Vortex Generators

[Madhatter]

For several months I have been working on trying to reduce stall speed with the use of vortex generators.  I installed vgs on wings and stabilator at various % chord and even used two types of vg's however was unable to provide a significant reduction in stall speed. 

About a month ago I was having a conversation with Tom Peghiny about vg's and he brought up a possible issue of something not having to do with stall speeds. It involved possible boundary layer separation in cruise on the top of the cabin. I installed string tufts along the top of the cabin and with a video camera mounted on the vertical stabilizer and found that no separation occurred in cruise. Later that day I ran the full video and saw that the boundary layer completely separated during the flare to landing. I guessed that this might affect the tail due to significant disruption of airflow on the stabilator so I added 22 vg's about 2 inches behind the windshield. Flight tests showed no boundary separation during flare  to landing with the vg's. Cabin vg's definitely would be a positive addition due less drag and maybe better stabilator control when landing to avoid sudden descent that we all are aware of on a CT. I discussed this issue with Anni Brogan, owner and test pilot for Micro AeroDynamics and came to the conclusion to add vg's to the stabilator. I added 40 vgs to the stabilator at 4% chord and tried them in a convergent pattern and a divergent pattern. I settled on the vg  pattern NASA did in a study and is recommended on stabilator type aircraft. I installed string tufts on the stabilator before and after installing vg's and videoed them with a camera mounted on the gear leg.

Then the flight tests started and all I can say it was a game changer  when it came to landings, like night and day comparison. There was total stabilator control all through te flare  to the landing. Over several dozen landings over a number of days I tried to force the plane to stall out of the flare with no luck, I flew on calm days and also with 11 kts gusting to 17 kts. Sometimes a gust

 would balloon the plane up and still had effective stabilator control. All landings were done with engine at idle. Once I prolonged the flare so long I got the angle of attack so high the bottom of the tail scraped the runway and the plane continued to a nice landing. If you flare at 6 ft above the runway I'm sure nothing will help except some power.

I also performed approach, departure and accelerated stalls at altitude and found no change in handling. I generally don't do full power stalls in my plane as I have an E Prop which gives me a higher angle of attack and with the vg's my concern is a possible tail slide if I hit turbulence at the wrong time.

My vg's are now permanent.  If anyone decides to do this change the vg kit is available from  Micro AeroDynamics for $395 and includes everything.  I would suggest that you should attach the vg's temporarily with the double faced tape which is included and evaluate your plane's performance before making them permanent. Do your own analysis for your aircraft as some planes may differ. Mine is a CTSW.

The videos I have included are not geat and I had the mike on so they are noisey,  they are my first attempts for a film career

Edited April 23, 2021 by Madhatter
Clarification of convergent and divergent

[Mike Koerner]

Another very nice piece of work, sir.

Thanks!

Mike Koerner

[Runtoeat]

Well done study and thanks for posting.  Do you think the double back foam tape would hold DG's?  Thinking it would be nice if one wanted to remove without destroying them.

[Jacques]

very interresting....I think I will copy that.......have a question about VGs orientation

why are the stabilator  VGs in opposite orientation than the ones on the cabin ?

thanks

 

 

[Madhatter]

The Micro Aerodynamics kit comes with temporary adhesive. The vgs are reversed due to a NASA study that recommends a divergent pattern  for stabilator type aircraft. 

[AGLyme]

Best post in a long time... the outcome is superb.

[Mike Koerner]

To my eye, I would call the pattern shown on top of the cabin divergent and the underside of the stabalator convergent.

Mike Koerner

[Madhatter]

 

Just now I used 40 degree flaps and was able to hold the flare for a while.

 

[Madhatter]

I guess it depends on whether you are looking at the small space between vgs or large space. I guess that makes us both right. I try to use this logic on my wife but it never seems to work. 

[Madhatter]

I believe you are correct, I'll ask Anni tomorrow. Good observation.  The terms divergent and convergent are my interpretation of the pattern. The actual pattern is applied with the templates provided by Micro AeroDynamics.

[GrassStripFlyBoy]

Madhatter - Good stuff.  I'd have a hard time accepting VG's along the whole wing, but these areas look to be "bug friendly" and not nearly as bad from appearance view.

I'll very likely install these. 

[GrassStripFlyBoy]

22 hours ago, Runtoeat said:

Do you think the double back foam tape would hold DG's?

I believe so, other brand VG's rely on tape for install.  I didn't hunt it down but recall reading during the preflight some number of VG's are allowed to be missing (talking the whole wing high quantity install).  I think with good surface prep and quality tape it would work fine.  Worst case you'd have to fix one every so often.

[Madhatter]

I tried some Stolspeed vgs but they are plastic and are twice as tall. Micro Aerodynamics look better and do the same thing.

[FlyingMonkey]

Makes perfect sense, the Stab provides downforce instead of lift, so it makes sense everything would be backward (VGs on underside and reversed orientation).

Very nice work and clearly a lot of research and testing went into this.  Did you get any stall speed benefit once the fuselage VGs were installed and everything dialed in, or was it primarily a pitch control authority increase?  Either way, very cool. 

[Madhatter]

No stall reduction. 

[Philip Welsch]

Any chance vg's on stab will increase stress on pivot assembly?

[Madhatter]

No drag, no more load than normal.

[FlyingMonkey]

9 hours ago, Madhatter said:

No drag, no more load than normal.

That's what's amazing about VGs, they are essentially "free" performance.  The drag is so minimal as to make no difference, and on some airplanes the results are really dramatic.

[Anticept]

Vortex generators in the best of circumstances will reduce cruise by about 3%, but that is in high velocity applications.

For our little CTs, there's practically nothing that will register.

[Madhatter]

I saw no reduction at all with the vgs even when I was evaluating all the wing and tail vgs together, over 100 of them.

[Anticept]

Yep. We're too tiny to really suffer any significant parasitic drag effects that aren't already present.

 

That said, it is entirely possible that your vortex generators have reduced drag in cruise. The spine of CTs are a pretty aggressive slope, and its possible that boundary separation was already occuring near the boom. That would set up additional turbulence in the wake. The vortex generators might have reduced that effect and so compensates for any parasitic losses. Just a theory.

[Madhatter]

Vortex generator's drag varys by the geometry of the vg itself and by % cord location. The closer to the leading edge the greater the chance of drag issues.  Either way it is mostly negligible unless you get carried away. 

 

[Anticept]

I'm not going to pretend to know as much as an aeronautical engineer, but I have found little interesting articles on VGs over the years. The stabilator VGs was absolutely one that I was thinking of before, but putting them only on the top of the fuselage was not something I thought of.

Anyways, check this article out. This is why I stated my theory: https://phys.org/news/2012-09-scientists-purpose-vortex.html

It does mention how critical the design and placement are or it can work against you, just like you said.

[Madhatter]

I also put the vgs on the stabilator,  not just the cabin top. I found the boundary layer separation on top of the cabin only by accident when looking for something else brought up by Tom Peghiny.

[Madhatter]

I just want to add one thing. You don't have to be an Aeronautical Engineer to change or improve things in aviation, you just have to be persistent even when you fail. I know a lot of people who have done incredible things that they were not trained on. Even a person who repairs bicycles for a living can make a difference in aviation.