.

We had one pilot (also with an S7) that bought a difference brand of VG's and he installed his 5" back from the tangent off the leading edge and his plane turned out to be a real laggard. His cruise speed was impacted negatively. Lost nearly 10mph on the cruise speed. We attributed it to the place he installed his vg's.

We kind of thought of it as more of the sweet spot to install vg's. You want the low stall speed and control for the slow flight but you don't want a large drag negatively impacting your top end or cruise speed.

[jtpitkin06]

This is always a lively topic. It's why we build experimental airplanes.

There are so many variables – The size and shape of the VG’s, the position, the angle, the spacing. Trying to give someone a “best” position is difficult without a wind tunnel and extensive flight testing.

A while back I remember one very good video on a tufted Kitfox with before and after observations of the upper wing surface during the stalls. It was a very professional approach. In that video there appeared to be a much better stall progression at a higher angle of attack after the installation. I don’t remember what effect it had on cruise or if it was stated in the video.

The thing that makes me wince is when builders go to great lengths to build their wings with smooth leading edges and fill in every bump. Then some install a bunch of beer can pop top shaped things on the wing thinking it will fly better. Well, maybe yes in certain regions, and maybe no.

When walking around the Boeings I marvel at the placement of VG’s. There are some in specific locations on the wing, but not all the way out to the tip. There are some on the rudder and curiously, some on the fuselage near the tail cone. Plus, there are some on the engine nacelles.

But the Boeings have high speed wings trying to fly slowly. On the other end of the scale is the Cessna Citation I. It has a low speed wing trying to fly fast. The Cessna has no VG’s. Clearly, there is not a standard position and they are not installed willy nilly.

So what’s this mean for the Kitfox? If you like experimenting, go for it. As the installer of the VG’s you become the experimenter and the test pilot. But you may not get the results you like the first time. If I were to put VG’s on a Kitfox, I think I would put them on with double stick tape so I could reposition them. I’d try different angles and different distances from the leading edge. Start with the recommended positions and work from there.



Adding VG’s is a major modification to the airframe which may put you back in the FAA sandbox while you test fly it. Check with your DAR.

That’s the fun of experimental aviation. You get to try almost anything you want.

John Pitkin
Greenville, TX

[rwaltman]

A while back I remember one very good video on a tufted Kitfox with before and after observations of the upper wing surface during the stalls. It was a very professional approach. In that video there appeared to be a much better stall progression at a higher angle of attack after the installation.

http://www.youtube.com/watch?v=bIsWseMbDQU (before)
http://www.youtube.com/watch?v=gor7LhsAILs (after)

Roberto.

[t j]

War Eagle wrote:

We counted the flaperon into the wing cord...

I would like to hear more about including the flaperon in the kitfox wing cord.

Is there flow seperation between the wing and flaperon? Does the flaperon fly by its self after the wing has stalled?

[wadeg]

War Eagle,
You mentioned that per the PNW Aero instructions you installed the VGs 6"-7" back from the LE. Is this accurate? My KF7 instructions from them indicate 4".

Can you confirm how far back they are?

[wadeg]

An update on my VG installation and testing.

As the instructions describe, they were placed 4" back. Here are the results:

Clean stall went from 47 to 45.
1/2 flaps went from 44 to 43.
Full flaps remained at 40.

I felt no increase in stability anywhere in flight and the stalls were much more sudden with very little warning. The stall break was MUCH more aggressive. At full flaps, the elevator was unable to produce enough pitch so a small amount of power was added to get it to a break. And break it did!

I powered up and top speed is down by about 15!

Needless to say, I took them right off.

My assumption is that I have them way too far forward at 4". They are soaking currently in MEK to get the carpet tape off the bottoms for the next round.

[ackselle]

Thanks for the update... Keep us posted with your results.

Cheers,
Ackselle

[HighWing]

wadeg - Very good report and interesting results. War Eagle - Your results were good as well, except there was no mention of their effect on stall speeds. Since this is the primary reason for installing VGs, it would be nice to have some stall numbers to help overcome what history has already suggested on the typical Kitfox Riblet wing that VGs have minimal stall benefit. Most prior reports suggest better handling characteristics approaching stall. The fact that since the group build built approximately identical airplanes, the results would be significant. I suspect the lack of mention of stall improvement suggests it was nothing that really stood out. I would consider them on my new Model IV if there was more evidence that they improved stall speed significantly. For me the factors that influence me installing them and then keeping them include a possible negative affect on cruise speeds (which was addressed in both reports) along with their nuisance factor when washing the airplane - Thanks John Mc. for that thought. Both of these are major obstacles for me.
Lowell

[wadeg]

I look forward to futher testing but today's results were a 'bomb'.

At 4" back,
1. No improved stability.
2. More aggressive stalls.
3. Lost top speed (significant!).
4. Negligable improvement with clean stalls and 1/2 flap stalls.
5. No improvement at full flaps.

Sounds like I am ahead of the 'sweet spot'.

[War Eagle]

War Eagle,
You mentioned that per the PNW Aero instructions you installed the VGs 6"-7" back from the LE. Is this accurate? My KF7 instructions from them indicate 4".

Can you confirm how far back they are?

Here is a copy of the install spec with our kit. When we bought it is was listed as a series 6 kit.

It shows 5 inches on the drawing, not 4 inches as you are indicating.

We actually made a template out of aluminum to install them.

The dimension is not measured along the surface of the wing but it is the horizontal distance measured from the tangent at the leading edge. We actually have them set at just short of 6" back. This is the leading edge of the VG's not the centerline as listed on the drawing.

[War Eagle]

wadeg - Very good report and interesting results. War Eagle - Your results were good as well, except there was no mention of their effect on stall speeds. Since this is the primary reason for installing VGs, it would be nice to have some stall numbers to help overcome what history has already suggested on the typical Kitfox Riblet wing that VGs have minimal stall benefit. Most prior reports suggest better handling characteristics approaching stall. The fact that since the group build built approximately identical airplanes, the results would be significant. I suspect the lack of mention of stall improvement suggests it was nothing that really stood out. I would consider them on my new Model IV if there was more evidence that they improved stall speed significantly. For me the factors that influence me installing them and then keeping them include a possible negative affect on cruise speeds (which was addressed in both reports) along with their nuisance factor when washing the airplane - Thanks John Mc. for that thought. Both of these are major obstacles for me.
Lowell

Highwing,

Don't want to rain on your supposition but we infact did see very positive results in several areas:

1) For the trail draggers we saw much more low speed elavator control when landing;
2) Saw 6-8 miles reduction in stall speeds without flaps
3) No perceivable loss of cruise or top speed
4)No abrupt stall characteristics with or without flaps.

Wadeg,

Your vg install is too far forward. I think you are really disrupting the air coming over the leading edge and thus causing you some undesireable characteristics.



See the test data sheet below

[War Eagle]

I look forward to futher testing but today's results were a 'bomb'.

At 4" back,
1. No improved stability.
2. More aggressive stalls.
3. Lost top speed (significant!).
4. Negligable improvement with clean stalls and 1/2 flap stalls.
5. No improvement at full flaps.

Sounds like I am ahead of the 'sweet spot'.

Let me show you a few pictures of some of our installs.

Most of the vg's are set at about 6" back on the horizantal (from a tangent at the leading edge). We have one plane that has them set close to 7 inches backand his plane is still working well.

No one has removed their vg's and all the installs have seen at least 6mph reduction in stall and several have had 8mph reduction.

They have been on the planes since 2007. Both tail dragger and tricycle gear.

[HighWing]

I guess I need to quit supposing. I just thought that since VGs were introduced to help reduce stall speed and nothing was mentioned in the post but its non effect on cruise....

Thanks for the info. May be worth a try.
Lowell

[DAT]

I've had VG's on a lot of the certified planes I've had and have always gotten good results with significant reduction in stall speeds and VMC on twins. I added them to my Kitfox Series V. They definitely helped with stability in slow flight, in steep turns, and with elevator authority. They didn't have any effect on top end speeds. Stalls are maybe 2 or 3 mph slower but nothing significant ... just more stable. Installed them to the specs that came in the kit from John Mc.

[wadeg]

Well,
Here is the update for positioning the VGs at 6":

1. Top speed down about 13.
2. Clean stall went from 47 to 50....yes, up!
3. 1/2 flap stall went from 44 to 47...yes, up!
4. Aircraft ran out of elevator (gap sealed) and power was needed for the 1/2 flap stall.
5. Aircraft needed more rudder input approaching stall...became generally unstable.
6. Aircraft was pitch sensative at higher speeds (cruise).
7. Aircraft in climb felt like a fully loaded Kitfox on a warm day.

Very perplexing activity. My thought it that it is somehow moving my center of lift. My CG is within range (actually a little bit toward the nose heavy end of the scale).

I will be shelving the VGs for now but may test them in the future again. I miss the stability of the plane as it is and the inherent gentle stalls. I better not look a gift horse in the mouth with a full flap stall at 40. I also wish not to be testing too much with a young daughter at home.

While the VG kit is on the shelf, I take away 2 things:
1. The Kitfox airfoil and design are fantastic as they are. Build by the numbers and you will have a great flying airplane as it is.
2. If you consider VGs, NEVER adhere them with the glue until you have done testing using the temporary double stick tape.
I do not mean to denounce VGs in any way. They have proven themselves in certain applications on certain aircraft. Some Kitfoxes do indeed get great results. Why that is?....well, we might need a wind tunnel!

And thank you to those of you who have been so helpful.

[GDN]

Same question for me is anybody install VG on a kitfox 2 and what was the optimum position.
Mine are stolspeed generators models

Thanks you

[War Eagle]

In the September 2012 Kitplanes magazine there is an article on vg's for GA.

It was written by Baraby Wainfan and discusses general designs, theory, placement, drag and size.

It won't give you the placement positions for the various Kitfox models but it might be useful to those that are experimenting with their placement and use.

[GoingHawgWyld]

I just finished installing the VG kit from Pacific Northwest Aero. Installed the kit specifically for the Series 7. Prior to install I was getting the following stall speeds:

Cruise at 5000 RPM: 105
Stall - No flaps: 49
Stall - 1 notch: 47
Stall - Second notch: 45

When I say stall I am bringing the plane to the point of when buffeting starts. This is full stick back and as soon as the nose starts to drop it flys again instantly then buffets a little and nose starts to fall again. Can't get the nose to fully fall with full stick back but mostly very noticeable buffeting.

After the VG install per the directions given (My VG's are 5 1/8 behind the tangent of the leading edge) I get the following:

Cruise at 5000 RPM: 103
Stall - No flaps: 48
Stall - 1 notch: 46
Stall - Second notch: 43

The main difference I notice is there is no longer any buffeting. Full stick back and it just holds at the above airspeed with no nose drop. I am descending about 800 - 850 FPM and plane is actually controllable with the VG's but never a break and nose drop.

I do not like the results, but now I am second guessing as to if there is an issue with how the plane may be set up... My C.G. is at 13.98"... Should I be seeing a more pronounced break in the stall?

Ken