How to chose the right airfoil

[RookieOne]

How to chose the right airfoil for a model?

I'm planning some new models still for a non experienced pilot (me).

One of the airfoils I'm using for the sailplanes is an Eppler 195 because it was used on my first bought kit and I had good result.
But I would like to do some experimentation with other kinds maybe more floaters and less fast than E195.

For my second self-design scale models I don't really know what to use. I would like to build some twin engined foam warbirds keeping the easy handling and reasonable low speed (Eppler 205 maybe??).

I know basic performances of airfoils (thick wing & flat bottom like Clark-y gives high lift but low speed, undercambered are used on sailplanes and slowflyers, simmetrical must be used on acrobat models, thinner are used on high speed models and .......blah blah blah).

The thing I just need to know is: which ones are commonly used in model aircafts and why?

Some of them look exactly the same but small differences are really important. That drives me crazy!

.................

[fregon45]

First off, you should know that airfoil selection is not an exact science, and to do a really good job still requires a lot of trial and error.

I'm no expert on the subject, but I'll try to at least set you in the right direction. First you need to define the basic geometry of your model, such as wingspan, wing area, and weight. You also need to decide what kind of conditions you will be flying in, and specific flight characteristics you are interested in. (slow flier, aerobatic, etc.)

Next, from all of the info above, you need to calculate the approximate Reynolds number (Re) for your flight conditions, as well as a target Coefficient of Lift (Cl). Then you can look at airfoil polar plots to see which foils will give reasonable performance (low Coefficient of Drag) at the desired conditions.

In general, polar plots are better than nothing at all, but in reality are not very exact. They can be generated by experiments in a wind tunnel, or whith predictive modeling. Ususally one doesn't agree exactly with the other, so you have to decide which polar you believe.

If you really want to go this route, you'll need to study up on basic aerodynamics. If this all seems too complicated, then you may be better off to just go with the more popular foils for models similar to what you are building. You'll probably do just as well.

Nathan

[GYROGEARLOOSE]

Being something less than an airfoil expert myself, I can share my "secret", -I cheat! Watch other airplanes until you see one that displays most of the characteristics you want and COPY!
Most kit manufactures list the airfoil used. Read reviews of the models and look for the information you need.
Find a free copy of the airfoil program "PROFILI" and down load it. You can print out templetes from it to cut ribs or foam cores, it even gives you skin allowance.
Ofcourse airfoils are not the only factor to consider and several other factors interact and effect the over all performance, but the airfoil is important.

[Paul]

E195, E205, SD3037, etc…

In the wind tunnel there may be a difference, but in the real world I don’t think it’s worth your time to search for the perfect airfoil. The advice Nathan gave is fine in theory, but who can build a wing to the tolerances required to see the difference between two very similar airfoils.

Take into account the errors inherent in your software, printer, wood thickness, and the average persons building skills and most of this science is out the window. And if you’re building an open structure balsa wing forget it, you have 10 different airfoils for every few inches of wingspan.

Copy the airfoil of another aircraft whose flight characteristics you like, and get to the building board. Or look at a bunch together and try to draw your own, incorporating the things you like most about each one. In my opinion a good eye and a French curve will produce just as good an airfoil as anything else.

[Dsegal]

When modelers worry about choosing airfoil types I remind myself that Maynard Hill set the world's RC model endurance record using a "French curve" airfoil he whipped up in about 30 seconds. BTW, the flight was for over 30 hours!

Dave Segal

[Bill Glover]

Yes, don't worry about it too much. Copying a section from a kit or plan that is similar in style & performance to what you want will work well. Remember even 'flat plate' wings work, so anything with a bit of a section to it has got to be better than that

For my own-design models (all sports/aerobatic type) I've always stuck to the simple NACA 4-digit series. You can get these from any of the rib plotting packages, or as the formulae are relatively simple, even calculate the sections yourself (I wrote a Lotus spreadsheet to do this, many years ago).

[Bare]

Perhaps you should consider a simple foolproof Clark Y airfoil.
I'm led to believe that Keith Shaw uses that profile almost exclusively.. with apparently significant effect I might add.

[Joe Yap]

I agree with Bare. Besides, it's the simplest wing to built.

[RookieOne]

Clark Y for a sailplane? I though undercambered airfoils were better.

I'm already using ClarK Y on trainers.

Anyway, thanks to all for your suggests.

[Joel K. Scholz]

I am now starting my 23 rd scratch built and have used this foil on the last two. It is called a JT-88. This foil was suggested to me by a long time modeling friend. The planes I have used it on, a Rearwin speedster and an STOL biplane of my own design, each weight over 60 oz. they will get off the ground in less than 5 feet with a 10 mph head wind. I have not been able to stall either plane but each is highly aerobatic with the exception that they will not fly upside down, they right themselves no matter how hard you try to stay inverted. Theywill not do an outside loop. It would be a great foil for a trainer type plane.These planes both utilize a trinty car motor and astro gearbox on 9 cells.

[Joe Yap]

Hi Rookieone,

Undercamber airfoils produce more lift when flying upright. It's usually used on gliders which are seldom flown inverted. Clark-Y does tolerate a little inverted although not quite good at it. Semi-symmetrical airfoils like the Eppler 374 tolerates inverted flight better and I used that for aerobatic gliders. If you're talking about WWII warbirds, for gentler flight, Clark-Y is good, if you have enough power, maybe a semi-symmetrical one will be better for aerobatics. BTW covering an undercambered wing is quite tricky as the covering film tends to stretch across the TE and the thickess part of the wing for the underside which sometime turn it into a flat-bottom airfoil. You'll probably need some kind of jig to build a wing with anything other than flat-bottom airfoils.

Hi Joel, that's a HUGH airfoil!!!!! Is that for your full-size microlight????

[Joel K. Scholz]

Sorry Joe,
Looks like I sized it wrong.

[martin richards]

I build badly so prefer flat bottom as they are easier to set up on the board. Anyway, with traditiona balsa and film/tissue covering the section's never the same at the ribs as between them.

[RookieOne]

Joel,

That’s a very thick airfoil (and big picture too!!) Thanks.
From my little knowledge, this will require lot of power to keep flying but it will give lot of lift. Correct?

Joe, I’ve seen the eppler 374. Even if semi-symmetrical airfoils are designed for aerobatics, the thickness gives more lift. Isn’t it ??

Both are nice. I’ll test JT-88 with a J-3 Cub and the E374 with a warbird (B-25 maybe).

[Joel K. Scholz]

I will say that this foil does give a tremendous amount of lift. It is probably true that it's thickness will add to the drag. I don't know how much power is needed to fly it. The two planes I am using it on are around 550 sq. in. I use a Trinity Monster max wild wind with astro 2.38 gearbox on 9 cells. Depending on prop used this gives 40 t0 48 oz thrust at 25 to 30 amps. Both of my planes are over 60 OZ, yet get airborn in less than 15 feet with no headwind.If you are worried about power here is another foil to try. It is low drag.