Airfoil Selection...

[Arbo]

How do you guys go about selecting the proper airfoil for a design? There are so many it seems it would take forever to go through most of them and find the right one (in prolifi, which I just registered)...

Any secrets? Good sites that show what kind is good for high speed/ floaters, massive lift, etc?

Paul

[Sparky Paul]

I look at what has worked in the past for me, or for similar designs.
All airfoil wheels have been invented.
The bad ones get zero notice, the others show up in the listings, such as Profili or UIUC.

[Arbo]

I figured as much... the problem I was having is prolifi has 2000+ airfoils... loads to weed through...

Paul

[vintage1]

Unless you are looking for ultimate performance there are only two basic airfolils you need to know,

(i) draw round edge of shoee, and make it flat bottomed. This does well with most planes that only fly one way up. Its generally called a 'Clark Y' after a well known UK brand of shoes... :D


(ii) draw round edge of shoe, then trace and invert for symmetry. This does very well on models that fly both ways up.


You can find lots of examples of all these shoes in Profili, but the brand names escape me. Probably dutch clogs or sandals or something.

Don't be tempted to use a wellington boot unless the plane is designed to take off from water.

There is a third more advanced type of airfoil

(iii) take sheet of balsa, and plane and sand it till it looks like what most other wings look like.

This is advanced stuff tho, and requires use of 'bionic eyeballs'.

Unless you ae pylon racing or competition sailplaining, this is all you need to know.
:D

[Frank B]

Another thing to be aware of, is the thickness of the airfoil. Use thin airfoils for speed, and thicker airfoils for slower flying and lower stall speed.

[vintage1]

Thats right. Use more of a smaller shoe for slow flyers! Obvious innit?

:D

[victor hull]

It's what we used to call Zip, Zip. If it looks good, I't will fly.

[steve wenban]

here's the site I use
http://www.aae.uiuc.edu/m-selig/ads/coord_database.html

[Andy W]

My sailplane buddy tells me about the huge arguments that go on in a sailplane mailing list about airfoils. Fact of the matter is - airplane in lift goes up, airplane in sink goes down. :)
..a

[Frank B]

But you still need some general guidelines for selecting a reasonable airfoil. You don't use a 16% symetrical airfoil for a thermal glider.

[flieslikeabeagle]

It seems there might be a third type of airfoil: Draw along the edge of a small shoe, remove shoe, place a larger, longer shoe just below the first line, draw along the edge of it. :). Draw along the upper edge of the big shoe and you get an undercambered airfoil, named after the Undercamber brand of shoes. Draw along the lower edge of the big shoe and you get a semi-symmetrical airfoil, named after, uhhh, the Semisym shoe company.

A while ago I built a Graupner Mini Piper. This plane got some very good reviews, and the writers of those reviews attributed some of the flight performance to the use of the Wortmann airfoil, which is based on the first two-shoe method described above.

I seem to recall hearing similar things about the Wingo a while ago.

Is this just journalistic hogwash, meaning that airfoil choice for models is quite uncritical, or is this simply because most modellers don't care too much about efficiency and go by the maxim "if it flies, it's good enough"??

-Flieslikeabeagle

[Sparky Paul]

Beag, there's really not much difference in performance between a Florsheim, Thom McAn, Nike, K-Swiss wing shape.
But compare the Nike to a Selig or a Heppler airfoil say ,which has been properly selected for the task, flown by a good pilot,and the performance difference will be noticeable.
For the "fly that way, yank and bank, fly this way, yank and bank" any shape will do.
For the "my plane can fly higher, longer, faster than yours", finessing the parameters beginning with the airfoil -does- make a difference.

[BMatthews]

It's what we used to call Zip, Zip. If it looks good, I't will fly.

But this technique presupposes that you are wordly enough to know what looks "right" in the first place.

For built up construction it really doesn't pay to get too exotic. Look for zip-zip guidance from such all time favourites as the Clark Y (not the Oxford type :D) NACA 2415, NACA 0015, RAF30, Selig 3021, Selig 4233, Selig 8022 and other similar simple shapes.

With the advent of Xfoil many folks are designing airfoils with simple and practical shapes that work well if not perfectly. Mark Drela's AG35 from the Allegro Lite is a case in point. Made to work with real world balsa construction and to minimise the covering sag problems this is one that is on my "must do soon" list. The HN series of airfoils in Profili2 is another series that looks like it would be good for this as well.

In the end thickness and camber are the two key elements that determine how the airfoil will fly and why you choose to use one over the other.

[DLC]

What kind of plane do you want? If you want a high lift floater, look at under-cambered airfoils. If you want an aerobatic plane, look at symmetrical airfoils, and if you want a general purpose flyer, look at Clark Y type airfoils. Profili lets you pick an airfoil and then identifies other similar airfoils. Since my flying skills and reflexes are not the greatest, one thing I always look for is benign stall characteristics. Based on my experience, Profili does a pretty good job of predicting stall characteristics.

[rcjetpilot]

Hi Arbo,

EDF?? I'll assume you looking for something in "jet like" performance.

In any event, I'll offer my 2cents on airfoils.

Avoid airfoils with very pointed leading edges as they are designed for high speed and low angles of attack.

The percent thickness relative to cord is important, and is refered to as thickness. I'd go no less then 8% and no more than 15% As you go thinner, you increase landing speeds, and have reduced drag so "slowing down" could take some time:)

If the wing has a fair amount of tapper ( tip less then 1/2root) you'll need to build in some washout OR... use a variable percent thickness over the span. For example, you could use a 9% at the root and 12% at the tip. This creates a mild amount of washout without the need to build in a twist. This is what I do.

Flat bottoms are ok, however Id suggest something with 10 to 30 percent camber position. The more camber you add, the more stable the slow flight handling.

As for the selection of airfoil, I use NACA 6 digit types. (63A series)

Bob

[Arbo]

Not looking for anything in particular.. but I have a few ideas I've started to sketch out and got to thinking about wings.

Thanks for all the info/input everyone

[vintage1]

The long and the short of it is, mostly, its not that critical, unless you are flying competition.

In partuicular very low rates of sink and or very high speeds.

All airfoils are compromises between low and medium speed operation, and rate of airflow breakaway near the stalled condition. The more perfect an airfoil is for a given condition, the less perfect it is likely to be for another.

An ultra efficient airfoil that gives amazing glide characteristsics but won't pick up speed in a shallow dive is not as useful for a sailplane as a less efficient one that will speed up and allow thermal hunting.

And a wing that tips stalls viciously is a pain to fly as well.

[Highflight]

I never thought about airfoil selection myself until I got into designing my own aircraft (with CAD). Then I saw that there were somewhere around a zillion different available so I started to pay attention.

Without going through the entire thought process I did, what it all came down to for me were some rather simple choices for whatever I wanted to design. And those choices were:

Start by taking a look at whatever airfoil the full size aircraft used and consider using that. However, only "consider" using it because we have to deal with Reynolds numbers and a lot of times, the airfoil used on the full size aircraft would be a terrible airfoil for the model. At the same time, if the full sized aircraft used a Clark Y or something already common to R/C use, just do it.

If I choose not to use the full size airfoil (which is usually the case), I then decide how I want the model to perform, and I have narrowed down a short list of airfoils that I use pretty much exclusively for my intended projects.
Keep in mind that it's almost impossible to accurately determine what airfoil you're looking at on a finished aircraft, so even in scale projects, it's not terribly critical which one you choose for the purpose of "proper" scale.
That gives you a lot of latitude for choosing an airfoil for reasons of intended performance rather than for looks.

The short list of airfoils I use have been chosen for overall appearance, but more importantly for the way they perform with the particular design I use them for. The short list is:

1. Clark Y - For ANY "just go flying" kind of model that I want to be a gentle flyer with at least "some" airbatic capabilities. Note that contrary to what most people think, the Clark Y is NOT a flat bottom airfoil. When I use the Clark Y, I usually use just flat "airfoils" for the empennage.

2. NACA-0012 and NACA-0015 would both be a good choice for all-out airbatic flying being that they are both fully symmetrical. These two airfoils are essentially the same except that the 0015 is thicker which would allow slower flying. They are a BAD choice for most Scale models and should be limited to models designed for pattern or airbatic flying.

3. NACA-0009 is a "single use" airfoil and is used for the horizontal stab AND the vertical stab of any aircraft where I want an airfoil empennage rather than just flat. Don't ever use if for the wing. I almost always use the 0009 in my designs without even thinking about it.

4. My "Scale" favorite is the NACA-2415 for virtually ANY aircraft that I want to fly scale, and look scale too. It's thick enough to allow scale looking flight speeds for almost any aircraft and generates a ton of lift. If I were to design and build a large bomber of some sort, I might even consider the NACA-2418 which would allow slower flight than the NACA-2415.

In all the above cases, be aware that dihedral, proportion of airfoil surface areas, CG, design incidence, and washout all come into play and can drastically affect the performance of ANY airfoil.
Whether designing and drawing a new design or doing a fresh design of an existing aircraft, you NEED to know all of these parameters before you even start.

I know that other people may have their favorites as well, but life sure got simpler for me after I settled on just those airfoils noted above.

Note that I'm not into gliders, and they're a different animal altogether. However, if I were, I would probably start exploring the various Selig's as a place to start.

Highflight

[AndyKunz]

I've fallen in love with the S3016 for my S-400 planes. Easy to build (yes, slightly more than a flat bottom but not enough to worry about), not as power sensitive as flat bottoms, thin enough to let them zip, and easy to fly inverted.

Andy