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!

.................:confused:

[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???? :p

[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.

[Joe Yap]

Yes Rookieone,

Thicker airfoil does produce more lift. To be fair, if you compare E374 and Clark-Y with same thickness, the Clark-Y will produce more lift.
Thicker airfoil does produce more drag but not necessary requires more power to fly the plane. You can simply play around with M-dot(mass flow) and V-dot (velocity). Higher mass flow means higher static thrust, and higher velocity means higher top speed. If you have thicker airfoil (higher drag), change the prop to a larger diameter and less pitch to overcome it (more mass flow),or change the gear ratio. If you have thin airfoil (less drag), use a small prop with higer pitch to achieve more velocity as thin airfoil needs to fly fast to create lift. The good examples are WW1 Biplanes/triplanes and pylon racers driven by a Speed 400.
There aren't much airfoil that can totally satisfy all flight envelope and that why most aircraft uses flaps to change its airfoil(camber).You can have a ultra thin airfoil to fly supersonic and use flaps to increase camber for TO/landing.

[EZ-Bee]

I like Paul's answer.

You see the 205 used a lot in both e-power an sailplanes, it was in its time a moderately agressive choice for a foil with a bit of penetration.

I saw an article once that plotted the history of hot sailplane aerofoils.

Over time camber has lowered along with thickness ( apart from a series of very thick aerofoils to survive Gorrilla Winches ).

Hand Launch movement bought back more camber for a while , but thin caught on here too.

Thin foils give more penetration, it was found better to be able to cover lots of ground efficently to find the next thermal.

Thin foils do put the emphais on designing good structures and perhaps exotic materials.

Older foils like 205 give not too thin and hard a structre to build but still a bit of penetration. They may also play nicer.

Indeed the Clark Y may have a good following for its ability to Play nice and support strong easy to build structure.

GYROGEARLOOSE idea is a great one, find a plane that flies and behaves the way you would like yours to ( Fits your Flying Style ), use that areofoil with similar wing loading.

Copy something that works:

For the Right Aerofoil, Copy the Left :-)

Always worked for me .....

Peter

[fregon45]

I propose we change the name of this board to "Modeling Technology" since hardly anyone wants to discuss the real science of flight. One thing that I always disliked about the Mag "Popular Science" is that it never discusses anything scientific. It's just a review of current gadgets and technology.

Anyway, now that I'm done with my rant I'll go on to say that all of the opinions here on copying other designs is really the best advice. It will get you to where you want to go the quickest and with the least amount of headache. Let's not discourage anyone from actually learning something about the theory of flight, though.

Nathan

***Flame shield engaged****

[markrduckworth]

What is the definition of a Clark-Y airfoil? From one's I've seen that claim to be Clark-Y, it looks like flat bottom from spar to trailing edge, but curved up to the leading edge. Is there a more precise definition?

I have plans for a biplane with symmetrical airfoil wings. But I was thinking of changing it to something else that would fly better with less power. I would intend to spend much less time inverted than not. It sounds like the Eppler 374 would be good?

Mark.

[EZ-Bee]

Will the real Clark Y stand up !

There is probably a lot of 'Clarks' that have been thinned and otherwise manipulated.

There is also the age old system of using the edge of your shoe for the main curve and filling in the rest by memory.

An excellent site to see all the Aerofoils you could ever want is:

http://amber.aae.uiuc.edu/~m-selig/ads.html

In the Aerofoils section is a list of more than 1000, with .dat files of co-ordinates to import into your Aerofoil Plotting progam.

There are also .gif file images so you can preview and see what they look like.

Links to free analysis software are also provide if you want to get into the technical suff and see the physical efffect you aerofoil has in the air around it.

Picture of Clark Y according to UIUC attached.

Peter...

[RookieOne]

Would you use a thinned Clark-Y on a sailplane?

[EZ-Bee]

The Clark Y is better suited for the B25 .

Something like the SD7037 , may fit the bill for a glider.

Floaty type, that always flies right side up.

Wing loading really effects the Floaty bit.

Peter...

[Joe Yap]

IMHO, it doesn't mean Clark-Y is useless on sailplanes whether altered or not. You mentioned sailplane but did not say what type. It all depends on the condition of the slope lift, if it's slope soarer and the flying characteristic desired. If you are flying over small slope where lift is concentrated in a small area, a standard Clark-Y should work well since it tends to fly slower and can stay in the same lift zone longer. If the place you are flying has lift zones all over, you'll need a newer or thinner airfoil to move around and hunt for the lift. Standard Clark-Y can also tolerate higher wing loading. My DAW TG-3 uses a altered Clark-Y which flies very well on small slope ( 2m high and about 10m wide ) since it can stay around it and stay in the small lift zone.

[Joe Yap]

This a pic of my DAW TG-3.

[Humbug]

Hi Guys,

I'm just starting in r/c and an interested rookie in aerodynamics. I just had found Martin Hepperle's great site for airfoils and modeling aerodynamics (link below). His page which discusses the effects of rib and spar construction on wing aerodynamics was quite interesting and explained Paul's point above that a wing like this has "10 different airfoils for every few inches of wingspan"; i.e., because of "droop" of the covering between the ribs along the span.


http://beadec1.ea.bs.dlr.de/Airfoils/theory.htm

This site should also satisfy Nathan's desire for a little theory! :)

Steve

[Sparky Paul]

Here's four common and good shapes..
Clark-Y
Eppler 205
NACA 2411
Selig 2032
Within usual construction standards the shapes are the same.

[RookieOne]

All in one wing!

[Spam]

Originally posted by Humbug
Hi Guys,

I'm just starting in r/c and an interested rookie in aerodynamics. I just had found Martin Hepperle's great site for airfoils and modeling aerodynamics (link below). His page which discusses the effects of rib and spar construction on wing aerodynamics was quite interesting and explained Paul's point above that a wing like this has "10 different airfoils for every few inches of wingspan"; i.e., because of "droop" of the covering between the ribs along the span.
Steve

That's a small part of why I like building foam wings! There are other advantages to foam: you can build them as cheap or as nice as you want. Most of my prototypes are tape covered, while finished birds are either glassed or sheeted with balsa. Once you build your templates for cutting the wing, you can make new wing cores in a matter of minutes.

re: selection of glider airfoil: Everybody here has posted good advice: it's just a matter of how technical you want to get. I agree on the SD7037 airfoil for a thermal-hunter, good lift, low drag and a good speed-range at light wing loadings. I used Clark-Y airfoils almost exclusively at first, and was always very happy with them on a powered plane. The only reason I don't use them much anymore is I like to fly inverted. You can fly inverted with a clark-Y, just not very well. For fully symmetrical airfoils, the NACA series are great, you can adjust the thickness to your flying style.

The one thing that nobody mentioned is the effect of thickness on AOA (Angle of Attack). A thinner airfoil will stall at a lower AOA than a thicker one. (take a look at a gas-fun-fly type plane, or a 3-d plane--very thick airfoils). There is always a tradeoff, though: the thicker an airfoil, the higher the coefficient of friction. Since friction is a function of the speed squared (If I remember my physics right), you can have a huge speed difference between the same airfoil with differing thicknesses...

Best of luck

[Joe Yap]

I have also use the S3021 and S4083 on my gliders and have good results. However, you don't need to go crazy about choosing airfoil since theory and practical are sometime quite far apart. What I mean is that it also depends on your worksmanship. Sometime, a stroke of the sanding block can effectively change the airfoil. If you shrink the bottom covering of an undercamber wing too tight, you may ended up a flat bottomed wing. It's quite difficult to produce a 100% accurate wing. Choose one which has the desired characteristic and within your building capability.

[EZ-Bee]

Joe....

The 2m high slope... Interesting...

In Auckland if you want to find the slope flyers you go to the top of one of our many 'Extinct ?' volcanic cones.
Though they can lack a nice flat landing spot.

Lucky hardly anyone looks up otherwise it would be regulated out of existance.

Apart from one , I think you have to travel some distance for flat feild flying.

Where are the flying spots in Singapore ?

No problems with fines ?

Peter

[Joe Yap]

Hello Peter,

Right here in Singapore, there's only one popular slope at Bedok reservoir which is shown in the above picture. However, slope flying there is only possible during the NorthEast monsoon season during December to March. Around June to September, we have the SouthWest monsoon wind and I sometime fly my TG-3 over the 2m high( Yes! Only 2m ) slope on an eroded part of the East Coast beach. It's not a legal flying site since it's quite close to the International Airport ,but I'm pushing my luck since I restrict my flying to tree top height and fly within a 50m zone. No trouble so far. You can see why I chose the Clark-Y wing TG-3 for the job.It's like a "park soarer".I got this idea after reading a Dave Garwood's article titled "Small hills, big air" from the March 2000 issue of the MAN. The best time to fly there is when the tide is low.

No fines so far.

[kbosak]

In Poland we use the CRD family of profiles which contributes significantly to local development of aviation activity.
Their numbering vary, but the acronym could be directly translated into WHG - "Whatever Hand Gave" ('Co Ręka Dała') ;)

[Bill Glover]

You know this thread is more than 5 years old?!