Pollywog Airfoil

Have deceided to build a Fierce Arrow, after seeing it a while ago, posted in this forum.

I would like to know about the pollywog type airfoil it uses.
What is the basis of this modified symetrical airfoil ?
What is the concept behind it.

Tight Lines to all.

Glen: I am restoring a Fierce Arrow right now. The airfoil has only the slightest suggestion of "pollywogness". You can barely detect any concavity aft of the airfoil high point. It is nothing like the early Veco or Kenhi kits.

The pollywog was created because kit manufactures wanted to get more ribs on a sheet of balsa.

It is not a very good airfoil for Stunt.

LAST EDITED ON Dec-14-03 AT 02:22 PM (CST)
 
Sorry Bob, but they did it so the trailing edge would line up. It was not a tapered piece. I have the 1949 Veco Chief with the pollywog ribs and there is a lot of balsa between the ribs, so no savings there. Bob Palmer mentioned in one of his interviews it was for the trailing edge only. Either way, they were and are a pain to cover and have the paper stick to the profile. We used Ambroid or Duco mixed in with the pure clear dope. Now rejuventor does the job.

I asked the same question (sort of!) in post #6977. The Peacemaker has this tadpole shaped wing and IMHO stunts well - whether it is this or not is open for debate...
Adrian

Adrian, I don't think the Peacemaker has a true pollywog section. The Peacemaker has a completely convex section with a thin T.E. extension stuck on the back? A pollywog sectn. has a true reflex portion aft of the high point, so that the t.e. of the wing tangents with the surface of the flap. I wondered about this in the previous corres.

LAST EDITED ON Dec-16-03 AT 00:15 AM (CST)
 
Phil said he did an experiment and a Combat plane with a polliwog airfoil "definitely turned better". Can't find that particular thread, but here's something else:

http://www.clstunt.com/cgi-bin/dcforum/dcboard.cgi?az=read_count&om=2835&forum=DCForumID1#39

Riley Wooten's Quicker had the polywog airfoil. It flew well.

>
>Riley Wooten's Quicker had the polywog airfoil. It flew
>well.

I'm sure Al's comment related to it's use on flapped stunt planes. His experiments indicated that shaping the aft part of the airfoil to promote smooth flow over the flaps was a large improvement, The pollywog does exactly the opposite.

LAST EDITED ON Dec-19-03 AT 07:12 AM (CST)
 
>>
>>Riley Wooten's Quicker had the polywog airfoil. It flew
>>well.
>
> I'm sure Al's comment related to it's use on flapped
>stunt planes. His experiments indicated that shaping the aft
>part of the airfoil to promote smooth flow over the flaps
>was a large improvement, The pollywog does exactly the
>opposite.
>
> Brett

When my father and I returned to stunt we decided to try lots of different things that we had read about or had forgotten and we had a ravenous appetite for trying new stuff. Dad built a polywog airfoiled Coyote. It was a combo of ideas he had been saving up since the 60's and was very noticeably reflexive in shape with the high point forward and a blunt rounded LE as well as swept wash out tips top and bottom. He flew it for a season in competition. Frankly it flew fine and cornered quite well with no flaps but was a tad heavy and or underpowered. Its greatest detriment was the drag of the blunted LE. While it flew fine under power its glide ratio was a little quick.

I tested one of the Wortman FX airfoils, cut from foam as close as I could with templates. These were a series of reflex airfoils designed for full size glider tails- relatively low Rn(~500,000) trying to get the max Cl with low drag, needed for a glider. The differences in profile among the related airfoils, just looking at the sections, is negligible. You won't get the best one just by drawing something freehand!!!

If the original FA had a reflex section go for it. You won't notice any flying differences between a reflex and a flat back section. That big flipper on the back totally disrupts the airflow over the wing in turns anyway, so section lift coefficients don't mean anything. That's one thing the combatteers have learned over the years. Putting the elevator on the trailing edge is a bad idea. It can be made to work OK, but a modest boom is much better.

LAST EDITED ON Dec-16-03 AT 01:48 PM (CST)
 
Glenn, I'm flattered you chose my Gladiator as your avitar. Cool, thanks!!

well, you gonna be more flatered, I'm also gonna use tit.

that Gladiator is covered with Oz Cover, ain't it?

Yup, Oz cover on the wings, silkspan everywhere else. Thanks Ion!

I still have my CHIEF 1 with pollywog airfoil. I built it from Tom Dixon plans many years ago. It started life with an Enya 35, then with a Fox 35. It now has a Brodak 40 with only 1 flight on the new motor. It has the signature of Bob Palmer on the wing, obtained at a recent VSC. People say, "don't fly it because it has Bob's signature on it". I say, NONSENSE! Bob wouldn't want it to become a "hanger queen" just because of his signature.

Floyd aka: Dr. Spark

I have a question.

What is the difference between a "pollywog" airfoil and a so-called "non-pollywog" airfoil that has essentially a flat flap (either stationary or moving) attached to its rear? They both look like a "pollywog" to me. As far as air moving over the surface, the air sees in either case a concave reflex in the section.

This is not intended to be a trick question. I would like someone to be able to explain why a "typical" airfoil section on most stunt ships (using sheet flaps) or the airfoil like on a Flight Streak with a fixed flap should not be considered a pollywog section.

Keith,

I can only offer my thoughts, surely someon. else will see it differently. The "pollywog" airfoil such as on the Chief and the Ringmaster to a lessor degree actually have reflexed upper and lower cambers the result in the "waist" of the airfoil being farther forward and the transition to the flaps or trailing edge smoother. Looking at it from my free flight background I would say the "pollywog' airfoil is basically undercambered and thhus produces more lift them the conventional airfoils we use.

Whatever--

Most stunt airfoils and flaps still look like pollywog airfoils.

While waiting for the weather to clear (snowed in again today) I went to my pile of books and found MODEL AIRCRAFT AERODYNAMICS by Martin Simons. It shows the NACA 63-series symmetrical airfoils with discernable "pollywog" in them. The NACA 64 and 65 series have even more pronounced reverse curve. There is one called FX-71-L-150/K20 an obvious "laminar flow" type with 15% thickness at the 34% point with darn near as much "pollywog" as the Chief 1.

So even though Veco used the pollywog to work with a rectangular trailing edge piece, there seems to be some historical precedent for this type airfoil.

(Temp is 27 this afternoon, and can't go anyplace because my driveway at 1/3 mile long, with a pretty drastic slope, is iced up so my Toyota tends to slide right off into the ditch- as it did on Saturday).

Gee Floyd,

Temp here is about 27C - Jim in a tropical climate, for now anyway!
I can't fly here though, i'm surrounded by about 1 billion square miles of oecan!

I would gladly trade my Farenheit with your Celsius (or Centegrade, as I was taught in school). Wouldn't be so bad if I lived in the city with streets and all! No one is going to plow my 1/3 mile driveway so I can get out.

Who ever thought Oregon would be so downright unfriendly?

Well, at least when the "big one" hits, it won't fall into the Pacific. However, those volcanos are down right unfriendly. Out here we have tornados, but no snow or earthquakes.

Ty,

That's right - You wern't around in '93 were you!!

30" of snow overnight on the 12-13 of March, a Friday night buried just about everything in North Central and North Eastern Alabama.
Luckily I was in Fort Wayne at the time. I had to delay my
return to Anniston for about 3 days...Sooo - Don't say it
never snows in Alabama!

P.S. It was still mighty deep when I got back but they did manage to get all the main roads (Interstates) cleared off.

Snow, right, once every twenty years!! We got that every year in St Louis, Idaho, Newport, RI, northern Nevada. We got more in Memphis than Alabama. Lets face it, winters down here are a joke, and I'm not complaining. I love the fact that I can wear short sleeve shirts outside at least 345 days a year.

Hi Floyd

It's 6.30 pm in Sydney and the sweat is dripping off me due to a 32ºC maximum today.

You are quite correct about the 63 series NACA airfoils. However, the max thickness point is at 35%. Pull the max thickness forward to 30% and the reflex curve almost disappears. Move it further forward and the reflex disappears completely.

>However, the max thickness point is at 35%. Pull the max
>thickness forward to 30% and the reflex curve almost
>disappears. Move it further forward and the reflex
>disappears completely.
>

.... Or splice the aft end of a NACA 00 series airfoil to the end of it. Makes a very nice airfoil.

Bob-

>.... Or splice the aft end of a NACA 00 series airfoil to the end of it. Makes a very
>nice airfoil.

Bob

I was trying this just the other day, since you also get a very user-friendly airfoil for I-Beam style rib slicing. While I really like and am using the forward-highpoint 6-digit section for my flapless wing (for which I'd expect 'pollywog' sections to work well), rib slices based on it would not line up well (level) spanwise behind the high point as their trailing edges are clipped to accomodate l.e. sweep on a flapped wing. The more convex 00XX aft sections do allow a pretty uniform and level aft slope across the span (probably better for flap hinge flow control too). The high point moves back only moderately in % chord as these NACA 00XX slices are clipped. You can get a nice l.e. radius for forward-squeezed high point, with the six-digit section nose applied to the 00XX rear slope.

So I think you can create an I-Beamer or derivative with forward high points across the span with this section and not have an extremely blunt leading edge. So far I've tried a 23% root section (no flaps included) with maximum thickness at about .29c. It becomes about 24-25% at .33c - .34c at the tip on a wing with straight hinge line and .75 taper ratio. I had't yet tried to join them at a further-forward maximum thickness. The 50% mix of the two airfoils isn't too bad either. The "pollywog" shape aggravates attempts to achieve forward high points on traditional sliced-rib I-Beamers, since the concave part of the root sections transition spanwise to the more convex part of the tip sections, making the aft tip sections actually thicker than the root.

So a question:

Have you found a good way to combine the 63A0XX nose with the 00XX aft section in Profili II? I haven't done it without resorting to amending the ordinates, and those listed are insufficient to create a really smooth section. Only other way I see to try it is to list the myriad NACA 00XX ordinates from the high point aft, but this may superimpose the listed 63A0XX nose ordinates on the result anyway.

SK

Geoff and all. The plots for these airfoils are mostly impractical for model construction. Fer instance, the trailing edge is mostly too thin, meaning you have to "fudge" thicker to get some strength at the T.E. That destroys the whole idea. I sometimes have to laugh at some of the glider plans which boast Selig or other foils. They cannot be built without compromise. Does that destroy the whole idea of a "perfect" or "modern" airfoil?

Where is Bill Netzeband when we need him?

>Fer instance, the trailing edge is mostly too thin, meaning you have to "fudge" thicker to get
> some strength at the T.E. That destroys the whole idea.

Yeah, I just set them for chord lengths that give the trailing edge thicknesses I want when chopped off at the desired chord, Then I calculate the adjusted maximum thickness and percent chord positions to give me back what I want. It might take an extra iteration. But the only data I know for 6-digit NACA sections is irrelevant to our uses anyway. The sections themselves don't seem particularly suitable, unless used with modified maximum thickness positions (the Horten brothers discovered how dangerous application of such "laminar" sections can be at low RN, when they built their high-A/R, narrow-chord tipped Ho IVb sailplane with 'Mustang' sections during WW II). I just use these as bases for developing something quite different that I like.

Some of the low-RN NACA 0009, 0012, 0015, 0018, and 0021 data I've cited may be OK, although, as Igor has pointed out, it's usefulness is limited, since very little of it concerns sections with deflected flaps, and few of these are like the flat ones successfully used over the history of CL stunt. Then there's always the question of smoothness and its effect on transition, and so on...Still, I agree with Larry C. that at appropriate thicknesses these should work well in our applications - again based on what they allow the air to do near the leading edge and near the hinge line.

It's all "eyeball engineering" for me at this point, taking into account what I've seen, what others have reported, and how I think air behaves. I probably won't ever put in the research time Al has devoted, nor Igor's computational theoretical effort. But I pay attention to their results and still think airfoils are fun.

As for Selig, Lednicer, et al, they seem pretty sharp to me, and probably have their methods of compensation for physical reality. Lednicer for sure would be doing a high-level CFD analysis with software that's probably not readily available or AT LEAST applying a sophisticated knowledge of how to adjust for his programs' shortcomings. My opinion here is based on his once frequent posts on another forum. The RC soaring folks, especially the flying-wing designers, seem to find these sections an improvement over older sections.

I'm just happy to get a shape I think will prove advantageous and then have fun finding out. High science, eh?

SK

Serge. I hesitate to get into technical discussions on aerodynamics. I only observe the following:

1. The Pollywog airfoil on my Chief I seems to be just fine!

2. The NACA 0018 on my own "WILDMAN 60" is also just fine!

3. Other strange foils, like on my FANCHER DOCTOR are just fine!

Perhaps there isn't all that much science involved at all!

>Perhaps there isn't all that much science involved at all!

But don't overlook the fact that, for instance, certain Werwage, Fancher, Rabe, Burger, Cartier, and NACA airfoils have differing reported performance characteristics, even though they all have been judged good in their specific applications. There are documented advantages and trade offs for each, and I think you can correlate the performance characteristics to their appearances (appropriately chosen dimensions). So, while they may all be good, they are also different. I agree that in CL, the airfoil choice can be significantly less critical than in model sailplanes or full-sized aircraft.

LAST EDITED ON Jan-09-04 AT 01:34 AM (CDT)
 
Serge et al

I export the co-ordinates of the sections that I want to splice together and do a cut and paste in Notepad or Wordpad. Then to get a TE at 1/16 for a wing without flaps, I truncate the section the required amount and move the Max thickness point forward to compensate. I use the same process to create TEs 1/4 or 5/16 deep for wings with flaps.

The last step is a trial and error process at present but I am hoping that Winfoil Ver 3 will do it all for me. Don't know yet.

Cheers, Geoff

My main objective is to get something that is repeatable and has a logical basis. I doubt that there is much aerodynamic rationale in what I am doing.

In times past, the outside of a shoe was just as valid but less repeatable.