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'.