I am trying to choose an airfoil for a sport model. This is pretty difficult without know some specifics about the model, but design is an iterative process so I need to start somewhere... What are some of the more popular sport airfoils and where can I find CL vs. angle of attack plots (also CM vs. angle of attack)? I have polars on some of the "typical" NACA airfoils and the 2415 looks reasonable. Any comments?

Andy Lennon has a great book on Designing RC airplanes. It is laid out so either an engineer or a layperson can read it and glean the knowledge they seek to design a good to great flying airplane. It has a great section of how to select an airfoil. I think Tower carries it. There are books that deal with just airfoils available. You can check at your local library. When I lived in Tulsa, Ok. the public library had a copy. I was teaching at Spartan at the time, and one of the other instructors liked to use it to design his planes. He would show up with some weird design and everyone would say, "it wont even get off the ground" but they would fly, and fly great.

Glad you found the airfoil that will give the performance you want,

Merry Christmas,
Soj

I also have a BS in Aero Engrg, as well as many years of modeling design and flying. If you have found published airfoil data at the appropriate reynolds number, I would be suprised. Considering the extremely low RN at which models operate, extrapolation from the published data is highly suspect. Another practical consideration is how accurately the typical construction methods reproduce the selected airfoil surface. A foam core wing can hold the shape OK but is only as accurate as the cutting process, and a built up wing can be drastically off due to sags even though the ribs are accurately cut. With these and other such considerations of trying to apply full scale data to model design, I've found that based on sound aerodynamic principles, the TLAR approach can yield very good results. I have recently completed two models using this intuitive approach, and the flying is exceptional. One has a high wing with a rectangular planform that has a setup similar to a "stick", but with a cabin, spinner and wheelpants, it dosen't look like a stick. The other is a low wing with a tapered planform and real outboard ailerons. If you just enjoy doing all the calculations, by all means have at it, but the TLAR method is a perfectly good design tool.

Unless you're getting into sizes like 1/4 scales, TLAR works as well as anything.
Your 2415 or the 2412 will do everything you want for a sport plane.

You guys gotta be kidding me! I suppose the Navy's using TLAR's on all of it's unmanned aircraft too? I can agree that it is difficult to get airfoil data in the appropriate reynolds number ranges, but it is possible. And yes, the construction method will dictate how successful you are at reproducing the shape, but you as engineers should know as well as anyone that garbage in equals garbage out.

Just a little more information I dug up....

RE can be calculated using the following:
68459*V*c
my Re is 2,200,000 ...

I went to:
http://www.nasg.com

and used to following:
http://www.nasg.com/afdb/show-polar-e.phtml?id=869

Definitely not kidding you. Many years with the flight test department of a major full scale manufacturer, and the many suprises I encountered merely illustrate the fact that even with the resources available there , airplane design is not nearly the exact science some think. Just a couple of instances come to mind, significant errors in engineers range calculations for one of the early jumbos due to attempts to extrapolate wind tunnel data to reynolds numbers where empirical data did not yet exist, and flying around for several months with 1400 pounds of lead pigs in the forward area of the cargo compartment until the envelope could be expanded as the airplane was quite tail heavy. ( a subsequent stretched model was made with a much longer nose to correct). To a greater extent than the layman suspects (or the engineer will admit) there is a bit of TLAR in much full scale design. The two aircraft I mentioned in my earlier post further illustrate my approach and are excellent fliers. If you are trying to set new world records (i.e.Burt Rutan) get as deep into the science as you like, but if you are planning a sport fun machine, it's a lot of effort for little gain.

<blockquote>Quote
<hr>Originally posted by: ksechler
You guys gotta be kidding me! I suppose the Navy's using TLAR's on all of it's unmanned aircraft too? I can agree that it is difficult to get airfoil data in the appropriate reynolds number ranges, but it is possible. And yes, the construction method will dictate how successful you are at reproducing the shape, but you as engineers should know as well as anyone that garbage in equals garbage out.<hr></blockquote>
.
Sport planes as specified in the original toss-up do not perform in the Re areas where the typical UAV does.
When looking at the chaotic distribution of Cl and Cd at typical model Res, a rounded front flat plate with a tapered trailing edge will do the job.
When the design requirements get specific, THEN the airfoil choice can affect the performance; but for flying that way for 15 seconds, do a 180, return... it machs very little nichts as to the wing's profile.
"..reproducing the shape."
Fabricating a sport model in a CNC generated female mold with a fully composite layup wing construction isn't only gilding a sow's ear, it's a pretty good waste of time.
.

Thanks for the reply. I think people were missing the point. I'm not trying to calculate max turn rates or speeds or anything requiring very precise data. What I was looking for was some data for existing airfoils so that I could choose one and not end up tracing my shoe. I chose a Selig airfoil for the reasons you sited.
I didn't do any real sofisticated calculations I merely selected an airfoil that was sufficiently thick to let me build a strong wing, looked about right (semi-symetrical) and had a reasonable polar.

<blockquote>Quote
<hr>Originally posted by: ksechler
I didn't do any real sofisticated calculations I merely selected an airfoil that was sufficiently thick to let me build a strong wing, looked about right (semi-symetrical) and had a reasonable polar.<hr></blockquote>


And he ends up with... TLAR. [img]i/expressions/face-icon-small-wink.gif[/img]

Oh crap.... What a s.a.

I guess you got me to one extent. In my own defense I didn't want to TLAR the actual airfoil. I wanted a proven section so at least had some idea of what I was going to end up with....

I hate getting burned with my own words....

I just couldn't resist after all the complaining about TLAR... [img]i/expressions/face-icon-small-wink.gif[/img]

Oh man!! I enjoyed those posts to the limit!!! [img]i/expressions/face-icon-small-smile.gif[/img][img]i/expressions/face-icon-small-smile.gif[/img][img]i/expressions/face-icon-small-smile.gif[/img]

Science brings a lot to any discipline, but...having TLARd a fair number of planes....I can attest to the fact that they quite often work very well.

I can also attest to the difficulty of transferring the data on a computer screen to REAL modeling materials. We start out with "X" airfoil...and after assembly, and sanding...and covering....what is it then?

Unless we're using precisely sized, dimensionally stable materials and sophisticated manufacturing processes, I feel the end result is a mere interpretation of the original airfoil.

I've seen folks at the field with horribly warped surfaces....SQUARE (you read that correctly: SQUARE) leading edges...entire sections of covering vanishing in flight, and the stuff still flies halfway decent.

Go figure.

Do your best work, and when you can say "That looks about right"......You're golden.

'Nite.