Turbulent Query For Ted

Mr. Fancher I've recently been poring over your SN article on the Doctor/Medic and I have a question or two. You aver that the turbulation enabled by the multispar LE is an important part of the Doc's flapless equation. Just how much do you think that the turbulation contributes to the stall delaying characteristics of the wing design? In the modeling world I've mostly seen intentional turbulation of various sorts applied to slow flying models such as free-flight ships where absolute maximum lift from their thin and sometimes undercambered airfoils is desirable because of the poky gliding speeds used. Most are aware that turbulation artificially roughens the surface of an airfoil so that boundary layer flow will remain attached as far back along the surface as possible thereby enhancing lift. How important is this effect at the 50 m.p.h. plus flight speeds of a CL stunter? Important enough for you to stress in the design obviously but is it as important as the very blunt LE of the airfoil with it's inherent stall resistance? If one wanted to fool around with a flapless design with the Doctor's basic numbers but with a conventional tapered C-tube construction of equal size and airfoil would you consider the effort pointless if aggressive turbulation was not used? Idly wondering---Mark

That doesn't sound all that idle...

On a related subject, and only to air my views on turbulator spars on stunt ships, I'll offer the following.

For the last several years, I've been flying Dick Mathis's Excalibur in Classic competition. I don't know if you are familiar with this plane or not, but it sports a rather thin wing with a rather sharp leading edge. This goes agianst all that I've been lead to believe a stunt wing should look like. But, in the air, it proves itself to be a capable stunter, thin, sharp wing and all. I know it only weighs 37 oz's, but it still flies better than it should. I know of several others that fly equally as well.

My only possible explanation is the wing uses turbulator spars. In my experience, even flying slower than normal lap times, it doesn't stall.

Ted will likely shed much more light on this subject when he gets a chance to post, but my opinion, for what it's worth is yes, they do help.

>For the last several years, I've
>been flying Dick Mathis's Excalibur
>in Classic competition. I don't
>know if you are familiar
>with this plane or not,
>but it sports a rather
>thin wing with a rather
>sharp leading edge. This goes
>agianst all that I've been
>lead to believe a stunt
>wing should look like. But,
>in the air, it proves
>itself to be a capable
>stunter, thin, sharp wing and
>all.

Re-thought this once more after reading Leonards comments below. Perhaps it is this thin sharp wing that's recieving the benifits of turbulators. Mayhaps a thicker blunter wing may not need or recieve any help from turbulators. Leonards' comparison idea may indeed have merit.

>Mr. Fancher...
>You aver that the turbulation
>enabled by the multispar LE
>is an important part of
>the Doc's flapless equation....
>If one wanted to fool
>around with a flapless design
>with the Doctor's basic numbers
>but with a conventional tapered
>C-tube construction of equal size
>and airfoil would you consider
>the effort pointless if aggressive
>turbulation was not used?
>Idly wondering---Mark

Again, as we await Ted's answer, might I offer a suggestion? (Well, I will anyway.) I few years back Ted had installed turbulators on his "big" ship. I don't know all the reasoning, and I don't know the outcome, but I do know they aren't there now. Basically, he had taped some string down to the wing in several places along the span to serve as the turbulators.

Now, I have never used them or felt the need for them. And, certainly, there are thousands of designs out there not using them. So, if you are inclined to build a plane with the Doctor's basic numbers and airfoil with a conventional C-tube construction, I would say go for it. And then, if you wanted to see what the difference would be (or what you might be missing), you could do as Ted did and tape some string down over the wing at the appropriate spots to act as turbulators to see what, if any, difference these would make.

I would be interested in hearing your results. The whole stunt world would be waiting with baited breath (I think that is the right expression.) I feel that it is only when someone does a true experiment such as this that we really learn what the difference is and what improvements can and cannot be made in our designs.

I am not accusing Ted of this, but some designers have made statements (such as incorporating moveable flaps into the airfoil of the wing, or double undercamber in the airfoil) of improving the design or lift characteristics when experiments have proven this to be just the opposite. I truly would be interested in someone doing the tests as I have suggested (and maybe Ted already has.)

>The whole
>stunt world would be waiting
>with baited breath (I think
>that is the right expression.)
>Leonard Neumann
Hmmmm.....My wife calls me "Bait breath" sometimes, is that the same thing?
Kiiski

Sardines can't be that bad, my dogs eat them!

>Again, as we await Ted's answer,
>might I offer a suggestion?
> (Well, I will anyway.)
> I few years back
>Ted had installed turbulators on
>his "big" ship. I
>don't know all the reasoning,
>and I don't know the
>outcome, but I do know
>they aren't there now.
>Basically, he had taped some
>string down to the wing
>in several places along the
>span to serve as the
>turbulators.

It was flying wire, .018 stranded cable to be exact. It may still be there. The point was to improve the turbulation on the tan/maroon original Trivial Pursuit. The model has a conventional foam wing with a glass-smooth and perfectly "fair" surface finished with PPG DAU75/DXR80 clear. This finish is *much* smoother than even a good dope finish - none of the rubbed-out look dope look - it's literally like a mirror.

This airplane flies pretty well, even at 67 oz and 650ish square inches. But sometimes, particularly when it's raining, it tends to stall in corners. The theory is that the rain causes the flow to separate in places. To keep the flow attached, Ted added the turbulators. The idea of course is to make the flow turbulent instead of laminar, which adds drag in most circumstances but improves the ability of the flow to stay attached to the surface.

This was a *tremendous* improvement, and the airplane proved almost impossible to stall no matter what the conditions happened to be.

It didn't seem to be a problem with David's virtually identical airplane (first yellow/green Star Gazer. made with he same batch of parts), probably because David's is 1). not as glass-smooth and 2). weighs 62 oz.

Ted can confirm this (I think he's supposed to get back today or tomorrow) but I recall that the second batch of TPs* used built-up wings in order to produce a surface that's not as "fair" and thus contains natural turbulation effects. Debate it if necessary, but even 2 oz heavier, and despite it's recently-solved other problems, I've never once seen this airplane stall in any conditions. Of course, after the refinish job dropped 5-6 oz, it's no longer an issue.

>
>Now, I have never used them
>or felt the need for
>them. And, certainly, there
>are thousands of designs out
>there not using them.
>I would be interested in hearing
>your results. The whole
>stunt world would be waiting
>with baited breath (I think
>that is the right expression.)
> I feel that it
>is only when someone does
>a true experiment such as
>this that we really learn
>what the difference is and
>what improvements can and cannot
>be made in our designs.

We've done this experiment many times. The best example was on Jim Aron's "Frankenstunt" This was a small 35-sized airplane with one of the "flat aft section" airfoil, I would guess it's around 540 squares and about 56-57 oz with a Stalker 46. Around here, while he couldn't do it, I was able to get it through respectable patterns without stalls. It took some care, but it could be done. When he went to Muncie, the thin air made it completly impossible to get through certain parts of the pattern, most specifically the third corner of the hourglass, and the outside center corner of the second pass through the square 8. It would occasionly crap out in other places, like the intersection of the vertical 8. By Wednesday it was clear that something had to be done or it was going to get crashed. I decided to put wire turbulators on the high point. Immediately, it would make it through the manevuers again. It still wasn't a good airplane, but it would do patterns.

>I am not accusing Ted of
>this, but some designers have
>made statements (such as incorporating
>moveable flaps into the airfoil
>of the wing, or double
>undercamber in the airfoil) of
>improving the design or lift
>characteristics when experiments have proven
>this to be just the
>opposite.

Maybe. In this case it's a well-proven idea that works well when necessary. The reason you don't need it on most models is that most models don't need any more lift. If you are not running out to begin with, then more doesn't help. Most models of reasonable size/weight/power don't get anywhere near the limit of lift capability. That's why 65 oz 650 square inch airplanes can keep winning despite the presence of the super-light's. Knocking off 10 oz doesn't tighten the corners, and the other, second-order performance effects can easily be overcome with the overkill power that's now available. Of course they are able to turn tighter from a technical standpoint, but it's irrelevant because the pilot is the limiting factor, not the airplane.

I'll let Ted answer the Doctor/Medic question, but this is a completely different situation in that non-flapped airplanes can easily run out of lift. And they always benefit in "quality of turn" if not "radius of turn" from being lighter, at least in decent air.

Brett

*his Great Expectations/Final Edition (same airplane), whatever he's calling it today, and David's faded Blue/Yellow&Blue "shakemaster"/Yellow and Maroon Trivial Pursuits

What do you suppose is happening, Brett? I could imagine this would work at the Reynolds number of a gnat, but I have seen nothing good come from bumps toward the front part of combat wings. Do you reckon it's sticking the flow down over the flaps?

LAST EDITED ON Aug-26-01 AT 12:00 PM (CDT)

>What do you suppose is happening,
>Brett? I could imagine
>this would work at the
>Reynolds number of a gnat,
>but I have seen nothing
>good come from bumps toward
>the front part of combat
>wings. Do you reckon
>it's sticking the flow down
>over the flaps?

I was afraid you were going to ask somthing like that. I was extremely skeptical about the idea to begin with for just that reason. 400000 sounds on the high side for the turbulation effect to work. But I cannot deny the results.

The effect may be to make sure the air turns the corner at the hinge line.

The "Frankenstunt" airfoil has a lot of surface curvature right around the high point, then is dead flat from just past to the TE. Just picking it up, is feels like its too discontinuous to work properly - it's like a sharp edge that doesn't "feel" right. I'm no Kelly Johnson, but it just seems wrong somehow. The requirement to turn two pretty sharp corners, one at the high point and then the exaggerated angle at the wing/flap joint have to be bad, somehow. Maybe it's bad enough to make the turbulation work even at high Rn.

This doesn't really explain the Trivial Pursuit. It may be that it's getting laminar flow, which separates at raindrops. I can't over-emphasize that the surface finish is completly flawless. It probably looks smooth under an electron microspope. The only other one that I've seen that even comes close is yours.

Neither explanation is too satifying. And don't even begin to ask me what's really going on with the wires on the tail.

> Neither explanation is
>too satifying. And don't even
>begin to ask me what's
>really going on with the
>wires on the tail.

On further consideration, in all the experiments I've seen or done the wires are acting exactly like turbulators would, Rn notwithstanding. They seem to be keeping the flow attached. It helps as described on the wings. On the flat tail with round LE, it seems to help the air turn the corner near the transition from radius to flat. This would also tend to explain why the wires don't help on the semi-pointy, airfoiled tail, with it's reasonably continuous pressure profile.

Barring another, better explanation (and bearing in mind, my best work is done in a vacuum), I am forced to conclude that the turbulators are helping.

Brett--Two mo' questions if I may. Would running a wire or two under the covering on the LE do the same job especially if the covering is not a mirror finish? Also do you know if anyone has done a Doc with a minimalist full fuse? Seems like another few oz. could be saved over the beefy slabs needed for profiles.

>Brett--Two mo' questions if I may.
> Would running a wire
>or two under the covering
>on the LE do the
>same job especially if the
>covering is not a mirror
>finish?

Hard to say. Since I don't know why it works even when it does, I don't know how to assess the variations on it!

> Also do you
>know if anyone has done
>a Doc with a minimalist
>full fuse? Seems like
>another few oz. could be
>saved over the beefy slabs
>needed for profiles.

Not that I know of. But it's funny you should mention that. I really like the way the Doctor flies when the engine runs properly, and in good air. It's got that perfect fingertip control that we've been trying to get on the "full stunt" models.

On the original, the engine runs have been pretty unreliable. I don't think it's a design issue, but just this particular model. The resonant frequencies of the nose just happened to come out about where the engine runs.

It also doesn't care for turbulence too much. This is almost certainly due to the straight wings. So I've been playing with the idea of building a full-fuse, tapered wing version. If someone wants to beat me to it, I suggest 1" more root chord, and 1" less tip chord. Power should be a high rev/low pitch motor. Unflapped airplanes are way too sensitive to speed variation to use a 4-2 break motor sucessfully.

Of course, this defeats most of the purpose of the original design, that is, simple to build but very high capability trainer. The "easy to trim" should still be true.

What about using an engine from our esteemed SSW host Leonard? They are supposed to run in a fairly constant speed and grunt out tons of torque with big props. This might work as well as a high winder for constant airspeed. How does this sound? Simple box fuse cross-grained planked top & bottom, upright engine with accessible tank ala L. Neumann, Doctor areas & moments, c-tube wing with wire "trip strips", and your recommended 1" greater root chord and 1" less tip chord? Turbulence is a big problem around here (and I guess everywhere) most of the time so the tapered planform sounds good.

LAST EDITED ON Aug-25-01 AT 04:37 PM (CDT)

Leonard wrote:

".......The whole stunt world would be waiting with baited breath (I think that is the right expression.).........."
====================
Leonard,

I would not have written this if you hadn't included your paranthetical comment. The phrase is actually "with bated breath." The word "bated" is a shortened form of "abated."

I one actually baited his breath for any length of time, he might not have many friends.

I said, "I think" that was the right expression. Perchance I was wrong. (I shouldn't think, right?

Whoo! Boy am I glad I came back from England sooner rather than later. Sorry to not have contributed but I've been gone for six days earning the daily bread and wouldn't have come on tonight if not for a call from Brett alerting me to serious discussion of things aernautical (or not, depending on the source).

don't get me wrong. I love this sort of discussion and stand as ready to learn as to pontificate. Nonetheless, here's my feeling on the subject at hand.

I think a careful reading of the Doctor article will show that my utilization of a multi-spar wing was the result of personal experience with other ships which tended to support the possibility that a surface other than glass smooth tended to inhibit airflow separation (stalls) better at stunt speeds. Brett has pretty accurately elaborated on the experiences with, first, the original Trivial Pursuit (which stalled uncontrollably in triangle and hourglass bottoms during some modest, high temperature rains early in the week of the '93 team trials but, after the addition of the "turbulators", went on to win the event)and later with Uncle Jimby's ship.

Leonard has mentioned that he hadn't seen them on the ship since and that is probably right since the Great Expectation was finished in time for the Shanghai WCs and in that guise or later, as the refinished Final Edition, has been my primary competiton ship since. I did fly the T.P. at one later nats while the F.E. was being refinished and it had the turbulators removed. This was solely because the weather didn't include rain. If it had they would have been put back on if for no other reason than they seemed to have worked "way back when".

It's worth noting that the F.E. sans turbulators had earlier finished second twice at the nats behind the ubiquitous Paul Walker without the turbulators...and without the rain. It is my personal opinion that this ship was right at the maximum wing loading to allow excellent performance (lift available well balanced to lift required under the demands of the pattern) under most conditions but when the additional weight of the water was added the high G induced weight exceeded the lift of which the wing was capable and the wing simply exceeded its critical angle of attack trying to do so. Ergo, the stall. It took only a modest increase in the critical angle of attack to allow the wing to produce the lift necessary and, it would seem, the turbulators seemed to provide what was required.

To reitereate another comment from Brett, the Great Expectation with a higher wing loading but a built up wing (with the comensurate surface irregularities) has never stalled even though I fly it with much greater abandon (I bang corners when they seem called for). Of course, there is no way to quantify exactly what makes the difference but the circumstantial evidence of the efficacy of the turbulators on the earlier ship and the improved performance of the later but more heavily laden ship would certainly bear investigation.

To get directly to the original question from Mark. I wouldn't have a problem with a Doctor built with a C-tube wing. I don't for a minute think the turbulated surface produced by the multi-spar wing is a make or break deal. The Final Edition in the earlier discussion was not a multi-spar but a sheeted leading edge exactly as Mark proposes. I'd say go for it if you're more comfortable with that type of construction and, quite frankly, if you intend to cover with monokote type covering the improved rigidity would probably more than balance out any possible decrement in performance from the lack of the postulated superiority of a more aggressively turbulated surface.

I'd like to add re the tail wires about which a lot has been written. I'm less inclined than my buddy David Fitz (who beats me regularly it should be noted)to have found benefit in the wires we located on the upper/lower surfaces of tails. I won't refute his findings since his experiments were much more detailed than my own. However...

What I did feel was proven to my satisfaction was that in my particular case (again the Trivial Pursuit/Great Expectation/Final Edition evolution) great benefit resulted from the placement of a single wire precisely on the forewardmost point of a previously rounded leading edge to establish a precisely located stagnation point. This resulted in the elimination of the tendency of the ship to hunt badly in level flight and much improved the quality of its exit from hard corners.

To wit: the original Trivial Pursuit had an airfoiled stab and elevator with a comparatively sharp leading edge. (David Fitz' original Stargazer had an identical tail cut from the same foam jigs and sheeted identically) Both of these ships flew just fine with no significant hunting or problems in exits from corners. The T.P. had two or three Nats seconds and several other top five placings as well as a win at a team trials and David's Stargazer had a nats victory and other high placings. These were good solid stunters.

The follow up ships for both were built with flat stabs and rounded leading edges and both had the aforementioned hunts and corner exit problems. Both were innoculated with turbulator/stagnation point wires and turned into competent and occasionaly victorious stunters. The effect was so dramatic on the Great Expectation that when it was refinished and christened the Final Edition the leading edge of the entire stab was redone to make it longer in chord and sharper. This is the best flying stunt ship I've owned.

It's important to note that many excellent flying ships have been built with the flat stab/rounded leading edge configuration. I've no doubt that there may be some other design anomaly between the T.P series and those successful designs that makes them less...or non...susceptible to this phenomenom. Suffice to say that with this design both David and I have spent considerable time and effort to determine that this is a viable fix for a problem that didn't exist until we went to the flat/rounded configuration.

Again, way too long. Sorry 'bout that. Once again, my advice to Mark is to build the ship as he wishes and as long as the wing loading is kept low (the real secret to a good flying unflapped design) it'll do just fine. Gotta remember that us "designers" (and I use the word with my hard hat on) have to employ all the "trick stuff" when we put our wares on the public marketplace to show everyone how smart we are. Still up to the consumer to separate our wheat from our chaff.

Good luck, Mark.

Any airplane the color of the FE that can win the Nats has to be pretty good.

I can't speak for others but I think I am going to create a binder and start printing and collecting these collective ruminations. I'm not at a level to offer meaningful contributions to discussions such as these but I enjoy reading them greatly -and I've learned much along the way. Keep it up-the Joe bellcranks of the world are eating it up......

Ted,

Were you able to verify the supposition that the turbulators were responsible for keeping the aircraft (Smooth Foam Wing TP) from stalling by flying it again in the rain when attached? What effect could have humidity water weight build up in the model have played? Do you think that could also have been a factor in the stall? I know, I'm just full of questions, but when something peeks an interest, I am just full of questions. Sorry to be a bother.

Thanks for all the input Ted--and everyone. Think I'll give it a try this fall just for the heck of it. Had a thought which might be helpful on a c-tube wing. Cover the LE only with one of the common fabric shrink materials (or maybe lightly filled silk?) with either paper or plastic from the spar back. This would save weight over covering the whole wing with heavy fabric but would give a little laminar flow encouraging help where it's needed most. The more I stare at the Doctor planform the more it seems that the combo of a super blunt airfoil and long modern style stab moment,not normally seen on flapless designs, is what makes it work so well. This 'foil might not work too well however on a tapered wing. The increased angle of the spanwise flow might dump too much lift to be completely effective although it would probably improve the wings turbulence behavior. So many tradeoffs--so much fun. FYI a few weeks ago I was out at Scobee Field in Houston watching the redoutable Dee Rice fly a Barnstormer he's built for OTS. The plane looked as solid as an M1 tank on the lines and it's turn was more than spritely enough for the average duffer. Good looking plane, lovely smooth flyer, really nice guy. Maybe I'll start a Flaps Temperance Union to encourage building well behaved competitive modern flapless stunters. Thanks again--Mark

Turbulator testing.

To find out if a wing is working better or worse (or just differently)turbulated, temporarily attach the turbulators on either the inside or outside wing and then go fly it. Work up to hard corners and watch which way the model tends to roll. You might try this on an old beater first to get a feel for the process. This is a technique the free flighters have used for years. Never heard of it being done on a C/L ship but might prove interesting. Anyone ever see a stunt ship snap roll?

Hutch

>Turbulator testing.
>
>To find out if a wing
>is working better or worse
>(or just differently)turbulated, temporarily attach
>the turbulators on either the
>inside or outside wing and
>then go fly it. Work
>up to hard corners and
>watch which way the model
>tends to roll. You might
>try this on an old
>beater first to get a
>feel for the process. This
>is a technique the free
>flighters have used for years.
>Never heard of it being
>done on a C/L ship
>but might prove interesting. Anyone
>ever see a stunt ship
>snap roll?
>
>Hutch

This is an interesting suggestion, Don (this is Don, I'm guessing). The only problem I can think of is that there is no suggestion being made that the turbulators (if that is even the proper term for what they are doing) are increasing the lift at a given angle of attack. Only that they delay the separation of airflow until a higher angle of attack. The test you propose would likely only prove valuable if you are able to drive the unmodified wing to a stall and be able to determine that the modified wing continues to maintain laminar flow at the same angle of attack. On a ship such as my Trivial Pursuit this could probably be done since it is always close to critical during hard turns.

One further comment on my casual suggestion that turbulator might not be the best nomenclature for the phenomenom we are postulating. Many Boeing airliners have utilized devices known as vortex generators. Most of the 707/720 series had these small vertically oriented squares of aluminum located at strategic spots on the upper surface of their wings, the alleged purpose for which was to maintain a laminar flow at higher angles of attack.

The most amazing use of these devices of which I am aware is their placement in the "S" ducting of the center engine on Boeing 727s. The original 727 on its maiden voyage belched fire from this center engine (No. 2) fore and aft at rotation because, it was determined, the airflow at high angles of attack separated in the intake ducting and a compressor stall resulted. The fix was to install vortex generators in the curves of the ducting to prevent flow separation. It is entirely possible that this is a more germane description for what is actually taking place with the strips taped to the wings of the planes we are discussing.

By the way, some have indicated they thought we were placing these strips on the leading edge of the wings. In actuality, their location was on or about the high point of the wing (in this case about 25% of the chord). It might also be interesting to know that we only added them to the top surface of the wing since the stalls we were combating only occurred on the hard inside corners at high G...the backside corners of the triangles and hourglass.