turbulator tape vs VG's

[Dplunkt]

Has anyone tried "turbulator tape" like that used on gliders instead of the traditional VG's like microaero's? Did it work? results?

thanks

[GothiUllr]

I haven't tried it myself, but i know of at least one military drone that uses those tapes and they're cruising at 170 knots so it seems they can take the higher speeds. I don't know why they wouldn't work.

[Ron Blum]

It depends on your airfoil design, your Reynolds number, your boundary layer thickness, how effective you want them to be, and what you are trying to do with them. Some gliders (very low Reynolds numbers) use them to intentionally trip the boundary layer to be turbulent (so a laminar bubble/separation doesn't occur ... that's higher drag). No matter what VG (tape or other mechanical device you use), it will increase drag. Why? Because a VG takes higher energy air above (off) the surface and swirls it into the lower energy air on the surface (in the boundary layer). The trick is to lower more drag than you create.

For example, if air on the aft end of your airfoil is separated (this can happen both at high speed and low speed), you can add a row of VGs before the place where the air is separating. This will give the boundary layer more energy to try to stay attached to the airfoil ... and therefore reduce drag.

As another example for better stall characteristics, if the air is separating before the aileron as you approach stall, a row of VGs ahead of the aileron (and ahead of where the flow is separating) will add energy to the boundary to keep the airflow attached to the airfoil and give you better control. You will see this on several production airplanes.

VGs aren't a silver bullet, but they do fix some airflow problems.

[Dplunkt]

It depends on your airfoil design, your Reynolds number, your boundary layer thickness, how effective you want them to be, and what you are trying to do with them. Some gliders (very low Reynolds numbers) use them to intentionally trip the boundary layer to be turbulent (so a laminar bubble/separation doesn't occur ... that's higher drag). No matter what VG (tape or other mechanical device you use), it will increase drag. Why? Because a VG takes higher energy air above (off) the surface and swirls it into the lower energy air on the surface (in the boundary layer). The trick is to lower more drag than you create.

For example, if air on the aft end of your airfoil is separated (this can happen both at high speed and low speed), you can add a row of VGs before the place where the air is separating. This will give the boundary layer more energy to try to stay attached to the airfoil ... and therefore reduce drag.

As another example for better stall characteristics, if the air is separating before the aileron as you approach stall, a row of VGs ahead of the aileron (and ahead of where the flow is separating) will add energy to the boundary to keep the airflow attached to the airfoil and give you better control. You will see this on several production airplanes.

VGs aren't a silver bullet, but they do fix some airflow problems.

Hi Ron, thanks for the reply. I've been intrigued by VGs for a while. I happen to fly a PA-22 which uses the cub airfoil and responds to VGs well. I've been reading some abstracts regarding micro VGs (Not Microareo brand but smaller designs) that are no higher and in fact lower than the boundary layer and thus don't create as much drag as the ones we see today. Also, they are shaped in a V with the apex down stream and apparently create a larger vortex than traditional VGs given the size, increasing the effectiveness.

On another note there have been studies, as you say, with putting either dimple tape or zigzag tape just ahead of the separation bubble reducing drag. Piper puts micro Vgs on the flaps of their Meridian for example to increase the flap effectiveness and reduce stall. I would never do this on a certified planehttp://eaaforums.org/images/icons/wink.gif but I have thought about micro VGs at about 10% of the cord from the leading edge, trying to determine the separation and using zigzag tape there and maybe trying it on or near the flaps, etc. just to see if it noticeably affects stall and drag. According to Microaero which has the STC for PA-22s their VGs reduce stall by 8%-11%. I'd like to see if the smaller VGs could match that, etc.

Dan P

[Ron Blum]

STC for PA-22s their VGs reduce stall by 8%-11%. I'd like to see if the smaller VGs could match that, etc.

Dan: This is cool that you are experimenting! I am not sure what all you can do on a certificated airplane before you need to hang an experimental ticket on it, but ... way cool. (in my line of work I always put an X ticket on the airplanes).

Since your airplane is fabric covered, laminar flow is not possible; you will always have a turbulent boundary layer (not always bad). Tufting the airplane will show you where the airflow transitions from attached to separated, remembering that each little tuft is also a little VG. A GoPro camera (I am not endorsing a brand name, but you get the idea) on top of the vertical will show you flow over the top of the wing and flaps.

Height of the VG is solely the amount of energy that you need to keep the flow attached (if that is even possible). Flaps in the landing position are almost always separated on the upper surfaces. Although this doesn't increase lift, it does increase drag ... which in a landing scenario is good because you can approach more steeply and get rid of it (the excess drag) quickly in a go-around situation.

Be cautious of large performance claims: 1) Production airspeed indicators get less accurate at very, very low speeds because the differential pressures are very low ... on the order of 0.04 at stall. 2) Due to the higher angles of attack, fixed, production pitot tubes are less in alignment with the airflow and are not optimized for off axis flow. In other words, Flight Test departments use calibrated, very expensive, often swivel-head pitot tubes (to align better with the airflow) to get calibrated numbers and 3) OEMs have to be conservative when they draw the curve through stall speeds because every airplane must meet the numbers.

Have fun!

[Bill Greenwood]

I don't know much about this myself, but recently read an article from several years back in SOARING magazine, where they experimented with turbulator, which was a layer of tape perhpaps 2 in wide or so. I don't have the article, but recall some points.
First, gliders are different from most powered planes in several ways.
1. The are very slow, stall speed is often about 30 knots and best thermalling speed might be only 50.
2. The high performance ones are almost aviation freaks, with very long thin wings, maybe 60 feet and small fuselages.
3. Glide ratios, with normal wings, range from 22 to 1 for a basic Cub like 2-33 trainer to 60 to 1.
4. Trying the turbulator did work, and even hit glide ratios of 100 to 1. BUT, it was only that effective at one speed, in these tests it was right at 50 mph and fell off sharply at other speeds.

[Ron Blum]

You're right on, Bill. Sailplanes live in a VERY point designed world at VERY low Reynold numbers. People will say that there is a difference between 80:1 and 100:1, and there is. The pilot flying the 100:1 paid out the wazoo for that little extra, but he probably missed the thermal, didn't center the thermal properly (or stayed in it too long or too little) or didn't interpret the weather well enough to fly the right track to win the contest.

That's why Bob Hoover is one of my favorites. He didn't have the best _____ of anything, but what he does with what he is flying is just ... well ... awe-inspiring.

[Dplunkt]

Hi Guys,
Yea, I fly gliders too and I'm not sure that the turbulator tape would be the choice to replace VGs on the leading edge. As you guys noted the Reynolds number, aspect ration, etc. of gliders makes the comparison to powered apples to oranges. having said that I'm looking for performance enhancements in a couple areas one being slow flight and stall delay which puts me in that 45-60 mph range but high angle of attack.

Tell me if this makes sense; rather than relying on my ASI what I plan to do is to stall or slow flight maneuver noting my GPS speed in all the directions of the compass and then averaging. This is ground speed but averaged in all four directions should give me a consistent basis for comparison.

regarding performance claims; the thing that interests me with boundary layer manipulation is that it has been tested by everyone from NASA, the military, wind turbines, to auto racing with quantifiable results. I've read articles on using these things on everything from props to struts. There are new shapes that claim to equal traditional VG's without intruding into the free stream, etc. Also, given the cost and difficulty of dealing with the FAA i would imagine that has inhibited some adaptation and use of newer ideas. for example these are shapes I've never seen tested in GA:
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the small triangular one next to the penny claims to create a stronger vortex than a traditional VG yet is below the boundary layer. We'll see. One more thing...it's just fun messin around with this stuff.

[Jim Heffelfinger]

Good reading here as well..... http://phys.org/news/2012-09-scientists-purpose-vortex.html

[Ron Blum]

Both of the VGs you show want the airflow to go straight into the device. The black one is used on semi tractor-trailers (and I'm not sure what the center raised piece does). The blade VGs on airplanes (either an inverted "T" or an angle), the flow must come into those at an angle. In other words (on airplanes), the optimum is to have the VGs aligned with the flow in cruise (minimum drag) and then be at an angle (due to outflow or inflow on the wing) as the airplane approaches stall.

Boundary layer thickness is a function of the length of the chord and where you are on that length. As real rough estimates on a small GA airplane, it is zero at the leading edge and 1/4" to 1/2" at the trailing edge. The tape is about the only thing smaller than that. As you can tell, we're only touching the surface of this topic.

As for the comparison between VGs on an airplane and dimples on a golf ball, both try to reduce drag under certain conditions, but that is where the comparison ends. Dimpling an airplane will not make it go faster (sorry). On a golf ball, though: 1) it does go further (less drag on a low Reynolds number sphere) 2) it does go further with a more vertical descent path (on a good shot the ball initially rotates bottom side forward, creating lift as the stagnation point moves down and aft on the ball and 3) (for the "these guys are good" - LPGA and PGA folks) Intentionally tailoring (slicing and hooking) your shots are a great way to improve your game.

As you mentioned, this stuff is fun!

[Dplunkt]

Good reading here as well..... http://phys.org/news/2012-09-scientists-purpose-vortex.html

I read that article too, not bad. To Ron's point there is really limited comparison from a golf ball to a wing for a variety of reasons including the ball is spinning. Where dimples made sense to me was not in dimpling the entire surface but like zigzag tape, putting a couple rows along the length of the airfoil at a strategic location just in front of the separation bubble to keep the air attached longer. For example Aircraft spruce sell a turbulator tape kit for cessna struts.

To your points Ron, if the boundary layer is only say 1/8 or less at the location where VGs are applied why do they need to be 1/4"+ tall? The other thing that i like about the delta shaped vg is that deltas create a strong vortices at higher angles so this shape might be effective (I've not tested it though). I'm thinking that, as you noted, that the airflow goes into the VG and thus "compresses" a little causing a stronger "exit" over the top of the delta shaped sides creating a stronger vortex than it would otherwise. i have no clue if I'm right about that though.

When the weather turns better (we got 5 inches of snow last night) I'm going attach temporarily some of these as try to document any change in flight characteristics. if I see any good results I'll post those back here.

[Ron Blum]

Sounds like you have a good plan. I am interested to know the results. Strength of the vortices are a result of the airflow energy. Looking at a typical profile of the boundary layer, most of it is at a higher airspeed (it's only the flow really, really close to the surface that is greatly slowed.

[raytoews]

I tried dimple tape on my Grumman Cheetah (don't panic it's in the OM category) and found no or virtually no difference. I put it at the high point of the wing as per instructions.
Now I have a set of VG's I'm going to try. Grumman guys don't think it will help but nobody has ever tried.
First I'm going to do some tuft testing to see what it really does, the videos are pretty impressive.
Has anybody tried tufting the bottom of the wing, I know there is no "lift" generated there but tufting looks for turbulence,,,,right?
If I get time I will try it and screw a gopro to the bottom of the wing and have a look see.
The same for the back of the canopy, there is about a 1/8 transition between the back of the canopy and the fuselage.
I understand the VG's are supposed to re-attach the laminar flow, is this not correct?

Ray

[Ron Blum]

Now you're talking! If you find anything on the bottom of the wing, I would be shocked (except behind gear legs, etc.). If something like that happens, it would be in cruise and not near stall. BTW, contrary to popular belief (low pressure on top of the wing and high pressure on the bottom of the wing), both sides are actually lower than ambient pressure (for most of the surface) until you get to some pretty high angles of attack. I am jealous of what you're doing. Again, have fun ... safely.

[Ron Blum]

Just looking real quick, but I see 4 draggy things on the Cheetah (besides the back edge of the canopy like you mentioned): exhaust stack, nose gear strut, round antennas (you would be surprised) and the beacon on top of the vertical.

[Bill Greenwood]

There is a great article on Vortex Generators in the June 2012 issue of AVIATION CONSUMER magazine with the subtitle Real Bang For The Buck. It covers 5 brand on 16 different airplanes, and is especaially recommended for extra safety on light twins. Also says, "not an option for modern high performance aircraft like a Cirrus SR 22", but as to why, it really seems that no one has tried it on Cirrus. They did try them on a Lancair IV which is very high performance and stall speed came down so that it really would not stall, but cruise speed lost 11 mph. This was with the vg at 11 % back on wing chord.
If the vg is too far forward, it will lower stall speed but penalize high speed, and if too far rearward the reverse so that it may not help stall speed.
The main company seems to be Micro Aerodynamics.

[Ron Blum]

Yes! You don't get something for nothing ... except if you're fixing a problem (which new airplanes, designed with high performance CFD programs should not have. This is a great forum. Thanks!

[Victor Bravo]

In another lifetime, in a previous century, I was a sailplane racer. Although not a trained aerodynamicist by any means, I do know that the difference between the "turbulator tape" and the "VG's" is the intention and effect of the modification, and an order of magnitude difference in the amount of air they affect.

VG's are there to put additional energy into the boundary layer, to prevent or delay airflow separation over adverse gradients. The VG's do create parasite drag, which was considered an acceptable loss in return for the delayed separation. The fact that VG's wound up not costing anything (or much) in speed was an unexpected benefit, icing on the cake.

Turbulator tape is there to force the transition between laminar flow and turbulent flow at a certain point, in a controlled manner, so as to prevent random separation bubbles from forming. These act like TINY little VG's, but they only affect the air within the boundary layer, maybe 1/16 to 1/8 inch. We used the stuff on the bottom of glider wings mostly. The differences were very small,and the performance of a racing glider was one of the few areas where you could actually measure some benefit.

VG's will delay separation on just about any airfoil, although they will instantly turn a low drag airfoil into a high drag airfoil. VG's on a high drag airfoil (Cubs, Cessnas) will deliver a noticeable benefit, but not make the high drag airfoil any more draggy.

Turbulator tape needs a really smooth, laminar airfoil to accomplish their job. Gliders, maybe laminar power-planes like a Lancair, Glasair, etc. Put them on a Cub wing and you would probably never see a difference.