Airfoil for a Flapless Stunter

I have revisited the archived flapless stunter and related threads (sections, turbulators, etc.). I believe a currently misplaced NACA report indicated that at low Reynolds Numbers, the 0018 was about optimum regarding stall aoa in that series, and I think I remember Jim Thomerson posting the opinion that he wouldn't go much over 18% on a smallish stunter. My approximations for successful flapless stunters include thicknesses of about 15% for Bill Netzband's "Doodlebug", about 16.5% for my Mike Pratt "P-Force", and 17% for Bob Baron's OS .32 powered "Humbug 91", but I also remember Ted posting his well explained preference for thickish sections with the high point forward. Still, the 20% doesn't "look" all that bad either, even at 25% chord (moved forward from 30%). All in all, I haven't decided what mods to try.

SO...here are the questions. For a stunter in the 380-400 sq. in. area range, with root chord of 10.0"-10.5", higher than normal aspect ratio (5.8 - 6+ for a 48" - 50" span), and intended to fly with an LA .25 at high rpm/low pitch...

1) What max thickness would you choose, and at what chordwise position would you place it?

2) For the best compromise for calm vs. windy conditions, would you opt for a less blunt leading edge than the old NACA 4-digit sections?

I might try an elliptical chord distribution with 1/4-chord line perpendicular to fuselage, or a straight taper with straight or swept back 1/4-chord line - I drew a lot of them. Tail moment is not particularly long, and stab/elevator (blanketed) would be 60/40 or 70/30, with as much as 25% of the wing area. This is not particularly orthodox, but makes for a neat little plane that I'd like to give a fighting chance to.

What do you think?

Re the airfoil question: this is a boring answer, but, based on my experience, I would stick to something pretty close to 18%, 30% back from the LE.

A flapless wing can't carry as much weight. I'd shoot for 28 oz. or so. That will probably be the biggest challenge.

The aspect ratio probably affects windy weather performance more than anything. Get much over 5 and it will be harder to trim.

Do whatever you can to keep the centrifugal force up. The harder the plane pulls the less the lines bow and the easier it is to trim. Just shortening the lines a couple of feet can make a big difference.

If you want a better airfoil than a modified NACA airfoil, look at data in Stuttgarter Profilkatalog I or Profilplaren fuer den Modellflug by D. Althaus.

I built a flapless stunter a few years ago. I used the Pathfinder airfoil which is about what Jim describes. In theory, it looked good, I mounted a fixed extension, similar to what is on a Flite Streak after recalling some reading I did some years before. The author was trying to explain his theory that a fixed extension going straght back on the wing chord centerline, would act like a flap at 7-10 degrees deployed, when the AOA was increased. The reasoning seemed to be good, but nothing to back it up, test wise. Anyway, it couldn't hurt I felt.

The plane wound up at ~380-400 sq's and I powered it with an OS .25fp.

The results were mixed. Under power, the plane was over powered with the .25, and a .20 was installed. It would likely have been fine with a .15fp. It flies a pretty good pattern. But, the weight came out way too heavy, and the airfoil was 20% or greater. The worst problem occured when the motor quit. This plane just won't glide well. The weight , combined with the thick airfoil and it's drag makes the plane drop like a rock. I have to be at 4 feet or lower, when the engine quits, to make any kind of decent landing, and that better be done quickly.

My impressions and conclusions, based on this one example?

The airfoil entry is the most impotant part of the airfoil.
Too thick an airfoil can be a problem.
Too heavy is a big problem, combined with the above, a major problem.
If I'd have designed and built around a .15 for this size wing, it might have ended up lighter. Lighter would be better.
Fixed flap extensions might be benificial, and don't hurt.

If I was to do this again, and I planned on using the same engine, I would make it at least 500 sq's. I would use somewhere between 16 and 18% on the airfoil. I would keep serious control of the weight.

I hope this helps you avoid the mistakes I made.

John

I'm wondering why everything is spinning around?

The challenge is another reason for some of us. Overcoming the obstacles, and the creative process. Lot's of reasons. The plane in my case is a twin boomer, 'cause I like the way they look.

John (a lid for every pot) Miller

I'm wondering why everything is spinning around?

I'm working on a flapless .25 plane too.

>Profili, Stuttgarter Profilkatalog I, Profilplaren fuer den Modellflug >

Serge, Howard, Any websites associated with these? Or could I be the only one who doesn't already have them?

> the plane was over powered with the .25, and a .20 was installed. It would likely have been fine with a .15fp >

John, How did this show up? Just too fast even with low-pitch props, violent 4-2 breaks, or did you experience adverse yaw/roll that went away with the .20? This last is a subject with widely divided opinions, so I'm curious what your experience was.

> the weight came out way too heavy, >

John, Do you recall what it weighed? Marvin mentioned 28 oz as a good target. Mike's Primary Force prototype is 35 oz. (with 500 sq in).

> thicknesses of about 15% for Bill Netzband's "Doodlebug", about 16.5% for my Mike Pratt "P-Force", and 17% for Bob Baron's OS .32 powered "Humbug 91" >

Nothing that Wild Bill or Bob Baron designed would come as any great surprise. The slender 16.5% on Mike's highly regarded Primary Force is interesting. Mike, are you inclined to enlighten us here, or would that be like asking the magician for free lessons?

Thanks,

Kim:

Profili Site (where you can download the program):

http://www.baronerosso.net/software/profili/profili1_2.htm

George:

I just like to try out ideas - more fun for me. This one also has a slightly different appearance that I'd like to realize in a model.

http://www.nasg.com/afdb/list-airfoil-e.phtml

Check out the Eppler E479. I've been meaning to try it.

Go to the NASG home page for some more interesting stuff, particularly the Birdman Rally.

Sure looks a lot like the airfoil we use on the Pathfinders Howard.

Neat site. I bookmarked it to check out later.

John Miller

I'm wondering why everything is spinning around?

It was plain too fast with the .25 on a 3 or 4 pitch prop. It's still too fast on the.20, but more manageable.

>
>> the weight came out way too heavy, >
>
>John, Do you recall what it
>weighed? Marvin mentioned 28
>oz as a good target.
> Mike's Primary Force prototype
>is 35 oz. (with 500
>sq in).
>

Itook it off the wall and weighed it. It's 35 oz's. There is one other item that I've been meaning to try, and that's more nose weight. I have this hunch that it might be too tail heavy for it's configuration and is stalling early without power. In any case,I still believe the airfoil would be better slimmed done a few percentage points.

John

I'm wondering why everything is spinning around?

Serge;

Profili 2.0 is out. Nice package. The author asks for a small donation.

As for flapless stunter airfoils, I think that compared to a flapped ship you are always faced with a compromise. If it is thick enough to generate sufficient lift and blunt enough to retard windup, landings are going to be harsh as the plane will shed speed rapidly after the engine quits. Mike Pratt has had good luck with the NACA 00 series on his ship. I have used a modified version of the NACA 00 as well. My mods included moving the high point forward to about 27% of the chord, fairly blunt radius, etc. If interested I will do a CAD extract and post here. Same problems as John Miller had. It is all right when the engine is running, but once the prop stops turning you better get it to the ground fast. Go thinner and you get into real windup situations with any wind.

The airfoil that I have liked the best was the one Pat Johnston used on his Shark 402. I have tried to improve on it, but failed miserably. I just blatantly copy it now (and give Pat the credit he deserves).

There is another aspect of flapless ships that are as important as the airfoil - the engine/prop setup. Years ago I found that my flapless ships flew better with engines that prefered a slow, steady two cycle rather than a 4-2 break. So, my OS Max 30 and Enya 35 always went on flapless ships. I really didn't know why. Years later Ted Fancher's Doctor article had the most logical explanation that I had read as of yet - as flapless ships do not have changing geometry to generate lift, you must keep speed up during maneuvers to continue to generate lift. If speed falls off, so does lift. With the advent of schneurle ported engines, keeping a flapless ship in the air got a lot easier. These engines generate sufficient thrust to keep a flapless ship moving fast enough to generate sufficient lift better than a 4-2 break engine.

For the airfoil, I would think the NACA 0018/19/20 as it comes out of the equation would be very hard to beat. I don't think that it's really crucial to move the high point forwardHowever...

Some of the rest of it sounds a little off of what I would do. 400 squares is definitely on the small side for a 25. I'd be afraid that you won't be able slow it down enough. Given that the wing loading will need to be low (like 10 oz/square foot), that's only 27ish ounces, max. That's certainly dooable, but it will take some pretty good attention to weight saving, given that the engine and muffler will weigh something like 9-9.5 oz all by itself.

The engine will certainly pull a much larger airplane, and that will be a lot easier to build to the required weight. For a 25, I probably wouldn't go below 450 square inches, and I wouldn't be a bit concerned about 500 with a NACA 0018. That should be pretty easy to build to 35 oz. as long as you don't go crazy with the paint job and over-construction.

Given that the wing loading will need to be light, I am also concerned that the high aspect ratio may not be all that good an idea. These light flapless airplanes fly a lot like kites to begin with, and they really bounce around in turbulence. The aspect ratio will exacerbate this. Tapering the wing will help, but I don't know if one will offset the other. If you want strictly ultimate dead-air performance, this may be a good idea, but it's liable to geta little "jumpy" in bumpy air.

The tail seems a little larger than it needs to be, but that won't likely hurt anything. The tail moment is another interesting item. I think it's pretty easy to make it too long, in addition to too short. I think you are on the right track, there.

Just some thoughts, take it for what it's worth. One thing to consider - cut-and-try is a legitimate development tool. Given that you could probably put together a tapered-wing profile along the lines of your idea in a week or to, maybe its worth doing it as a simple experiment. A month or two of test-flying will answer your questions.

>http://www.nasg.com/afdb/list-airfoil-e.phtml
>
>Check out the Eppler E479.
>I've been meaning to try
>it.
>
>Go to the NASG home page
>for some more interesting stuff,
>particularly the Birdman Rally.

Howard;

The Eppler 479 has a little bit of "pollywog" to it. I am not sure that this is optimal.

I have taken the Epple 479 nose and grafted it to a NACA 00 series trailing edge with CAD. Nice result. Here are a couple of examples.

Eppler 479 nose, NACA TE, 20% thickness, max thickness at 30% chord:

[photo not recovered: eppler_stunt_30_20P.jpg]

Eppler 479 nose (compressed), NACA TE, 17.5% thickness, max thickness at 25%

[photo not recovered: eppler_stunt_175_25_9_5.jpg]

Here is another - don't really remember how I put it together, but it was in my Eppler airfoil directory...

20% Airfoil, Eppler 862 top extract mirrored, airfoil peak at 28% chord.

[photo not recovered: eppler_862_stunt_1.jpg]

CAD and Profili let you have a little fun with airfoils... .

Almost everything was already written, so only few notes:

1/ The area 400 is really too small. You must see that our Middle is flapped model so you must not go under its area until you are not sure you can do it lighter. I would really go up to 450 - 500.

2/ Unflapped airfoil for us depends on blunt LE. I would not afraid to use the airfoil from full-scale stunters – elliptical LE and straight aft side. Maximal thickness at 20% and 18 - 20% thickness at root. You can draw it yourself – make a rectangle 100x18. Fit in an ellipse of dimensions 18x40 to front of it. Connect lines from middle of aft section to that ellipse. That is all. You can do it for tip the same way – it is good idea to go down to 14 - 16% thickness. Smoothly shaped airfoils without flaps like those Naca or Epplers have less drag but it is not our case, and especially not with such engine. The Suchoi or Sportster like built by Beringers will fit.

3/ Too large tail can only help but be sure to use thin flat airfoil at around 3%.

If you want to experiment, can I suggest that you look at NACA 63 series or NACA 63A series with the high point moved forward to something like 25 or 30%.

They both have a slight reflex curve in the back part of the section which I think, for ease of building, should be converted to straight. Others may have a different opinion

NACA 63A is fuller behind the high point than 63 so you would finish with less flat section when you remove the reflex curve.

I have several well regarded flapless model plans in my collection that use sections just as I have described.

And as we debate airfoil sections, let's not forget the 'secret' flattened section that Todd Lee campaigned quite successfully last year.

The Eppler section is much fuller in the front portion that the Pathfinder and fuller behind the high point. When I redraw the Eppler section to move the high point to match the Pathfinder exactly, the two sections are quite different, the LE radius, the high point and the TE are the only bits that match.

I will send you a drawing if you want a direct comparison.

Regarding series airfoils- the profile Giles I built last year used a NACA 0018 with no changes. It works fine with an LA40 at 700 square inches, but is a bit heavy at 49 ounces. It makes for a very thick(2.9 in.) wing and is very draggy. It doesn't fall out of the sky when the motor quits. I can easily whip it a lap or more. It just flys very slowly, about 10 mph, so it won't glide far without whipping.

A polliwog or airfoil/fixed flap combination will generate more lift. I built some planes using an Eppler 479(has a bit of a pollywog section, concave behind the high point). It flew well and would carry about 20% more weight than a blunt, flatback airfoil. It had to, since the structure to keep the shape of the rear of the airfoil added 20% to the weight. A flat, 1/8 in. sheet flap on the back of a more standard airfoil(ala' FliteStreak) would be easier and work just as well.

[photo not recovered: 3da5bbe702e649db.jpg]

Geoff and I have discussed this section, and I believe we both think it's a great airfoil. Almost identicle to the Patternmaster sections, which are used in a lot of modern stunters by several designers. Of course, they are using flaps, but I tend to believe that with the fixed flap extension, ala, Flight Streak, this may well be a great section for a non-flapped stunter.

Best, John

I'm wondering why everything is spinning around?

You are right about the slight bit of reflex. However, if you move the high point forward from 35% to 25%, and lengthen the tail section from 65% to 75%, the relfex is almost unnoticeable:

[photo not recovered: naca063_1.jpg]

This example is 20% thick.

This may not be as accurate as what you generate, as I used a combination of Profili, Intellicad, and Turbocad to generate, split, compress and stretch, and rejoin.

I think that Phil Cartier is right by saying that an airfoil with the peak this far forward will need some accomodation to keep the paper from excessive sagging and possibly some type of internal bracing.

So it wouldn't matter if it is curved or flat?

No flames please, just trying to learn.

O Hirsute Flattener of Greater Metropolitan Areas,

Did you step on your rib pattern? I trust that the zero-curvature section at the high point of the zillafoil is an artifact rather than an actual feature of the beastie in question.

Kim Mortimore

"O flatten me not, on the lone prairieeee...."

Airfoil: In it’s self I don’t think the outside shape of the airfoil is all that important (at the low speeds we fly at). It can be flat from the high point back or rounded both work quite well. At the same time you must consider the airfoil for other reasons to help define the mission of the model. For example, you don’t want a really blunt 24% thick airfoil on a .25 to .35 powered airplane. Too much drag for the engine to overcome and the model won’t glide worth a “tinkers” darn. Or if you pick an airfoil that is too thin it will stall in tight turns.

The airfoils used for the P Force are the NACA 16% at the root and the NACA 18% at the tip. When the airfoils were trimmed within the airfoil program they came out slightly thicker. The high point of the airfoil was moved forward to the 25% point to get the high point closer to the C.G. (16%) and the leading edge was kept a constant radius from root to tip. I would not call this airfoil really blunt but more in the midstream.

Wing Loading: Wing loading is one of the most important elements in a flapless stunter. In my opinion the wing loading should be around 10.5 to 11 ounces per square foot to be competitive. Since we do not have flaps the model needs to belightweight to make smooth and crisp turns.

Wing Pan Form: The sweptback leading edge ads a little more stability in the turns and looks way better than a constant cord wing. The P Force is lightly loaded so the aspect ration was kept at just over 5. I was concerned that it would get bounced around in the wind so I stayed away from the higher aspect ratios.

Tail Moment: A long tail moment is also essential for a flapless stunter to turn properly. To make it easy to understand just imagine that the P Force was equipped with 3” flaps (same wing area) and you measured from the flap hinge line to the elevator hinge line it would be 16-1/2”.

Tail Area: The tail area of a flapless model should not be as large as some of the flapped models. The tail area of the P Force is 19% of the wing area with the stabilizer at 60% and the elevators at 40% (of the tail area). In my opinion this is still too big and the total tail area should be about 18 to 16% of the wing area. The tail area of the Skyray is 16% and it turns very well.

To my way of thinking you have to define the mission of the model before you ever start drawing and/or cutting wood. Like all airplanes everything is a compromise and all aspects should be carefully considered and then you decide what you think is important. Part of the fun of designing is to find out if your ideas really did work (or not).

See Ya!
Mikey

I am not sure, Currell. With our low Reynolds numbers, I am not sure that it really matters. Again the example of Todd Lee comes to mind.

I believe the reason Ted Fancher favored having a blunt LE and an airfoil max thickness closer to the LE was to generate consistent lift regardless of the angle of attack. He says it works. I don't argue.

When I first took a crack at designing my own ships as a kid, some of my early airfoils were football shaped with an airfoil peak way back in chord - maybe as much as 40%. These did round maneuvers beautifully, but anything approaching a square was a disaster. Later designs pushed the peak forward in the chord and came with a blunter LE. Round maneuvers were a little less smooth, but I could do squares without the ship falling out of the sky. I don't know why. BTW - these were all flapless ships, most of them a modified I-Beam design.

Thanks for your input. Re: Trainer 60:

Look on this URL for Selig (UIUC) data on the NASG site:

http://www.nasg.com/afdb/show-airfoil-e.phtml?id=1176

They have data for four RN's from about 1 to 4 million and ordinates. I was unable to get the UIUC airfoil database to come up this morning, and my Profili version doesn't list it. Anyway, it looks as though the CL max is close to 1.2 at 4 x 10^6.

Randy-

Thanks.

Thanks much for your generous input; this is just the kind of information and perspective I wanted. It's instructive to hear the designer's own views. Now, knowing some of the thought that you put into it, I'll have to finish my P-Force, which has been delayed by completion of another, older project (meanwhile, I have NOT wrecked the "SkyRay" I'm flying!).

Everyone-

Thanks for the overwhelming responses. I have found your various views interesting and informative. It has also been interesting to hear of your personal experiences and to see some ideas Bob, 'zilla, et. al. have come up with. Based on Forum response and your private e-mails, it looks as though several of you too are enjoying this. Thanks!

Mikey, a tinker's dam is similar to a rivet. It was used to repair leaking pots and pans in the past, before they became "throw-away".

Thanks for the tips. I had some fun playing with these on Profili (am not fluent in CAD). I was surprised to find that you can also effectively increase the l.e. radius some in Profili by choosing to move a point somewhat behind the max thickness forward. While it apparently squeezes all points between it and the l.e. forward a bit (?), I was surprised to find that in this case it did not cause a reversal of curvature behind the high point. For instance going "from" the 40% point "to" the 25% point on the 63A010 moved the maximum thickness to about 23%-chord, with a correspondingly raised/more convex surface ahead of that point, but retaining a nice continuous convexity. I guess there are ways to trick the program into (crudely) adjusting additional parameters.

Now I hope that there is further comment on the effect of attaching stationary flaps to any of these sections.

The polar data is only as good as the model used to measure it. Changes of as little as .005 in. in thickness on a 10 in. or so model wing can make a measurable difference. On some airfoils this can make a big difference and change how the model flys, on other airfoils it barely makes a measurable difference. I personally have no clue how to tell what might happen. As I remember the article, they didn't either.

The classic D-tube wing is just about the worst design, from an airfoil standpoint. The D section trips the airflow and detaches it making the wing perform poorly. There is an article either in the UCIC site or in one of the soaring digests on this phenomenon.

If that little aft edge on the D-tube leading edge is so critical, can we imagine all sorts of nefarious effects of something like edges of diagonal ribs!

I always assumed that the diagonal ribs might actually benefit the air flow separation problem by turbulating the airfoil. And recently we saw a nice example of a Mathis Excalibur, which has 3 spars across the front of the airfoil instead of being planked. If I remember right, Mathis himself mentioned the possible benefits of turbulation of airfoil with that arrangement. (Heck, even the Twister airfoil has a pretty good bump where the its top and bottom spars are - the LE is not planked..)

I don't doubt the veracity of what is said. What I might question is just how often the typical stunt ship breaches this territory where these problems actually come into play.

It also seems that we could help solve such problems with the foam wings, with their very smooth planking.

L.

"You're just jealous cause the voices only talk to me."

John, the little stunter I'm going tso send you the plans, has an aorfoil made by my frrend Eneas.

It's notflapless, but it has, as I told you, an "awsome corner"..

He just sat down, got inspiration and "made up" the airfoil, no theory, nothning...

He also has another nice small stunter design, the "Cação".

I'm flying it nose heaavy, and still it can turn very tight, w/out yaw...

He has some kinda gift ...