What is a turbulator?

In Christ,

Ion, the ignoramus...

>In Christ,
>
>Ion, the ignoramus...

A turbulator is a deliberate protrusion placed on the upper surface of a wing to induce turbulent flow into the boundary layer of air near the surface of the wing. It can be as simple as a piece of tape or string attached spanwise along the upper leading edge of the wing at anywhere from 5-20% of the chord. The reason for introducing turbulent flow is to keep the boundary layer of the air flow attached to the wing upper surface. Literally to make the airflow "stick" to the wing upper surface rather than separate. Keeping the flow attached can delay the stall and greatly enhance the lift produced, since the wing can be flown at a higher angle of attack, and hence higher lift, without separation of the flow occurring. This is usually done on high performance Free Flight wings, particularly Nordic gliders (F1A) and Wakefield rubber (F1B). I've never heard of it having any practical application in Control Line, since the goal is to increase the efficiency of gliding flight at low Reynolds Number (where flow separation is typically a problem).

> I've never heard of it having any
>practical application in Control Line, since the goal is to
>increase the efficiency of gliding flight at low Reynolds
>Number (where flow separation is typically a problem).

Turbulators work VERY WELL on stunt planes. Flow separation can happen very easily at the combinations of Reynolds numbers and pressure differentials we run.

I've seen turbulators made of flying lines, taped to the high point of the airfoil, take an airplane that simply couldn't get through a corner, fly through with no problems at all. And we just had the discussion about turbulators on the tail. Of course, it has to be having a problem in the first place for a "fix" to show any effect.

The wire turbulators create a very sharp discontinuity. I am a lot less convinced that the "turbulator spars" on some stunt planes actually do anything useful.

One form of turbulator that I think would have good potential for use on stunters is the outrigger type. A strand of elastic, thread, or wire would be mounted on struts ahead of the LE. For Free Flights, these are typically small plywood riblets. For our use, perhaps wire or tubing struts would be more aesthetically pleasing. As you can imagine, the length of turbulator between the struts vibrates madly in the breeze, and create a nicely turbulant flow of air and a *bunch* of drag. More drag would require more power, so the weight should become a less significant factor. Leftover .015" cable would be good turbulator material, with a spring tensioner or turnbuckle inside the fuselage to keep it tight. I'd use aluminum tubing for the struts. How far ahead of the LE? Good question. I seem to recall some rules on that issue, but not what those were. Steve

> Turbulators work VERY WELL on stunt planes. Flow
>separation can happen very easily at the combinations of
>Reynolds numbers and pressure differentials we run.
>

This occured to me as I continued to ponder the question last night. I had concluded that turbulators on a stunter could prevent stalling during hard manuvers and could be of benefit on some occasions. They would have to be placed symmetrically on the top and bottom wing surfaces, of course. I also concluded that it would be an interesting area to do some research and experiments. "Turbulator spars", like on the Mathis design have long been popular in Free Flight. And of course, Dick Mathis was a prolific Free Flight competitor. I was going to write a follow up post mentioning all this, but you've saved me the trouble. Can't get anything past you guys. Interesting stuff for sure.

Good Thread!

Andy Tomasch

P.S. What ever did happen to Fast Richard?

> I've never heard of it having any
>practical application in Control Line, since the goal is to
>increase the efficiency of gliding flight at low Reynolds
>Number (where flow separation is typically a problem).
>
>Andy Tomasch

Turbulators can come in many forms and have been used with varying degrees of success on contol line models. Reference the multi-spar wings that show up in photographs in this thread as well as discussions by Ted Fancher and Brett Buck in this forum and the great article by Dave Fitzgerald in the Mar/Apr 98 issue of Stunt News.

Thanks!!

But did you really have to quote my confession?

Like the Crusador rabbit said to the dragon, "Don't bve sadistic about it"...

God bless you!

In Christ,

Ion, still an ignoramus nut to a lesser extent...

Has anybody tried votex generators like the one in the photo below?
They work great on full size planes. Disregard the drooping leading edge.
Andrew Raney

I've read that, on some full size planes, they use a rough wing surface to achieve the same purpose.

That's what I use...a back row finish.

Ah Ah!!!

That's why my first ships flew so well...

In Christ,

>Has anybody tried votex generators like the one in the photo
>below?

Back in the early '60s my dad made thin card stock triangles in this pattern and attaced them to the leading edge of his Nordic glider. He never removed them, so they must have worked!

I've seen somewhat the opposite effect with combat models. These operate near Clmax at Reynolds numbers of about 700K. Once, for example, I had a Nemesis II that was a complete dog for no known reason. I found a Monokote wrinkle an inch or two long near the wing LE. I sanded off the wrinkle, and the plane worked fine. On another occasion, I had a Rushpuppy foamy that was similarly substandard. There was a 1/4"-chord dent for several inches where the upper spar was recessed below the surface of the wing. I removed the covering locally and filled the dent with spackling compound. The plane flew great thereafter. Haven't we discussed this before?

I've tried VGs near the LE, too. They were bad. They may have some applications to stunters, maybe in front of flap or elevator hinge lines, but probably wouldn't offset the bother they'd cause when you wipe off the airplane.

Howard, it is clear, the turbulator brings advantage only on low RE.

The reason is simple. The thickness of boundary layer (BL) grows down the chord. It is laminar from stagnation point (SP) till transition point (TP) and turbulent till trailing edge (TE). Its thickness grows from SP till TP. The thick laminar BL is not stable, so it converts to turbulent. Whirlpool works like a “ball bearing” and so BL after TP is thinner, but going to TE grows again.

The thick boundary layer is something what makes problems. The low pressure over surface tries to turn flow toward the airfoil surface. If the BL is too thick, the flow can be turned so much, that it does not run toward the TE anymore, it changes the direction and runs back to LE sticked to airfoil surface – this is what we call “separation” or “bubble” if it is small and stable.

So the turbulator helps, if makes the BL thinner. The high RE makes BL thin and early turbulent, while low RE makes big peek of BL at transition point. Properly designed turbulator at proper place can minimize thickness of BL by modifying of TP. If transition happens too late, the laminar BL is too thick before transition and if transition happens too early, than BL is too thick at TE.

It answers lot of questions.

1/ The natural TP at high angle of attack is very close to LE (because SP is well under the airfoil and air must run back to LE, continue around it to upper surface and it gives enough energy to make transition). Out tail is at RE=~200 000. So the clear position of turbulator which CAN help is LE – it s place of very high speed at high AoA. The elevator has very high negative AoA in corner and almost always separates at LE. Turbulator can modify the moment when it happens and thus push it to AoA where it does not hurt corner. In any case it makes the separation more predictable and thus it allows more repetitive corners.

2/ A blunt flat tail can make bubble where round LE goes to flat surface. The reason is, that LE has constant radius – it is wrong the radius on good airfoil should have decreasing radius. The result is, that the flow separates at some angle of radius and stick back where the radius change to flat surface – means the place where the pressure is much higher. The result is stochastic bubble, which can lead to troubles with stability in level flight or even to hunting. Little improved transition from radius to flat surface can help. But turbulator on LE can solve those problems at all.

3/ Our wing is at RE=~400 000 I do not think turbulator is able to do any change.

4/ If you try to add turbulator to LE of wing at high RE number, the BL at TE can be much thicker than without turbulator. It can happen on your combat wing – the reason is earlier separation at TE.

Here are some pictures. They show BL thickness (the black line). It is symmetric Naca airfoil 2412 at 0 deg AoA. The #1 and the #2 is at RE=10 000 the #3 and #4 is at 400 000. The #1 and the #3 is without turbulator (means natural transition), the #2 and #4 is with turbulator of optimal thickness – both at 20% of chord.

You can see several important things:

- #1 has very thick BL, it is very unstable, so it is very easy to separate at LE at very low AoA – it will completely lose lift
- #2 – the BL is much better on almost whole airfoil, the only problem is still at TE, but separation at TE is not so fatal – the stalling will be much smoother.
- #3 RE 400 000 makes the BL very stable – there is no reason to play with turbulator.
- #4 shows what turbulator does, the transition is earlier, but the only result is, that the BL at TE is thicker so it is rather negative. Higher RE numbers will be even worse.

Igor wrote:

>3/ Our wing is at RE=~400 000 I do not think turbulator is
>able to do any change.
>

and later

>- #3 RE 400 000 makes the BL very stable – there is no
>reason to play with turbulator.
>- #4 shows what turbulator does, the transition is earlier,
>but the only result is, that the BL at TE is thicker so it
>is rather negative. Higher RE numbers will be even worse.

I won't directly challenge this analysis. However, in actual practice, I have seen turbulators on the wing work absolute wonders on heavy airplanes in reducing the stalling tendencies.

The most dramatic was on an airplane built by Uncle Jimby, named "Frankenstunt". This airplane has about 560 square inches, and weighs somewhere in the upper 50's, with a 4-2 break Stalker 46. The airfoil is absolutely dreadful (I'll sketch it and edit it in later), but it would just about get through all the corners at sea level in reasonable temperatures, if you were really gentle on the controls. At Muncie, the high density altitudes pretty much killed it dead. It was completely unable to get through almost any corner, with the third corner of the hourglass a near crash, as it would go 15 degrees, stall, fall a ways, turn some more, then stall again, and again, until you just had to bail out.

There wans't a lot else to do, so Ted and I decided to try to put on turbulators (as had worked on several other airplanes that were in a lot less trouble). They were .018 cable flying wires, taped to the high point of the airfoil, or slightly behind. We just put them on between flights. Next flight, it was immensely better in most corners, and while it would still stall in the third corner of the hourglass, it only did it once, and it could be finessed.

This is just one example. I can think of several others right off the bat. So, despite the analysis above (which seems pretty definitive), turbulators on the wing definitely can help in marginal situations.

Well ….. what I mean by those pictures is, that our typical wing is somewhere where turbulator does not change too much. But thick and blunt airfoils are more sensitive to changes in boundary layer. So if you take such a thick blunt airfoil, and you get problems on top of hemisphere – means at relatively low speed, and especially with deflected flap, the turbulator can make some small difference. You say sometimes it works well sometimes not, so it needs only very little difference.

However, I do not think you did typical turbulator. I think the boundary layer at that place is already turbulent. Also turbulator past the high point cannot work well. I think the wire did not work like turbulator, it was rather spoiler. It is known effect on low RE airfoils. The trick is to kill fraction of lift at TE what makes more stable flow at LE. It is typically done by small stair at or aft of high point. The flow at high AoA separates at TE and the separation quickly propagates till the stair. This lowers the low-pressure peak at top of LE and it secures sticked flow till the stair. So the stair kills lift at TE but without stair you get immediate separation at LE.

I do not know if I see well that airfoil. There are two such type developed for full size aerobatic plains. One has blunt LE and second newly designed has sharper LE followed by that blunt and thick middle point. That sharper LE is necessary for some figures, where the wing must stall. That airfoil is not dedicated to be used with flaps. But is very clever airfoil. It is able to make lot of lift thanx it thickness and high point very close to LE. The sharp LE allows very well defined separation at some particular AoA and relatively flat segment between sharp radius of LE and high point allow sticking the flow back very easily – it means it kills the hysterezy. But it is big mistake to combine this airfoil with flaps. The flap deflection makes low pressure at LE and it can be reason of unexpected separation at LE – it is very bad stall. I recommend looking at this video – it shows how the flap deflection changes pressure at LE and also stagnation point without change of AoA:

http://www.netax.sk/hexoft/stunt/images/f240x180

This trick has nothing to do with RE number and works well also on full size wings. The best example are flaps for landing. They are not sealed – if you would seal them, the wing will produce lot of lift, but at one point it abruptly separate at LE – it means immediate crash instead of landing.

I use this trick on my new unflapped model for children in club. It has nice smooth foam LE that changes to triangular section. It allows easy construction; because the LE can be prefabricated for children while backside can be easily cut from one piece of balsa as triangular pieces. But the idea for this topic is, that it makes small inconsistence in surface what works exactly like your wire at high point.

So I did few nice pictures explaining that trick with stair at high point. There are two airfoils. They differ only in one point. The second has “something” at place of maximal thickness (18% at 20%). The flap is 17% deflected to 30 deg. It is at RE=400 000. The transition to turbulent boundary layer is at 12% (far front of max. thickness) in worst case, so turbulator does not change situation.

The polar says all. The first airfoil makes bad stall at 9 deg AoA. It is over our usual AoA in corner, but it can easily happen on heavy model in thin air.

The second makes small stall bump at 0 AoA, but it is not important as the flap and thus also elevator is deflected and AoA=0 cannot happen. But lift in positive AoA is very consistent. It is because that small “something” causes very strong pressure gradient leading to early separation already at little positive AoA. That separation aft of high point makes little higher pressure and more stable situation on leading edge where is lift concentrated. The result is little less lift, but it does not make any stall troubles till high angle of attack, and so the model does not terribly fall down if you reach some particular AoA.

Ugly!!!

Looks like Ted's airfoil after a tragedy...

God bless you!

In Christ,

Hi Walter,

(I'm AMA247439 all right..)

That's HER! I don't seem to have any digital pix of that ship anywhere. And haven't seen any regular photos recently either.

Tony Drago took some pictures of this ship at VSC several years ago. Is this where you got them?

Thanks for posting them. I didn't remember that ship looking so good, must be the photograph effect!

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"The sense of uselessness is the severest shock which our system can sustain." -Thomas Huxley

>>Strangely, the Excalibur I built had a twist in the aft
>>fuselage, which resulted in the stab LE being about 1/8"
>>drooped. As a result, the ship wanted to tighten and get
>>slack on the lines whenever an inside loop was flown. It was
>>horrible.
>>
>>When my friends pointed out the problem, I cut out the stab,
>>filled the slab slot, and cut a new one that was properly
>>aligned. After replacing the stab, I had a wonderful
>>airplane.
>>
>>[photo not recovered: 3e69bb3870f12ad5.jpg]
>>
>>"Experience is what allows us to repeat our mistakes, only
>>with more finesse!" -Derwood Fincher
>
>
>Is this the pretty little lady your referring to?

Larry,

you don't need no StuntRib!

Just look at that baby...

God bless you!

In Christ, (and drooling)

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God bless you!

In Christ,

Great Ion, how much?

Como se dice, Ion, en Engles?

[photo not recovered: 3e69bb3870f12ad5.jpg]

"Middle Age is when your age starts to show around your middle." -Bob Hope