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Old Dec 19, 2012, 09:21 PM
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Sources for Peanut and Bostonian Airfoils

When I'm building Peanuts or Bostonians from plans by recognized designers like Walt Mooney or Don DeLoach, I don't worry about where the airfoils come from. Lately, I've been looking at the UIUC airfoil site and realizing that the prototypes, especially the Golden Age ones, used quite different airfoils. I realize that the low speeds and small chords used for rubber powered models force differences.

Are there recognized examples for best airfoils in the 2-3 inch chord regime? Do they have any counterparts in the database?

Some examples are pictured below. Walt Mooney's Peanut of the Curtiss-Wright 15C Air Sedan has a flat bottomed 11% airfoil pictured below. The prototype used a Goettingen 593 airfoil according to data on UIUC. That's somewhat thicker, has a rounder nose and a bit of undercamber about 2/3 back. The airfoil for Bostonian Celtic by DeLoach is a bit thinner and sharper nosed. I had a template for that airfoil that I used on my latest more conventional Bostonian, and I get good climb.

Two other pictures compare the Goettingen 593 with a Goettingen 385, which is a 10% flat bottomed airfoil. I don't see full sized planes using it, but it's a lot closer match to the model airfoils, IMHO.

I guess what I'm trying to get to is a set of coordinates that I can use to plot various sized model wings. I could just use photoshop or copier scaling, but knowing the numbers just feels better.

Any other candidates for the winning airfoil?
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Old Dec 19, 2012, 10:19 PM
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Your concerns can be easily tempered by a few simple facts.

First off is that at the VERY low Reynolds numbers found on small and slow flying rubber models the airfoils simply don't work at the same sort of level as they do on larger and faster flying models.

Second is that the realities of stick and tissue is that any airfoil you try to use is doomed to be wildly distorted by the time you look at the reality of the covering sag as little as 1/8 inch away from the ribs.

So all in all I would not lose any sleep over this issue. Build 'em thin and build 'em light and the air the wing passes through will take care of itself.
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Old Dec 20, 2012, 09:10 AM
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Any other candidates for the winning airfoil?
Joe, the curved plate is close to optimum for these very low Reynolds numbers (for a Peanut/Bostonian Re10-20,000). Something as thick as 11% is almost definitely "below it's critical Re number" on a peanut. This means that the airflow is separated with no reattachment, a kind of permanent stall at all angles of attack. This is very draggy and not all that lifty so a bad idea all round, especially for rubber power. The properties of the airflow change quite dramatically at these small sizes.

The curved plate that has been most tested is Gottingen 417A . But anything similar will work well like on a No-cal.

Of course there are structural issues with something this thin and it doesn't look very scale. But it works. However the thicker stuff has been used for years so our performance expectations have been influenced by what we see. But Clark Y types are really not optimal below something roughly Parkflyer size.

The nose radius is significant: a sharper nose is better for low Re, a blunt LE absorbs too much of the limited energy in the airflow and promotes the separation problem. (It raises the critical Re of the airfoil.)

As Bruce says, stick and tissue construction makes serious airfoil analysis almost meaningless. But thin as possible with modest camber is the way to go. A good compromise between fly-ability and build-ability is the Neelmeyer type flat bottomed airfoil at around 9%.

And wing loading helps a lot, a light model can often bring gains that mask any airfoil issues.


Jon

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Old Dec 20, 2012, 05:23 PM
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I wonder if only covering the top would be as good or better aerodynamically.
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Old Dec 20, 2012, 05:37 PM
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Bruce
So I guess if the Goettingen 385, or the thinner Neelmeyer, looks right to me, it won't bother performance of these planes too much. Certainly not as much as a warp.

Jon
Thanks for the Neelmeyer airfoil. I copied the coordinates into Excel and it graphs just fine. I've added a rib pattern from the Anna jr. Embryo by BMJR I've been flying this Fall. It climbs fine after R.O.G. outdoors. Nothing like a 7 inch prop and 4 strands of 3/32 to yank a little plane like that off the table. By the way, BMJR recommends 4 strands of 1/8. It'll really blast off on that.

The microfilm indoor guys use a "Simplex" airfoil based on a logarithmic spiral, and a table for those can be generated mathematically. They look better down around 4-5% camber. If you're building a wing with mixed fore and aft and diagonal ribs, you can get compatible ones of same height fairly easily.

I have found the calculator on RCGroups from May 2005 to generate a Simplex airfoil from Chord and % camber so I can get the 2 or 3 different templates for a mixed F1L or EZB wing. I opened the *.txt file in the post listed below with Excel and was able to get tables of X and Y for airfoils. I used the Graphing function of Excel to plot them, after adjusting the axes of the graph to give a 1/1 printout. It takes a little trial and error, or a simple calculation, to match the height of straight and diagonal ribs by adjusting the % camber.


RC Groups, Aircraft - General, Modeling Science, Simplex airfoil formula?

Simplex airfoil calculator by JMP_Blackfoot

http://www.rcgroups.com/forums/showp...7&postcount=15
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Old Dec 20, 2012, 07:22 PM
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The indoor single surface arc shapes seem to vary from designer to designer. I know some are using a Simplex style with the high point a little forward of the mid chord while others use a simple circular arc which obviously puts the high point right at the 50% chord.
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Old Dec 20, 2012, 09:42 PM
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I've been building a Gary Hodson designed Wart A-6 that uses the Simplex airfoils, so I got interested in looking up the Simplex calculator. Haven't looked at how to calculate the height of an arc of a circle in quite a long time.

At one time, even the best used a French curve for this. I posted plans for a Carl Goldberg microfilm tractor from 1938 that has an obvious French curve airfoil.

http://www.rcgroups.com/forums/showp...&postcount=779
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Old Dec 21, 2012, 05:23 AM
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I did some tests using single surface airfoils nothing special just made up some undercambered wings and swapped them out on the same fuselage. The winner was 4% camber at 40% chord, (nocal type airfoil)
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Old Dec 21, 2012, 08:34 AM
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Goettingen 417A

I've looked up the Goettingen 417A airfoil. It looks great for No-Cals, covered one side, and for Peanuts of veterans, like a Fokker Eindecker. Maybe a little hard to cover 2-sided, as needed for a competing Bostonian. Also a bit too far off scale for most planes past 1919. It should be fine for the top of a sliced rib wing structure for peanuts, though.

Thanks for the help and Happy Holidays.

Joe
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Old Dec 21, 2012, 11:27 AM
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Quote:
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Also a bit too far off scale for most planes past 1919.
Joe, funny you should say that, the airfoils developed during this period (such as RAF 15, common on WW1 bipes) were developed in wind tunnels at model sizes before the effect of Reynolds number was fully understood. So there's no surprise they work well on models

I had thought about a thin balsa sheet wing on a Bostonian but it doesn't seem quite right somehow, aesthetically I mean...


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I wonder if only covering the top would be as good or better aerodynamically.
That's how No-cals are done, the Jedelsky wing is another example. Obviously there is some drag from the structure but the thinness of the airfoil compensates at low Re. At higher Re where thicker airfoils work ok then it's better to streamline the structure.


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I did some tests using single surface airfoils nothing special just made up some undercambered wings and swapped them out on the same fuselage. The winner was 4% camber at 40% chord, (nocal type airfoil)
That's interesting and sounds about right. The Gott 417A is 6% camber but 4% may well be better. Camber is good for going slow and duration on low power but you can certainly overdo it and cause separation on the lower surface of the wing. Especially on a micro RC model where some speed range is required.
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Old Dec 22, 2012, 01:42 PM
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Yak, the all sheet and Jedelsky wing idea might be good for a casual park sport model. But if you stop and consider the volume of wood in a curved plate 1/16 sheet wing for a Bostonian you'll see that for the same volume of wood you can build 3 or 4 built up wings. So the idea dies simply on the merits of weight for a model of this size in a contest class.

Trisquire, there is a serious issue with covering regular airfoils on the top side only. Due to the shape the point of maximum camber then shifts well forward and the amount of camber rises horribly. This creates a harsh and deep cup like shape along the lower front side of the airfoil which is going to generate a lot of drag even at low speeds.

The Goetingenn 417A is a unique case in that it has no real thickness to it. It is so thin that it's basically a single surface airfoil even if it's covered on both sides. So IN THAT CASE it's no wonder it works well at our size and speed range.
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Old Dec 22, 2012, 03:13 PM
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Originally Posted by BMatthews View Post
Trisquire, there is a serious issue with covering regular airfoils on the top side only. Due to the shape the point of maximum camber then shifts well forward and the amount of camber rises horribly. This creates a harsh and deep cup like shape along the lower front side of the airfoil which is going to generate a lot of drag even at low speeds.
So why do the indoor endurance guys cover the top side only? F1B, Easy-B, Penny Plane. As far back as the 30s, some of the Comet dime scale plans advise you to cover the top side only. A bunch of guys come to our indoor trade show every year with big RC scale models - 60" wingspan, etc. The flying surfaces are covered on the top only. They are the slowest, most realistic planes in the dome. I doubt that any of them use special airfoils. Like most model planes they were designed using the TLAR principle.
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Old Dec 22, 2012, 03:45 PM
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Yak, the all sheet and Jedelsky wing idea might be good for a casual park sport model. But if you stop and consider the volume of wood in a curved plate 1/16 sheet wing for a Bostonian you'll see that for the same volume of wood you can build 3 or 4 built up wings. So the idea dies simply on the merits of weight for a model of this size in a contest class.
Actually if you use decent wood it could be done. At 4lb/ft^3 a 48in^2 wing of 1/16" balsa weighs 3.1g. My built up Bostonian wings are around 3 grams. And 1/16 is pretty thick, my Frog Speedy has a 10.5" span and 1/32 sheet flat plate (not so stiff as a curved plate could be)

Not saying I would use Jedelsky though, too draggy. But a balsa sheet cambered wing could be done on a 14 grammer. However I don't think I will bother

Trisquire, you are on the right lines but (I think) what Bruce is saying is that you can't just use a normal thick airfoil and cover the top (You could but it wouldn't be great). It needs to be drawn with single surface covering in mind.
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Old Jan 05, 2013, 04:14 AM
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Just came across the Eppler 376 airfoil which is nearly a curved plate but with a lot of camber, 8.9%. In fact covering the top surface of a 'normal' 9-10% airfoil would look a lot like this.

This is a good example of way too much camber for low Reynolds numbers. If you look at the drag polar for Re50,000 you can see that there is separation above Cl0.8 and masses of drag. It's not that good at Re100,000 either

So this airfoil would be a no no for a Bostonian or Peanut, it might come into its own on a big slow flyer but it would have to be very big! At high Re it's a very high lift airfoil, but not at low Re
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Old Jan 05, 2013, 01:31 PM
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It looks a lot like the sort of airfoil that could be generated on a sail if the mast was given a foam filler sleeve of the entry shape for the airfoil. Then the rest would be done using film tension and battens.

Trisquire, I missed your reply that you posted back before the Holidays hit. To compare apples to apples I was actually referring to only cover the top side of a normally double covered shape. The resulting single surface formed by doing so is quite differernt from the shallow circular arc or Simplex style curve used on the single surface duration class models.
In the one case the surface is using the strong curves found along the upper side only of a "thick" airfoil. In the case of the duration models the arc shape is the same as the camber curve of a regular airfoil where the thickness was reduced to effective 0%. When done in that way the shape doesn't have a strong leading edge lower side cup like we are seeing in that E376 airfoil in Yak's post.

You can see this for yourself if you have a NACA airfoil generator. Take the upper side shape of something like a 4412 and then reduce the thickness down to 4401. You're effectively left with the 4% camber at 40% median line of the thick airfoil. And the two curves will look radically different.
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